Bulletin No. 237 Series j ft ?e?criPtlv« Gwl°^ 43 , 90

\ D, Petrography and Mineralogy, 29

DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIRECTOR

PETROGRAPHY AND GEOLOGY

OF THE

IGNEOUS ROCKS

OF THE

HIGHWOOD MOUNTAINS, MONTANA

BY

LOUIS VALENTINE PIRSSO1ST

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1905

CONTENTS.

Page.

Letter of transmittal. .......................^............................ 11

Introduction__._.._._.____..--.-----_--.------..--.---...---- .-.------ 13

Bibliography ___.._______. __.__._____..---.._..-------.--.-..----..-..- 14

Chapter I. Geography and history ........----.-....-..--...-.----..----- 14

Location ..__._.._.._____..---..._------- --------------------. 14

Bibliography .. ...-.-...-..----.-_.._. ...... ....................:...... 15

Topography and geography .......................:.-.------.------.- 15

History................................................................. 16

Chapter II. Geology of the igneous stocks ................................ 20

Introductory ................................................... ------ 20

Highwood Peak stock....-.-.-..........-._.....--...-.---_.-.--._... 20

Character of the border contact.-..-.-.-..-..-.--...-----..-.--.- 22

Middle Peak stock.................................................. 23

Contact phenomena. -..---.......---------. -..-..-..--------.-.-- 23

Endomorphic contact phenomena ..........^..................... 24

East Peak stock....................................................... 24

Shonkin stock ...................... ....-'---.--...--".-...-.---.-.---.- 26

Occurrence of missourite.______.--.,._._.._--_.- --_-------- 27

Arnoux stock........................................................ 28

Chapter III. Geology of the sheets, dikes, and extrusive rocks............. 30

Intrusive sheets.............--.-........-.----.-.-...-....--...-.---_. 30

Dikes.... ..-....._.____..-_.....__..._..-_. . .....-.-- _....... 31

Introductory ._____._.._.:_________. ...____'_._______..____.____ 31

Radial disposition .................^........:..................... 31

Character .-.................................................-.. 32

Rocks composing the dikes ....................................... 34

Light-colored f eldspathic (salic) dikes............................ 34

Dark basaltic (femic) dikes ......... ..............--...----...., -35

Relative age of the dikes .......................................... 35

Extrusive flows and breccias ...........'............................... 36

Areal distribution ...... ........ .................................. 36

Feldspathic extrusives of the first period.......................... 37

Basaltic extrusives of the second period........................... 38

Distribution of basaltic extrusives. ........................... 39

Sources...............................-......................_... 39

Chapter IV. Geology of the laccoliths .................................... 42

Introductory ............ ......................................... .... 42

Minor laccolith of the Shonkin Sag .._..-..-.............-..-.......... 43

Major laccolith of the Shonkin Sag ................................... 43

Ends of the laccolith wall.............................. ....... 44

The laccolith rock............................................ 44

Interior of the laccolith ...................................... 45

Causes of dissection.......... .... ............................. 46

Internal structure .............................................. 46

3
4 CONTENTS.

Chapter IV. Geology of the laccoliths Continued. Page.

Palisade Butte.... .........................................r ......... 48

Rock variation .............................................. 48

Laccolithic character ........................................ 49

Square Butte --.-----.I--..--..--.....................-....-.---. ...... 49

Introductory.........._................................---... 49

General description__________________________________________ 49

Laccolithic origin . ._____________._.._____.._.__._._._.__.._.__ 50

Lower zone of dark hoodoos.. .....................r.......... 50

Upper zone of white rock .................................... 51

. Origin of the platy parting................................... 52

Diagrammatic section.......-.-....---...-..--.........-. ..v 53

Chapter V. The sedimentary platform...-......--.....--.-..---.-.---.... 55

Introductory _____..__.._._._____._..._.____.___.___.___.____ 55

Cascade formation ______________.________ .______..____.._.... 55

Dakota formation............................................ 55

Colorado formation .......................................... 55

Eagle formation .................... ......................... 56

Montana formation ._-.......-.....---... --.---.-.--.-.-.---- 56

Summary. ---------................-.--..---.....---...-.-... 56

Chapter VI. Petrography ................................................. 57

Introduction........................................................... 57

Summary of Lindgren's work ....._........--...----...-..... 57

Previous work of the author. ................................. 58

Classification ........................................................ 59

Stocks and laccoliths.-.---.................-.....-.............-----. 60

Grano-pulaskose (syenite var. pulaskite) of Highwood Peak ...... 60

Megascopic characters -.---..--..---...---.--.-... .--..-..... 60

Microscopic characters.-------------- ..-..---.'-.---.------.-. 61

Chemical composition........................................ 63

Mineral composition or mode..-....------...:..---..---..-.-. 64

Classification in the new system............................... 64

Classification in prevailing systems...----....-.-.---.--...... 65

Pulaskose (sodalite-syenite) of Square Butte......-----.-..--.-.-. 66

Introductory ................................................ 66

Megascopic characters ...........................!........... 66

Texture ..................................................... 66

Microscopic characters. -....--..-....--.--..-.....-......-..... 66

Mineral composition or mode.._._._---.--------.----....---.-- 67

Chemical composition........................................ 68

Classification in prevailing systems.----..-----.--.------.. .. 69

Classification in the new system ^............................. 69

Comparison with related types -......-....-.-...--.---.-...... 70

Tracho-highwoodose (nosean-syenite) ....----.-....-----...-.---- 71

Occurrence ____.....__.._...-.__.__......_..__.__.___.._.____ 71

Megascopic characters ...........................'............ 71

Microscopic characters..-----.-------.---.--.-.-----.-.- ... 71

Chemical composition ----...--.-..-----..-..----..----------. 72

Classification in the new system......-..,........-..--..----.. 75

Classification in prevailing systems..-.--..--.--...-.-.----..- 76

Grano-shoshonose .(monzonite) of Highwood Peak .__..___.______. 76

Introductory ................................................. 76

Megascopic characters ---.-..---....---..------.--..-.-..-..- 76
CONTENTS. 5

Chapter VI. Petrography Continued. Page. 

Stocks and laccoliths Contintied.

Grano-shoshonose (monzonite) of Highwood Peak Continued.

Microscopic characters. --- _-____._._.....__ _._____....__..... 77

Behavior with acids.......................................... 78

Chemical composition..._-_-._..-_-...._._..__.......-.__.... 78

Mineral composition or mode...-.......-.-.-..-.-.-..--.....- 80

Texture ....................................................... 80

Classification in the new system............................... 81

Classification in prevailing systems. ------.-..-.--..-.-------- 82

Dikelets..................................................... 82

Contact f acies .-.-..-.--..---..-.-....----.--...-...---.-..-- 83

Fergusose (fergusite) of Arnoux stock............................ 83

Occurrence.................................................. 83

Megascopic characters -.....--....-.......-...-..-....-- --.---- 83

Microscopic characters. --.-.-..-,--.-.-.-.---..--..-.-......- 84

Chemical composition..............--..-...-..'......----.-.-. 85

Texture ..................................................... 87

Classification in the new system -......-.-....-..-.-.-.-----.. 87

Mineral composition or mode................................. 88

Classification in prevailing systems. -------.--.--.......-.---- 88

Grano-borolanose (basic syenite, shonkinitic type) of Middle Peak. 89

Megascopic characters -.-....-.-..--.-..-.......-----.--.--.. 89

Microscopic characters---....-----.----...---_-.-..---.....-- 89

Chemical composition..........-....-.--.-..'..-..-....--.--.- 91

Mineral composition or mode........................... ...... 92

Classification in the new system.............................. 93

Classification in prevailing systems --------------------------- 94

Border f acies ...............----.. .---...-----.----.-.-.--.-- 94

Borolanose (syenite) of Palisade Butte .,.........!................ 95

Borolanose (syenite) of Shonkin Sag laccolith-.____..__-...-....... 96

Shonkinose (shonkinite) of Square Butte -.....-.-------_.-----..-. 97

Megascopic characters -..--.-...---.-----.----.----.--------. 97

Microscopic characters..... .-..-----..-----..-.....--- -.----. 98

Chemical composition..........-.-....-...-.-..-....-....-.-. 102

Mineral composition or mode...--.--.---......-.-..-.-..----.- 103

Classification in the new system.....................---...... 104

Shonkinose of other Highwood localities.....-..--_.-..-.------.-- 105

Leucite-shonkinose (leucite-shonkinite) of East Peak.............. 105

Introductory.............-.--.------.--...-.--.-.-..-----..-. 105

Megascopic characters-----.--.--..-.-------.----.--.-------- 105

Microscopic characters. ....................................... 106

Chemical composition of white components................... 107

Occurrence of analcite -.-..-...---.----...-.........---.- .--. 107

Chemical composition of the rock............................. 108

Mineral composition or mode.------_.---.--..---......---.--- 110

Classification in the new system .............................. 110

Classification in prevailing systems...._--.. .-_-._----.---.--- Ill

Montanose (shonkinite) of Shonkin Sag laccolith.........---...-.. Ill

Introductory- .---.--..-.....----.-.-.---.-..--.--.-------.---- Ill

Megascopic and microscopic characters...----,--.-----------. 112

Chemical composition........................................ 112

Classification in the new system.............................. 114
6 CONTENTS.

Chapter VI. Petrography Continued. page. 

Stocks and laccoliths Continued.

Missourote (missourite) of the Shonkin stock...................... 115

Introductory .....____ ___.._.______.____._.._..______.___..___ 115

Megascopic characters ------------_----....__..___.____.__ ... 115

Microscopic characters. ---.--.---------...._......_._...-_..._.__ 116

Chemical composition......................................... 117

Mineral composition or mode......---..-..:.-.--....---------. 118

Classification in prevailing systems........................... 118

Classification in the new system.............................. 119

Intermediate rock types.-.-.-.--....--_-..-..--.--..----...... 120

Dikes and sheets.--_------__._.__.-_-__.--._.._.____._._.._.__.__-_.. 121

Introduction. ................................................... 121

Trachiphyro-pulaskose (sodalite-solvsbergite-porphyry)....... .... 121

Introductory................................................... 121

Megascopic characters.............---........-.-...---,.-... 122

Microscopic characters..................... ........':......... 122

Chemical composition. -..........--.-..-.....------.-.--..--. 122

Classification in the new system.............................. 128

Mineral composition or mode.--_-------..__-_-----------. -... 125

Classification in prevailing systems............................ 125

Trachiphyro-highwoodose (Highwood tinguaite-porphyry)......... 126

Introductory....--................--..--.......-.....-.-'.-- . 126

Megascopic characters --.---.-.'-.---.-.......-.........-...-. 126

Microscopic characters. ..:.....'.............................., 126

Chemical composition........................................ 127

Mineral composition or mode................................. 129

Classification in prevailing systems. ----..-.----.---.-.---.--. 129

Classification in the new system. .................;........... 129

Eocks of tinguoid habit (grorudite-tinguaite series)..-.......--.-. ' 130

Trachiphyro- monzonose (gauteite variety of bostonite) ........... 132

Occurrence.----.-...--.-.-.----.-...----...-.----.-.-----..- 132

Megascopic characters ......--..-..---.-.....-.--...---.-.--- 132

Microscopic characters. ........-.-..-..- .--..---.--.-----..... 132

. Mineral composition or mode.....-.-.-...---..--...'-......... 133

Chemical composition........................................ 133

Classification in prevailing systems. --.-.. ..................... 135

Classification in the new system..-.-.----.---...--..-.-.----- 135

Trachiphyro-borolanose (syenite-porphyry)....................... 136

Occurrence > --.--.-......---....---,-.-.-.--.--.-.--.-..------ 136

Megascopic characters _.---------.---.---..-..--.-. ..-------- 137

Microscopic characters........----.-.-.--....---..-.-----..--- 137

Chemical composition...._........................--.--.---..- 138

Mineral composition or mode..---.--..-.....----------.------ 140

Classification in prevailing systems..... ...-...-.---.--...--. 140

Classification in the new system.-.-.-------................i- 141

Phyro-biotite-cascadose (rninette of High wood type) .............. 142

Occurrence .---.--...----.. _...-..-.-.- .-------.....--.-.---. 142

Megascopic characters ................... ^....'............... 143

Microscopic characters. ....-........-.--,..-..-......-..----. 143

Chemical composition.. ..........-----....-...... ..........- 144

Mineral composition or mode. ........^....................... 146

Classification in prevailing systems........................... 147
CONTENTS. . T

Chapter VI. Petrography Continued.. Page. 

Dikes and sheets Continued.

Phyro-biotite-cascadose (minette of High wood type) Continued.

Classification in the new system.._____.___......'.-..--_.---.. 148

Texture and name -.__.._____.__...___...-...-.----.-----.-.- 149

Monchiquose (analcite-basalt)....--.-.-.-.........-....-.-..----. 149

Introductory......._.......___._._.___._.._...----..-..--.-.. 149

Megascopic characters --._._---.-------...--..-- -------.. .. 150

Microscopic characters.-... .................................. 150

Discussion of analcite ...........................---...--...... 151

Chemical composition.........-......-.:....-..-..--.----.-.. 155

Mineral composition or mode.--..--..---.-..--.---.--.-----.- 157

Classification in prevailing systems..-....-...-.-..---.-.--... 157

Classification in the new system ............................... 157

Extrusive flows, breccias, and tuffs....---..-.-.-.....----.----...-..-- 158

General petrographic description .....-..^................. ...... 158

Feldspathic lavas and tuffs -.....-..-...-.....--..-.........-. 158

Basaltic lavas................................................. 158

Amygdaloidal basaltic lavas. -................................ 159

Scoriaceous basaltic lavas ..........-.....-...--..-.--.-.--... 159

Basaltic tuffs and breccias......... ............................ 160

Trachiphyro-hornblende-adamellose (latite or trachyandesite)..... 160

Occurrence................................................... 160

Megascopic characters .---..-..--.........-.-.--.-...-....-.. 161

Microscopic characters..-__...._.._...____.___..--....._....._ 161

Varieties of the type -.-.--...-.................--.--......... 162

Mineral composition or mode. .........................^...... 162

Chemical composition......................................... 163

Texture ..................................................... 165

Classification in the new system ................................ 165

Classification in prevailing systems........................... 166

Phyro-shonkinose (analcite-leucite-basalt) ........................ 166

Occurrence ..--...........-..-...--...........-..-.--.--...... 166

Megascopic characters .......................................... 166

Microscopic characters....................................... 166

Chemical composition ._.________.____.....__'..__...___...._._ 167

Mineral composition or mode................................. 169

Classification in prevailing systems ..-..--.--.-...-.--.-...... 169

Classification in the new system ......,........--.....-.---..... 169

Chapter VII. General petrology of the Highwood region................. 171

Introduction............. ...... ...................................... 171

Chemical characters of Highwood magmas ........................... 171

Norms of Highwood rocks ...._......_._'...__.____.._.__-......_._' 175

Geologic occurrence of the different magmas --.--...-.-.-........-.-. 178

- Stocks and laccoliths............................................... 179

Relative volumes of the different magmas -..-...-...--.-..-..-...-.-. 180

Shonkin Sag laccolith ............................................. 180

Square Butte ......-..-..----....-.....:.. ....................... 181

Palisade Butte.................................................... 181

Differentiation in laccoliths .......................................... 181

Osmotic theory .................................................. 182

Theories of differentiation.. ........................................\ 183

Differentiation produced by crystallization.................. ..... 185
O - CONTENTS.

Chapter VII. General petrology of the Highwood region Contimied. Page. 

Differentiation in laccoliths Continued.

Electricity.................... .................................. . 187

Combined effect of convection and crystallization ..._..........'... 187

Differentiation in the stocks..............................--.-.-.....-. 190

Composition of the original magma -.---.....----.-.-..-.-....... 193

Differentiation and derivation of dikes.--.---.....-..--.---.--.---..-. 193

General differentiation of igneous rocks ............................... 195

Mathematical relations of magmas shown by graphic methods..... 197

Arrangement of volcanic centers _.........--...--------.-.-----...--.. 198

Age and order of succession of the igneous rocks -.----.-----.........- 199

Index......-.-.......................................'-...............--. 203
ILLUSTRATIONS.

Page. 

PLATE I. Topographic map of the Highwood Mountains .........-..-.. . 14

II. A, View northward from Highwood Gap, showing slopes of vol- ' 

canic debris; 13, Upper Davis Creek, mountains of volcanic flows 

and breccias -..---.-...-.--....--..-.-..-...--....-.-....-... . 16

III. Geologic map of the Highwood Mountains ...................... 20

IV. A, Columns of shonkinose, east side, of Palisade Butte; B, View

in the zone of erosion monoliths, Square Butte................ 48

V. A, Pulaskose (syenite) resting on shonkinose, southwest side of

Square Butte; J5, East end of Shonkin Sag laccolith........... 54

VI. Fergusite from Highwood Mountains._-...-._..__..__...------. 82

VII. Shonkinose of Square Butte .................................... 100

FIG. 1. Index map, showing location of Highwood Mountains............. 14

2. Locality of missourite............................................ 28

3. Dikes at Highwood Gap......................................... 36

4. Plan of Shonkin Sag laccolith .... 1... ............................ 44

5. Stereogram of Shonkin Sag laccolith.............................. 46

6. Cross section of Shonkin Sag laccolith ............................. 47

7. Former section through Square Butte ............................ 54

8. New section through Square Butte ............................... 54

' 9

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY,

Washington, D. C., May 11, 1904.

SIR: I have the honor to transmit herewith a manuscript entitled 

"Petrography and Geology of the Igneous Rocks of the Highwood 

Mountains, Montana," by L. V. Pirsson, and to recommend its publication as a bulletin of the Geological Survey.

This paper is a valuable discussion of one of the interesting centers 

of igneous rocks in a province already celebrated through the work of 

the author in conjunction with Mr. W. H. Weed. 

Very respectfully,

C. W. HAYES,

Geologist in Charge of Geology. 

Hon. CHARLES D. WALCOTT,

Director United States Geological Survey.

11

PETROGRAPHY AND GEOLOGY OF THE IGNEOUS ROCKS OF THE 

HIGHWOOD MOUNTAINS, MONTANA.

By L. V. PIRSSON.

INTRODUCTION.

In the following Avork will be found the results of field and laboratory studies of the igneous rocks of the High wood Mountains of Montana. The field work was carried out by Mr. Walter H. Weed and 

the writer, chiefly during the latter part of the summer of 1894, though 

the eastern part of the area was again revisited for a few days in 1896.

The north-central part of the area, around the SJionkin and Arnoux 

stocks, was not visited by the writer, as this part of the work was done 

by Mr. Weed after the writer was called from the field. As the field 

work in petrology was incidental to the areal mapping of the region 

on the base map of 4 miles to the inch for the Fort Benton folio of the 

United States Geological Survey, under charge of Mr. Weed, the time 

that could be devoted to a careful study of details was necessarily 

limited. Future studies of the district may therefore bring out minor 

features which were not seen or which are but briefly treated.

As the result of this work during recent years a number of papers 

dealing with features of especial interest in the area have been published by Mr. Weed and the writer, including a summary account of 

the geology of the district by Mr. Weed in the Forb Benton folio. A 

list of these papers will be found in the bibliography (p. 15). It was 

also our intention to prepare a complete memoir on the geology and 

petrography of the region, but pressure of work in other and more 

important directions prevented this, and finally the writer was 

intrusted with the task of preparing a report on the geology and 

petrography of the igneous rocks. Since, however, the Highwoods 

are a group of eroded volcanoes, rising through almost undisturbed 

Cretaceous strata, the main problems of geologic interest connected 

with them are necessarily of a petrologic character and are therefore 

treated in this report. In carrying out the work the writer is under 

great obligation to Mr. Weed, who has freely tendered not only the 

material collected but also his field notes, maps, and photographs, and 

who has made many valuabe suggestions concerning the geology. The 

value of the work is therefore in large measure due to him.

Thanks are also due to Dr. H. S. Washington, who kindly allowed 

the writer to use, in advance of publication, the results of his collected tables of analyses, which proved of service in comparing a

number of the types described with those of other regions.

13
CHAPTER I. 

GEOGRAPHY ANJ) HISTORY.

LOCATION.

The mountain group whose igneous rocks are described in this bulletin is one of the series of detached isolated areas which lie scattered 

about on the great plains of central Montana. Far to the west rises 

the great and continuous wall of the main chain of the Rocky Mountains, while to the east for a long distance stretches the level plains 

country. To the traveler going westward by the Great Northern 

Railway, these mountain clusters, rising in the distance blue and 

cloud-like from the level plain like islands from the sea, are the first 

mountain elevations seen after crossing the great basin of the

FIG. 1. Index map showing location of Highwood Mountains.

Mississippi-Missouri system. The Highwood Mountains lie within 

the great bend made by the Missouri River as it flows across the 

plain after it issues from the eastern ranges of the Rocky Mountain 

system. They are definitely located by the meridian of 110° 30' west 

longitude and the parallel of 47° 30' north latitude, which intersect in 

the center of the group. The nearest mountain ranges are the Little 

Belt Mountains, about 20 miles to the south, the Bearpaw Mountains, 

about 50 miles to the northeast, and the Judith Mountains, about 50 

miles to the southeast.

The area is reached on the north and east from Fort Benton on the 

Great Northern Railway, about 20 miles from the foothills. The 

stage route from Fort Bentou to Lewistown passes by the eastern side 

.14 . .
U. S. GEOLOGICAL SURVEY

TOPOGRAPHIC MAP OF THE HIGHWDOD MOUNTAINS, MONTANA.

TriangulationbyE.XvDouglas

Topography by NorthernTranscontinenta! Survey 

Surveyed in 1883-7

i

, 10 miles

Contour interval 200 feet

- 19O4 .

. 

19 kilometers
PIKSSON.] GEOGRAPHY AND HISTORY. 15

of the area, and there are relay .stations and post-offices at Steele, 

about 4 miles east of Mallard Lake, and at Campbells, east of Square 

Butte. On the south and Avest the region is reached from Belt and 

Armington, towns on the Little Belt branch, of the Great Northern 

Railway.

BIBLIOGRAPHY.

1. History of the Lewis and Clark Expedition, by Elliott Coues, vol. 1, p. 342.

2. Exploration and Surveys for a Railroad Route to the Pacific; vol. 12, Near 

the Forty-seventh and Forty-ninth Parallels, by I. I. Stevens, pp. 128,173, 

239.

3. Report on the Exploration of the Yellowstone and Missouri Rivers in 1859-60, 

by W. F. Raynolds, Capt., U. S. Engineers, Washington, 1868, p. 162.

4. Report of a Reconnaissance from Carroll, Montana, to the Yellowstone Park 

in 1875, by Col. Win. Ludlow, Washington, 1876, (War Dept.), p. 14 and 

map.

5. American Journal of Science, 2d-series, vol 31,1861, p. 233.

6. On the Geology and Natural History of the Upper Missouri; Report made to 

G. K. Warren by Dr. F. V. Hayden; Philadelphia (Sherman & Son, pub.), 

1862, p. 119.

7. Geological Report of the Exploration of the Yellowstone and Missouri Rivers, 

by Dr. F. V. Hayden, assistant under direction of Capt. (Brvt. Brig. Gen.) 

W. F. Raynolds, in 1859-60, Washington, 1869, p. 93.

8. Tenth Census of the United States, vol. 15, Mining Industries, Washington, 

1886; Relation of the Coal of Montana to the Older Rocks, by W. M. Davis, 

p. 709; Eruptive Rocks, by W. Lindgren, p. 724.

9. Eruptive Rocks from Montana, by W. Lindgren: Proceedings California Academy of Sciences, series 2, vol. 3, 1890, p. 39.

10. A Sodalite-syenite and other Rocks from. Montana, by W. Lindgren, with 

analyses by W. H. Melville: American Journal of Science, 3d series, vol. 45, 

1893, p. 286.

11. Highwood Mountains of Montana, by W. H. Weed and L. V. Pirsson: Bulletin of the Geological Society of America, vol. 6, 1895, p. 389.

12. Missourite, a new Leucite Rock from the Highwood Mountains of Montana, 

by W. H. Weed and L. V. Pirsson: American Journal of Science, 4th series, 

vol. 2, 1896, p. 315.

13. Geologic Atlas of the United States, Fort Benton Folio, Montana, Washington, 1899; Geology mapped by W. H. Weed, assisted by L. V. Pirsson; 

Descriptive text by W. H. Weed.

12. Geology of the Shonkin Sag and Palisade Butte Laccoliths in the Highwood 

Mountains of Montana, by W. H. Weed and L. V. Pirsson: American Journal of Science, 4th series, vol. 11, 1901, p. 1.

TOPOGRAPHY AND GEOGRAPHY.

The elevated tract comprised in the Highwood Mountains in its 

greatest extension is about 25 miles long from east to west and 16 

miles wide from north to south, and has a total area of 250 to 300 square 

miles. The outer foothills are rather low and rounded, with few 

craggy or broken tops. Toward the center the country becomes more 

rugged and the highest elevations are sharp peaks which rise 3,000 to 

4,000 feet above the plains country and 6,500 or 7,500 feet above the 

sea. On the south side the descent to the plain is much more abrupt
16 IGNEOUS ROCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

than on the north. The two highest mountains, High wood Peak 

(7,600 feet) and Arrow Peak (7,420 feet), are separated by a deep pass 

known as High wood Gap, which, with the valleys descending from it, 

divides the mountains into two portions. One of the main roads 

across the mountains runs through this pass. A view of the mountains, looking north from near the divide in High wood Gap, is "given 

in PL II, A.

On the south the streams drain into Arrow Creek, which heads in 

High wood Gap; on the west they are tributary to Belt Creek. The 

northern portion of the area is drained by the heads of Highwood and 

Shonkin creeks, which flow directly into the Missouri. Within the 

mountain tract the streams are bright running brooks of clear cold 

water, generally with abundance of trout and whitefish; but as they 

debouche upon the plain they are apt to become sluggish and alkaline 

from the Cretaceous clays, and in summer are sometimes dried away 

to standing pools. The upper stream valleys, cut in the rather soft 

Cretaceous beds or volcanic breccias, are of typical V form, with 

rather sharp descent. The general form of the drainage is radially 

outward from the mountain group and of branching pattern.

The slopes, except jvhere broken by the projecting craggy walls of 

protruding dikes or sheets, are rather smoothly modeled, talus heapings and screes being somewhat uncommon, and are usually carpeted 

with a thick growth of grass. The upper northern slopes, however, 

are covered by heavy forests of small pines and often by dense 

thickets of the lodgepole pine (Pinus murrayana). There is no doubt 

that these thick blankets of pine on the northern slopes caused the 

mountains to receive the name "Highwoods."

The outer foothills, and especially the openings of the valleys upon 

the plains, are generally utilized as ranches, and the available water 

is used for irrigation. ' The higher slopes and the semiarid stretches 

of plains country are given up to pasturage.

All parts of the area.are very accessible, as roads run up all the valleys and in one or two places cross the higher ridges. The generally 

smoothly modulated mountain slopes are easily traversed on horseback.

The temperature shows the same range that generally characterizes 

Montana, although extremes of heat and cold are greatly moderated 

by the dryness and vigor of the atmosphere. As the mountains stand 

isolated upon the plain they are condensers of moisture, and in summer time are frequently the focus of local thunder storms that help to 

keep green the vegetation of the higher slopes.

HISTORY. -

So far as known to the writer, the Highwood Mountains are first 

mentioned in the reports of the Lewis and Clark expedition, a though 

they are not given a definite name. They are mentioned several times

aCoues's History of Lewis and Clark Expedition, vol. 1, p. 342,
U. S. GEOLOGICAL SURVEY ULLETIN NO. 237 PL. II

A. VIEW NORTHWARD FROM HIGHWOOD GAP, SHOWING SLOPES OF 

VOLCAN 1C DEBRIS.

li. UPPER DAVIS CREEK; MOUNTAINS OF VOLCANIC FLOWS AND BRECCIAS.
riussoN.] HISTOKY. 17

by Governor Stevens in his report/' and it is evident from his narrative 

that nearly all the mountain groups and streams of this region had 

then, in 1853, received the names they now bear. The Belt Mountains 

are called by him, however, "Girdle" Mountains, and a variant of 

Shonkin is Shonkee Creek, which is probably a misprint. The pine 

timber on the High woods and its probable usefulness to the surrounding region in the future is commented upon by him. Lieutenant 

Mullan of the Stevens expedition, b with a detached party, ascended 

Shonkin Creek and passed to the east of the Highwoods into the 

Judith basin.. He speaks of the first spurs of the Highwoods as being 

a thousand feet in height.

In 1860 Lieut. John Mullins c passed alqug the same route on his way 

from Fort Benton to Fort Union. His first camp was in the foothills 

on the northeast side of the mountains, and the next day he followed 

the Shonkin Sag down to Arrow Creek. On the map accompanying 

the report the position of the mountains is roughly indicated, but their 

name is not given. The mountains were not visited by the Ludlow 

expedition in 1875,'^ but on the route map their position is shown and 

their name is given, together with those of the prominent peaks and 

the main streams.

The earliest mention of the geology of the Highwoods is by Prof. 

F. V. Hayden.* In a summary of' the geologic results of Reynold's 

expedition, which he had accomp-mied, he says:

In the Belt, High wood Mountains, and indeed all along the eastern slope in this 

region we find continual evidence of the outpouring of the fluid material in the 

form of surface beds or in layers thrust between the fossiliferous strata. These 

igneous beds thin out rapidly as we recede from the point of effusion. A large 

number of these centers of protrusion may be seen along the slope of the mountains west of the Judith Range. The erupted material sometimes presents a vertical wall 300 feet high, then suddenly thins out and disappears.

Almost the same wording is used in his report accompanying that 

of Warren's explorations./ This statement applies very well to the 

laccoliths of the region. From Kaynold's^ report we know that Mullins, whom Hayden accompanied, passed down the valley of what is 

now called the Shonkin Sag to Arrow Creek, and thus passed the cliff 

wall of the Shonkin Sag laccolith. He probably had this in mind 

when writing the above, and also in his report on the geology of the 

country traversed by the expedition, which appeared about ten years 

later. In this he says:

From Fort Benton we crossed the prairie country in an easterly direction not 

far from the foot of the mountains. We find the cretaceous beds predominate,

"Explorations and Surveys for. a Railroad Route to the Pacific, vol. 12, pp. 123,-173, 239.

b Ibid., p. 123.

cRaynolds, "W. P., Exploration of Yellowstone and Missouri Rivers in 1859-60,1868, p. 93.

rfLudlow, W., Reconnaissance from Carroll, Mont., to the Yellowstone Park, 1876, p. 14.

e Am. Jour. Sci., 2d series, vol. 31,1861, p. 233.

/On the G-eology and Natural History of the Upper Missouri, 1862, p. 119,

ffOp. cit., p. 163.

Bull. 237 04 2
18 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

with here and there indications of eruptive rocks, and we know that the mountains that surround us on every side are very largely composed of that material. 

The country is covered with saline lakes, which add much to the desolateness of 

the scenery. We have near the Arrow Creek a bed of erupted material thrust 

between cretaceous rocks, which presents a vertical wall of 150 to 200 feet at 

one point and then suddenly ceases. These small centers of effusion of melted 

rock seem to cover this whole region. The most conspicuous examples of ejected 

material are the Square Buttes, which is a general name for numerous peaks with 

broad, flat upper surfaces and with a tendency to a lofty, square, columnar form. 

The cretaceous rocks, so far as I can see, seem to extend quite closely up to the 

mountain elevations, and everywhere present the lithological character of No. 2. 

Arrow Creek is a small stream with a narrow fringe of cottonwood, surrounded 

with high bluffs forming very rugged features, properly called Bad Lands. On 

Arrow Creek I found ammonites, cardium, baculites, inoceramus, etc. The cretaceous rocks in this region seem to belong entirely to No. 2, though Nos. 1 and 2 . may be included. It is mostly a black plastic clay, with'now and then a bed of 

sandstone. The igneous rocks in this region show very distinctly the origin of 

the vast quantities of saline matter which covers the ground and mingles with the 

waters of the streams. These rocks seem to contain large quantities of this saline 

material, which gathers upon their surface, giving to the igneous peaks a whitish 

appearance. This may account for the great quantities of it which pervade the 

formations in the West.

Beyond the few observations of these early explorers, the first geologic examination of the district was made by W. M. Davis and W. 

Lindgren, at that time attached to the Northern Transcontinental 

Survey, in 1883. They traversed the mountains through Highwood 

Gap, collected material, and the results of their reconnaissance, showing the essential geologic features of the mountains, have been published. 0 In this report Lindgren gives a brief resume of the petrography of the igneous rocks, showing what interesting types the region 

affords. This sketch of the petrography of the area will be alluded 

to more in detail later on. Lindgren continued his petrographic

studies of Highwood material, and in 1890 published a paper on the 

analcite-basalts, 6 which was followed in 1893 by an article on the syenite of Square Butte, c with included chemical-analyses by the late Dr. 

W. H. Melville, this last paper being based on material collected by 

Dr. C. A. White.

In 1894 the area was explored and mapped by W. H. Weed and the 

writer for the purpose of studying the areal geology and mapping the 

Fort Benton quadrangle for the United States Geological Survey, the 

work being in charge of Mr. Weed.

The writer was not .able to complete the field season on account of 

other duties, and hence a portion of the area around the head of 

Shonkin Creek and the Shonkin stock was not seen by him, the work 

being completed by Mr. Weed, and the material collected by him. 

In 1896 a bri^f visit was made by Mr. Weed and the writer to the

a Tenth Census, vol. 15, p. 709.

&Proc. California Acad. Sci., ser. 2, vol. 3,1890, p. 51.

c Am. Jour. Sci., 3d ser., vol. 45,1893, p. 286.
PIRSSON.] HISTOBY. 19

laccolitliic area on the eastern side of the mountains, at which time 

the Shonkin Sag laccolith was studied.

As results of this field work and collection of material a preliminary sketch of the geology of the mountains, accompanied by a detailed 

study of the petrology of Square Butte, was published by Mr. Weed 

and the writer in 1896, a and this was followed by two papers, one 

dealing with the description of missourite, & a new type of leucite rock 

occurring in the Shonkin stock, the other with the geology of the 

Shonkin Sag and Palisade Butte laccoliths. 0

The results of the areal work and mapping have been presented in 

the Fort Benton folio of the Geologic Atlas of the United States, with 

descriptive text by Mr. AVeed.

« Bull. Geol. Soc. America, vol. 0,1895, p. 389. 

b Am. Jour. Sci., 4th ser., vol. 2,1896, p. 315. 

cIbid., vol. 11,1901, p. 1.
CHAPTER II.

GEOLOGY OF THE IGNEOUS STOCKS.

INTRODUCTORY.

Briefly stated, the general geology of the High wood Mountains is 

that of a group of extinct and greatly eroded volcanoes. Beyond this 

the details are largely of local and in only a few particulars of general 

interest. On the southeast, of the mountains, and scarcely separated from them, is a restricted area of intruded sheets and laccoliths. 

In the eroded volcanic area proper occur all the concomitants of 

violent extrusive volcanism. There are central cores or stocks representing the main canals to large bodies of magma below; there are 

great masses of piled-up breccias mingled with lava flows, which are, 

however, only remnants of former lofty cones; finally, there are networks or systems of dikes surrounding and dependent upon the 

central cores, cutting sediments and breccias alike, and generally 

showing a remarkable radial arrangement around the centers of eruption to which they belong.

Brief accounts of the general geology of the High woods have been 

already given by Mr. W. H. Weed and the writer, a and in somewhat 

more extended form by Mr. Weed.^ It is intended here to describe 

only such salient features as are necessary to an understanding of the 

petrology of the area, the main purpose of this bulletin. In essential

points the descriptions here given are similar to those in the works 

just cited, though presented in more extended form. The geologic 

map (PI. Ill) is taken from the map given in the Fort Benton .folio.

HIGHWOOD PEAK STOCK.

The highest peak in the mountains marks the location of an approximately circular body of granular igneous rock about a mile in diameter. The igneous mass is in contact in places with Cretaceous 

sandstones and shales which are disturbed and metamorphosed and in 

places with breccias which have been indurated by it. It consists of 

two very distinct and separate types of rock, one a syenite of Albany 

type (pulaskose in the new classification), the other amonzonite (shoshonose). As one is so feldspathic and devoid of dark minerals that

a Bull. Geol. Soc. America, vol. G; 1894, p. 389.

6Description of the Fort B ton quadrangle, Geologic Atlas U. S., folio 81

20
U. S. GEOLOGICAL SURVEY BULLETIN NO. 237 PL. Ill

LEGEND

SURFICIALROCKS

Glacial drift 

and till

SEDIMENTARY ROCKS

Montana Conn at ion 

(clay-shale witfisandstone 

iraerbedtlcdafldatthe top!

p'Ke !

Ea£le formation 

(whilesandstone with 

lignite seams and day)

Colorado formation

(blavkshalf at tfic btute

gray shaJe above, arid

santtstone interbedded;

contains an. ash. bed, a b

Dakota formation

{sandstone and tnterfieddedcUty shcd&j

Cascade .form all on

( sandstone and shale

wtih coff-i seam, at tfie top)

Ellis forma lion 

(Uniestonegrading upward 

intovafidstone and-pebbly 

sandstone at the base/

no°i5'

^^^j&fe

Henry Gannett,Chief Geographer 

A.H.Thornpson. Geographer in charge 

Triangulation by E.M.Douglas

Topography by NorthernTranscontinental Survey 

Surveyed in 1883-7

GEOLOGIC MAP OF THE HIGHWOOD MOUNTAINS, MONTANA

Oi23i5678aio 

Scale

mile s

o 5 10 kilon eters

Geology by Walter Harvey Weed 

andLouisV. Pirsson 

Surveyed in 1894

Contour interval 200 feet 

1904

JULIUS BIEN & CO.N.Y.

LEGEND

(continued)

Quadrant formation

i sand-si one, art-en and red 

shale, and -white limestone}

IGNEOUS ROCKS

Moiizonite

/also basic syenite of

JVIiddlfPeak and leucite -

3hon.kinite of East Peak)

L

sh

. Sbonlviiiite 

( including m is-soarite and. 

fergusite ofArnom: stock J

Basaltic sheets

and dikes 

(Highwood mine tie, ana-lcite - 

and leuciie - b

bbr

Basal tic breccia, 

f'lo\vs,and scoria

-and; leucite, basalts)

Tr achy tic dikes 

and'sheets 

( syenite - and.

tingiLaite-porphyri&sj

abr

Trachyande s i tic 

breccia and tuffs
rmssoN.]. HIGHWOOD PEAK STOCK. 21

it appears almost white in considerable masses and the other is very 

dark, the contrast between them is pronounced. On the southern 

portion of the peak the syenite forms the exposed crags, and outcrops 

' on the slopes and in the heads of the valleys. Masses of upturned 

and altered Cretaceous beds are seen lying against it, but they are not 

large and, so far as can be told, its intrusion was not followed by any 

extensive or important displacement or rupturing of the sediments. 

The coarsest-grained rock, of a somewhat miarolitic type, is found in 

the highest masses exposed, but nearly everywhere the rock is moderately coarse.

Along the divide running to Middle and South peaks are occasional 

outcrops of massive rock above the smooth slopes of gray indurated 

ash. It appears probable that they represent apophysal tongues of 

somewhat differentiated character, which extend in this direction perhaps as irregular dike-like masses, and, as they occur at irregular 

intervals, may possibly extend to and connect with the Middle Peak 

mass.

On the summit of Highwood Peak the light-colored rock soon gives 

way on the north to dark-colored monzonite. The contact between 

the two was not observed in place on the summit, but among the sliderock debris of the west slope are found not infrequently pieces shoAVing the two types in contact. In these the white syenite holds small 

angular chips and fragments of the monzonite, torn and displaced 

from a wall of the dark rock, and it is inferred that the syenite was 

intruded later than the monzonite.

At the north end of the peak the monzonite becomes very dense and 

trap like, and it is supposed that the contact is close by in this direction. The peak is sharp on all sides, and the exposures*of the crest 

give way rapidly to a rolling talus of small fragments, which in turn 

is succeeded by the smoothly modeled grassy contours of the lower 

parts. In such places tire structure is not exposed, but can only be 

inferred from the crest above.

Taking into consideration the rudely circular form of the mass, the 

grain of the rock, the small amount of displacement of the inclosing 

strata, the numerous radiating dikes, and the surrounding breccias 

and flows of lava, it appears almost certain that the core of igneous 

granular rock forming Highwood Peak represents the plugged canal 

of a former volcano. This core of igneous rock was once of much 

greater height and size, and the column of liquid lava rose through 

the conduit of sedimentary rocks into a cone of mingled ashes and 

breccias. The size of this cone can not now be accurately determined, 

but it was certainly considerable as compared Avith the present size of 

the mountain mass, as is indicated by the portion still remaining west 

of Highwood Peak and forming the high, narrow ridge called Pinewood Peak. This consists of dark-colored, basic breccias overlying 

lighter, more acidic ones, indicating variations in the character of 

the material erupted at different periods.
22 IGNEOUS KOCKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

Since the cessation of igneous activity a large amount of erosion has 

occurred, removing the overlying breccias and flows in great part, 

leaving masses of them here and there and in other places cutting 

into the sediments, so that now the core is well exposed, and being 

the central and most resistant rock mass of the complex, it now forms 

the highest peak.

CHARACTER OF THE BORDER CONTACT OF HIGHWOOD PEAK STOCK.

On the southwest slope of I-Iighwood Peak the character of the contact at the periphery of the mass is such that it deserves especial 

mention. Here outcrops of igneous rock run tongue-like down the 

mountain slope, with strips of inclined, much altered, sediments 

between them. The main rock is the syenite of the crest, mentioned 

above, but it con tains some mica in addition to the augite. It is here cut 

by or is in contact with stringers and masses of a kind of '' niica trap " 

a dark ferromagnesian rock at times containing much biotite in a 

dark-gray groundmass. This phase passes quickly, in many of these 

stringers and masses, into a leu cite facies. The change may take 

place within a few inches, often in the distance of an inch. The large 

biotite phenocrysts disappear, the mica flakes become much smaller 

and more intimately mixed in the groundmass, and spots of leucitelike character appear until the rock closely resembles well-known 

varieties of leucite-basalt. A remarkable feature is that when the 

actual contacts of the light and dark rocks are examined it is found 

that the light incloses angular fragments of the dark, while, vice versa, 

the dark incloses the light. This occurs in a large number .of cases, 

so that it is a common and persistent feature of the contact. It is 

also noticed, especially on somewhat weathered surfaces, that the 

white syenite appears somewhat streaked and eutaxitic. Observation 

does not reveal any change in granularity by which it may be told 

whether the light or the dark is the older. "*

Movements may have taken place in the conduit among masses of 

magma already differentiated, as at Square Butte, and in a highly 

viscous condition, or the dark rock may have been ejected through 

the light one and carried fragments of it upward, while the light rock 

held pieces of a previously solidified basic magma. The latter would 

seem on the face of it the more reasonable supposition, since around 

the peak are basaltic lavas resting on feldspathic ones. The order of 

succession of the igneous rocks is discussed in Chapter VII.

As one follows around the slope to the west the dark rock is seen 

to replace the white entirely, and the projecting masses and crags 

among the upturned sediments may be the roots of former effusions 

breaking through the side of the cone. This idea is supported by the 

fact that as the saddle east of Pinewood Peak is approached the rock 

becomes less and less granular and denser, assumes in places an
JMHSSON.] GEOLOGY O.F THE IGNEOUS STOCKS. 23

amygdaloidal structure, and finally on the saddle and on Pinewood 

Peak has the character of a true effusive and is even strongly pumiceous.

MIDDLE PEAK STOCK.

As previously mentioned, small intrusive masses frequently break 

through the level-bedded sandstones or indurated beds of ash which 

mostly compose the high ridge connecting Highwood Peak with Middle Peak and extending to South Peak. These are of granular rock 

ranging from extremely feldspathic syenites to dark basic types composed chiefly of angite and other ferromagnesian minerals. These 

small' intrusives and also the dikes along the ridges indicate connections with deeper-seated masses below. At Middle Peak, however, is 

a large body or stock of granular igneous rock. The intrusion, as 

seen on the map, is of roughly circular outline. Its eastern edge follows the crest of the ridge along the contact with the Cretaceous

sandstones, and its western boundary is on the western slope, where 

its presence is shown by crags outcropping among the slide-rock 

debris. To the south the intrusion is continued in a long tonguelike apophysis of no great breadth which runs south for 2 miles or 

more on the eastern side of the ridge a little below the crest. The 

thickness or breadth of the apophysis is, however, somewhat variable 

and is difficult to make oiit, as the lower edge is so commonly masked 

by debris; its outcrops are well seen on some of the spurs extending 

east from South Peak.

The rock composing this intrusion varies somewhat in fineness of 

grain and relative amount of feldspathic and ferromagnesian minerals, bat in general is of medium nature. It is dark gray, moderately 

even granular, resembles many diorites, and in the hand specimen is 

extremely like the monzonite of Highwood Peak, though, as shown 

later in the petrographic description, there are several important 

points of difference. At the contact the feldspars are apt to be of 

pronounced tabular habit and arranged in parallel iluidal structure. 

In the apophysis to the south a varietal phase replaces the type of 

the main mass, biotite becoming a prominent ingredient. 0wingto its close resemblance in the common variety to the monzonite of 

Highwood Peak, it has been given ihe same color and pattern on the 

map.

CONTACT PHENOMENA OF MIDDLE PEAK STOCK.

Along the crest of the Middle Peak-South Peak ridge the contact between the stock and the sedimentaries is clearly and beautifully shown. In fact, at this point the hard, tough, and resistant 

altered beds appear to determine the crest of the ridge, since they 

resist erosion better than the granular igneous rock on the one hand
24 IGNEOUS BOOKS OF HIGHWOOD MOUNTAIN'S. [BOLL. 237.

and the softer unaltered shales and sandstones on the other. The 

igneous rock itself does not appear to be modified at the contact. 

The full size of grain continues directly to the contact, wall, which is 

irregular, the igneous rock penetrating the sediments in stringers, 

bulbous masses, and veins for a short distance, producing a narrow 

mixed zone. The sediments at such places, on the contrary, are 

greatly altered, and where they contained lime-bearing material a 

greenish pyroxenic rock is produced carrying garnets and chlorite. 

Where they were clay slates a dark, tough, dense hornstone or adinole 

is produced, similar to that observed at Castle Mountain a and in the 

Crazy Mountains.6 Changes into spotted slates, audalusite, or mica 

hornfels were not observed, as described by various writers at other 

localities.

The width of the adinole zone is probably about a quarter of a mile, 

not greater. The metamorphic effect dies away at South Peak, 

although on the crest it is still strongly marked, and while the lines 

of bedding are not wholly obliterated, they are twisted and gnarled, 

and original lime-bearing nodules are converted into hard concretionary masses, often hollow and lined with pyroxene crystals. These 

rocks weather purple, gray, and green "on the surface. Where the 

metamorphismis most marked the pure sandstones have been changed 

into hard quartzite'. The beds show great shrinkage, and in places 

on the ridge where they are exposed to a thickness of 15 feet they 

have split into prisms often several feet in diameter. They roughly 

resemble basaltic columns and form large blocks in the sharp-edged 

and chippy talus. Such shrinkage and cracking of the sediments by 

metamorphism about an igneous core may have an influence in permitting the intrusion of dikes into them at a later period.

ENDOMORPHIC CONTACT PHENOMENA OF MIDDLE PEAK STOCK.

The igneous rock, as previously said, holds its full size of grain, as 

well as could be told, to the contact, but changes somewhat in texture. The feldspars become of a more distinct tabular habit and are 

arranged more or less roughly parallel to one another and to the contact, so that the rock splits more easily in this direction. The effect 

is somewhat similar to that seen in the well-known syenite from 

Plauen near Dresden, and is probably the result of fluidal movements 

in the crystallizing magma which tended to "set" or orient the developing feldspars.

EAST PEAK STOCK.

The piled-up masses of flows and breccias that form Arrow Peak are 

continued to the south and east by two mountain ridges, inclosing a

a Weed and Pirsson, Geology of the Castle Mountain mining district: Bull. U. S. Geol. Survey 

No. 189,1896, p. 94. 

> I) Wolff, J. E., Geology of the Crazy Mountains: Bull. Geol. Soc. America, vol. 3,1892, p. 451.-
FIKSSON.] EAST, PEAK STOCK. 25

basin in which are the headwater tributaries of Davis Creek. From 

the open country below, the view of this basin and its encircling 

mountain walls is very picturesque. The smoothly modeled slopes of 

the foothills are bare of vegetation save for the pale-brown nap of 

grass that covers them in summer time. In the stream bottoms is a 

luxuriant growth of cotton woods and smaller trees masses of vivid 

green against the brown. Above these rise the mountains, the darkchocolate-brown of their rugged sides and crags softened here and 

there by forests of dark pines. The general effect, looking northward 

from Davis Creek at East Peak, is shown by PL II, B, from a photograph by Mr. Weed. At this point the flows and breccias composing 

the ridge are interrupted by a body of granular igneous rock approximately a mile in diameter and of the general shape shown on the map.

On the mountain slope this mass of igneous rock has been eroded 

into massive crags, pinnacles, and precipices that are noticeable at a 

considerable, distance and from below are very striking. It ascends 

nearly to the top of the ridge, and on the northeast and west sides 

passes under the breccias which compose the ridge and which have 

been baked and altered by it. Its exposure slopes to the south and 

is terminated by a shallow, narrow valley running parallel to its face. 

The low ridge forming the south side of this drain also affords exposures of the massive rock in place in contact with tuffs and breccias. 

These latter on the slope to the south are soon replaced by the underlying Cretaceous sediments, on which it is clear they form only a 

shallow patch, and with which on the south side the igneous rock 

must be in contact at no great depth. From this exposure on the 

southern ridge was taken the specimen analyzed (760, p. 109).

A marked feature of this rock is the heavy massive jointing, which 

produces hugh angular blocks in the talus, which then weather and 

break down into smaller masses. The jointing causes the crags and 

pinnacles to have the form of great prisms or nearly rectangular 

masses. It gives in places a decidedly columnar aspect to the exposures. The columns are not vertical, but inclined in the same direction as the general slope of the exposed face of the mass. The 

breccias, beneath which the igneous rock disappears, are tilted away 

from it, as shown by their rough'bedding lines, at angles of 10° to 20°.

All this goes to show that the stock or core in its general direction 

is not vertical, but-dips somewhat to the north. It extended upward 

into previously piled-up masses of extrusive material. Whether it 

reached the surface and became in itself a conduit for effusions can 

not now be told. If effusive rocks existed at all, erosion has long 

since swept them away and has cut down into the stock itself and 

the surrounding breccias until on the southern edge they have almost 

disappeared.

The rock composing this stock is medium granular and pearly gray
26 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

to dark gray in color. It contains, so large a proportion of angite, 

chiefly with other ferromagnesian minerals mingled with its feldspars 

and feldspathoids, as to be of rather basic type. In the specimen 

analyzed, taken from the exposures of the low frontal ridge mentioned above, it contains among the feldspathic components a considerable quantity of leucite, and in the prevailing terminology would . 

classed be as a basic leucite-syenite. It is described in the petrographic portion of this work under the name "leucite-shonkinose."

SHONKIN STOCK.

This locality was not visited by the writer, and for the following 

notes and the specimens which illustrate the variations in the rock 

mass he is indebted to Mr. Weed, who studied the occurrence after 

the writer had left the field.

The Shonkin stock is the largest body of intrusive granular rock in 

the mountain area, being about 2| miles long and 1£ miles in its greatest breadth. It is irregular in shape, being broader at each end than in 

the middle. It is in places in contact with the sedimentary strata 

Cretaceous shales and in places with volcanic ejections which lie 

upon them. The igneous material has been intruded into the sediments and along the contact between the sediments and the extrusive 

volcanic material. All this material is greatly altered by the intrusion, and through the hardened rim there depart radially outward a 

great number of dikes. The igneous rock, whose average type is a 

rather coarse, dark, granitoid-looking rock, resembling many gabbros 

in appearance, from place to place shows great variation in grain 

and a slight difference in the relation of feldspathic and ferromagnesian minerals. This is most noticeable at the south end, where a 

coarse agglomerate of the massive rock in blocks has a cement of 

finer material, the mineral composition of both rock and cement being 

the same as that of the main type, but showing great textural variations. This is supposed to be the actual locus of volcanic activity 

the throat, in fact, of the former volcano at this place. The mass is 

so eroded and weathered that it seldom forms heavy outcrops, acting 

in this respect like many coarse-grained stocks which readily break 

down. Conspicuous exposures may be seen near the contact, but the 

rock is apt to be hidden by the talus-like material which results from 

the breaking down of the mass. The jointing is platy at the contact, 

but massive elsewhere.

The rock composing this mass is made up mainly of dark ferromagnesian minerals augite, olivine, biotite, and iron ore but a considerable amount of a light-colored mineral is present. In some places the 

latter is alkali feldspar, forming shonkinite (shonkinose); in other 

places it is leucite, forming niissourite (missourote), described on page 

115. These two minerals, both white, formless and granular, look so
rrnssoN.] SHONKIN STOCK. 27

much alike in the hand specimen that it is almost impossible to distinguish between them, and therefore between the two rock types, by 

mere inspection. Moreover, there are intermediate types in which 

alkali feldspar and leucite are intermingled in the white portion. As 

these facts were unknown when the area was mapped, and as the types 

could not be discriminated by ordinary means in any case, it is impossible to say how much of the Shonkin stock consists of missourite and 

how much of shonkinite. In the specimens collected by Mr. Weed 

over a large portion of the stock the shonkinite greatly predominates, 

and it is therefore inferred to be the chief type. At some places in 

the Highwoods where intrusions occur near the contact, the rock 

mass, which consists of shonkinite, is filled with large, coarse pseudoleucites, as large as the end of one's thumb, which contain many 

grains of ferromagnesian minerals. This fact renders it uncertain 

how much of the shonkinite in this area may be original and how 

much may be pseudoleucitic after missourite. This uncertainty also 

is all the greater when one takes into account the rock forming the 

neighboring Arnoux stock, whose white mineral, describe'd on page 

83, consists mainly of pseudoleucite.

Occurrence of missourite. The pure unchanged leucite type (missourite) of this stock has been already described by the writer from 

Mr. Weed's specimens. The rock is of importance on account of the 

place it fills in the prevailing systems of classification, and because 

until this occurrence was known, unchanged leucite had never been 

found in an intrusive granular igneous rock, a fact of great importance in theoretic petrography. It was supposed that the \vhole mass 

of the Shonkin stock was made up of this type, but since subsequent 

study has shown that this is not the case, the exact locality where 

typical missourite occurs will be described. For the accompanying 

sketch map (fig. 2)-the writer is indebted to Mr. Weed. The type 

specimen was taken from the talus blocks at the foot of a low cliff 

of a reddish color above the ranches on the east side of the stream, 

which is the head fork of Shonkin Creek. It will be noted that the 

place is at the southwest foot of the mountain between the main upper 

mountain forks of Shonkin Creek, and is on the border of the intruded 

masses of the stock. The leucite of the rock at this place is a pale 

greenish-gray on a freshly broken rock surface, while in those places 

where it is replaced by feldspar the latter is white to pale pink, if one 

can judge from Mr, Weed's specimens.

The adinole zone of contact-metamorphosed shales is largely filled 

with intrusive sheets of varying thickness. These sheets are coinposed of types of leucite-basalt dense dark rocks spotted with phenocrysts of augite and whitish ones of pseudoleucite, similar to those 

types .which occur in the sheets around Square Butte and the Shonkin 

Sag laccoliths and whose petrographic characters are described on
28 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

page 166. The sheets and dikes are described on page .30. It should 

be remarked in conclusion that the edge of the stock is cut by dikes, 

one of which is a light-colored feldspathic porphyry (syenite-porphyry 

or trachiphyro-borolanose).

ARNOUX STOCK.

The Arnoux stock is a small, rudely circular mass of granular 

intrusive rock perhaps a mile in diameter. It is about 2 miles northContour interval 200 feet 

FIG. 2. Sketch map showing locality at which missourite was found.

west of the large Shonkin sto'ck. It is intrusive in the previously 

ejected basaltic breccias, which have been altered by it as at the 

Shonkin core. It has been cut into by a northward-draining tributary of Shonkin Creek, and although the exposures are not marked, 

they are good in places. While there is some variation in the rock 

composing it, the main type, as shown by Mr. Weed's specimens, has
AENOUX STOCK. 29

a very interesting composition and consists chiefly of pseudo-leucite 

and a smaller quantity of augite. The rock is fnlty described on 

page 83 under the name fergusose (fergusite), as it is a new and 

distinct variety of igneous rock.

At some places this rock contains a larger proportion of the ferromagnesian minerals, and at others, near the contact, the outline of the 

pseudoleucite may be lost, and the rock has then the appearance of 

the shonkinite of the other localities in the region.

This stock is entirely isolated at the surface, and as it was not 

visited by the writer he has no suggestions to offer concerning any 

possible connection with the near-by larger Shonkin stock, although 

this naturally presents itself as a possibility.
C PI AFTER III. 

GEOLOGY OF THE SHEETS, DIKES, AND EXTRUSIVE ROCKS.

INTRUSIVE SHEETS.

In the region of active volcanic outbreaks in the Highwood Mountains intruded sheets of igneous rocks do not form a prominent feature 

of the geologic structure. Only in the eastern laccolithic area are 

they important. They are not entirely wanting, however, for they 

have been noted in several localities in the foothills or lowest slopes 

and in the open country surrounding the mountains where the trenching of the streams has revealed them, and in the altered strata surrounding the Shonkin stock. In the highest parts of the mountains 

they seem to be wanting among the sediments, except in one or two 

rather doubtful cases.

Near the Thornton ranch, on the head of the South Fork of Williams 

Creek, a sheet of minette of High wood type (phyro biotitic shonkinose) is intruded into sandstones. It is about 12 feet in thickness, 

the upper portion- being of coarser grain than the lower, has a brownish color, and weathers to a coarse sand. The lower, denser portion 

has a very massive parting, giving rise to large blocks which tend to 

weather to rounded bowlders. The sheet is full of large, welldeveloped mica phenocrysts, sometimes half an inch in diameter, 

which are more or less parallel to the general plane of the sheet and 

tend to cause parallel fracturing or lamination in its upper portion.

Several similar sheets were noted in the bench lands to the west of 

this occurrence. They commonly determine the position of the upper 

surface of the bench, forming, together with the baked sedimentaries, 

a hard layer in the soft Cretaceous strata. In one case the rock, on 

weathering, has developed a pronounced onion structure of concentric 

shells. The inner kernels may be lifted out, leaving bowls an inch or 

so in thickness and smooth and regular in appearance. In this 

instance the rock is badly weathered and the structure was evidently 

the result of concentric cooling, aided perhaps by a previous rolling 

and kneading of the viscous intrusion, which has been brought out 

by weathering. In other cases the sheet may have a thin platy parting at top and bottom, so that at a short distance it resembles a slaty 

sedimentary bed.

One of these sheets is cut by the long dikes crossing the bench 

lands north of Mann, on Otter Creek. In this area the beds at times 

show small local anticlinal dips, suggesting that they were disturbed 

30
PIKSSON.] INTRUSIVE SHEETS. 31

below by intrusion of sheets or small, flat laccoliths, which erosion 

has not yet exposed at the surface.

The rock forming the sheets is the minette of Highwood type (phyro 

biotitic shonkinose). Sheets of feldspathic porphyry so common in 

the Little Belt Mountains to the south are seldom found in the Highwoods. These basic sheets break down into coarse soil and rotten 

rock, in which the micas turn to greenish plates of chlorite. An 

excellent example of this may be seen on the little hill in the open 

country south of the mountain slopes a few miles east of the Curry 

ranch, which is situated where Arrow Creek debouches from Highwood Gap.

Sheets of such rock in the black shales occur above Fitch's ranch, on 

Little Belt Creek, near its upper forks. They were also observed by 

Mr. Weed in the adinole zone around the Shonkin core at the head of 

Shonkin Creek, where they reached 8 feet in thickness. A heavy

intrusive sheet found by Mr. Weed on the creek between Alder and 

Birch creeks, on the eastern side of the mountains, is about 20 feet in 

thickness and produces a waterfall in the course of the stream; a 

5-foot dike runs into it and is thought to be the feeder of the sheet. 

The main rock of the sheet is a shonkinite, full of rather automorphic 

augites, while that of the dike is a much denser- basalt.

Specimens of fine compact shonkinite or basalt collected on Alder 

Creek above the ranches were derived from intruded sheets in the 

Cretaceous beds above and were mapped later by Mr. Weed.

The most striking instances of intrusive sheets in the Highwoods, 

however, are those found in connection with the laccoliths of Square 

Butte and the Shonkin Sag, and will be described in connection with 

these laccoliths.

DIKES. 

INTRODUCTORY.

Beyond question there is no feature of geologic structure in the 

Highwood Mountains of greater interest than the systems of dikes. 

Except, perhaps, the laccoliths on the east, which are so well dissected 

and exposed for study, the dikes are the most striking and characteristic phenomenon of this A^olcanic area. They were noted by Lindgren and Davis, a and their systems and character were mentioned in 

previous works of Mr. Weed and the Avriter.

RADIAL DISPOSITION OF DIKES.

The most characteristic feature of the dikes is their general radial 

disposition around the mountain mass. This is clearly seen by a 

glance at the geologic map, but further study shows that the great

"Tenth Census of the United States, vol. 15. .Mining Industries, Washington, 1880; Relation of 

the Coal of Montana to the Older Rocks, by W. M. Da vis, p., 709; Eruptive Rocks, by W. Lindgren, p. 724.
32 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

majority fall into several distinct systems; that they radiate not only 

in a general way about the mountains, but also about distinct loci, 

and that each locus is a body of granular igneous rock the former 

canal of a volcano and a center of upward force and eruption. These 

facts are shown on the accompanying map (PI. Ill), on which the 

actual directions of the exposed dikes are shown by the colored lines. 

Most of the dikes cluster about the Shonkin and Highwood peaks as 

centers, especially the former. Around the outer flanks of the mountains the radial arrangement of the dikes was noted as they were 

mapped, and the existence and character of the Shonkin stock suspected and its place located by Mr. Weed and the writer before it was 

visited by Mr. Weed, who was then able to prove the correctness of 

the supposition. It has long been known that dikes radiate about 

eruptive centers, especially on the dissected flanks of the volcanoes. 

The value of this arrangement as a means of locating eroded eruptive 

centers was shown by Iddings in his work on the Crandall and Haystack a areas of the Absaroka Range to the south.

A radial arrangement of dikes and the principles of magmatic differentiation which have been evolved in recent years may be useful 

instruments in determining the geologic structure in -certain cases.

If all the dikes in the High woods could be carefully mapped on a 

large scale this arrangement would be much more pronounced than it 

appears on the map. On account of the small scale of the map, the 

limited time available, and, more especially, certain reasons connected 

with the exposures to be mentioned later, only a part of the dikes 

actually existing are shown.

CHARACTER OF THE DIKES.

In many cases the dikes have proved more resistant to erosion than 

the Cretaceous beds they have penetrated, and now protrude as rough 

stonewalls or "reefs." This is more especially conspicuous in the 

outlying foothills and open bench lands, where there is no vegetation 

to obscure them save a nap of short yellow-brown grass. An especially noticeable locality is near the head of Byrnes Creek, north of 

Mann on Otter Creek, and south of the mountains. Here a dike with 

a north-south trend toward Highwood Peak runs across the open 

country for miles. It is divided into massive blocks 3 to 4 feet on a 

side, which have the regularity of placement often found in dikes, 

which gives them an appearance of artificial construction, like masonry. 

Although only 8 to 10 feet wide, the wall often has a height of 10 or 

15 feet for some distance. The rock is an extremely dense and heavy 

Highwood minette (shonkinose).

Similar dikes forming heavy projecting walls are found on the 

bench lands south of the mountains, in the basin of the south fork of

a Geology of the Yellowstone National Park: Mon. U. S. Geol. Survey, vol. 32, pt. 2,1899, pp. 

224-231.
nussoN.J DIKES. 33

Aspen Creek, and on the grassy slopes of the divide in Highwood Gap, 

where they constitute the chief geologic feature. Other excellent 

examples are the great wall dikes east of the Shonkin Sag laccolith 

and near the stage road from Fort Benton to Lewistown and the great 

reef dike running from Palisade Butte to Square Butte. The greatest 

number are found on the open foothills and bench lands and upper 

slopes east of the Shonkin stock, where they were studied and mapped 

by Mr. Weed. From the top of Palisade Butte 70 are' in one view. 

In all the cases mentioned the rocks composing these dikes are black, 

heavy traps of various kinds, as described later. The width of the 

dikes in these cases is usually 6 to 10 feet. It is rarely so great as 20 

feet.

In some cases the dikes have weathered more rapidly than the sediments, and the courses of a few are shown by shallow trenches. 

In these instances the soil produced by weathering is often richer 

than that of the adjoining sediments, appears to hold moisture better, 

and supports a heavier, greener growth of grass or a different kind of 

vegetation. The course of the dikes is then marked on the open, 

brown hill slopes or level bench lands by greener bands. To one 

looking down from the height of South Peak they appear like long 

green pen lines on the surface of brown paper, and their radial trend 

is clearly seen.

These dikes are usually composed of mica traps or Highwood minettes, which are less resistant to weathering than the other varieties 

of the rocks found in dike form. They have produced a greater or 

lesser degree of inetamorphisin, depending on their size and number. 

In some cases, as in a weathered dike near the Thornton ranch on 

Williams Creek, the sandstones have been altered in narrow zones to 

quartzite, which resists erosion so that it gives a line of two parallel 

outcrops on either side to mark the course of the weathered dike. 

The same phenomenon was seen also in the dikes cutting outward 

through the adinole zone surrounding the Middle Peak stock, where 

trenches or gaps in the metamorphosed rocks mark the site of the 

weathered dikes.

On the upper mountain slopes there are undoubtedly great numbers 

of dikes which could not be mapped with certainty. Their occurrence is marked only by small rock heaps and outcrops here and 

there. Since the exposures of the dense rocks of the extrusive flows 

are so nearly like those of the dike rocks that the difference between 

them can be told only by careful field study and comparison of hand 

specimens, the" accurate plotting of a network of these dikes would 

have taken an amount of time which would not have been warranted 

by the results to be attained. This refers especially to the dikes of 

analcite-basalt (monchiquose) occurring on the slopes covered by 

breccias and flows of similar rock. Even in many cases where the

Bull. 237 04 3
34 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

dikes occur among sedimentary rocks they are often mingled with 

small patches and remnants of flows.

It is certain, however, that the greater part of those occurrences, 

which are clear and undoubted, have been mapped, and they are so 

numerous that the addition of even a considerable number could add 

nothing more to what is already known concerning their character 

and geologic significance.

ROCKS COMPOSING THE DIKES..

For field purposes and mapping the dikes may be considered as 

composed of rocks of two main groups light-colored feldspathic 

rocks, feldspar porphyries (salic type), and dark-colored basaltic 

ones, "traps" or basalt porphyries (femic types). Under each of 

these groups there are several distinct varieties, whose petrographic 

characters are given in detail in a later chapter, but which are here 

briefly noticed as an aid to their recognition in the field and to an 

understanding of their geologic relations. = 

LIGHT-COLORED FELDSPATHIC (SALIC) DIKES.

As in the intrusive sheets, the light-colored feldspathic dikes are 

few in number, not of great size, and, except from the petrologic point 

of view, of relatively small importance. Near the Curry ranch, where 

Arrow Creek debouches from the mountains, and in the basin of 

Aspen Creek they are composed of pale-brown rocks of trachytic 

character with hornblende phenocrysts. They occur also on the edge 

of the Shonkin stock on the main divide between Highwood and 

Shonkin creeks, where the north ridge of Twin Peaks begins, on the 

point above the Arnoux stock, on the slopes leading up from Highwood Gap to the High wooden-South peaks ridge, and at the edge of the 

Middle Peak stock. These are light-gray or brown porphyries with 

embedded phenocrysts of feldspar, usually large, flat, tabular in shape, 

and with small black augite prisms. They are syenite-porphyries, 

largely composed of alkali feldspars. The dikes vary from 6 to 10 

feet in width.

The most important feldspathic dike is the great wall dike in the 

valley of upper Highwood Creek, about a mile below the divide. This 

dike is gray, and although somewhat variable, averages about 12 feet 

in width. It trends toward Highwood Peak. It is massive but somewhat plate-like in structure, and has a sort of contact-like crust composed of very short, small prisms, under which the rock is somewhat 

crumbly. It contains numerous inclusions of dark-colored rock an 

inch or so in diameter, brought from below, and cuts dark basaltic 

breccias which are considerably indurated. At a distance it appears 

as a great light-colored wall running down the eastern slope to the 

Stream, and is a very noticeable object. The rock is a light gray-
PIRSSON.] DIKES. 35

green with numerous small white feldspar phenocrysts. It is a peculiar tinguaite-porphyry (highwoodose).

DARK BASALTIC (FEMIC) DIKES.

For field purposes the dark basaltic dikes may be divided into three 

classes according to the character of the prominent phenocrysts. In 

each case the base or groundmass is more or less dense and ranges in 

color from dark stone gray to black. The first class contains prominent phenocrj^sts of biotite up to half an inch in diameter and is composed of mica traps or minettes of Highwood type (phyro biotitic 

shonkinose). The second class has prominent phenocrysts of rather 

large, well-formed black augites and is composed of augitophyres or 

augite-basalt-porphyry. The third class contains prominent phenocrysts, showing round, octagonal, or hexagonal cross sections of a

white mineral which may be altered leucite, but in some cases is analcite, which is held to be often of primary origin. a This type may be 

termed, in accordance with Lindgren's suggestion, analcite-basalt. 

It is monchiquose.

 There are few feldspathic dikes, as previously stated, and their 

occurrences have been given. In this section what is said of the dikes 

in general refers to the basaltic dikes, which are found in such numbers that it would be useless to try to give specific details of particular occurrences. In so far as individual occurrences present matters 

of petrologic interest these are treated in the petrographic portion of 

this work.

RELATIVE AGE OF THE DIKES.

There have undoubtedly been several periods of dike intrusion, and 

the different centers have had their own periods of formation. This 

renders the relative age of the dikes more or less doubtful, but in 

general ib may be said that the feldspathic dikes cut basic breccias; 

that in one instance on the west side of Highwood Gap an acidic dike 

is cut by a basic one, and that the basic dikes cut flows and breccias 

of all periods and also one another, as at Highwood Gap, as shown 

in the accompanying rough sketch map (fig. 3). Another excellent 

example is seen in the bluff on the east side of Highwood Creek below 

the mouth of the branch creek which drains the north slopes of Arrow 

Peak. The dikes here cut basic (femic) breccias and one another.

Other examples of the intersecting basic (femic) dikes are found 

in many places, as on the southern slopes of South Peak, where three 

intersect at one point and form a star. It is evident that the dikes 

not only occur, in the main, radially disposed around the centers of 

eruption, as previously described, but that locally also they may

"Lindgren, W., Eruptive rocks from Montana: Proc. California Acad. Sci., ser. 2, vol. 3,1890. 

Pirsson, L. V., Monchiquites or analcite group of igneous rocks: Jour. G-eol., vol. 4,1896, p. 679.
36 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

trend in different directions and intersect in a variety of ways. 

Further conclusions regarding their origin and relations are deferred 

until the general history of the district is discussed.

3&$Pine rfootfs 1H_

' {»"" ~"j. V,-T- .J2_£- ^ ~:, .:,: ^ iii?^^: >nr%?riitiiii

Open divide

llf HIGH WOOD

FIG. 3. Sketch map of dikes at Highwood Gap.

EXTRUSIVE FLOWS AND BRECCIAS.

AREAL DISTRIBUTION.

In a very rough way the lower limit of a major part of the piled-up 

extrusive material is defined by the 5,000-foot contour around the 

mountains. Above this, with the exception of Highwood Peak, all 

of the highest summits and ridges of the group are composed of flows 

and breccias. The ridges, such as Pinewood Peak on the west and 

the great main mountain mass running northward through Lava and 

Arrow peaks and terminated at its northern end by Twin Peaks 

and North Peak, are almost entirely made up of these ejections. In 

some places, however, cthey reach down to 4,000 feet, as at the 

debouchment of Highwood Creek on the north side and in the valley 

of Davis Creek on the southeast. The sedimentary strata are found, 

however, also in the higher mountains, as in South Peak. These 

facts are of importance because they show that the eruptions have 

taken place over a country of irregular relief and that a great amount 

of erosive dissection has taken place.
rinssoN.] EXTKUSIVE FLOWS AND BRECCIAS. 37 

FELDSPATHIC EXTRUSIVES OF THE FIRST PERIOD.

Volcanic activity began by the extrusion of heavy flows and explosive breccias of feldspathic type. These flows are petrographically 

between trachytes and andesites; in fact, they are trachyandesites and are described on page 160, under the name of adamellose. 

They occur not only in the flows as firm, dense rocks of felsitic character, but also as explosion breccias made up of angular lapilli with 

cement of finer material, and as tuffs and compacted volcanic ash. 

The greater part of this feldspathic material is found around Highwood Peak. A particularly prominent flow of it occurs on the north 

side, its lower part forming a marked bench. Highwood Creek just 

before it debouches out into the'wider valley, cut in the soft Cretaceous beds, cuts through the lower end of this flow and forms a small 

canyon, through part of which the road passes. Other excellent 

exposures of these heavy flows are found on the north side of Pinewood

Peak, especially on the slope leading down into the valley of North 

Willow Creek, where was collected the material which has served for 

the analysis of this type. Feldspathic flows and breccias occur also 

on the south side of Highwood Peak, in the basin at the head of Little 

Belt Creek. These rocks are light gray to brown in color, dense and 

felsitic in appearance, with small phenocrysts of mica or hornblende; 

sometimes by oxidation of the iron-bearing components they are of 

a deep mahogany red. The breccias are in part explosion breccias 

and in part, as shown by their structure, flow breccias. The finer 

materials produced by explosion the tuffs are found in the 

higher ridges; for example, on the ridge between Middle and Highwood peaks and on the ridge around the heads of Davis and Aspen 

creeks.

The natural deduction from the distribution of the trachyandesite 

(adamellose) lavas and breccias is that the center from which they 

were ejected was at Highwood Peak. The arrangement and dip of 

the flows and heaviest material immediately around Highwood Peak 

and of the finer and thinner at a distance point clearly to this 

conclusion.

Succeeding this period of eruptive activity came one of quiescence 

and erosion, during which the core that had undoubtedly been 

formed was largely cut away. This is shown by the fact that the 

succeeding basaltic ejections fell upon a country of irregular relief, 

spotted here and there with patches or erosion remnants of the 

trachyandesitic lavas and breccias. For instance, at the north foot 

of Highwood Peak, on the north side of Highwood Creek, the basaltic 

extrusives rest directly upon the Cretaceous beds, while a short distance to the south, across the creek, are heavy flows of the trachyandesitic lavas. The latter and tlie breccias and tuffs could not have
38 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

extended so far in other directions and not have covered the surface 

so near the center of activity. If the basaltic outbreaks had'immediately succeeded the earlier feldspathic ones the feldspathic rocks 

should be found in this locality under the basaltic flows. The feldspathic rocks are absent, however, and consequently they must have 

been removed by erosion before the outbreaks of basaltic material 

took place. The same is true in other localities.

BASALTIC EXTEUSIVES OF THE SECOND PERIOD.

 The second and last period of eruptive activity gave rise to material 

very different in character from that ejected in the preceding period. 

While there is a great difference between the types of this period, due 

partly to mineral composition and partly to texture, they have common features which sharply distinguish them from the feldspathic 

lavas and breccias of the first period. They are dark, heavy rocks, 

rich in ferromagnesian components, in augite and olivine especially, 

and sometimes in biotite. They belong distinctly to the basaltic group 

of rocks. They vary somewhat in feldspathic components, but none 

possess plagioclase feldspars and none are of the type of common 

basalt. Commonly, when they are compact rocks, there are embedded 

in them more or less rounded white phenocrysts, which often have 

more or less distinct octagonal or hexagonal outlines. In such cases 

they appear like the dike and sheet rocks previously mentioned, and 

indeed in .many cases it would be impossible to distinguish the two 

rocks. They have also a very striking resemblance to the leucite 

rocks from Italy and from Lake Laach, in the Eifel district in Germany. In some cases the white component is a very fresh analcite, 

and these rocks are analcite-basalts, like those in the dikes. But in 

many other cases the white portion is composed of variable white

minerals of feldspathic nature, and the rock may then be pseudoleucite-basal.t. A very careful search was made for unaltered leucite 

in these rocks, but with the exception of one or two doubtful cases 

it could not be found. It seems most probable from the shape of 

the crystals and their general relationship to these magmas, rich in 

potash, that they were originally leucite, but further consideration of 

this question is deferred until the petrography of the basaltic lavas is 

taken up.

Like the intrusive rocks, the extrusives of this period generally 

carry numerous phenocrysts of well-formed black augites and olivine, 

the latter often altered to a red pseudomorphous mineral.

The breccias and tuffs of this group are dark, chocolate colored, or 

brown rocks running into distinct dark-purple shades in the finer 

tuffs. They carry fragments of basaltic rocks and are generally 

greatly altered and the iron-bearing components oxidized. The fragments are sometimes green, sometimes purple, and mingled with them 

are pieces of black shale occasionally hardened to slaty forms. In
PIBSSON.] EXTEUSIVE FLOWS AND BRECCIAS. 39

some cases the fragments are angular, true lapilli forms; in others 

they are rounded, producing volcanic-conglomerate. In the localities 

visited there was no evidence that they are water-laid.

Slaggy scoriaceous forms were found in the lavas, especially on the 

higher crests, as at Lava, Arrow, and'Pinewood peaks. On the LavaArrow peaks ridge great quantities of very vesicular lava and of 

rounded "bread-crust" bombs were observed. Amygdaloidal forms 

are also common, the amygdules reaching "half an inch in diameter in 

some occurrences. The amygdules are usually compact, with fibrous 

structure, consisting of various zeolites, natrolite, stilbite, etc. Some- 

. times, as on Pinewood Peak, the vesicular rock is full of the pseudoleucite mentioned above, and the vesicles are filled with small 

amygdules of about the size of shot. In such cases it is not easy to 

distinguish them; the rock then appears crowded with white minerals.

Distribution of basaltic extrusives. As may be seen by reference to 

the geologic map (PI. Ill), most of the higher peaks and ridges are 

made up.of basaltic extrusives. High wood Peak and' the ridge running south to South Peak are practically the only exceptions to this 

rule. In Arrow Peak the basaltic extrusions reach their highest elevation. They form two large areas, one on the east, the other on the 

west of Highwood Peak, and at these places rest on trachytic (felclspathic) extrusives of the former period. They form the great central 

area of the mountain group, resting in part on the earlier lavas and 

breccias and in part on Cretaceous strata. They have been greatly 

eroded and are also found to the north in isolated patches of considerable size. A view of the mountain masses at the head of Davis 

Creek, composed of these volcanic ejections, is shown on PI. II, B, 

which is reproduced from a photograph by Mr. Weed.

SOURCES OF THE EXTEUSIVE ROCKS.

In considering the possible sources of the various flows, breccias, 

and tuffs of the two periods, there are certain general facts which 

must not be lost sight of. In a volcanic area which has suffered little 

or no erosion the cones give as a general rule all the testimony that is 

needed in this direction. In the Highwoods, however, great erosion 

has taken place and the cones have been so much cut into and carried away that the remnants afford only partial evidence for their 

reconstruction, and additional data must be sought in other directions.

When a column of molten magma rises through its conduit to the 

upper portions of the earth's crust it gives rise to volcanic action. 

After-the volcano has become extinct and erosion has progressed far 

enough there will be uncovered a stock of massive rock the solidified 

and crystallized magma column of the conduit which is generally 

surrounded by a complex of radial dikes and intruded sheets. This 

complex cuts into, or at a greater distance is surrounded by, still une-
40 . IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [HULL. 237.

roded masses of flows, breccias, and tuffs. Such phenomena have been 

described by Iddiugs at Electric Peak" and on Crandall Creek 6 in the 

Absaroka Range and l>y Mr. AVeed and the writer at Castle Mountain, 

Montana. 6' Examples in other regions are also well known.

In the Highwoods there has been apparently a number of such centers of eruption, and it is difficult to determine what share each may 

have had in this work. As previously mentioned, the weight of evidence tends to show that the feldspathic lavas were erupted at Highwood Peak, as at this place the stock of granular rock most nearly 

agrees in chemical and mineralogic characters with the lavas, as may 

be seen by reference to the petrographic descriptions (pp. 60,160,191). 

The correspondence is not exact, of course, but an absolute chemical 

correspondence between a stock rock and the related extrusives is 

scarcely to be expected, although it may occur. Considerable divergences are noted between successive outflows of the same period, and 

may be caused by progressive processes of differentiation in the liquid 

mass below. The discussion of this subject is postponed to a later 

chapter. Both field evidence and petrographic relations indicate that 

the Highwood stock was the source of the feldspathic extrusives.

The basaltic extrusives probably were erupted from several centers. 

The chemical correspondence is here very close indeed if the rocks 

composing the Shonkin, East, and Arnoux cores are compared with 

the basalt selected for analysis. It seems almost certain that a large 

part of the basaltic extrusives of the central and northern part of the 

area have had their origin at the Shonkin stock center. The arrangement and bedding of the stock and the character of its south portion, 

which consists, according to Mr. Weed, of a tumultuous agglomerate 

of coarse-grained blocks in a finer-grained cement, are strongly confirmative of this. The evidence in regard to the other stocks is not

so clear, and they may or may not have been centers of eruptive

activity.

The basaltic flows and masses around Highwood Peak point to a 

renewal of activity at this center after the period of feldspathic lavas, 

since it seems difficult to refer those of Pinewood Peak, for example, 

to the distant stocks of the central area. The flows might be the 

surface outpourings of later dikes, but the character of much .of the 

material indicates explosive action. If they are referred to this center there were, first, outbreaks of feldspathic lava, followed by eruptions of basaltic lavas, and the intrusion of the monzonite stock was 

succeeded by the intrusion of syenite. The chemical characters of 

these magmas and their bearing on the petrology of this center are 

discussed in Chapter VII, p. 190.

"Eruptive rocks of Electric Peak and Sepulchre Mountain: Twelfth Ann. Rept. TJ. S. GeoL 

Survey, pt. 1,1892, p. 569.

t> Geology of the Yellowstone National Park: Mon. U. S. Geol. Survey, vol. 33, pt. 2, p. 215. 

o Geology of the Castle Mountain mining district: Bull. U. S. Geol. Survey No. 139,1896," p. 56.
EXTKUS1VE FLOWS AKD BKECCIAS. 41

One point in regard to the origin of the basaltic eruptives remains 

to be noticed.- On Arrow Peak heavy flows of basalt dip downward 

along its sides in several directions. This and the general character 

of the peak and its material suggest that the peak may not be a mere 

remnant of a once lofty cone, whose center of activity was at some 

distance, but may itself have been a center of activity. Otherwise 

these flows, whose character and attitude are unmistakable, would 

have to be attributed to outbreaks of dikes on the sides of an erosion 

remnant a not altogether probable supposition. If Arrow Peak represents a center of activity, it is probable that there is, in some part 

of its base, a mass of granular rock, which is covered by flows and 

breccias and which erosion will one day bring to light.
CHAPTER IV. 

GEOLOGY OF THE LACCOLITHS.

INTRODUCTORY.

The laccoliths of the High woods have been described by Mr. Weed 

and the writer in two previous papers, mentioned in the accompany- ' 

ing bibliography, on the Shonkin Sag and the Square Butte laccoliths. 

It has seemed best, however, to repeat in abstract the essential portion of these descriptions, partly to make this paper complete and 

partly because the study of the Shonkin Sag laccolith, made after the 

paper on Square Butte had appeared, has furnished a key to the 

interpretation of the structure of this latter laccolith, which the facts 

obtainable at Square Butte itself do not entirely afford. This has 

caused a modification of ideas regarding it in several respects, though 

it has furnished a remarkable confirmation of the views previously 

expressed. Moreover, while the majority of petrographers have 

accepted the conclusions drawn in these former papers, in their bearing on theoretic petrology, some have advanced views of their own 

which appear to the writer to be founded on misconceptions of the 

facts. It is desired to discuss these and to advance some new conclusions which farther study has brought out. To render the discussion 

more intelligible and convenient a rather full abstract of the descriptive parts of the former papers is embodied in this chapter.

In this eastern extension of the Highwood Mountains the four principal masses of igneous rock are of intrusive laccolithic nature. In 

addition there are a number of dikes and intrusive sheets. There is no 

evidence that the forces which elsewhere in the mountains found outer 

vents and gave rise to extrusive material did so here; the facts all 

point to the opposite conclusion.

The sedimentary beds into which these rock were intruded are 

arenaceous shales or fissile sandstones horizontally disposed, except 

where the intrusions have disturbed them. They are of yielding 

nature and admirably suited for the intrusion of sheets and laccoliths.

As may be seen by reference to the map (PL III), they are cut by 

two drainage valleys which meet each other at a nearly right angle, 

that of Arrow River and that of the Shonkin Sag, in whose flat floor a 

feeble stream, Flat Creek, meanders. The sag is an abandoned channel of Missouri River and is easily traced on the map by the contours, 

the course of Flat Creek, and the scattered alkaline lakes which now

42 -
PIRSSON,] GEOLOGY 01? THE LACCOLITHS. 43

occupy it. The dissection of the land between these two drainages 

has exposed the laccoliths and sheets.

There are four laccoliths in this area, the major and the minor in 

the Shohkin Sag, Square Butte, and Palisade Bntte..

MINOR LACCOLITH OF THE SHONKIN SAG.

About 2 miles due north of Square Butte, as may be seen by reference to the geologic map (PL III), is a mass of igneous rock intruded 

into the sediments. It is shown as a semicircular area of shonkinite 

exposed in the west wall of the sag. From the valley below, it appears 

as a dark cliff, perhaps 100 feet in height and a few hundred yards 

long. It has a columnar structure, and from the foot of the cliff a 

talus slopes down to the valley. It is clearly intrusive in the sandstones, and although not a symmetrical laccolith it is evidently of

laccolithic nature. The rock composing it is similar to the shonkinite

(shoiikinose) of Square Butte. While not of great importance it lias, 

a certain interest and value, as it is another example of laccolithic 

intrusion, and by its confirmative testimony helps to explain the 

nature of the other masses, such as Square Bntte and Palisade Butte, 

where the evidence concerning the character of the intrusion is not 

so clearly shown.

MAJOR LACCOLITH OF THE SHONKIN SAG.

The Shonkin Sag turns west at the smal'l laccolith just mentioned, 

and about 3 miles from it in the north wall of the valley is the section 

of the main laccolith described by Mr. Weed and the writer. From 

the opposite side of the valley an excellent view of it is obtained, and 

it is seen as a columnar cliff about a mile long, with an even and 

regular face, interrupted about the center by a deep canyon-like 

gulch which cuts through the whole cliff. The columnar structure is 

pronounced, the polygonal columns having a diameter of 2 feet or 

more and an estimated height of over 100 feet, which is also the thickness of the laccolith along the cliff front. All along the front are 

seen the horizontal light-brown Cretaceous sandstones which form 

the floor upon which the laccolith lies. The regular, even level of 

this floor is very noticeable. Extending from the foot of the cliff to 

the valley is a great talus which is cut at intervals by rain-washed 

ravines. The detrital material reaches nearly or quite to the foot of the 

columns, and the sandstones beneath the laccolith are in places nearly 

concealed, but in the intervening ravines a considerable thickness of 

them is seen. On top of the laccolith are the horizontal, unbroken 

Cretaceous sandstones into which it is intruded. Their thickness is 

variable, as from each end of the cliff wall it gradually diminishes; 

about the middle, where the laccolith is cut by the intersecting gulch, 

the sandstones disappear and leave, especially on the eastern side, a
44 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

portion of the top of the laccolith bared. On the opposite side of the 

gulch, however, a considerable thickness of them rests on top of the 

columns of igneous rock.

Ends of the laccolith wall. From what has been said above, it is 

clear that this cliff wall is the cross section of a flat mass intruded 

into and lying between the beds of sandstone, its laccolithic character being seen only at the outer borders.

The main body of the laccolith at the eastern end thins out until it 

becomes only an intrusive sheet not more than 10 feet in thickness, 

which on the same horizon extends a great distance into the sandstones. In addition to this main lower fringing sheet there are two

FIG. 4. Plan of Shonkin Sag laccolith. A, former course of Missouri River; B, laccolith; C, gulch.

or three lesser ones, which do not extend far and quickly die away in 

the sedimentary beds.

'At the western end the relations are less clear. The valley wall has 

been broken down and eroded somewhat, so that there is not the same 

vertical cut, but the exposures are sufficient to show that the structure is less typical, the beds having been affected to some extent by 

rupturing. The laccolith, however, frays out in thin, fringing sheets, 

and the basalt sheet, to which it thins down, has the same thickness 

and character as on the other side, and may be traced, always at the 

same level horizon, among the sandstones several miles up the valley. 

The persistency of the thin sheet of which the laccolith is a greatly 

thickened portion is remarkable.

The laccolith rock. The rock composing the outer fringing sheets 

is dark colored, has a dense texture, and is dotted with crystals of a 

black augite and round white spots of an altered leucite. It may be
MAJOR LACCOLITH OF THE SHONKIN SAG. 45

called a leucite-basalt. The lower 12 to 15 feet of the great columns 

of the main body of the laccolith are also composed of this rock, which 

then passes into a fully granular rock of granitic texture, consisting of 

augite, olivine, biotite, and orthoclase, like the shonkinite of Square 

B.utte. At the top the columns again pass into the porphyritic leucitebasalt.

Interior of the laccolith. It has been mentioned that the central 

portion of the laccolith wall is broken by a stream gorge which has 

cut through the laccolith into the underlying sandstones. The stream 

gradually rises above the sandstone nntil it flows, over the igneous 

rock, and at the same time it is joined by tributary gulches, so that 

the whole interior of the laccolith is thoroughly dissected and laid bare.

Ascending the gulch, one soon comes to the contact of the igneous 

rock and the underlying sandstones. The fissile, platy sandstones at 

the contact are changed to a dense, flinty, blue rock, which generally 

has a thickness of a few inches and never more than about a foot. 

The igneous rock at the contact is dense and dark and is filled with 

augite and altered leucite phenocrysts; it is similar to the rock in the 

outer columns described above. It maintains this character for 

about 15 feet and then passes into an evenly granular, coarse-grained 

shonkinite. This has a columnar parting, which, as one approaches 

the center, is less evident than in the outer cliff wall. The drainages 

now lose their canyon-like character and widen out into V forms with 

broader spurs between them, often showing wide expanses of naked 

rock.

The shonkinite has a thickness of about 75 feet and is succeeded 

by a rock of different character. There does not appear to be any 

contact between the two, but the shonkinite in a little distance passes 

into the new type. This is a much lighter colored rock of a coarser 

grain. It is composed of large augites, many of them 1 to 2 inches 

-long, often radiating from a common center in such a manner as to 

form stars, and of equally long but slender foils of biotite, with the 

interstices filled with white feldspars or feldspathic material. This 

rock was always found in a weathered and crumbled condition. This 

layer or zone is about 15 feet thick and passes above into a white 

syenite of medium grain, specked with augite crystals. This has a 

rather thin, horizontal, platy parting, by which it splits and weathers 

into piles of plates. On a clear day the decided contrast makes the 

syenite seem white and the dark shonkinite appear black, especially 

when the two rocks are seen in large masses.

This syenite resists weathering much better than the crumbly 

transition rock mentioned above, and has therefore determined the 

shape of curious and fantastic rock piles along the line of the outcrop. These piles commonly take the form of a mushroom or stool, 

in which the outspreading top is formed of the syenite, while 1Jie stem 

is composed of the transition rock.
46 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. £37.

The thickness of the platy white syenite is about 25 to 30 feet, and 

above it passes within a short space into the same coarse, crumbly transition rock that occurs below it. Here, however, the transition rock 

is only about 5 feet in thickness; it then passes into a coarse shonkinite like that below, which in about 5 feet begins to be denser and 

blacker, and in about 5 feet becomes a porphyritic rock spotted with 

augites and altered leucites the same leucite-basalt as at the bottom.

The top of the laccolith is an elevated plateau, which has a marked 

turtle-back form and is cut in the center by the gulches which merge 

into the dissecting.gorge. Except around this basin it is covered with 

the overlying sediments, but here the top of the igneous rock is 

exposed.

Cause of dissection. From the presence and character of the rnorainal drift it is evident that during the Glacial epoch the continental 

ice sheet in this region pushed its way as far south as the lower slopes

PIG. 5. Stereogram of Shonkin Sag laccolith.

and foothills of the Highwood Mountains. The Missouri River was 

driven southward and forced to flow around the foot of the ice front. 

In doing so it excavated the valley of the Shonkin Sag and thus sawed 

down and through the outer circumference of the laccolith. The 

position of the intersecting tributary was previously determined and 

the ice moved over the already eroded surface of the laccolith. At 

the close of the Glacial epoch the river retreated to its present position, and since then erosion has deepened and extended the dissection. 

The relative positions of the former Missouri Valley, the laccolith, and 

the dissecting stream are shown in fig. 4, which represents them in a 

diagrammatic ground plan. In fig. 5 is given a stereograni of the 

laccolith, in which the various points mentioned are shown in a 

diagrammatic way. The dotted surface, at the top represents in a 

general manner the area bared by erosion.

Internal structure. -From the description which has been given, it 

is clear that the outer portion of the laccolith is different from the
PIRSSON.] MAJOK LACCOLITH OF THE SHONKIN SAG. 47

interior in character and in the type of .rocks composing it. The differences may be concisely summed np in the following section, which 

is approximately correct:

Section of laccolith.

Dense shonkinite . . ...-......-.....--.......---.----.....----

Shonkinite ................ --. .............................

Syenite .......-.-.......-....---.--.-.-.--.------..-.--...--

Transition rock .............................................

Shonkinite ...................----.----.-.---------...-----

Total (approximate) --.....-........----.---.--..-.....

Center.

Feet. 

5

5

5-6

3

25-30

15

60-75

15

140

Outer wall.

Feet. 

 10-15.

75

15

100

From these data a cross section of the laccolith can be constructed 

which will have the appearance shown in fig. 6.

FIG. 6. Cross section of Shonkin Sag laccolith. Vertical and horizontal scales the same; white 

indicates syenite and transition rock; black indicates shonkinite.

Tn fig. 6 the vertical and horizontal scales are the same, and the 

sheet-like or flattened shape of the laccolith is shown. The transition rock and the syenite are shown in the white portion of the section. 

Their vertical thickness is known, but their horizontal extension 

is from necessity largely conjectural, since there is known only the 

distance from the center to the outer cliff wall, as revealed in the 

dissecting gulch.

The writer's conception of the laccolith and the structural relation 

of its interior parts would be represented by the figure of revolution 

which would be generated if the cross section were revolved upon 

a perpendicular drawn through its middle point. It is true this 

would cause the laccolith and the successive shells and syenite kernels to be also circular, to have a common center, and to become a 

true circle in ground plan. It is not known that this is exactly the 

case. The laccolith, the interior shells, and the kernels-may be more 

or less ellipsoidal or irregular in outline. They probably do not have 

exactly common centers and the same thickness everywhere. Nevertheless, these are mere details which are believed to be of little importance in comparison with the idea that the figure would express the 

generally circular, concentrically zonal arrangement of the parts.
48 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [HULL..237.

The discussion of the bearing of these conclusions on theoretical 

petrography and the origin of the various rock types is deferred 

until a later chapter.

PALISADE BUTTE.

Palisade Butte is less than 2 miles west of Square Butte. It stands 

isolated upon the open country, and forms, like Square Butte, a 

prominent landmark. It rises about 800 feet above the plain', and its 

outline against the sky resembles the weathered stump of a tree. On 

all sides is a long talus slope, in great part covered with soil and grass 

and broken here and there by low outcrops and masses of rock, which 

leads up with increasing gradient from the plain to the bare cliffs of 

massive rock which compose the main mass. Above these cliffs are 

again steep slopes interrupted by cliffs.

The walls are remarkable for the regular columnar structure of the 

rock. The hexagonal columns are, on an average, about 18 inches in 

diameter, though often greater, and divided by regular cross joints. 

Where they occur they extend the whole height of the exposures, in 

places a distance of 100 to 150 feet, but it is evident that originally 

they must have extended through the whole of this portion of the 

igneous mass, as in the Shonkin Sag laccolith, and therefore may 

have been several hundred feet in length. Their character is shown 

in PI. IV, A, from, a photograph by Mr. "Weed.

The rock composing the columnar portion of the butte and the outcrops in the talus is shonkinite. On exposed surfaces it weathers to 

a dark color, giving the cliffs a somber and gloomy character.

The butte is crowned at the top by a large mass of a light-colored 

rock of platy, laminated character. This is an augite-syenite, much 

like that in the Shonkin Sag laccolith. It weathers with a lighter 

color than the shonkinifce on account of the preponderance of feldspar 

over augite, and the contrast between the two, while not pronounced,

is striking.

The mass of syenite, which has a considerable thickness, is roughly 

wedge-shaped, with a cliff of some height toward the south and a 

slope toward the north. On the south side the rock, with its platy 

horizontal parting, is clearly seen resting on the massive columnar 

shonkinite, but toward the north the relations are less clear, because 

the butte to a great degree loses its precipitous character and slopes 

down in talus masses toward the plain, with the broken-down rock 

masses intermingling.

Rock variation in Palisade Butte. The rock composing Palisade 

Butte shows the same variation as that found in Square Butte and 

. the Shonkin Sag laccolith, only it is not so sharply expressed. In the 

low outcrops which rise through the talus slopes and are farthest from 

the butte the rock is very rich in augite and therefore dark and femic 

like that of Square Butte. As one approaches the butte an increase 

in feldspathic components is observed, and the rock of the columnar
U. S. GEOLOGICAL SURVEY BULLETIN NO. 237 PL. IV

A. COLUMNS OF SHONKINOSE, EAST SIDE OF PALISADE BUTTE.

7>. VitW IN ZONE OF EROSION MONOLITHS, SQUARE BUTTE.
PALISADE AND SQUARE BUTTES. 49

cliffs is perceptibly different from that of the lower outcrops. In the 

syenite of the platy mass on top this difference is, of course, much 

more marked; the rock is not so strikingly salic as the syenites of 

Square Butte and the Shonkin Sag laccolith, but it is still a syenite.

Thus the same order holds as in the other laccoliths a basic outer 

and lower mass, and above a more feldspp,thic upper inner portion. 

There has been, therefore, the same kind of differentiation as in the 

other laccoliths, only it is more gradually expressed.

Laccolithic character of Palisade Butte. The position, structure, 

and rock relations show that Palisade Butte is a remnant of a laccolith formerly of greater size.

It is to be noted- that a heavy dike of dark fernic rock of monchiquoid habit (shonkinose) runs out from the eastern side toward Square 

Butte. As shown elsewhere, this dike has the same chemical character as the shonkinite, of which the greater part of the butte is composed. It is thus possible that this dike may have been the source 

of the igneous material, especially since the present mass is evidently 

only a remnant of a much greater one which formed the original laccolith. In this connection it may be observed that the sedimentary 

beds of the plain on this side are hardened and toughened, with fragments and patches of black, dense femic rock,-which may possibly be 

remnants of the laccolithic floor in this direction.

SQUARE BUTTE.

Introductory. In size and conspicuous position Square Butte is the 

most important of the laccoliths of the area. That it was known 

to the early explorers may be inferred from the remarks quoted 

in the historical summary given in the beginning of this bulletin. 

The exploratory work of Davis and Lindgren evidently did not lead 

them in this direction, since they give no description of it. Previous 

to the former paper by Mr. Weed and the author, the only article relating to Square Butte of which the writer has any knowledge is one by 

Lindgren and Melville," in which is given a petrographic description 

of material collected there by Dr. C. A. White. The butte is described 

as being composed of a light-gray eruptive rock having a distinct 

lamination, with several sheets of a dark volcanic rock intruded in the 

sediments around its base. This is presumably from a note furnished 

the authors by Doctor White, who had passed under its base a number of years before, while on a trip collecting fossils in company with 

the late Prof. Jules Marcou.

General description of Square Butte. Square Butte is a circular 

mass resting on the point of the table-land at the junction of the 

Arrow Creek and Shonkin Sag valleys. The platform consists of 

nearly horizontal shales and sandstones, which on three sides are

« Am. Jour. Sci., 3d ser., vol. 45,1893, p. 286. 

Bull. 237 04 4
50 IGNEOUS EOCKS OF HK4HWOOD MOUNTAINS. [BULL. 237.

deeply trenched by stream gulches descending from the base of the 

butte into the larger valleys below. On the western side an open 

plain stretches from the base of the butte toward Palisade Butte and 

the breccia foothills of the main mountain group. It is the dominating feature of the scenery in this part of the region, and forms a 

prominent landmark from the open level country to the north and 

east, its dark base and white crown making it conspicuous for a long 

distance. Its name is derived from its flat top.

The trenched table-land on which the butte' rests has a height of 

4,000 feet above sea level, and the mass of igneous rock rises 1,700 

feet above its pediment, making the altitude of the upper surface 

about 5,700 feet. The slopes are at first gentle, but become steeper, 

and at the top, on all sides, is a precipice perhaps 200 feet high. In a 

few places this escarpment is cut by small, narrow gulches. The 

summit is nearly level, elliptic in outline, and nearly 1 mile across in 

its greatest length.

The symmetric form of the butte is rather remarkable. It presents 

from nearly every point the appearance of a very short section of a 

huge cylinder resting on a low, broad, truncated cone. This regular 

arrangement is interrupted only on the southwest side, where a short 

tongue-like protrusion of the mass occurs.

Laccolithic origin of Square Butte. Square Butte is composed 

entirely of igneous rock. Above the sandstones of the table-land no 

sedimentary rock whatever is seen. Near the immediate contact of 

the igneous rock with the sedimentary strata the sandstone beds 

curve up sharply on all sides. Below this, where the trenching by 

the streams has gone on, the sandstones have been cut into and the 

intrusive sheets which form a peripheral fringe around the mountain 

are brought to light. These relations are shown on the map, PL III, 

and in the cross section, fig. 8.

The form of Square Butte, the ring of upturned sediments around 

it, and details of structure, which will be presented later, make it evident that the butte is a laccolith stripped of its sedimentar}7 cover, but 

not yet sufficiently eroded to lose its general form. This interpretation 

of its origin is also supported by the occurrence to the east of Square 

Butte of the laccoliths in the Shonkin Sag previously described.

Lower zone of dark monolitlis at Square Butte. Square Butte, from 

everj^ point of view, presents, first, a base of dark, somber slopes, 

extending nearly halfway to the summit, which in turn are capped by 

light-colored ones that over great areas are often white. From a distance of a few miles the dark base is seen to be fantastically eroded 

into jutting towers and spires of rock. This series of strangely shaped 

monoliths, which surround the lower slopes of the mountain on all 

sides, die put at about a given height. Above this fringe of pinnacles 

are seen the white upper slopes, composed of masses and walls of rock 

which are in marked contrast to the black base.
PIRSSON.] SQUARE BUTTE. 51

As one approaches nearer and enters the region of black monoliths, 

it is found to be a maze of small, partly wooded glens, separated by 

towering masses and pinnacles of rock which have a height of from 

100 to 150 feet in many places and of but a few feet in other places. 

The attention is immediately arrested by a peculiar and regular 

platy structure. The masses of rock are built of a series of inclined 

disks, each a few inches in thickness and oval to subangular in shape, 

with rounded edges which accentuate the disk-like form. Generally 

the disks decrease in size from bottom to top, but there are exceptions to this rule, and in these cases strange and weird figures are 

produced. The plane or hade of the disks is not horizontal, but 

inclines to the outside in all directions around the mountain, approximately parallel to the prevailing slope, which, indeed, is determined by this platy parting. Their character may be seen in PI. 

IV, 5, which is reproduced from a photograph by Mr. Weed.

The disposition is precisely like the dip and strike of sediments in 

a domed anticline, and the resemblance at times to sedimentary strata 

is striking.

Upper zone ofwliiie rock at Square Butte. The monoliths are found 

over a distance of a mile up the slope. They diminish in size as 

one goes upward, and a horizon is reached where the rock changes 

abruptly from the dark, nearly black augitic phase to the white syenite described by Lindgren. In many places the monoliths continue 

higher, but are made of the white rock. They are smaller in size, 

but possess the same remarkable disk-like, platy structure, and the 

disks are perfectly parallel to those of the black variety below.

The transition line between the two rock varieties is extremely 

abrupt, but it is not of the nature of a contact. The even grain continues throughout, but in the space of a few inches or a foot or so the 

black augite begins to diminish and finally disappears, hornblende 

occurs, the rock assumes a more feldspathic character, and rapidly 

passes into the syenite which was described by Lindgren and which 

constitutes the main inner mass of the mountain. There is thus a 

narrow mottled zone between the black and the white rocks.

The monoliths which lie near the transition zone are sometimes 

black disks resting in place on white rock below; the transition band 

sometimes passes through them and they are black disks resting on 

white ones, or it passes through the disks almost vertically, so that 

one part of each disk is white and the other black.

The facts just presented are to be carefully noted, because they 

show that however much the two varieties of rock may differ and 

however abrupt may be the change from one into the other, they were 

not formed by two separate intrusions, but on the contrary are a geologic unit, and that the mass as a whole Avas intruded at one and the 

same time, and cooled and crystallized under the same conditions,
52 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. ?37.

and that the explanation of the peculiarities which it presents must 

be sought in another way one which has an important bearing on 

theoretic petrology.

As one approaches the top no more black rock is seen; the remainder of the mass is of white or pinkish syenite and presents everywhere 

the same even grain. The same platy structure continues, and at 

times there are no talus slopes, vegetation, herbage, or even soil, only 

smooth, white surfaces of naked rock, on whose almost polished 

slopes it is impossible to climb. Toward the top the average thickness of the plates increases somewhat and their dip gradually becomes 

less, until eventually they are horizontal. The ring-shaped precipice 

which forms the top has been caused by breaking off of the horizontal 

plates. The regularity of this platy jointing, together with the even 

rounding of the corners through weathering where the joint planes 

cross, gives a likeness to colossal masonry in the upper walls.

Origin of the platy parting at Square Butte. From what has already 

been said in regard to the platy parting which forms so marked a 

feature of Square Butte, it will be seen that it bears the same relation 

to the mass as a whole as do the enfolding leaves of an onion to the 

bulb cut in half by a horizontal plane.

The parting planes are thought to represent parting surfaces parallel to the former covering of the laccoliths, from which the isothermal planes of cooling descended into the mass. The writer can 

conceive of no other hypothesis which would give a reasonable 

explanation of their arrangement and disposition; and since Square 

Butte is unquestionably an intrasive mass, they are regarded as one 

of the strongest proofs of its laccolithic nature.

Such an arrangement of the parting planes of a cooling igneous mass 

is by no means unknown, however, as it frequently occurs in the 

great phonolite domes of central Europe. Rarnsay a has also described 

a similar kind of parting in the great mass of alkaline syenitio rocks 

of Umptek, in the peninsula of Kola, in Russian Lapland, which he 

regards as of laccolithic character. He also attributes this parting to 

the fact that the cooling planes caused shrinkage parallel to the outer 

cover of the mass. He also believes that between the rock layers thus 

formed later injections of magma of different composition have been 

forced, like intrusive sheets in sedimentary strata.

Johuston-Jjavis, & in a short criticism of the former paper on Square 

Butte by Mr. Weed and the writer, suggests that this parting is not 

due to shrinkage caused by cooling, but to initial shearing stresses 

which were due to the forcing of a viscous .mass into its present position and which are like that which produces the well-known lamination observed in many lava flows of highly siliceous magmas. This 

view the writer can not accept, because this platy parting parallel to

aFennia, II, No. 2,1894, p. 81.

6 Brit. Assoc. Adv. Sci., Eept. Liverpool Meeting, 1896, p. 792.
PIKSSON.] SQUARE BUTTE. 53

the upper outer surface is extremely common both in intrusive masses 

of all kinds of igneous rocks and in cases where the magmas must 

have been very fluid at the time of movement and shearing stresses 

could not therefore have occurred in them. In many cases such 

masses have at the top aplaty parting, which grows thicker below and 

finally changes to the columnar parting in conformity with the law^ 

that the initial major parting takes the direction of least resistance. 

The mass in which is the celebrated Fingals Cave, and the Shonkin 

Sag laccolith, previously described, are examples, and if Square Butte 

were deeply dissected by erosion toward its center it might show an 

outer platy parting and a deeper inner columnar one. Palisade Butte 

offers strong evidence on this point, as it has the same structure and 

thus unites all these three laccoliths together iato a similar group. 

In view of the various stages of dissection in the different laccoliths, 

it being least in Square Butte, and of the facts just presented, there 

is no good reason for assuming that the platy parting of Square Butte 

was produced otherwise than by the usual and well-known one of 

contraction due to cooling. Square Butte is the largest, cooled the 

most slowly and regularly, and therefore possesses the platy parting 

with the most regularity.

The fact that the same parting planes pass through syenite and 

shoukinite alike is also strongly opposed to the view that they are due 

to a shearing lamination. And, moreover, since the general study of 

all the facts presented -by these three laccoliths has led to the belief, 

as shown elsewhere, that the two kinds of rocks found in Square Butte 

have been produced by differentiation after the intrusion of the 

magma in a homogeneous condition, it is also believed that the magma 

was then in a far too liquid condition to have permitted shearing 

stresses to be set up within it.

Diagrammatic section at Square Butte. It has been already said 

that the study of the Shonkin Sag laccolith has caused the writer to 

modify in some degree his conception of what is the probable interior 

constitution of Square Butte. The same facts are also to be seen in 

a less pronounced degree at Palisade Butte, but until after the Shonkin Sag laccolith was studied their true significance was not understood. Square Butte supplies the key and unites all the laccoliths in 

one group. As the laccoliths are in different stages of dissection, each 

supplies evidence that is wanting in the other two.

In the previous published section, which may in a simplified form 

be seen in fig. 7, the syenite was given a very great volume, and the 

transition zone was placed at the bottom of the laccolith, thus making 

it consist chiefly of this rock. At that time the conception as to the 

relative amounts of the two rock types was wholly theoretical, the dissection being no greater than the diagram indicates. Since, however, 

all three laccoliths consist of the same kinds of rock, i. e., similar 

magmas intruded near one another at the same time and under si ml-
54 IGNEOUS BOCKS OF HIGHWOOD MOUNTAIN'S. [BULL. 237.

lar conditions, and since they are of similar structure, so far as shown 

by erosion, it is reasonable to conclude that they are essentially 

similar throughout, though they may differ in minor details of form, 

size, completeness of differentiation, etc. This being the case, the 

writer now believes that shonkinite comprises the greater part of

Scale */.,

FIG. 7. Former section through Square Butte. a, Syenite; b, shonkinite; c, sandstone and 

shale; d, underlying sheet. Dotted line shows restoration of laccolith. Vertical and horizontal scales are the same.

Square Butte and underlies the syenite as in the other two laccoliths, 

and that the relations of its parts are correctly shown in fig. 8. In 

this connection the geologic map should be consulted.

The protrusion to the southeast on a projecting rock tongue also 

confirms the above hypothesis, as it shows the shonkinite below, with

FIG. 8. New section through Square Butte. New line, a, Syenite; 6, shonkinite; c, sandstones 

and shales; d, underlying sheet. Dotted lines show restoration of laccolith. Vertical and 

horizontal scales are the same.

white syenite above, as seen in PL V, A, from a photograph by Mi1 . 

Weed.

Consideration of the bearing of these facts on the question of the 

origin of the laccoliths and their relation to the other igneous rocks 

of the High woods is deferred to the last chapter, on the petrology of 

the area.
U. S. GEOLOGICAL SURVEY IULLETIN NO. 237 PL. V

A. PULASKOSE (SYENITE) RESTING ON SHONKINOSE, SOUTHWEST 

SIDE OF SQUARE BUTTE.

B. EAST END OF SHONKIN SAG LACCOLITH.
CHAPTER Y.

THE SEDIMENTARY PLATFORM.

INTRODUCTORY.

 The sedimentary beds on which the extrusive masses of the mountains rest, and which are cut by the intrusive rocks (see PL III), have 

been described by Mr. Weed in the Fort Benton folio of the Geologic 

Atlas of the United States. The part of his description which relates 

to the area shown on the map is here quoted, in order to show the 

character of the material in which the igneous rocks were intruded. 

The nature of the sediments has so often influenced the manner in. 

which these intrusions took place that this is an important part in the 

discussion of the latter. The beds are all Cretaceous.

Cascade formation. The Cascade formation consists of alternating beds of 

sandstones and shales. The lowest bed of sandstone, 160 feet thick, is distinguished from the underlying Ellis sandstone [top of the Juratrias] by its dull 

brownish color, its weathered appearance, and its lamination. Above these sandstones is the coal seam, which in some places occurs near the top of the formation. The sandstones lying immediately beneath it are impure, micaceous, 

and of a lavender tint. The coal seam is from 5 to 12 feet thick. A massively 

bedded sandstone that caps the coal seam and forms a sloping table-land over 

large areas is taken as the top of the formation. The total thickness on Belt Creek 

is 520 feet. * * *

The fossil leaves found with the coal seam are of Lower Cretaceous age and 

resemble those of the Kootanie formation of Canada.

Dakota formation. The Dakota formation consists of beds of sandstone alternating with red and gray shales and clays. Certain purplish sandstones are of 

variable composition, carrying large amounts of purplish and red shale in streaks 

or disseminated through the rock. Buff-colored sandstones are purer and are 

often cross bedded. Some sandstones pass horizontally into clays, and these and 

the interbedded clays are mostly of a reddish or lavender color, containing lumps 

of yellow sandy material. At a horizon 140 to 300 feet above the coal of the Cascade formation the red shales contain bowlders and lenses of limestone varying 

from a few inches to a foot or more in thickness. The rock is dense, blue gray in 

color, weathering light buff, and contains numerous fresh-water fossils. The 

sandstone above these fossil-bearing beds is assumed to be the uppermost bed of 

the formation, giving a total thickness of about 300 feet along Belt Creek.

Colorado formation. The Colorado formation consists chiefly of-leaden-gray 

shales. The base is not readily separable from the Dakota, as the sandstones of 

that formation become shaly toward the top and alternate in their layers with the 

dark clay shales of the Colorado formation. The Colorado comprises two formations, previously distinguished as the Benton shale and the Niobrara limestone. 

The former is typically developed about the town from which it is named [Benton 

on the Missouri, about 20 miles north of the High woods]: the latter is also a 

shale formation in this quadrangle, but contains limestone concretions. * * *' 

[In] the Belt coal field the Benton shale consists of alternating strata of gray

55
56 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

shale and impure shaly sandstones, a good section of which is seen in Belt Butte. 

At this locality and along the south base of the Highwood Mountains the formation holds a white ash bed whose rock resembles porcelain and breaks into shaly 

fragments. The sandstone over this ash bed contains fish-scale impressions, and 

100 feet beneath it is a sandstone bed which generally holds pebbles of a black 

chert. * * * West and south of the Highwood Mountains it [the Ben ton] consists of dark-gray or black shales in beds 50 to 200 feet thick, alternating with 

yellowish, rather massively bedded sandstones. * * *

Eagle formation. The Eagle formation * * * consists of a sequence whose 

most prominent bed is sandstone. The whiteness of the formation is in marked 

contrast to the dark-gray shales above and below it. The formation consists at 

the base of thinly laminated sandstones stained light brown by lignitic material 

and containing concretions and nodular masses of iron ore. These beds grade 

upward into a pure white sandstone which . * * * forms extensive bluffs 75 to 

100 feet high. * * * Exposures * ' * * occur in the bluffs of Arrow Creek 

and northeast of Square Butte, where its thickness is 235 feet. At these localities 

the beds are nearly level or but slightly tilted and their relation to the leaden-gray 

Colorado shales beneath is clearly seen. * * * Fresh-water shells are found in 

white sandstone beds in the High woods « which may represent the formation, but 

the strata are flexed and disturbed, while the species identified are forms commonly found in the Laramie. The beds on Arrow Creek are clearly capped by a 

conformable series of marine beds 2,000 feet in thickness, which are in,turn overlain by the Laramie to the east (of the map).

Montana formation. The Montana formation is composed principally of 

leaden-gray clay shales which are much like those of the underlying Colorado. 

The formation contains much sandstone interbedded with the shales in the Highwood Mountains. * * * The subdivisions of the Montana recognized farther 

east are not found here, and it is doubtful whether the top of the formation exists 

within the limits of the Fort Benton quadrangle. &

Summary. On top of these formations are bench gravels and 

alluvium in the stream bottoms, and over a part of the area outside 

of the foothill country there is spread the continental glacial drift 

and till, carrying bowlders from distant sources. The total thickness 

of the sediments in this area, as given by Mr. Weed, is as follows:

Thickness of sedimentary rocks near Highwood Mountains.

Surficial materials I.--...-.....-----..-.-.-.....-......... variable.

Montana.._...______.-_._____._-...__-_..__...._'__._ 1, 200-1,600

Eagle.-...-.....-....-..................'-... ................ 235 

Colorado .......^.............................................. 1,850

Dakota................................................... 180

Cascade -....--.. ...-...--.-.--..---......--....--.........,.. 500

This gives a total thickness of from 4,000 to 4,500 feet of soft sandstones and shales of the character described above. These strata are 

weak, unresistant, and easily ruptured, and without doubt this fact 

has had great influence in determining the character of the intrusions 

and the great number of dikes which in the Highwoods form so 

marked a feature of the structural geology. Except for local disturbances in the mountains, the beds are horizontal or nearly so.

« That is, in the mountain area itself. L. V. P.

I' Consequently it is not found in the area shown on the accompanying geologic map (PI..III). 

L. V. P.
CHAPTER VI. 

PETROGRAPHY.

INTRODUCTION.

The Highwood Mountains have proved a very interesting field for 

the petrographer. Some of the rock types that they have furnished 

have proved of importance in systematic petrography, and all of them

are more or less novel in character and their study and description

has added to the theoretic side of the science. Rosenbusch a early 

called attention to the importance of the igneous rocks of central Montana, and in concluding his brief report upon the Highwoods in 1885 

Lindgren says: "Future examination will doubtless reveal rich harvests to the petrographer in the Highwoods and in the mountains of 

the upper Musselshell River." The truth of this remark has been 

shown in "his own paper on the syenite of Square Butte and the various memoirs on this region published by Mr. Weed and the writer, 

and it will receive, we belieA*e, a further illustration in the present 

work.

The field can not be considered, however, as exhausted. The expenditure of the amount of time necessary to make complete collections 

for petrographic purposes would not have been warranted under the 

circumstances by the results to be attained. Without doubt, in the 

future, petrographers will find many matters of interest which were 

overlooked or only touched upon in the necessarily rapid field work. 

Ifi is believed that the larger features of importance which the district 

affords and which tend to the elucidation of the many problems of 

petrology that confront the student of igneous rocks have been discovered and are sufficiently treated in the present work.

Most of the analyses here given have been already published in 

Bulletin No. 228 of the United States Geological Survey, with'very 

brief notes or lists of their minerals and under the provisional field 

names first used by the author. Those who have used the analyses 

will find no difficulty in recognizing them in the present work.

Summary of Lindgren's petrograpllic work. Lindgren 6 regarded 

the Highwoods as the basement complex of an old volcano whose 

pipes and dikes had resisted erosion and given form to the moun-

« Mikr. Phys. Mass. Gesteine, 3d ed., 1895, p. 128.

6 Tenth Census, vol. 13, pt. 15, Mining Industries, 1885, pp. 724-730.

- 57
58 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. .[BULL. 237.

tains. In describing the rocks petrographically he divides them into 

trachytes and basalts. Since they are considered post-Cretaceous in 

age he does not directly classify them as syenites and other granular 

rocks, but, following the prevailing usage of that time, he placed them 

under the headings mentioned above, and considered the granular 

types he met as the equivalents of syenite, etc; It should be recalled 

that at that time the age distinction with respect to igneous rocks 

still held a strong position with the leading systematists.

Under the trachytes three types are described: (a) Coarse granular, 

(6) porphyritic with feldspathic groundmass, and (c) porphyritic with 

a strongly augitic groundmass. The first type, so far as one can 

judge, seems founded on the syenite (pnlaskose) of High wood Peak. 

The second is clearly a description of the highwoodose (tinguaite var. 

highwoodite) of the great dike, about 1 mile north of Highwood Gap, 

in the valley bottom. The augite he describes rather fully, and considers it to be the same throughout the whole range of Highwood 

rocks. A series of specimens are then briefly described, in which 

there is a gradual increase in the ferromagnesian components, especially the augite, until they become dark basaltic types or basalts 

with sanidine instead of plagioclase. The appearance of olivine in 

these is noted. This progression brings us to the third type, with 

strongly augitic glassy groundmass. These are dense dark-green or 

gray rocks, with phenocrysts of augite, feldspar, and nosean (or analcite), and isotropic groundmass full of augite needles. Of these 

types just mentioned no locality is given, but from the general course 

of the party through Highwood Gap they appear to Have been collected in this part of the mountains, and would seem to be some of 

the basaltic dikes of this part of the area. The last type appears to 

be a rock of tinguoid habit, judging from Lindgren's description and

his correlation of it with a Judith Mountains type, where rocks of

tinguoid appearance (judithose) are common, as described by the 

writer. In this case it can be referred to the tinguaite (pulaskose) 

of South Peak, as described later.

Lindgren's account of the Highwood rocks is concluded by a rather 

full description of the analcite-basalts, the first announcement of this 

interesting group. This was followed by a later paper dealing especially with them and discussing the origin of the analcite, which is 

held to be of primary origin. This part of his work on the Highwood 

rocks will be referred to in detail later, when the analcite rocks are 

discussed.

Previous work of the author. As mentioned in the bibliography of 

the liighwoods, several papers dealing with particular phases of the 

rocks or descriptive of special types have been published ~by the 

writer, with field notes upon the occurrences by Mr. Weed. In this 

report that material has been drawn upon so far as is necessary to
PETKOGRAPHY. 59

give completeness to the work, and the results of additional study 

and correlation will be added.

CLASSIFICATION.

For purposes of petrographic description the Highwood rocks will 

be classified according to their geologic mode of occurrence. It is 

admitted at the outset that this represents no scientific system of 

classification, since by it things quite diverse in a petrographic sense 

might be brought together. Under the head of dikes, for instance, 

might be included rocks of all kinds mineralogically and rocks of 

granular, porphyritic, and even glassy texture. The classification is 

made solely for convenience in local use and reference. According 

to this classification, the rocks will be divided into the rocks of the 

stocks, of the laccoliths, of the dikes and sheets, and of the extrusive 

flows and breccias. They are named arid described according to the

new system of classification devised by the author and others," and it 

is hoped that the work will serve as an illustration of the use of this 

system. According to it, the following classes, orders, rangs, and 

subrangs are represented in the Highwood Mountains:

Rocks represented in Highwood Mountains, according to neiu classification.

Class.

1 

Persalane ...........

i

Order.

Canaclare ---------

Bohemare -...-.-.-

Rang.

Pulaskase .........

Dacase ....--...-..

Umptekase.... ....

Monzonase .........

Andase .......---.

jLanrdalase ........

lEssexase ..-...--..

| Wyomingase ......

I Monchiquase ........

Kamemnase ......

Albanase '..........

Subrang.

Pulaskose.

Adarnellose.

Highwoodose.

Monzonose.

Shoshonose.

Fergusose.

Borolanose.

Montanose.

rSlionkinose. 

\Monchiquose.

Cascadose.

Albanose.

To one acquainted with the new system and its precise definition of 

magma units this assemblage of subrangs gives a general idea of Highwood magmas and rocks and their genetic relations with one another 

that it would be difficult to convey except by long descriptions.

Since this new system is as yet hardly old enough for petrographers 

to have become thoroughly acquainted with it, the rocks in the following descriptions are also given the usual names; the reader will thus 

be able to make comparisons between the systems and to correlate the

a Cross, Iddin<?s, Pirsson. and Washington, Quantitative Classification of Igneous Rocks, Chicago, 1903.
60 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

types with those of other, regions. According to the prevailing systems the rocks to be described are as follows:

Rocks represented in Higliwood Mountains, according to prevailing classification.

GRANULAR ROCKS.

Syenite, Augite (Albany type of Rosenbusch). Higliwood Peak.

Syenite, Augite (basic type approaching shonkinite). Middle Peak.

Syenite, Sodalite. Laccoliths.

Syenite, Nosean (new type). Intrusive near High wood Gap.

Monzonite. Highwood Peak.

Shonkinite. Shonkin stock and the laccoliths.

Shonkinite (with leucite). East.Peak-and Shonkin stock.

Missourite. Shonkin stock.

Fergusite (new type of leucite rock). Arnoux stock.

PORPHYRITIC ROCKS.

Syenite-porphyry (Higliwood type). In dikes. 

Trachyte-porphyry (hornblende-bostonite-porphyry;1 ). In dikes. 

Tinguaite-porphyry (Higliwood type). In dikes. 

Trachyandesite (latite). In flows and breccia. 

Minette (Highwood type). In dikes and sheets. 

Analcite-basalt (monchiquite). In dikes and flows. 

Pseudoleucite-basalt. In dikes, sheets, and flows.

PETROGRAPHY OF STOCKS AND LACCOLITHS 

GRANO-PULASKOSE (SYENITE VAE. PULASKITE) OF HIGHWOOD PEAK.

A light-colored feldspathic rock composes the southern half of 

Highwood Peak in its upper portion and extends in talus slides and 

outcrops down to the southwest. It has a jointing which produces 

large, heavy slabs several feet long and a foot or more in thickness, 

which are best seen on the summit of the peak, where they occur in 

the rather massive outcrops. At this point the rock is very light colored, almost white, with a pinkish to pale brownish tinge. Lower 

down on the southwest in the gulches and southward on the divide 

the color changes to a light smoke gray.

Megascopic characters. In the hand specimen the white rock from 

the top is clearly seen to be almost wholly composed of feldspar having a tabular development, which gives the rock a trachytic habit. 

This is not so noticeable in the gray variety, which is more granitoid 

in character. Both varieties are dotted somewhat sparsely with ferromagnesian minerals, pale-green diopside, or glittering biotite in 

small crystals. In the rock at the top these minerals are in many 

places dull and lusterless or changed to little masses of limouite. 

The feldspar tables, which are the chief components, give cross 

sections which average about 8 mm. in length by about 2 mm. in 

breadth, and show the granularity of the rock. In the gray variety
PIKSSON.] PETEOGEAPHY OF STOCKS AND LACCOLITHS. 61

they are not so long and the fabric is more like that of a common 

granite.

Microscopic characters. Under the microscope there are seen in the 

sections the following minerals: Iron ore, titanite, apatite, diopside, 

hornblende, biotite, alkali feldspars, quartz, and secondary muscovite, kaolin, calcite, and chlorite.

Iron ore in small formless grains is rather uncommon and occurs 

usually with the diopside. The titanite and apatite are also rare. 

They occur generally in small masses, the apatite having the form of 

short prismoids.

The diopside is partly in small stout prisms, partly in very small 

scattered grains, and has a clear, very pale-green color without pleochroism. It has a well-marked but not perfect prismatic cleavage 

and a wide extinction angle, approaching 45°. The computation of

the rock analysis given beyond (p. 63) shows its approximate composition to be that given below.

An analysis of a pyroxene from Edenville, 1ST. Y., by Hawes a is 

quoted for comparison:

Composition of diopside from Highivood Peak and pyroxene from Edenville, N. Y.

&ot . .........................................................

AI n

FeA ....-......-...-..-.--.---...-..:....--.:.-.....-..-

Y^>. .........................................................

MnO... ......................................................

Mgo -.---.-..../-,--.--.-.--------.------.--..------.-.--.-.-.

CaO...... ....................................................

Ignition ....--........-...-.-.-..--.--......-....-....-...-...

Total......:..........'... ................................

I.

52.8

10.2

12.4

24.6

100.0

n!

51.05

2.02

1.30

12.28

12

10.02

22.07

.34

99.10

I. Diopside from Highwood Peak.

II. Pyroxene from Edenville, N. Y.; analysis by Hawes.

This is placed at the end of the diopside group by Dana. 6 The 

proportion of the different molecules is 12MgCaSi2 0(i to 5FeCaSi2 06 , 

and it therefore clearly belongs to the diopsides rather than to the 

hedenbergite group.

The diopside is altered in many places to a fibrous green hornblende, and the latter is also occasionally found alone; but unaltered 

cores of the diopside remaining in many of the bundles of hornblende

a Am. Jour. Sci., 2d ser., vol. 16,1878, p. 397. 

b System Mineralogy, 6th ed., p. 359.
62 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

show the original mineral. The hornblende does not appear to be 

anywhere original.

An ordinary brown biotite is. present in small amount. It is more 

abundant in the rock forming the lower southern slopes of the peak 

than at the top, and is often bleached or changed to a chlorite.

Of the dark-colored minerals the diopside is by far the most abundant, the others in comparison being limited in amount. In some 

areas these f erromagnesian components are partly changed to chlorite.

The feldspars are all alkalic, as proved by their optical properties 

and by "the study of the analysis, which shows all the lime demanded 

for the diopside. They are developed tabular on m (010), and the 

cleavage parallel to c (001) is very pronounced. Between crossed 

nicols they are very nonhompgeneous; are watered, waved, or moire in 

appearance; have perthite bands and included masses of a somewhat 

higher birefringence, and these areas and masses pass in spots into 

albite with distinct albite twinning. In some specimens it is difficult 

to find an area which appears really homogene.ous, but in such a section normal to the obtuse positive bisectrix, and hence approximately parallel to b (010), the direction of extinction a is 13° from the 

trace of the good basal cleavage (001) on b (010) and lies in the positive obtuse angle ft. Hence the feldspar is a soda orihoclase, and 

the large extinction angle would, in a general way, indicate a high 

content of soda. This is confirmed by the analysis, which shows that 

in general the feldspar content is in the proportion of OrjAbaWhile in some specimens the feldspar is fresh and clear, in others 

it is muddy and clouded with kaolin leaves, and when this occurs the 

nonhomogeneous aspect disappears, though whether this is merely an 

accident or there is some relation by which the more homogeneous 

feldspar suffers this change more easily could not be determined.

White mica is present in the rock from the top of the peak. This is

seemingly of secondary origin, in part after biotite, in which case it is 

in large crystals, and in part after feldspar, when it is in small nests 

and rosettes in the feldspar. Its amount at times may be considerable. 

In the rock at the top some quartz occurs filling fine angular interspaces between the feldspars. In the grayish variety along the south 

slopes it disappears, and the specimens taken here were tested chemically to ascertain if a small amount of nephelite might not be present, 

but without result. In one section an area of calcite was observed 

filling an angular interspace between feldspars in the same manner 

that Hawes ffl observed in the hornblende-syenite from Columbia, 

N". H. This is probably, as Rosenbusch 6 remarks, a product of infiltration into miarolitic cavities in the rock. The texture of the 

syenite lends additional confirmation of this idea.

aGeol. and Lithol. New Hampshire, 1878, p. 208. 

bMikr. Phys., Mass. Gesetine, 3ded., 1895,'p. 56.
PIBSSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 63

Chemical composition. The chemical composition of the rock is 

shown by the first analysis in the following table:

Analyses of pulaskose of Highwood Peak and related rocks.

Si02 .......

Al O

FeA ......

FeO. ------

MffO

CaO .......

Na2 0------.

K2 0........

H2 0+110°-

H.,0 110°-. 

Ti02 . ......

PA- --

pn

F ..........

Cl .........

ZrO2 -.-.--.

MnO. -.-...

SrO--.-...

BaO .......

Li2O .......

FeS2 .......

Total.

I.

65.54

17.81

.74

1.15

.98

1.92

5.55

5.58

1 .54 

.11

Trace.

Trace.

Trace.

(?) .

99. 92

II.

65. 43

16.11

1.15

2.85

.40

1.49

5.00

5.97

f .39 

I .19 

.50

.13

Trace.

.08

.05

.11

.23

.03

.07

100. 18

ill.

66.60

15. 05

1.07

4.42

.86

2.21

^4.03

5.42

I «

.76

Trace.

None.

100. 33

IV.

66.13

17. 40

| 2.19

.04

.81

5.28

5.60

1.22

.74

(?)

.13

(?)

99.54

V.

60.03

20.76

( 4.01

I .75

.80

2.62

5.96

5.48

.59

(?)

.07

Trace.

(?)

(?)

(?)

101.07

VI.

64.64

l.&. 27

2 42

1.58

1.27

2.65

4.39

4.98

( .27 

I .09 

.51

.37

.37

.05

Trace.

.08

.18

100.12

VH.

68.34

15.32

1.90

.84

.54

.92

5.45

5.62

.30 

.15 

.21

.13

.04

.07

.04

.08

None.

99. 95

VIII.

1.092

.173

.005

.016

.024

.034

.089

.059

} .030 

.001

I. Pulaskose (syenite) from top of Highwood Peak. Highwood Mountains, 

Montana. L. V. Pirsson and W. L. Mitchell, analysts.

II. Phlegrose (syenite) from Mount Ascutney, Vermont. W. F. Hillebrand, 

analyst. Bull. U. S. Geol. Survey No. 148, p. 68.

III. Toscanose (syenite, akerite) from Prospect street, Gloucester, Mass. H. S. 

Washington, analyst. Jour. Geol., vol. 6,1898, p. 798.

IV. Phlegrose (syenite, porphyritic akerite) from between Thinghoud and 

Fjelebua, Norway. Mauzelius, analyst. Brogger, Zeit. Kryst., vol. 16, 

1890, p. 46.

V. Pulaskose (pulaskite) f^om Fourche Mountain, Arkansas. R. N. Brackett 

and J. P. Smith, analysts. Ann. Rept. Geol. Survey Arkansas for 1890, 

vol. 2, p. 70.

VI. Toscanose (syenite) from Hughsville, near Barker, Mont. W. F. Hillebrand, analyst. Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, p. 19.

VII. Liparose (quartzose syenite) from head of Beaver Creek, Bearpaw Mountains, Montana. H. N. Stokes, analyst. Am. Jour. Sci., 4th series, 

vol. 1, 1896, p. 354. 

VIII. Molecular ratios of No. I.
64 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

In the prevailing systems of classifications this rock is a typical 

syenite in which the silica reaches the upper limit of the group and 

the rock stands next to the granites. The alumina and alkalies are 

high, the ferromagnesian elements low, and the rock is therefore in 

the alkaline series of' syenites. A number of analyses of similar 

rocks from other parts of the world are introduced for comparison, and 

attention will be drawn to some of them later.

Mineral composition or mode. By considering the molecular ratios 

given in column vin of the table of analyses and the minerals disclosed by the section, the rock may be shown to have the following 

mineral composition by weight:

Mineral composition or mode of pulaslcose of Highwood Peak.

SiO2 . ..,-..-

Al D

Fe2 03 ----------

FeO... .........

MgO.. ------ .- -

CaO.. _-.:_-..-.

Na,O... ........

K20.-- .........

H20...... ......

Total. ....

Iron 

ore.

.0.74

.36

1 10

Pyroxene.

4.08

72

.98

1.92

7.70

Orthoclase.

15.84

4.53

4.15

24.52

Muspovite.

5.40

4.63

1.41

.54

11.98

Albite.

32.04

9.17

5.55

46.76

Quartz.

8.16

8.16

Calculated.

65.52

18.33

.74

1.08

.98

1.92

5.55

5.56

.54

100. 22

round.

65.54

17.81

.74

1.15

.98

1.92

5; 55

5.58

.54

99.81

The occasional biotite crystals and the small amount of hornblende 

may be considered with the pyroxene. .All of the water has been 

assigned to the muscovite, whereas undoubtedly a partis hygroscopic. 

This makes the amount of muscovite somewhat too high, and also 

tends to lower the orthoclase. With these exceptions the calculation 

represents the composition of the rock very closely. That there is 

only 9 per cent of normative f eniic constituents in the mode shows the 

strong salic character of the rock.

Classification in the new system. From the analysis of the rock 

previously given, its position in the new system of classification may 

be determined by calculation of the norm, as shown in the following 

table:
PETROGBAPHY OF STOCKS AND LACCOLITHS. 65 

Calculation of norm of pulaskose of Highwood Peak.

sic...... .._;.__.__..

AI n

Pe.jO, ..............

PeO,.. ....... ......

MgO. ...... ........

CaO ... '.... .......

NajO...... .........

K.O... ...... .......

TiO

H.,O...... ..........

Total ........

Analysis.

65.54

17. 81

' .74

1.15

.98

1.93

5.58

 . 11

.54

99.92

Molecular 

ratios.

1.092

.173

.005

.016

.024

.034

.089

.059

.001

Or.

354

59

59

59

Ab.

534

89

89

89

An.

50

25

25

9.5

Qz.

Ill

111

Di.

18

3

ft

9

9

Hy.

25

7

18

9,5

Mt.

5

5

11..

1

1

. 1

Or...

Ab..

. An ..

Qz--.

Di...

Hy..

n .

HoO.

.... 32.80

..... 46.63

..... 6.95

..... 6.06

..... 2.04

..... 2.72

1.1(5 

.... .15

.... .54

Class, SSL=^°_4=15.3=I, persalane. .Fern. 6.07

9S 04 A f er Order, ^=H«i = .or7=perfelic=5, canadare.

F ou.Oo 

Rang, K2Q'+Na2O ] l^s.s-domalkalic-JJ, pulaskase.

\jB>\J &)

'6.07 Subrang,,?2 ^ 89 1.5-sodipotassic-pulaskose.

!N Q>$\) 59

Total... 99.65

Since the texture of the rock has been shown to be the broad trachytic one, it may thus be termed a tracho-pulaskose, which, as the 

fabric varies into the granitic, passes locally into grano-pulaskose.

Classification in prevailing systems. If account be taken of the 

tendency of the lighter colored feldspars to assume a tabular habit, 

giving the somewhat broadly trachytoid texture, and of the paucity 

of dark minerals, the rock belongs to the pulaskite type of syenites 

of Kosenbusch. It does not differ markedly from the pulaskite of 

Arkansas, whose analysis is given in the table on page 63, except in 

regard to silica and alumina. It might with propriety be termed a 

pulaskite type of syenite with accessory quartz.. 

" The gray variety does not show this tendency to the trachytoid 

structure, but is purely granitoid in type. It appears more nearly 

related to the akerites than to any other type of syenite given by 

Rosenbusch. It agrees very closely with the akerite in chemical composition, as is shown by the analyses On page 63. In whatever way it 

may be classified it clearly belongs in the alkaline series of syenites, 

which passes from the alkali-granites on the one hand and grades into 

the foyaite or nepheline-syenite group on the other.

-Bull. 237 04 5
66 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

PULASKOSE (SODALITE-SYENITE) OF SQUARE BUTTE.

Introductory. This rock was very fully described by Lindgren, with 

analyses by Melville of material collected by Dr. C. A. White, and 

was mentioned in the former paper by Mr. Weed and the writer. It 

has become well known in the literature of petrography, and here 

only such details will be given as are necessary to complete the discussion of the petrography of the region and of the position of the 

rock in the new system of classification.

Megascopic characters. On fresh surfaces the rock is white, often 

with a pale-brown or pinkish tinge. It has a very heavy jointing, 

and on top of the butte it often appears in huge, thick slabs several 

yards long. It is made up mostly of feldspars, which have lathr 

like or tabular forms and reach o mm. in length. Through it are 

freely sprinkled slender, black, glittering prisms of hornblende of 

about the same average length as the feldspar. There is not enough 

hornblende to detract from the light color of the rock at a distance. 

Small grains of a salmon or pale-brown colored sodalite are also 

present.

Texture. The platy form of the feldspars gives the rock a broad 

trachytoid texture that which in the nephelite-syenite characterizes 

the "foyaite" type of Brogger. Since the interspaces between the 

feldspars are not always completely filled, the rock has a more or less 

miarolitic habit.

Microscopic characters. The microscope shows the following minerals present, mentioned in the order of their formation: Apatite, 

hornblende, orthoclase and albite, sodalite, nephelite, and a.nalcite.

The hornblende has its prisms bounded by m (110) and 6 (010); 

terminations are wanting. It is frequently twinned on a (100). It 

is strongly pleochroic, c and & deep chestnut brown, a yellowish brown,

and absorption strong; fc c><t In the outer margins it is often 

changed to a deep greenish color, the brown merging into the green. 

The angle of c/yc is 13°. It is autornorphic against the feldspathic 

components. A few of the hornblendes contain iron-ore grains, but 

these do not occur in the rock except as inclusions of this character. 

The hornblende is similar to the brown hornblende found in the 

nephelite-syenite of Red Hill and other alkalic rocks, and is near 

barkevikite in composition, as shown by the following analysis by 

Melville:
PIBSSON.] PETKOGKAPHY OF STOCKS AND LACCOLITHS. 67

Analysis of hornblende of sodalite-syenite of Square Butte.

Si02 ................................... ..................... 38.41

TiOj,-..--.----..-........... ................................ 1.26

A12 03 ........................................ ................ 16.39

Fe A ................................... ^....................... 3.75

FeO......................................................... 21.75

MgO ...--.--..-......-... .................................... 2.54

MnO ..............................'.......................'.... .15

CaO..............:.........................................'. 10.52

Na2O ........................................................ 2.95

K2O .........._............_.._--._.-.-..-......_........... 1.95

H,O-.--........--.-..-..-...-.------.-.-------.----..--..-.. .24

Total................................................... 99.91

. The marked feature of this hornblende is the large amount of aluinina and iron it contains.

The alkali feldspars are chiefly orthoclase in lath-shaped sections, 

and associated with them is a certain amount of a triclinic feldspar 

shown to be albite. In the interstices between them is the colorless, 

isotropic sodalite, and the study of sections from different portions of 

the mass shows that in addition to this component there is present a 

variable amount of nephelite, which in some cases is abundant and 

in others is nearly or wholly wanting. In Lindgren's -material there 

was also considerable analcite, and this occurs generally. So far as 

can be told, the analcite appears to be wholly secondary, in part after 

nephelite and in part after feldspar; in the latter case it occupies 

baj^s, tongues, and areas eaten into the feldspar. The sodalite is at 

times partly or wholly replaced by cancrinite, and in other cases bundles of fibrous natrolite occupy angular interspaces and, judging by 

the manner of occurrence, are secondary after sodalite or nephelite. 

The sodalite and analcite were separated and analyzed by Melville, 

with the results given in the table on the next page.

Mineral composition or mode. From the bulk analysis of the rock 

given below, and from those of its components, Lindgren calculated 

that its mineral composition is as follows:

Mineral composition of pulaskose of Square Butte.

Hornblende.................................................... 23.00

Orthoclase.................................................. 1 50.00

Albite---.-...--.....---.----.---.-.---.---..........--...... 16.00

Sodalite.---------.-----------.-.----.-----.---.--..-..-..-... 7.96

Analcite ...................................................... 3.03

Total.................................................. 99.99

These results would be modified somewhat if the material studied 

had contained the unchanged nephelite. Lindgren remarks that the 

sodalite may contain some hydroxyl. The amount present is calcu-
68 IGNEOUS KOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

lated at practically 8 per cent. The rock contains 0.43 per cent of 

chlorine, which would give 5.4 per cent in the mineral. In theory 

normal sodalite requires 7.3 per cent.

Chemical composition. In the following table are analyses of this 

rock and of its mineral components, by Melville. An analysis of a 

related rock is added for comparison.

Analysis of pulaskose of Square Butte and component minerals.

Si02 . ...............

A12O3 ............

Fe,O3 ..............

FeO.... ............

MgO. ...... .........

CaO. _-.--._-_...-..

Na2O.. .............

K,O_... ............

H2 0+.... ..........

H2 0-.. ............

CO2 1. ..............

TiO2 ...............

p o

S03 ................

01*. .....---.----...

MnO. ..............

0=01.............:

I.

56.45

20.08

1.31

4.39

.63

2 14

5.61

7.13

1.77

.29

.13

.43

.09

100. 45

.10

100. 35

II.

0.941

.197

.008

.061

.015

.038

.090

.076

.004

.001

.012

III.

38.41

16.39

3.75

21.75

2.54

.10.52

2.95

1.95

24

1.26

.15

99.91

IV.

0.640

.161

.023

.302

.063

.188

- .048

.021

.016

V.

41.56

29.48

.49

. .15

.49

19.21

.91

3.73

.45

4.79

101.26

1.08

100. 18

VI.

49.54

25.07

.40

.20

22

15.32

.89

7.49

1.67

100. 42

.42

100. 00

VII.

49.06

16.07

7.92

2 41

2.65

8.21

5.17

3.18

2.27

.1.21

.81

.61

Trace.

Trace.

.98

100. 55

I. Pulaskose (sodalite-syenite) from Square Butte. W. H. Melville, analyst.

II. Molecular ratio of I.

III. Barkevikite from above. W. H. Melville, analyst.

IV. Molecular ratio of III.

V. Sodalite with some analcite from above. W. H. Melville, analyst.

VI. Analcite with some sodalite from above. W. H. Melville, analyst.

VII. " Sodalite-syenite "fromSchlossberg, near Grosspriesen, Bohemia. F. Hanusch, analyst. Hibsch, Tscher. Min. Pet. Mitt., vol. 21,1902, p. 522.

Since hornblende is the only ferromagnesian mineral in the rock, it 

contains all the iron, magnesia, and titanic acid. The feldspars 

appear purely alkalic, and if so the lime is also all in the hornblende. 

In this case the ratio of the oxides to one another should be the same
PIKSSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 69

in the hornblende analysis as in the rock analysis. These ratios are 

as follows:

Oxides in hornblende and pulaskose of Square Butte.

FeA .............................................

FeO. .......... ...................................

MgO..... ..--:2-. _..--. .................. ..I.......

CaO ........................................

TiO, ........I.....................................

Hornblende.

8.75

21.75

2.54

10.52

1.26

Pulaskose.

1.81

4.39

.63

2 14

.29

Ratio.

2.86

4.95

4.03

4.91

4.03

The titanic acid in the hornblende was calculated from the rock 

analysis and can not, therefore, be taken into account. The amount 

of magnesia in the rock is so small that the ordinary variations of analytical work might render this ratio not very exact. The ferrous irons 

and the limes show close agreement, based on larger quantities, but 

if they are correct the ferric irons can not be. Taking everything into 

account, it seems probable that in either the rock or the hornblende the 

amount of ferrous to ferric iron has not been very correctly determined, and that the error is in the rock determination, which should be 

about 0.86.

Classification in prevailing systems. This rock has been called a 

sodalite-syenite, and Rosenbusch, mindful of the rather small amount 

of feldspathoids present, has placed it under the alkaline syenites 

rather than in the nephelite-syenite family proper. Under this 

system of classification, especially if the variable nephelite and 

sodalite is considered, the rock forms a connecting link between the 

two families.

Classification in the new system. In the new system of classification the position of the rock is seen in the next table', in which its 

norm is calculated from the chemical analysis. It is of interest to 

note that this gives 70.56 per cent of alkalic feldspar, while Lindgren's 

calculation gives 66 per cent, the difference arising in the 3 per cent 

of analcite in his calculation. The 23 per cent of hornblende of the 

mode is replaced by anorthite, olivine, and iron ores in the norm, making all told 18.11 per cent, the difference being due to the fact that no 

femic alkaliQ constituents appear. This furnishes an excellent example of how the same oxides may be shifted to produce quite different 

mineral molecules, so that under a given set of conditions the oxides 

forming the usual component minerals may be united to form a single 

constituent differing in character from all of them.
70 IGKEOTJS ROOKS OF.HIGHWOOD MOUNTAINS. [BULL. 237. 

Calculation of the norm of pulaskose of Square Butte.

SiO,-. -------

A1A --------

Fe,03 ........

reO.... ......

MgO. ........

CaO .........

Na2O,--------

zso.. .........

Ti02 .........

PA ---------

Cl ............

'Rest .........

Total .-

Analysis.

56.45

20.08

1.31

4.39

.63

2 14

5.61

7.13

.29

.13

.43

1.29

100. 35

Molecular ratio.

0.941

.197

.008

.061

.015

.038

.090

.076

.004

.001

.012

........

Or.

0.456

76

76

76

Ab:

324

54

54

54

An.

70

35

35

35

Ne.

94

19

19,

12

So.

36

18

94

6

6

Co.

9,

2

01.

31

49

13

31

Mt.

8

8

8

11.

4

4

4

Ap.

8

1

1

Or....

Ab... 

' An...

Ne ... 

So....

Co....

Ol

Mt ...

n.....

Ap...

Rest..

.. 42.26

.. 28.30 

.. 9.73

.. 3.41 

5 96

20

.. 1.86 

-. .61

.. .34 

.. 1.29

89. 86 Class, ^L-§^-1L3=Ii persalane.

Order, p-=~Wll=perfeiic=5, canadare. 

WoTir, K,O'+Na2O' 166 , , ,. ,

K" O' 90 8. 72 Subrang, £j^=^=l.2=sodipotassic=pulaskose.

Total.. 99.87

Comparison with related types. When it was believed that this 

rock contained sodalite as its only feldspathoid (lenad component) 

it stood in a class largely by itself. The fact that it contains nephelite, however, renders it akin to the other alkali-feldspar rocks 

which contain small amounts of this mineral. It is thus closely allied 

with the brown hornblende-bearing rock of Red Hill, described by 

Bayley, a which contains a rather limited amount of nephelite and 

some sodalite. In the very able and valuable memoirs of Hibsch upon 

the interesting rocks of the Bohemian Mittelgebirge, brought out in 

recent years, he describes a "sodalite-syenite," the analysis of which 

is given for comparison. The similarity, however, ends with the sodalite, since its composition, as shown by Hibsch, is the same as that of 

the essexite of the Rongstock, and it appears indeed questionable if 

in any system of classification which takes account of the relative 

quantities of the minerals, rocks so rich in f erromagnesian components 

should be classed with the feldspathic syenites.

"Bull. Geol. Soc. America, vol. 3,1892, p. 231.
PETKOGBAPH? OF STOCKS AtfB LACCOLITHS. .71 

TRACHO-HIGHWOODOSE (NOSEAN-SYENITE).

Occurrence. This rock occurs as a dike or small intrusive mass in 

the Cretaceous strata south and somewhat east of South Peak, in the 

foothills and between two small head branches of Byrnes Creek. 

With the small scale and somewhat generalized character of this portion of the map, it is impossible to locate it more definitely. On the 

day it was discovered the country was covered with dense smoke from 

forest fires, which made it impossible to take bearings by which its 

position could be accurately given, and the necessities of the work 

did not permit of a second visit to this locality.

Megascopic characters. The weathered rock is a light brownish 

gray and has a somewhat cellular structure, owing to the relief caused 

by tabular feldspars arranged in trachytoid texture which have 

resisted weathering better than the other elements. Such sections 

attain lengths of from 2 to 5 mm.

On a freshly fractured surface the rock has a clear gray color with 

something of an olive tone, and the individual consjtituents are not 

very clearly contrasted. With the lens it is seen that the white feldspars, are dotted through and through with tiny dark-green specks 

and prisms of a f erromagnesian mineral, while the interspaces between 

them are filled with a much larger proportion of ferromagnesian 

grains. An occasional flake of biotite is also seen.

Under the microscope the following minerals are seen: Iron ore, 

apatite, titanite, pyroxene, biotite, alkali feldspar, nosean, kaolin, 

and probably analcite.

Microscopic characters. The iron ore is in small octahedrons, and 

is rather sparing in amount. Apatite, which is rare, is in short, thick 

prisms. Biotite is strongly pleochroic and shows slight peculiarities 

which recall its characters in the minettes; the outer border is deeper 

colored than the interior, and the crystal form is embayed or repeated. 

It is a comparatively rare component, as is also titanite, of which a 

few scattered crystals were seen.

The pyroxene shows considerable diversity of character; it is 

usually present in slender prisms which reach a maximum length of 

2 mm. In a few cases short, stout columns were noted. The larger 

crystals are composed of a pale-green diopside with high extinction 

angle and strong birefringence. Toward the periphery they assume 

a darker green from admixture of the segirite molecule, and are here a 

transition between segirite-augifce and diopside. Their outline is 

very apt to be beaded in the section by small attached grains of 

iron ore. In some places they are changed into a j-ellowish serpentine substance. In a few cases, especially in the vicinity of nosean, 

the augites become much richer in regirite and pass over into segiriteaugite. The smaller microlites are much more apt to be darker 

colored, and in some cases, especially in the interspaces enriched
72 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

in soda, they pass over into aegirite, or if of diopside the end projecting into the nosean is of segirite. This is similar to the occurrence 

of such pyroxenes in the phonolite of Cripple Creek as described by 

Cross a and in the analcite-basalt of the Little Belt Mountains 

described by the writer. 6 These small pyroxenes are neither numerous 

enough nor small enough to give the tinguaitic aspect, but in some 

of the interspaces they are rather thickly crowded. A somewhat 

similar structure of a syenite rock with thickly scattered small diopside prisms has been described by the writer from the Little Belt 

Mountains, Montana/

The orthoclase feldspar which makes up over half of the rock is in 

tabular crystals that produce the trachytoid structure so common 

in the alkalic rocks. It is somewhat kaolinized in places. It contains also at times the albite molecule by means of which it passes 

into soda orthoclase, or it may contain microperthite intergrowths with 

albite. The size of these feldspar tables as seen in the section is 

about 2 mm. long by 1 mm. broad.

In the angular interspaces between these feldspars there is a clear 

.colorless isotropic mineral of low refractive index. This is generally 

sprinkled through with the pyroxene microlites previously mentioned. 

It does not show any other positive characters by which it might be 

identified, and might be taken for glass if the character of the rock 

and its mode of occurrence did not utterly preclude such an idea.

Chemical composition. At first this substance was believed to be 

analcite, like that of the analcite-syenite from the neighboring Little 

Belt Mountains d, but when treated with dilute nitric acid the powdered rock gelatinized readily, and the acid solution after being filtered 

gave a strong reaction for sulphuric anhydride S03 and a mere trace 

of chlorine. Carefully treated with very dilute hydrochloric acid, the 

test for S03 gave the same result. The S03 did not come, therefore, 

from oxidation of metallic sulphides, but from a soluble sulphate.

The powdered rock gives but little water in the closed tube. The 

chemical tests show that a considerable amount of nosean must be 

present in the rock, and it must be a part of the colorless isotropic 

substance. Some analcite is probably present, and is perhaps indicated in places where the isotropic substance is particularly clear and 

limpid. In such spots a rough cubic cleavage can be seen, and at 

times faint optical anomalies. Here, also, some particles of calcite 

are found, in one case with distinct crystal form. It seems probable 

that the analcite is secondary after the nosean. A test by the method

a Geology of the Cripple Creek district: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, p. 35. 

6 Petrography of the Little Belt Mountains: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, 

p. 54-i.

«Ibid., p. 468 (No. 643). 

rflbid., p. 4C9.
IMRSSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 73

recommended by Osann* for distinguishing iiosean from other minerals of the feldspathoid group, by using acetic acid and barium chloride on the section, was partly successful; some areas had BaS04 

deposited upon them, while others remained clear.

No complete chemical analysis of the rock was undertaken, as it 

did not apjpear fresh enough to warrant the labor, but a partial one

Actual mineral composition or mode of traclw-highwoodose from near South Peak.

Or

 No

Di

Ac

Bi

Mt,

An

0,

Tn

Units measured. 

......... 819......

... ..... 100......

......... 329......

......... 28......

12

...---... 50......

,..:..... 25......

......... 14......

......... 13......

Total..!, 390......

Volume, 

per cent.

58.93

7.'20

23.67

2.01

.86

3.60

1.79

1.00

.94

100.00

v

v

X

X

X

v

X

X

X

Specific 

gravity.

2.6 =

2.2 =

3.8 =

3.5 =

3.0 =

5.2 =

3.2 =

2.7 =

3.3 =

158

15

78

7

2

18

R

fl

R

286

R

8

1

0

6

7

7

7

1

9

Weight, 

per cent.

= 53.6 orthoclase, albite.

= 27.3 diopsicle.

= 2.3 aegirite.

= .9 biotite.

= 6.4 magnetite.

= 2.0 apatite.

= .9 calcite.

= 1.1 titanite.

100.0

by Dr. W. F. Hillebrand will be given later. However, the rock 

appears to be a fair subject for microscopic analysis by RosiwaVs 

method, and this was undertaken, with the above results. Under 

aegirite is understood those pyroxenes which were deep green and 

pleochroic. Chance brought several titanites in the measured lines, 

and the amount may be rather high. To reduce this to a chemical 

composition it is arbitrarily assumed that at least 10 per cent of albite 

is present, the biotite is divided into 5 parts of leucite and 4 of olivine, 

the.pyroxenes are united, and their mass composition assumed to be 

that of a very similar one from the soda granite of Kekekabic Lake, 6 

whose composition is known. Making these assumptions, the chemical composition may be figured from the mineral analysis given in 

the above table. The titanic acid appears rather high, and there isprobably a more even relation in the iron oxides. The computed 

analysis is reckoned, of course, on a water-free basis. That it represents closely the composition of the rock is evident from its agreement, first, with the partial analysis by Doctor Hillebrand and, second, with the chemical analyses of closely related rocks of the area. 

These facts may be seen in the table on the next page, where all the 

analyses are given with water and traces omitted:

"Neues Jahrb. fill- Min., 1892, vol. 1. p. 222. 

'>Am. Geol., vol. 11,1893, p. 385.
V4 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Chemical composition of highwoodose calculated from mode.

Si02 . ........

AI o

FeA ......

FeO. .......

MgO ........

CaO .......

Na2O. ------

K2 0. .......

TiO

P n

SO, ....... .

CO.........

TotaL

Or. .

28.20

8.02

7.37

.43.6

Ab.

6.87

1.95

1.18

10.0

Lc.

0 98

18

11

.5

No.

1.86

1.58

1.44

.62

5.5

Di.

15. 75

.70

2.74

 1.52

2.79

5.27

.78

.11

29.6

Ol.

0 17

28

.4

Mt.

3 45

1 55

5.0

11.

0 66

74

1.4

Ap.

1 11

,84

2.0

Cc.

50

40

.9

Tn.

0 34

35

45

1.1

Total.

53,47

12.43

6.19

3.73

3.07

7.23

3.40

7.59

1.19

.84

.62

.40

100. 16

Analyses of higinvoodose and related rocks.

SiOa . ....................

AT n

FeA ----------------------

FeO .......................

MgO-......--.-.-...---....

CaO .......................

Na2 0 .._._..:.-._._..___._._

KjjO. ...........

TiO

PA o -----------------------

or\

CO.........................

Total ................

I.

53.47

12.43

6.19

3.73

3.07

7.23

3.40

7.59

1.19

.84

.62

.40

100.16

II.

53.98

3.94

7.39

.38

.64

III.

52. 05

15.02

2.65

5.52

5. 39

8.14

3.17

6.10

.47

.21

.02

IV.

51.75

14.52

5.08

3.58

4.55

7.04

2.93

7.61

.23

.18

V.

51.94

15.78

4.07.

3.17

3.48

6.04

3.44

7.69

.39

.59

.29

VI.

0.891

122

nqn

ft^i

.077

.116

f|FJK

. Uui)

.081

fil P;

.006

.008

.009

I. Microscopic analysis of highwoodose (nosean-syenite). L.V.Pirsson, analyst.

II. Partial chemical analysis of highwoodose (H2O; 1.92; Cl. 0.03). W. F. Hillebrand, analyst.

III. Chemical analysis of borolanose (basic syenite) from Middle Peak, Highwood 

Mountains, Mont. E. B. Htirlburt, analyst.

IV. Chemical analysis of ferguscse (fergusite) from head of Shonkin Creek, 

Mont. E. B. Hnrlburt, analyst.

V. Trachiphyro-borolanose (syenite-porphyry) from dike at head of Shoukin. 

Creek, Mont. W. M. Bradley, analyst.

VI. MoleciTlar proportions of I.
riussoN.] PETBOGRAPHY OF STOCKS AND LACCOLITHS. 75

In the analysis of the rock under description, as well as in the 

others, the general characteristic of the Highwood rocks the concurrent high lime and potash is clearly seen.

Classification in the new system. From the analysis derived from 

the mode of the rock may be calculated its norm and place in the new 

classification.

Calculation of norm of Mghivoodose from near South Peak.

Si02 ... -- 

AI n

FeA ..........

FeO.... ........

 MgO. ..........

CaO ...........

Na/X -------

KaO_... ........

TiO

PA -----------

SO, ............

CO.,.../.. ......

Total ....

Analysis.

53.47

12.43

6.19

3.73

3.07

7.23

3.40

7.59

1.19

.84

.62

.40

100.16

Molecular ratio.

0.891

.122

.039

.051

.077

.116

.055

.081

.015

.006

.008

.009

Or.

486

81

81

81

Ab.

150

25

25

9,5

No.

39

10

9,4

s

8

8

Qz.

9.3

9,3

Ac.

94

6

6

6

Di.

160

3

77

80

80

Mt.

33

qq

33

n.

15

15

15

Ap.

?,0

6

6

Wo.

16

16

16

Or....... 45.04

Ab ...... 13.10

No ...... 5.68

Qz....... 1.38

Ac....... 2.77

Di....... 1.7. as

Wo...... 1.86

Mt ...... 7.66

II........ 2.28

Ap...... 2.02.

COo...... .40

.,. Sal 65.20 ., TT , . Class, ~ =1" n' dosalane -

65.20

 33.97

^ , L, D.WJ Order, -^,"=rg i . = .09=perfelic=5, gerrnanare.

_ KoO'+NtuO' 1.30 r i ' * i Eaug, CaO' ="Q-=co=peralkalic=l, umptekase.

., ^ P K20' 81 , mn , ...... Subrang, jjTTn =49=^ ^=dopotassic=lnguwoodose

Total.. 99.57

/

The norm does not differ essentially from the mode, the main difference-being in the splitting up of 27 per cent of alferric augite into 17 

per cent of diopside and 10 per cent of feniic and salic minerals. The 

rock has then in essence a normative mode. It is in the lower end of 

dosalane, not far from s'alfemane, and is clearly per.fe.lic and in rang 

peralkalic. In subrang it is .just within the border that separates 

highwoodose from ilmenose. Hence it differs somewhat from the type 

highwoodose from High wood Gap, as may be seen by reference to the
76 IGNEOUS HOCKS OF H1GHWOOD MOUNTAINS. tflOLt. 237.

analysis of that rock, and approaches closer to salfemane, being much 

higher in lime, iron, and magnesia.

The texture is one common in granular rocks, consisting mainly of 

alkalic feldspars, a broad trachytoid one, produced by their lath shape, 

which, since it is megascopic, is designated "tracho" in the new 

classification. Thus the rock may be termed a ' c tracho-highwoodose." 

The rock is rather fine grained, and its appearance and character 

tends to show that it had crystallized rather quickly and in small 

mass and belongs to the "hypabyssal" types.

Classification in prevailing systems. In the prevailing systems of 

classification this rock would be termed a "nosean-syenite," which is 

a distinct and rare member of the foyaite family. Osann a has shown 

that nosean occurs in the granular intrusive rocks of this family in 

the same manner as nephelite andsodalite, and has the same function. 

Hackmann 6 has shown its presence in nephelite-syenite of Umptek, 

in Kola Peninsula.

In the above occurrences this mineral accompanies nephelite, which 

is the dominant feldspathoid; but in the present instance no nephelite has been seen. It is possible that some of the analcite is secondary after nephelite, but it seems more probable that it is after nosean. 

The nosean-syenite in that case would be an equivalent to the sodalitesyenites of the Montana region described by Lindgren c and Merrill. d

GKANO-SHOSHONOSE (MONZONITE) OF HIGHWOOD PEAK.

Introductory. This rock, whose field occurrence has been described 

in-the discussion of the geology of High wood Peak, has, as related, a 

somewhat different facies in different parts of its mass. The main 

type occurs next to the white pulaskose (pulaskite) forming the southwest part of the peak or intruded stock, and extends as the main portion of the mass until it merges into the fine,, dense type of the north

slopes and border. »

In the outcrops the rock appears very dark, a deep stone color. 

At a distance it is almost black and in striking contrast with its 

neighbor, the white pulaskose. Under the hammer it is tough and 

breaks with difficulty, there being no tendency toward jointing on a 

small scale. It is fresh and yields excellent specimens.

Megascopic characters. Examined on a freshly fractured surface, 

the rock is a dark stone gray with much the appearance of many 

rather fine-grained diorites or gabbros. It is composed of a mixture, 

in about equal parts, of blackish ferromagnesian and white feldspathic 

minerals. The average size of grain is from 1 to 2 mm. in diameter.

aNeues Jahrb. fur Min., 1892, vol. 1, p. 232. 

bKola, Fennia, H, No. 2,1894, p. 121. 

c Am. Jour. Sci., 3d sor., vol. 45,1893, p. 28C. 

<*Proc. U. S. Nat. Mus., vol. 18,1895, p. 671.
PIUSSON.] PETROGEAPHY OF STOCKS AND LACCOLITHS. 77

None of the minerals exhibit any crystal form, but occasionally there 

can be seen the biilliant surface of a small particle of biotite.

Close examination shows also that former joints have been recemented by material of a feldspathic nature, injected after their formation. This phenomenon is discussed in a later paragraph.

Microscopic characters. In thin sections the following minerals are 

found: Iron ore, apatite, biotite, diopside, labradorite, and soda 

orthoclase. The apatite is in the usual small, stout prismoids; the 

iron ore in small anhedra, generally associated with the biotite.

The pyroxene does not show any good crystal outlines, being in 

rough prismoids and grains. It has a pale-green color, and in places 

shows a faint but perceptible pleochroism, indicating probably a slight 

admixture of the segirite molecule; in one instance a large anhedron 

has a perfectly colorless core. There is cle'arly not enough of the 

segirite molecule in the mixture to classify the pyroxene as an segiriteaugite; it belongs in the diopside group, since a section of it cut nearly 

parallel to 6 (010) gives cAJC=40°. The thickness of the section, as 

shown by the feldspars, is about 0.03 mm., and the interference color is 

yellow of the second order, showing a maximum birefringence of 0.030. 

These are the properties of diopside. The mineral is spongy and 

incloses a large amount of iron ore and biotite. It is occasionally 

twinned on a (100). It is of a type common in the Highwood rocks.

The plagioclase is rather thickly scattered in the interspaces between 

the pyroxenes in rather small, short laths, ranging from 0.2 to 0.4 mm. 

in length. The laths are usually twinned according to both the albite 

and Carlsbad laws, and measurements according to Michel Levy's 

method, in the zone perpendicular to ft (010), show them to be labradorite of the composition Abj Anj; thus in one section the symmetrical 

extinctions of the albite twins are 9°, of the Carlsbad half 25°: the 

section is therefore that of a labradorite Abj An t cut about 42° from 

(100), or nearly parallel to the face y (201), and in covergent light the 

section shows the bisectrix ex nearly centered in the field.

The mineral of final consolidation is the alkali feldspar, which is in 

broad, shapeless plates, inclosing the labradorite laths in a poikilitic 

manner. In this respect it recalls the rock of Monte Mulatto figured 

by Brogger, a but the plagioclase laths, are smaller and more nearly 

like the type of Yogo Peak. 6 A section of the alkali feldspar was 

found parallel to fr (010) and showing the obtuse bisectrix jc. The 

cleavage parallel to c (001) was excellent, and a parting parallel to a 

(100) or in (110) permitted the orientation of the section, the angle 

between the two being 66°, while a (100) on c (001)=augle ft is (33° 54'. 

The bisectrix a lies in the obtuse angle /?; it is therefore positively

aPredazzo, 1895, p. 56.

bPirsson, L. V., Petrography of Little Belt Mountains: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, p. 476.
78 IGNEOUS KOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

inclined and is 9° 30' from the trace of c (001). These characters show 

that the mineral is a soda orthoclase or soda microcline (anorthoclase), 

the angle of extinction being that given by Fouque for anorthose. a 

It may be added that the rock appears very fresh and without any 

alteration products.

Behavior with acids. When the powdered rock is boiled with very 

dilute nitric acid, filtered, and the nitrate evaporated, a considerable 

amount of gelatinization ensues, indicating a mineral which yields to 

the acid.

Olivine has not been seen in the sections, although it may occur 

sporadically in the rock. The very small amount of chlorine, all of 

which is needed by the apatite, shows that not more than a mere trace 

of sodalite could be present. Nosean is also excluded by the test for 

S03 , giving a negative result. Moreover, no isotropic material has 

been found in the sections, which confirms the above tests and also 

excludes analcite and leucite. Leucite, which is decomposed by acids, 

does not give gelatinous silica. From all this it-is concluded that 

nephelite is probably present in a small amount in the rock, though 

it has not been found with certainty in the sections. It must be at 

any rate limited in amount, and, if present, plays an insignificant 

role. It will be noted that it occurs in the calculated norm.

Chemical composition. The chemical composition of this rock is 

shown in the following table. Included in the table are analyses of 

several other rocks of similar characters, from localities in Montana, 

which have been described in previous publications. There is added 

an analysis of the monzonite from Monzoni given by JBrdgger and of 

a Swedish rock classed by him as a monzonite. It will be seen that 

the High wood rock has similar features in its chemical composition, 

the rather low silica, rather high alumina, lime, iron and magnesia, 

moderate alkalies, and potash predominating over soda.

aVetude des foldspath: Bull. Soc. Fran. Min., vol. 17,1894, p. 148.,
PIUSSON.] PETEOGRAPHY OF STOCKS AND LACCOLITHS. 79

Analyses of shoshonose of Highivood Peak and related rocks.

No.

SiOa .........

AT n

Fe.2 03 . -------

FeO. ........

MP-O

CaO. ----..-

Na.,0 ----....

K.,0 .........

HZ 0+ .......

H20-. ...:..

TiOj. ........ it

P2O5 ---------

S03 . .........

Cl-_.. ....

Fl. ..........

MnO ........

BaO.. .......

SrO .........

Total . .

I.

51.00

17.21

2 41

4.23

6.19

9.15

2.88

4.93

.63

.13

.33

.03

Trace.

Trace.

.34

.14

99.60

II.

54.42

14. 28

3.32

4.13

6.12

7.72

3.44

4.22

.38

.22

.80

.59

.10

.32

.13

100.19

III.

52.81

15.66

3.06

4.76

4.99

7.57

3.60

4.84

.93

.16

.71

.75

.07

Trace.

.24

.09

100.24

IV.

54.20

15.73

 3.67

5.40

3.40

8.50-

3.07

4.42

.50

.40

.50

.70

1.00. 50

V.

50.35

15. 76

2.32

7.30

7.40.

10.12

2.75

3.89

.45

.30

.39

.35

101.38

VI.

52.05

15.02

2.65

5.52

5.39

8.14

3.17

6.10

.35

.47

.21

.02

.24

.42

.28

100.03

VII.

52.80

19.99

3.63

3.40

3.20

4.22

3.10

7.74

1.18

1.00

.70

100. 96

VIII.

56.25

20.50

1.85

4.23

2.54

3.62

5.91

4.80

.83

.63

101.16

IX.

0.850

.169

.015

.058

.155

.164

.047

.052

.001

.002

I. Shoshonose (inonzonite) from Highwood Peak, Montana. E. B. Hurlburt, analyst.

II. Monzonose (inonzonite) from Yogo Peak, Little Belt Mountains, Montana. 

W. F. Hillebrand, analyst. Am. Jour. Sci.. I'd ser., vol. 50, 1895, p. 473.

III. Monzonose (monzonite) from stock at head of Beaver Creek, Bearpaw 

Mountains, Montana. H. N. Stokes, analyst. Am. Jour. Sci., 4thser., 

vol. 1, 1896, p. 357.

IV. Monzonose (monzonite) from Tyrol. V. Schmelck, analyst. Brogger, 

Triad. Erup. Predazzo, 1895, p. 24.

V. Kentallenose (monzonite, olivine) from Sinalingen, Sweden. H. Santesson. analyst. Loc. cit., p. 46.

VI. Borolanose (basic syenite) from Middle Peak, Highwood Mountains, 

Montana. E. B. Hurlburt, analyst.

VII. Monzonite from Maros Peak, Borneo. Dr. Hinden, analyst. C. Schniidt

in Sarasin's Celebes, vol. 4,1901, p. 25. 

VIII. Micrornonzonite from Ambodimadiro, Madagascar, A. Lacroix, analyst.

Roches ale. d'Ampasindava, Nov. Arch, du Museum, 4th ser., vol. 1,

1902, p. 110. 

IX. Molectilar ratio of I.
80 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Included in the table is a grano-borolanose of the neighboring Middle Peak stock. This is inserted to show how closely it resembles shoshonose in general chemical composition, the chief difference being a 

higher content of alkali. This difference is sufficient to produce 

important mineral variations, however, as shown in the description 

of this type.

Mineral composition or mode. A consideration of the molecular 

ratios in the foregoing table and a study of the section give the following percentage composition by weight of the component minerals:

Percentage of minerals in shoshonose of Highwood Peak.

Iron ore 

Biotite .

.._-_.-._._.___._-------_;-_-_.---------------__- 2

...............................'.....-......... --.- 10

Pyroxene ..............................^........................ 30

Labradorite _.----.-----__----..--------.----------------. .... 25

Soda orthoclase ................................................. 33

100'

If it, is assumed that the pyroxene has the same composition as in 

the shonkinite of Square Butte, given elsewhere; that the biotite has 

the same composition as the dark-green biotite of Monzoni anatyzed 

by Rainmelsberg; a that the labradorite is a rather acid type Ab4 An3 , 

and that the soda orthoclase has about half as much soda as potash, 

as is often the case, the rock will have the chemical composition 

given in the following table :

Calculated and determined composition of shoshonose.

Si02 _. .................................... .......... ........

ALA ----------------------------------------------------

FeA -------------------------------------------------

FeO-. ---------..------------.---.-----------------------.

MgO ................................ ----------.- = -.---..

CaO---. ................................ .......... .......

NaA- ................................................. -.

K2 0...... _....... __.... .............. --.........-.--

Calculated.

pro 0

16.7

2 4

2.6

6.4

9.5

3.2

4.7

Found by 

analysis.

51.0

17 2

2 4

4 2

6.2

9.1

2.9

4.9

The agreement shows that the estimated amounts of the component 

minerals and their assumed composition must be nearly correct.

Texture. The texture of this rock is a normal granitic one. It is 

xenomorphic rather than hypautomorphic, for none of the older ferromagnesian minerals show good crystal form; only the smaller latha Min. Chem. Erg., 1886, p. 118.
PIUSSON.] PETKOGBAPHY OF STOCKS AND LACCOLITHS. 81

shaped plagioclases have automorphic development. The texture, 

indeed, is that one typical of most stocks, of which the High wood 

Peak mass furnishes a small but excellent example.

Classification in the new system. The calculation of the norm 

from the analysis shows that the rock is a shoshonose.

Calculation of norm of shoshonose of Highwood Peak.

Si02 . ............

Al O

FeA .-,._ 

FeO ...........

TVteO

CaO -..-......-

Na,0- ----------

KjO. -----------

TiOa ...........

P n

S08 ............

Eest ..-.-.-....

Total ....

Analysis,

51.00

17 21

. 2.41

4.23

6.19

9.15

2.88

4.93

.13

.33

.03

1.11

99.60

Molecular ratios.

0.850

.169

.015

.058

.155

.164

..047

.052

.001

.002

Or.

312

52

52

52

Ab.

114

19

19

19

An.

140

70

70

70

Ne.

56

28

?,8

?-R

Di.

174

19

fi8

87

87

Ol.

55

9R

87

55

Mt.

15

15

15

11.

1

1

1

Ap.

7

9,

9:

Or.......

Ab 

An ......

Ne ......

Di. ...... 

Ol. ......

TWt

11........

Ap ......

Rest.....

*M] n Sal. 66.28 9.96 Class, W(r =.TT-nQ=2.7=n, dosal

10 ,W66' 28

19.46 L 795

7 c)g] ' Order, p=gg-Tgj=.136=perfelic=5, germanare.

19.40 K20'+Na20' 99 ,, 

829 g' CaO' ^70^L4-alk

3.4831.99 Subrang, |^Q7=^=i.l.=sodipc

16 , P+O 1.9.40 , , ,.

1.14 ..,....., (MgFeO)+CaO" 300

alicalcic=3, andase.

=2, shoshonate.

.

Thus, in the new system, if the texture is considered and the classification be carried down into the grad and subgrad the rock is granoshoshonote. All that this name means, then, is indicated in the table 

above. Since the mode is not a normative one, and as, instead of 

olivine and feldspathoid molecules in the norm, biotite is actually 

developed in the mode, the rock may be termed a biotitic granoshoshonote. This name is a concise expression for a granular rock 

consisting of ferromagnesian and feldspathic minerals whicli has the 

following characteristics:

The ferromagn-esian minerals are present in notable quantity, but

Bull. 287 04 6
82 IGNEOUS EOCKS OF HIGHWOOD MOUNTAINS. [HULL. 237.

are exceeded in amount by the feldspathic minerals; the feldspars are 

not accompanied by any notable amount of feldspathoids; considerable anorthite is present molecularly, but is exceeded in amount by 

the alkali-feldspar molecules, in which the potash and soda are about 

equal; of the ferroniagnesian minerals, the iron ores are greatly dominated by the other ferromagnesian minerals which contain few or no 

alkaline molecules; olivine and a feldspathoid, which might have been 

expected to occur in the rock, are replaced by biotite, whose amount, 

however, is not large, not exceeding 12.5 per cent. " This name, then, 

surely gives a rather exact idea of the character of the rock, but it 

does not tell anything of its manner of occurrence, of the shape of its 

mass, of its relations to related rocks, and of its geologic age, for, 

according to the writer's belief, these things have no place in a systematic classification, although they may be of the greatest importance and of the utmost interest in other ways.

Classification in prevailing systems. In the prevailing systems the 

rock is a typical monzonite both mineralogically and chemically. 

Since the monzonites form a group of rocks of what may be termed 

mean composition, they vary in different directions according to the 

rock complex in which they occur and with which they show affinities. In the present case the monzonite is related to the special group 

of alkaline rocks occurring in the High woods, andits genetic relationship to this particular group is shown by the presence of augite similar to that in. the other members and by the occurrence of small and 

otherwise unimportant quantities of feldspathoid minerals. There 

are also other points of resemblance, but it is often as difficult to 

describe the minute and subtle features which indicate that a rock 

belongs to a certain geographic class or petrographic province as it is 

to portray the characteristics which distinguish one man from another.

The alliance between the monzonites and certain alkaline rocks has 

been already pointed out by Rosenbusch, a and the present case adds 

another example to the list.

Dikelets in shoslionose (monzonite}. As previously mentioned, the 

surface of the rock is everywhere seamed by very fine lines of lightcolored feldspathic material. This is due to veins or dikelets which 

have been injected into it. It appears to the writer that the only way 

in which these dikelets, which are very common in many granular 

igneous rocks, can be explained is by the hypothesis that they are 

later injections along joint planes formed by the contraction of the 

cooling rock mass. This seems confirmed also by the toughness of 

the rock body and its present lack of small joint planes. These 

previously existed, having been formed by the contraction of the 

cooling rock, but have been everywhere healed by a. later intrusion 

of feldspathic material. This, on the rock surfaces, produces lines,

aMikr. Phys. Mass. Gesteine, 3d ed., 1895, p. 124; Elemente der Gesteinslehre, 1898, p. 108.
BULLETIN NO. 237 PL. VI

FERGUSITE FROM
PIUSSON.] PETEOGEAPHY OF STOCKS AND LACCOLITHS. 83

at first glance scarcely noticeable, since the films, which have been 

broken across, are often as thin as paper. This healing of the initial 

jointing by later processes may be ascribed to pueuinatolytic action, 

and it would seem as if the material must have been carried in by 

heated liquids or gases, as the width of most of the cracks is so slight 

that it is impossible to imagine a viscous molten fluid forcing its way 

along them for any distance.

Contact fades of slioslionose (monzonite). As mentioned elsewhere, 

the main type just described passes on the north side of the mass into 

outcrops and rock piles of a dense, very dark, and somewhat flintylooking variety with a platy cleavage. On exposed surfaces it weathers to a dark gray, mottled by larger mineral grains. From its relations in the field this rock is believed to be a contact form of the type 

described above. In thin sections it shows the same minerals, but it 

has a somewhat different texture. The augite is in smaller grains,

but has rather better crystal form, while the feldspathic components 

are in very minute microlites and grains, the contrast causing the 

larger augites to appear like phenocrysts in a feldspar groundmass.. 

The iron-ore grains are commonly surrounded by a thin biotite mantle. 

These minerals .are not large enough to give the rock a porphyritic 

habit, and it might be called a grano-shoshonose (micromonzonite).

FERGUSOSE (FERGUSITE) OF ARNOUX STOCK.

Occurrence. :The main stock at the head of Shonkin Creek lies at 

the intersection of the parallel of 47° 15' north latitude and the meridian of 110° 30' west longitude. About 2 miles northwest of it is a 

much smaller stock intruded into the basaltic flows and breccias. 

This stock is in general circular in plan and is a little over a mile in 

diameter. It lies on the north slopes of the main ridge and is dissected by a ravine whose waters are tributary to Shonkin Creek. The 

locality was not seen by the writer, but the description of the main 

type composing this small stock is based on material collected by 

Mr. Weed.

Megascopic characters. The general character of this type is that 

of a light-gray massive rock of rather coarse grain. This general 

color effect is given by the mingling of a pale flesh-colored component Avith a much smaller amount of a dark-colored one. On closer 

inspection it is seen that the light component is in round grains running from 5 to 2 mm. in diameter, and averaging about 3 mm. These 

grains are clearly of a feldspathoid mineral, and in outline they are 

sometimes sharpty, sometimes more or less poorly, defined. In some 

places they are distinctly separated; in others they are closely crowded. 

Their general appearance is shown on PL VI, A, which presents .them 

of natural size. In PL VI, B, they are seen on a small portion of the 

rock surface magnified three times. It can be plainly seen in PL VI,
84 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

J3, that all of the light component consists of these rounded masses 

or of small anastainosing offsets from them. This is an important 

point in the classification of the rock and will be alluded to later on. 

As will be shown, they are pseudoleucit.es. The dark component 

which fills the interspaces between these round grains of pseudoleucite 

is a blackish-green augite in small irregular masses showing an occasional dull-lustered cleavage face. The largest of them are rather 

columnar and about 2 mm. long. Here and there at times a small 

bronzy-lustered plate of biotite is seen. These, intermingled and outlined in places. by little stringers of feldspathoid substance, form a 

sort of granular interfilling between the larger pseudoleucites. This 

appearance is shown on PI. VI, B.

Microscopic characters. In thin sections the minerals observed are 

apatite, iron ore, olivine, biotite, augite, orthoclase, nephelite, and 

zeolites with kaolin.

The apatite is in small, stout prisms of the character usual in basic 

rocks. The iron ore is in small formless grains. The amount of these 

minerals is small. Of the olivine, only a few scattered, mostly formless grains were seen in the sections. It is fresh and of the usual 

character. Its amount is entirely too small for it to be reckoned as one 

of the normal constituents of the type; its occurrence must be regarded 

as sporadic. B.iotiie is seen in scattered shreds without crystal form. 

It has the usual brown pheochroic character, and only a small quantity 

is found.

The augite is the only ferromagnesian mineral which is important. 

It is of a green color and very similar to the augites which have been 

described as characteristic of shonkinose (shonkinites). It is slightly 

pleochroic in tones of green, between a slightly grayish cast and a yellowish olive. Its angle of extinction is large, about 45°, and it is clear 

that the diopside molecule predominates and that it is not an segiriteaugite, though probably a little of the segirite molecule is present. 

In its cleavage and other characters it offers nothing of especial 

interest.

In plain light the areas of white feldspathoid mineral are colorless 

save for a brown tone here and there, caused by a partial kaolinization, which renders the material turbid. In polarized light they are 

neither isotropic nor do they show the characteristic twinning of large 

leucites. They are composed of irregular patches and rudely fibrous 

radiating bundles of minerals having the birefringence and properties of alkali feldspars and nephelite. They are, indeed, similar to 

occurrences already described from Brazil,0 Arkansas,* and Montana. 6

« Graeff, Neufes Jahrb. fur Min., 1887, vol. 2, p. 257.

b Williams, J. F., Ann. Kept. Arkansas Geol. Surv. for 1890, vol. 2, p. 268.

cPirsson, L. V., Am. Jour. Sci., 3d ser., vol. 50,1895, p. 395; 4th ser., vol. 2,1896, p. 194.
PIRSSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 85

The masses have a peculiar mottled, marbled effect, due to the 

intergrowth of the two minerals, and in a few very small spots are 

penetrated by clear areas of a colorless isotropic mineral which is 

held to be secondary analcite. In some places very small secondary 

feldspars with clean-cut crystal outlines seem to have formed. As 

the optical tests on these intermingled feldspathic minerals gave somewhat indefinite results, it was determined to confirm them by chemical tests. For this purpose a piece of the rock was broken into fine 

fragments and out of these was carefully picked about half a gram of 

material which contained only the white feldspathoid substance and 

in which was no dark mineral. Thus the possibility of any contaminating olivine was avoided. The fragments were reduced to powder, 

which Avas then boiled for a moment with very dilute nitric acid, the 

solution was filtered, and the filtrate evaporated. As the filtrate 

diminished in volume an abundant gelatinization ensued. The solution gave an abundant and powerful flame test for soda, showing clearly 

the presence of nephelite in some quantity. The solution gave no 

reaction for chlorine or sulphuric anhydride, which indicated thatthere was no sodalite or nosean present. The undissolved residue 

consisted of the alkali feldspars.

Chemical composition. The complete analysis of this rock is given 

in the next table. The noteworthy features of it are the rather low 

silica and alumina, the medium iron and magnesia, and the high lime 

and alkalies, with the strong predominance of the potash. Analyses 

of some other leucitic rocks are given for comparison. The silica, it 

is to be noted, is rather high for a leucite rock, owing to the fact that 

the greater part of the original leucite has been turned into orthoclase. 

It is not higher, on the other hand, than the rock from Brazil or the 

wyomingite, but these have higher alkalies and thus a higher amount 

of leucite.

In in and iv, typical leucitites, as that term is now used, the general relations of the oxides is very close to those in fergusose, the 

higher silica being the most marked difference. It thus represents 

very well in a chemical way the extrusive leucitic magmas.

In regard to analysis n, it would appear as if the separation between 

alumina and magnesia were doubtful, since Rosenbusch a remarks that 

it contains more pyroxene than in, while the analysis would indicate 

an extremely small amount of pyroxene. The amount of alumina is 

so large as to be almost marvelous in a rock with the mineral composition as described.

The comparison with the analysis of missourite is interesting. It 

is at once observed that missourite is a much more basic rook; it is 

lower in silica, alumina, and alkalies, and much higher in lime and 

magnesia, the iron remaining the same. Missourite is thus much 

richer in augite and has abundant olivine. Fergusose (fergusite) is a

aElemente der G-estemslehre, 1898, p. 350.
86 IGNEOUS BOOKS OF H1GHWOOD MOUNTAINS. [BOLL. 237. 

Analyses of fergusose from near Shonkin Creek and leucitic rocks.

SiO2 .. ........

Al O

FeA ........

FeO..........

MgO. ........

CaO .-..-.-.-

Na2 0. ........

KaO. . --------

H,O+1100 ---. 

HjO-1100 -.-

coa ..........

TiO2 .........

PA ---------

SO...........

01 ............

MnO. ........

BaO-. ........

SrO..........

I4O .........

Total ..

I.

51.75

14.52

5.08

3.58

4.55

7.04

2.93

7.61

| 2.25

.23

.18

Trace.

' .05

Trace.

.30

.07

Trace.

100.14

II.

52. 16

20/14

6.45

1.54

4.64

5.73

8.12

1.39

Trace.

(?)

(?)

Trace.

(?)

(?)

100. 17

III.

46. 51

11-. 86

7.59

4.39

4.73

7.41

2.39

8.71

r 2.45 

I 1.10

.83

..80

Trace.

.04

.22

.50

.16

99.73

IV.

45.99

17.12

4.17

5.38

5.30

10.47

. 2.18

8.97

1 .45

.37

.25

None.

100. 65

V.

46.06

10. 01

3.17

5.61

14.74

10.55

1.31

5.14

1.44

.73

.21

.05

.03

Trace.

.32

.20

99.57

VI.

53.70

11.16

3.10

1.21

6.44'

3.46

1.67

11.16

( 2.61 

i .80

1.92

1.75

.06

.03

.04

.62

.19

«100. 21

VII.

46.48

19. 00

4.74

2.30

2.49

4.35

8.46

6.78

| 3.31 

.36

1 22

.15

.19

.08

Trace.

. (?)

(?)

(?)

99.91

VIII.

0. 863

142

039

.050

.114

.125

.047

.081

.125

"Including traces of rare elements and deducting O=F1=0.44 per cent.

I. Fergusose (fergusite) from Arnoux core, head of Shonkin Creek, Highwood Mountains. E. B. Hurlburt, analyst.

II. Janeirose (leucitophyre) from Sierra de Caldas, S. Paulo, Brazil. F. W.

Dafert, analyst. Hussak, Neues Jahrb. fur Min., 1892, vol. 2, p. 149.

III. Chotose (leucitifce) from Bearpaw Peak, Bearpaw Mountains, Montana. 

H. N. Stokes, analyst. Weed and Pirsson, Am. Jour. Sci., 4th ser., vol. 

2, 1896, p. 147.

IV. Albanose (leucitite) from Capo di Bove near Rome, Italy. H. S. Washington, analyst. Am. Jour. Sci., 4th ser., vol. 9, 1900, p. 53. 

V. Missourote (missourite) from Shonkin core, head Shonkin Creek, Highwood Mountains. Weed and Pirsson. Am. Jour. Sci., 4th ser.. vol. 2, 

1896, p. 321.

VI. Orendose (wyomingite) from Leucite Hills, Wyoming. W. F. Hillebrand, analyst. W. Cross, Am. Jour. Sci., 4th sar., vol. 4, 1897, p. 130.

VII. Arkansose (leucitite) from Etinde Volcano, Cameroon Mountains, Africa. 

M. Dittrich, analyst. E. Esch, Sitzb., k. Preuss, Akad. Wiss., Berlin, 

 Math. Phys. Kl., 1901, p. 299. 

VIII. Molecular proportions of I.
PIKSSON.] PETKOGKAPHY OF STOCKS AKD LACCOLITHS. 87

leucocratic (salic) type in Brogger's sense, though not strikingly so, 

while contrasted with it missourite is melanocratic (femic); thus 

fergusose represents the leucitites, raissourite the leucite-basalts.

In regard to the surface lavas Rosenbusch a says': '' The two divisions 

of the leucite rocks, the leucitites, free from olivine, and the olivinebearing leucite-basalts, are distinguished not alone by the presence 

or absence of the olivine. The typical leucitites, in contrast to the 

typical leucite-basalts, are characterized much more by a far smaller 

content in iron ores and augite." Thus in the Highwood Mountains 

the intrusive representatives of leucite rocks, fergusite and mis-' 

sourite, have the same difference.

Texture. The texture of the rock is best seen from the photographs 

shown on PL VI. A glance at the picture of the hand specimen gives 

the impression that the rock is a porphyry. When the rock is examined, however, it is seen that this idea is not correct. By porph'yritic 

texture one understands that certain minerals possess a definite wellcut crystal form and that they are embedded in a distinct groundmass. 

This is not the case here, for all of the leucite is in approximately 

rounded grains with well-defined though crystallographically not 

well-bounded grains.of augite between them. There can not be distinguished two periods of formation of any mineral, nor two types of 

any mineral formed simultaneously. The structure is much more 

to be compared to that of certain syenites which are characterized by 

a tabular development of the feldspar. That is, the feldspathic minerals are dominant, and have attempted to take their own form and 

have approximately done so, compelling the ferromagnesian elements 

to crystallize in between them.

It is also to be noted that, whereas in a mineral which crystallizes in 

some form other than the leucitohedron and to which it tends to 

approximate the cross sections taken at random would have a variety 

of shapes, producing an effect of irregularity or of granular texture, 

in the case of leucite the endeavor to assume the form of the tetragonal trisoctahedron or leucitohedron results in the production of 

spheres or spheroids, and every cross section is either a circle or 

approximately one, thus tending to destroy the effect of irregularity 

and to produce a pseudoporphyritic structure. Aside from this, the 

rock has in the hand specimen the solidity, firmness of texture, and 

general appearance of the granitic types of rocks, as suggested by 

PI. VI, B.

Classification in the neiv system. In the new system this rock occupies a definite position in the subrangs under laurdalase, and as it is 

the only described analysis under this subrang its name has been 

given to the division. 6 This may be seen in the following calculation 

of its norm.

« Mikr. Phys. Mass. Gesteine, 3d. ed.,,1895-96, p. 1232.

b Cross, Idding.-<, Pirsson, and Washington, Quantitative Classiflcationof Igneous Rocks, p. 268.
88 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Calculation of the norm offergusose.

SiO2 ... ......... ..

AljOg..... ........

FeA-------------

FeO.. ............

MgO. ............

CaO. .............

Na2O. .............

K20. ....:........

TiO8 ... ...........

PA-------- --

01... .............

Best.............:

Total ......

Analysis.

51.75

14.52

5.08

3.58

'4.55

7.04

2.93

7.61

.23

.18

. .05

2.62

100.14

Molecular 

ratio.

0.863

142

.032

.050

.114

.125

.047

.081

.003

.001

.001

Or.

486

81

81

81

Ab.

49,

7

7

7

An.

98

14

14

14

Ne.

80

40

40

40

Di.

216

13

95

108

108

01.

10

9,

19

10

Mt.

39,

39

39,

n.

3

3

3

Ap.

3

1

1

1

Or....... 45.04

Ab...... 3.67

An...... 3.89

Ne ...... 11.36

Di....... .23.75

Ol....... 1.53

Mt ...... 7.42

n........ .46

Ap ...... .34

Rest..... 2.(

63.96

33.50

.,_ Sal. 63.96 TT ,

Class, yem =33 QQ=1.9 II, dosalane.

T 11 ^fi 

Order, =p=g<;rgQ=.21=lendofelic=6, norgare.

Rang, ~ QRQ/ 2 =yj=9.1=peralkalic=l, laurdalase.

Subrang,

~K O' 81

ic=2, fergusose.«

Total.. 100. (

Mineral composition or mode. The rock was too coarse grained for 

satisfactory measurement under the microscope, and it was therefore 

measured megascopically, with the following results:

Sal

Fern ............ ...:

Total .........

Units 

measured. 

............ 334

............ 141

............ 475

Volume, 

per cent. 

70.3

29.7

100.0

Specific 

gravity. 

X 2.6 =

X 3.3 -

1,827.8

980. 1

2, 807. 9

Weight, 

per cent. 

= 65.1

= 34.9

100.0

These results agree so closely with those calculated for the norm 

given above that we may say that the rock has a normative mode.

Classification in prevailing systems. In the prevailing systems of 

classification this rock has also a distinct place of its own and should 

be called fergusite. This is due to its mode of occurrence, mineral

a When this name was chosen it was supposed the Arnoux stock was in Fergus County. 

Recent maps show the locality a little north of the line, in Choteau County. Boundary lines in 

these thinly settled regions are not very accurately known, so the error, if it should be confirmed, is not a matter of importance.
PTRSSON.] PETBOGRAPHY OF STOCKS AND LACCOLITHS. 89

and chemical composition, and texture. It is defined as a granular 

intrusive rock consisting of dominant leucite with subordinate augite. 

Small amounts of accessory minerals maybe present, such as apatite, 

iron ores, biotite, sporadic olivine, etc., but the true determinants are 

the leucite and the augite: The rock is therefore the granular representative of the leiicitites, and it bears the same relation toward them 

that missourite bears to the leucite-basalts, and thus fills a gap in this 

system of classification.

It is true that in the present example the leucite has been changed 

to pseudoleucite, but that does not alter the validity of the type. 

In a large part, probably the major part, of the missourite of the 

Shonkin stock the leucite is also changed to pseudoleucite, but, as 

mentioned under the description of missourite, in places the leucite is 

still left unchanged and in other places it occurs changed and 

unchanged in the same specimen. A detailed study of all parts of 

the Amoux stock would probably show the same phenomena, or if 

the mass could be studied at various vertical levels it might show the 

same variations. In hand specimens of missourite and other altered 

leucite rocks of the Highwoods it is impossible to distinguish the 

unchanged from the altered leucite. Only when studied under the 

microscope does the real character appear. Therefore the type may 

be considered as a well-established one, although the specimen 

described is not a perfect unaltered example.

GRANO-BOROLANOSE (BASIC SYENITE, SHONKINITIC TYPE) OF MIDDLE

PEAK.

The rock forming the Middle Peak stock closely resembles the shoshonose (monzonite) of Highwood Peak in appearance and in chemical composition. It differs from it, however, in important particulars, 

as will be seen in the following description. It has a border facies 

which varies in several respects from the main type.

Megascopic characters. On a freshly fractured surface the rock has 

a medium-gray color and a moderately even grain, the individual components being in the neighborhood of 1 mm. in diameter. On inspection it is seen to be made up of light and dark minerals in about equal 

proportions. The light feldspathic minerals are devoid of distinct 

form, but the ferromagnesian component is chiefly a dark-green angite 

in short, stout, columnar forms without good terminations. The 

mingling of these two gives the rock at a short distance the general 

appearance presented by many medium-grained diorites.

Microscopic characters. Under the microscope the following minerals are seen: Iron ore, apatite, biotite, olivine, augite, and alkali 

feldspar.

The iron ore is at present in rather abundant small grains; a few 

la,rger ones were seen. It is generally surrounded with narrow 

mantles of a red-brown pleochroic biotite. There are also a few
90 IGNEOUS ROCKS OF HIOHWOOD MOUNTAINS. [BULL. 237.

isolated shreds of this biotite, but its total amount is small and its 

role insignificant. There is a moderate amount of a rather fresh 

oli vine present in rounded crystals which average about 0.4 mm. in 

diameter. In many places where it touches alkali feldspar there is a 

very narrow zone of a green pleochroic biotite separating them. 

. Two. varieties of augite are present, and it is by far the most abundant ferromagnesian mineral. The first variety, which is rare, is a 

colorless or faintly green diopside in long, slender, well-formed prisms. 

It has a wide extinction angle, strong birefringence, and is often 

twinned on (100). The other pyroxene, the most abundant dark mineral, is an augite of a green color with a tinge of brown. It is present in short, stout prismoids and grains, and also at times in larger 

columns, which are we'll crystallized. These attain a length of 4 mm. 

It has a good cleavage, is nonpleochroic, and often zonally built. It 

has inclusions of iron ore, biotite, and brownish glass, and the largest 

crystals are sometimes spongy and filled with these earlier products 

of crystallization. . ,

The light-colored feldspathic component appears to consist wholly 

of alkali feldspars. These laths are in rather thin tabular crystals 

which are almost always carlsbad twins. No albite or other twinning 

was observed. The laths do not contain any microperthite intergrowths, but are zonally built. In sections parallel to the-clinopinacoid, which are easily recognized by their nearly square outlines, 

lack of twinning, and medium birefringence, the cleavage parallel to 

the base (001) is seen as a series of fine lines with moderately high 

power. The direction of the vertical axis is told by a rather poor 

prismatic parting and the arrangement of minute inclusions parallel 

to the prism faces. The angle between these two directions was 

measured at 65°, which -is approximately equal to the angle ft of feldspar. On such faces the bisectrix £ emerges and the plane of the 

Optic axis gives au extinction angle of 5° with the base of the basal 

cleavage in the obtuse angle ft. These properties are found in the 

interior of the crystals, and they show that this portion of them is 

composed of a rather pure orthoclase. Toward the outer boundary 

the birefringence gradually rises a little and the extinction angle also 

increases to 12° and over. This shows that the inner kernel of orthoclase gradually changes to a soda orthoclase by assumption of the 

albite molecule, giving rise to. the zonal structure mentioned.

Between these feldspars lie the final products of crystallization^-a 

fine mosaic of interlocking granules of alkali feldspar. These small 

areas have an appearance which is much like that of the groundmass 

of some feldspathic dike rocks or of trachytoid lavas having a microgranitic structure. They were carefully searched for quartz or 

nephelite, and while no quartz is present, it can not be said that
rinssoN.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 91

nephelite is not. Under the circumstances and considering the chemical composition of the rock, it would be natural to expect nephelite, 

but the grains were too fine for any definite determination, and the 

presence of olivine precludes the usual gelatinization test. The 

writer has found that gelatinization and staining with eosin is an 

unsatisfactory and unsafe test where minute amounts of nephelite are 

concerned, and it was not tried. If a safe means of determining very 

small amounts of nephelite mixed in with alkali feldspars conld be 

discovered it would be of great seiwice to petrographers.

Some of the small areas mentioned above are rounded and in plain 

light appear pale brown from -incipient kaolinization. It is barely 

possible that they may be pseudoleucites, a suspicion aroused by the 

chemical composition of the rock.

Chemical composition. The chemical composition of this rock is 

shown in I of the table of analyses on page 92.

The characteristic chemical feature is the combination of high 

oxides of the alkaline earths and high alkalies, and more especially of 

lime and potash.. These give the rock a 'marked chemical individuality the Highwood stamp. The alumina is moderate, the silica 

rather low. These characters are shared by two other rocks of the 

district (li, III), which will be described in a later paragraph. There 

is a general resembance of these types in the distinguishing features 

to borolanite (iv), which is also low in silica, contains a good deal of 

lime, and is high in potash; its alumina is no doubt too high, in part it 

contains P205 and probably some MgO. Another rock which is similar in the same way is that variety of the nejphelite-syenite complex of 

Magnet Cove described by Washington, first as shonkinite and later 

as covite, whose analysis is given under v. In the new system of 

classification it falls in the same subrang. The chemical difference 

between these types and the shonkinoid ones is shown by comparison 

with the analysis of the montanose (shonkonite) of the Shonkin Sag 

laccolith given under vi. The lower alumina and alkalies and the 

larger amount of bivalent oxides produce a marked difference in the 

mineral composition, which is easily seen in a comparison of the hand 

specimens. The same is also true of the shoshonose. (.monzonite) of 

Highwood Peak; in the hand specimens, as previously mentioned, 

this rock very closely resembles that of Middle Peak, but the microscope shows, a considerable amount of plagioclase, which is produced 

by the higher lime and lower alkalies, and which is wanting in the 

Middle Peak rock.
92 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237. 

Analyses of borolanose and related rocks.

SiOa .........

A12 03 . ------

FeA ..---..-

FeO ........

MgO-----.

CaO ........

Na20. .......

K2 0 ........

H2 0+__.__..

HJ.O .......

CO,. ........

TiO2 ........

p o

csn

Cl .-...-...-

OA ------

NiO.. .......

MnO. .......

BaO ........

SrO. ........

C1=O_.----.

Total..

I.

52.05

15.02

2.65

5.52

5.39

8.14

3.17

6.10

.35

None.

.47

.21

.02

.24

None.

Trace.

.42

.28

100. 03

.06

99.97

II.

50.11

17.13

3.73

3.28

2 47

5.09

3.72

7.47

4.47

,.82

.67

.08

.07

.63

.35

100.09

.02

100. 07

III.

50.00

19.36

3.87

2.67

2.18

4.96

3.63

8.52

3.53

.46

99.18

IV.

47.8

20.1

6.7

.8

1. 1

5.4

5.5

7.1

2.4

.7-

(?)

.4

.5

.8

99.3

V.

49.70

18.45

3.39

4.32

2.32

7.91

5.33

4.95

1.09

25

1.33

.40

Trace

99.44

VI.

47.88

12.10

3.53

4.80

8.64

9.35

2.94

5.61

1.52

.70

.12

.77

1.11

Trace

.04

Trace

.15

.46

.13

99.99

VII.

51.00

17.21

2.41

4.23

6.19

9.15

2.88

4.93

.63

.13

.33

.03

Trace

.34

.14

99.60

vm.

0.867

.147

.017

.076

.135

.145

.052

.065

.006

.001

.006

IX.

0.835

.168

.023

.046

.062

.091

.060

.080

.010

.005

.001

.002

------

......

X.

0.833

.182

.024

.037

.055

.089

.059

.091

.006

.002

I. Borolanose (basic syenite) from Middle Peak. E. B. Hurlburt, analyst.

II. Borolanose (basic syenite) from Palisade Butte. H. W. Foote, analyst.

III. Borolanose (basic syenite) from Shonkin Sag laccolith. W. F. Hillebrand, 

analyst. (Partial analysis; TiO2 , P2 05 , etc., not determined; in part with 

A12O3 , which is high.)

IV. Borolanose (borolanite) from Lake Borolan, Sutherland, Scotland. J. Hort 

Player, analyst. Home and Teall, Trans. Roy. Soc. Edinburgh, vol. 37, 

pt. 1, 1892, p. 163.

V. Borolanose (covite) from Magnet Cove, Arkansas. H. S. Washington, 

analyst. Jour. Geol., vol. 9, 1901, p. 612.

VI. Montanose (shonkinite) from Shonkin Sag laccolith. W. F. Hillebrand, 

analyst, (Includes ZrO2 , 0.03 and V2O3 , 0.04.)

VII. Shoshonose (monzonite) from Highwood Peak. E. B. Hurlburt, analyst. 

VIII. Molecular proportions of I..

IX. Molecular proportions of II.

X. Molecular proportions of III.

Mineral composition or mode. In determining the actual mineral 

composition of the rock, it was found necessary in using Rosiwal's 

method to measure together the alkali feldspars and any nephelite 

that might be present, since, for the reasons given in the microscopic
TIRSSON.] PETEOGEAPHY OF STOCKS AND LACCOLITHS. 93

descripthDn,v it was impossible to discriminate between them. 

4,390 units were measured in 114 measurements, giving an average of 

38 units to each measured area. The following quantitative composition, is derived:

. Mineral composition or mode of borolanose of Middle Peak.

Feldspar ...............

Biotite ..................

Apatite ...._......-.-..

Total ............

Units 

measured.

_........--- 2,465

................ 1,275

............... 175

............... 60

............... 200

............... 35

............... 4,390

Volume, Specific Weight, 

per cent, gravity. per cent.

60.25 X 2.6 = 156.6 = 53.2

28.81 x 3.3 = 95.0 = 32.3

3.98 X 3.0 = 11.9 = 4.0

1.37 X 3.3 = 4.5 = 1.7

4.55 X 5.2 = 23.6 = 7.9

.89 X 3.2 = 28.4 = - .9

99. 85 294. 2 100. 0

Classification in the new system. The position of the rock in the

new system is given in the following calculation, which shows it to be 

borolanose near shonkinose. 

should be called grauo-borolanose.

Calculation of the norm of borolanose.

On account of its granular texture it

Si02 .........

ALA --------

Fe2 0, ........

FeO .........

MgO.........

CaO .........

Na2O ........

K2O .........

TiO,- --

P n

C12 ...........

Rest .........

Total ..

Analysis.

52. 05

15.02

2.65

5.53

5.39

8.14

3.17

6.10

.47

21

.24

1.07

99.97

Molecular ratio.

0.867

.147

.017

.076

.135

.145

.052

.065

.006

.001

.003

........

Or.

390

65

65

65

Ab.

84

14

14

14

An.

66

3R

33

33

Ne.

52

26

2fi

26

So.

18

9

1?,

3

3

Di.

218

31

78

109

109

Ol.

40

9,9

57

40

Mt.

17

17

17

11.

6

6

6

Ap.

3

1

1

Or.......

Ab ......

An ......

Ne ......

So.......

Di.......

01. ......

Mt ......

n........

Ap ......

Best......

Total..

36.14 _ Sal. 62.94 .. , ,

7.34

9.17

7 OQ

2.91

24.53

6.23

3.94

.91

.34

uiass, Fem ~35~95~J - 8:~ X1' aosaiane (near sairemane).

62.94 Order, p=gg1 gg=.19=lendofelic=6, norgare (portugare).

35.95

1.07

99.96
94 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

It is of interest to compare the norm as calculated above with the 

mode obtained by measurement. For this purpose the feldspathic 

components of. both are united under the symbol F, and ilmenite with 

magnetite.

Comparison of norm with mode of borolanose.

F ......................................................

Di

Bi ...................................................

Ol ....................... ...........:. .... ..........

Mt... .............I. .................................

An

I.

62.9

24.5

6.2

3.8

.8

II.

53.2

32.3

4.0

1.7

7.9

q

III.

53.7

33.7

6.2

3.8

q

Under I is given the norm and under n the mode. The chief difference lies in the relation of feldspar to pyroxene, and is caused by the 

fact that in the norm is included 9.2 per cent of anorthite. The lime 

silicate is, however, actually not in the feldspars but in the augite, as 

shown by the study of the section. If it were transferred to the 

augite the mode would assume the proportions seen in in and the, 

close agreement of the ratios of feldspar to augite is seen at once. 

The lime may not be present in the augite exactly as anorthite, but 

as a closely related lime silicate.

It is interesting to observe that this relation in the feldspars makes 

the chief difference between this rock and the shoshonose (monzonite) 

of Highwood Peak. There the anorthite has united with the albite 

to make plagioclase; here the lime as a silicate has gone into pyroxene.

Classification inprevailing systems. In the prevailing systems this 

rock does not have any very definite position. If the relative quantities of the components and its associations and chemical composition 

are disregarded and the fact that it consists in the main of alkali feldspars and augite is considered, it would be called an augite-syenite. 

There are certainly not enough feldspathoids present to place it in the 

hephelite-syenite family. If the basic character shown by the analysis, the relative quantities of the.minerals, aud its regional rock associations are taken into account, it appears as a type transitional 

between syenite and shonkinite, and might then be called a shonkinitic syenite.

Border fades. At the contact the rock just described has a rather 

distinctly marked endomorphic contact fades. It appears to be somewhat coarser in grain and has a slight tendency to a porphyritic texture. This is due in the main to the fact that the orthoclase feldspars 

appear in large, thin plates of considerably greater size than in the 

prevailing type and tend to be roughly oriented parallel to the contact 

in a rude flow structure which gives the rock a slight tendency to 

split in this direction.
nussoN.] ' PETROGRAPHY OF STOCKS AND LACCOLITHS. 95

Under the microscope the minerals of the general type just described 

are found with some little variation. In the contact rock the iron ore 

and apatite, and perhaps biotite, of a-rich pleochroic brown, are somewhat more abundant than in the type rock. The augite varies considerably in size and is a clearer green color, passing to a rich sea 

green on the border. It is, however, noiipleochroic in spite of this 

addition of the iegirite molecule, and hence it must still be chiefly 

diopside. The feldspars are in carlsbad twins, and in small angular 

interspaces are occasional clear isotropic areas of unaltered sodalite, 

whose occurrence serves to make almost certain the presence or former 

presence of a lenad (feldspathoid) mineral in the main type.

The.main feature of this border facies is the occurrence of horn- \ 

blende associated with diopside. The hornblende appears to some 

extent to replace the diopside, since in those places where it is abundant diopside is rare, and vice versa. It is a compact homogeneous 

variety, is not seen intergrown with diopside, and appears in all

respects like a primary mineral. It is much like arfvedsonite in a 

general way, and is at all events, considering its habitat and associates, one of the alkalic group of hornblendes. It is strongly pleochroic as follows:

a=Oclier-yellow.

b=Dark olive.

jC=Dark olive-green.

The absorption is very strong and the arrangement rather peculiar: 

li>jC>a. On account of the strong absorption, which is almost equal 

to a colored tourmaline, it was difficult to measure the angle of extinction on the clinopinacoid of c/\.C, but it is certainly as much as 30° 

and probably more a most unusual angle for a hornblende. The 

double refraction is rather weak. It occurs in well-shaped prisms 

with poor terminations. The prisms are stout rather than 'slender, 

and the prismatic cleavage is well marked.

This hornblende is unique in its characters and the writer knows 

of none exactly like it. It does not occur in any other rock in the 

Highwoods, its nearest affinity being the green variety in the socialitesyenite (pulaskose) of Square Butte, from which it differs in color, 

absorption, and angle of extinction. It does not agree with any of 

the alkalic series described by Brdgger." It may be that, although of 

alkalic tj^pe, the preponderance of potash over soda in the magma 

has had some effect in producing its novel characters. Unfortunately 

want of time and material have prevented the writer from investigating it chemically. '

BOROLANOSE (SYENITE) OP PALISADE BUTTE.

As mentioned in the geologic description, the upper part of Palisade 

Butte is composed of a rock which is much lighter and more feld-

« Grorudit-Tinguait Serie, 1894, p. 27.
96 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

spathic than the shonkinose or shonkinite of the main lower portion. 

There is a gradation between the two, but the top is of distinctly 

syenitic character.

Megascopically the rock is medium granular, of a light-brown color, 

and dotted with augite. In the section the same minerals are seen as 

described under shonkinose greenish augite, iron ore, apatite, alkali 

feldspars, and zeolites. The feldspars are in lath-like forms, which 

often radiate and produce rough spherulitic clusters. All the interspaces between them are filled with masses of a fibrous zeolite, which 

is largely natrolite. Probably nephelite or some other lenad (feldspathoid) mineral was originally present. The rock is not fresh 

enough to warrant more extended study, but an analysis of it was 

made to determine its systematic position and .for purposes of magmatic comparison. This analysis is given under II of the preceding 

table. In regard to classification in prevailing systems, it has a position similar to the rock of Middle Peak; it could be classified as a 

basic syenite, though if all the zeolitic areas represent a former feldspathoid mineral, such as sodalite, it would probably fall into the 

nephelite-syenite family.

In the new system of classification the analysis can be calculated 

into a norm composed of the following standard minerals:

Or....... 44.48 Ab 9 43 Class, w-^r=wns=2.6=II, doaalane.

An...... 8.03

Ne ...... 11.03

No ...... .71

Di. ...... 10.20

Ol. ...... 2.19

Mt ...... 5.34

II........ 1.52

Ap...... 1.68

Best..... 5.45

73. 76 Order, gr=gj-:g^=1.9=lendofelic=6, norgare.

^ 140 Rang, * 'J Qa ry '" =-gQ-=4.9=domalkaIic=2, essexase.

K O' 80 Subrang, -M- o/~Kn~^ 3=sodipotassic=3, borolanose.

Total.. Hto.14 

BOROLANOSE (SYENITE) OF'SHONKIN SAG LACCOLITH.

What has been said above in regard to the light-colored rock of 

Palisade Butte applies well to this rock in a general way. The lightcolored, grayish, moderately granular rock is dotted with considerable augite and less biotite.

In the section it is seer to be composed of apatite in small, pale 

violet-brown crystals, some iron ore, a few scattered olivines, some 

biotite in long foils of a dar. -brown color and strongly pleochroic, a 

good deal of augite, not very A. 3ll crystalized, pale brown in color and 

with good cleavage, a large amount of alkali feldspar greatly zeolitized, and some nephelite, which is fresher than the feldspar. The 

nephelite and feldspar are commonly intergrown in a poikilitic 

manner.

The rock is too much altered by zeolitization to warrant more
riussoN.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 97

extended description or a complete analysis, but to determine its affinities and for the purpose of comparing it with, the associated shonkinose for reasons given in the chapter on the petrology, Doctor Hillebrand made a partial analysis, which i's given under in of the 

preceding table. From this it will be seen that it closely resembles 

the rock of Palisade Butte just mentioned and also that of Middle 

Peak, both showing the characteristic high lime and potash of the 

High wood magmas. As to systematic position in prevailing systems, it 

is a little difficult, in view of the zeolitization, to say whether this 

rock should be esteemed a basic member of the syenite group with 

accessory nephelite or a member of the nephelite-syenite family; 

probably it belongs in the latter and near the covite of Washington. 

In the new system the analysis is sufficiently complete to calculate 

the norm and determine its position, which have been done as follows:

Or....... 49.48'

Lc........ .87

Ne ...... 16.76

An...... 8.90,

Di....... 10.781

01....... .83

Mt ...... 5.57

II........ .91

Ap ...... .67]

Rest..... 3.53

Sal. 70.01 An Class, g^Jf=;j]p7(;=4.0=I 

76.01 L 1763

Order, p=gg:gg=0.30=lenclofelic=6, norgare.

K30'+Na?0' 150 Rang, p- n/ =-00 =<U>=dofflalkalic=2, essexase.

91 Subraug, « " Q/-=gn=-1.5=3odipotassic=3, borolanose.

In making this calculation it has been assumed, from what is known 

of the mineral components of the rock, and the chemical character of 

the High wood rocks in general, that 0.50 per cent of Ti02 and 0.30 

per cent of P205 are present, and a corresponding amount has been 

deducted from the A1203 . While this is probably not absolutely correct, it is certainly more nearly correct than it would have been to have 

left these oxides out of account entirely. The result shows that the 

rock, like those of Middle Peak and Palisade Butte, is borolanose, 

where it evidently belongs. Considering its texture, it is, then, granoborolanose.

SHONKINOSE (SHONKINITE) OF SQUARE BUTTE.

The rock composing the dark-colored lower and outer mantle of 

Square Butte has already become well known as the original type 

of shonkinite, and for convenience and completeness the original 

description is summarized here.

Megascopic characters. The shonkinose of Square Butte is a dark, 

coarse-grained, rather crumbly rock, which is mottled by the contrast 

between the black augite and the light-colored feldspathic material. 

The augites are greenish black, columnar, well formed, and average 

about 30 mm. in length, being found occasionally three times as large.

Bull. 237 04 7
98 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

Considerable biotite is seen with bronze-lustered cleavage faces, which 

may be 1 or 2 cm. across, with irregular borders, and composed of 

smaller biotites in parallel growths which inclose poikilitically the 

other minerals. Filling the interspaces are. the white feldspathic 

minerals. Augite is the most predominant mineral and makes up 

half the bulk of the rock.

Microscopic characters. The microscope shows the following minerals : Apatite, iron ore, olivine, biotite, augite, albite, soda inicrocline, 

orthoclase, sodalite, nephelite, cancriuite, and zeolites.

Apatite is the oldest mineral and occurs in short, stout prisms. At 

times it is a pale red or violet brown and nonpleochroic. Other crystals are filled in the interior with a dusty pigment and are pleochroic; 

empale steel-blue, ft?=pale leather-brown. The prisms attain a length 

of 0.5 mm.

Olivine has its usual characters; it is generally fresh and colorless, 

but at times has a reddish ferruginous border that may be an alteration into iddingsite. In thicker sections some crystals have a faint 

pleochroism in tones of yellow and white.

Biotite is strongly pleochroic, deep umber-brown, and pale brownishorange. The extinction is parallel to the cleavage. Thin plates have 

a small opening of the cross that shows it to be meroxene. Where 

olivine touches orthoclase there is usually a reaction band between 

them of a deep-green biotite, which has very little pleochroism or 

absorption.

Pyroxene occurs in good crystals and, owing to the ease with which 

they may be detached from the matrix,, excellent material can be 

obtained for study. Measurements on the goniometer show the following forms: a (100), ft (010), m (110), and s (111); The pyroxene is 

somewhat tabular on a (100). The crystals were crushed, sifted, and 

separated by the silver-thallium nitrate fluid, and material of great 

purity obtained in this wajr was subjected to chemical analysis with 

the following results:

Analysis of pyroxene in shonkinose of Square Butte.

SiO2 -.-.-.._.._.._-.------.--.----.. ..--.-----..-..-_..--. 49.43

TiO2 _....................................................... .55

A12O;) ----------------------------------------------------- 4.28

FesO3 ------------------------------------------------------- 2.. 86

FeO-.._ ......^............................................ 5.56

MgO .:....._..............._............... ................ 13.58

CaO....... T ................................................ 22.35

MnO ......_.___._._....___...._______.__......_________;... .10

Na2O. ........:........................ .-.................. 1.04

K2O ...................._................................... .38 "

H.2O ........................................................ .09

100.21

A consideration of this analysis shows that the mineral has almost 

exactly this composition: 13 Ca (MgFe) Si2O6 + 2 (Na2R") (AlFe) 8 Si06 .
PLATE VII.

99
PLATE VII.

SHONKINOSE OF SQUARE BUTTE.

A. Shonkinose, Square Butte, High wood Mountains, Montana; 18 diameters: 

polarized light; nicols uncrossed. Actual field, 4 mm.' Apatite, iron ore, 

olivine, biotite, pyroxene, anorthoclase, orthoclase.

B. Shonkinose, Square Butte; 18 diameters; polarized light; nicols crossed.

Actual field, 4 mm. Iron ore, apatite, biotite, augite, and orthoclase. 

100
U. S. GEOLOGICAL SURVEY BULLETIN NO. 237 PL. VII

(A)

SHONKINOSK OF SQUARE RUTTK
rmssoN.] PETROGRAPHT OF STOCKS AND LACCOLITHS. 101

It is interesting to observe that if the analysis of this augite should 

be considered that of a rock, and the norm calculated, the angite 

would be composed of the following mineral molecules:

Calculated mineral composition of augite. 

Lencite..----------..----...--------------.---------.-----.--- 1.74

Nephelite-...-.-----.-....--..---.--...----.-...-----.-----..- 4.83

Anortliite --_.__--.______.-.__------.__-----_. --------------- 5.84

Diopsicle.------.---------------------.------------------.---- 73.80

Akermanite -_-------_._--_-_-------_---.--___---__--___-_..- 4.44

Olivine.-..--..---.--.-----..--------------.------------.---- 3.81

Magnetite................................................... 4.18

Ilmenite.. .................................................... .91

.......--......-.......--.--.:..--.------.---...--.---.-- .19

Total.................................................. 99.74

In a general way this shows that the angite is greatly influenced 

in composition by the magma in which it is formed. This fact 

becomes more striking when seen in this way than by a bare comparison of analyses. It will be useful, also, to observe this calculation 

in making the comparison between the norm and mode of the new 

classification.

In thin section the augite is of a dark-green color with an olive 

tone, good cleavage, and occasional inclusions of iron ore. Orthoclase is the dominant feldspar. It is fresh and generally xenomorpliic, occurs sometimes in rude columnar shapes, and contains 

inclusions of glass crystallographically arranged. The angle of the 

optic axes is variable, being usually small, sometimes nearly zero. 

In places the augite contains patches of intergrown soda microcline.

Albite is present in small amount, as shown by the specific gravity 

separation between 2.61 and 2.60. Qualitative analysis shows that 

the mineral is free from lime.

Nephelite has been found in the specific gravity separates and has 

been proved to be present in recent sections. It fills small interspaces, 

is commonly zeolitized, and the rock contains only a small amount 

of it.

Cancrinite was not definitely proved to be present in the sections, 

but was found and tested chemically in the products separated by 

heavy fluids.

Sodalite also occurs as an accessory mineral. It is seen occasionally in the section as a limpid isotropic filling of interspaces, but it is 

often zeolitized.

IsTatrolite occurs in fibrous patches and bundles secondary after 

feldspar, nephelite, and sodalite. The appearance of the rock in 

thin section is shown in PL VII, which also illustrates in a general 

way the character of the minerals in the Highwood rocks with feiuic 

norms.
102 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Chemical composition. The chemical composition of this rock is 

seen in the table below. Its characteristic features are the low silica 

and alumina, large amount of ferromagnesian oxides and lime, and 

the predominance of potash over soda.

Analyses of shonkinose and related rocks.

SiO2 --....:.-----

Al O

Fe2O3 .......... ...

FeO.. ............

MgO. ..........

CaO .............

Na2O---.. --------

K2O. .............

H20+_. ..........

H2O- ......i.-..-

r\r\

TiO2 .............

P205 .............

S03 ..............

Cl ...............

Cr203 ............

MO. ...... .......

MnO. ---------...

BaO ..............

SrO.. ............

Total.......

I.

46.73

10.05

3.53

8.20

9.27

13.22

1.81

3.76

1.24

.78

1.51

.18

.28

?

?.

100. 56

.04

100. 52

II.

50.00

9.87

3.46

5.01

8.31

11.92

2.41

5.02

1.16

.17

.31

.73

.81

.02

.03

.11

.07

Trace.

.32

.07

100.01

.08

99.93

III.

48.98

12.29

2.88

5.77

9.19

9.65

2.22

4.96

.56

.26

1.44

.98

Trace.

.08

.43

.08

99.99

.08

99.91

IV.

47.88

12.10

3.53

4.80

8.64

9.35

2.94

5.61

1.52

.70

12

.77

1.11

None.

Trace.

;04

Trace.

.15

.46

.13

99.99

V.

48.05

13.94

2.67

5.98

7.81

7.25..

2.72

6.56

1.66

1.10

1.15

98.89

VI.

50. 15

15.86

2 44

5.39

5.30

8.40

4.13

5.00

1.50

1.00

.86

100. 03

VII.

52. 09

11.93

1.84

7.11

12.48

7.84

2.04

3.01

.35

.16

.73

.34

Trace.

.10

.07

.15

?

?

100. 24

VIII.

0.779

.098

022

.114

.232

.236

.029

.040

.010

.011

.003

I. Shonkinose (shonkinite) from Square Eutte. L. V. Pirsson, analyst. (MgO 

corrected.)

II. Montanose (shonkinite) from Bearpaw Mountains. H. N. Stokes, analyst. 

Weed and Pirsson, Am. Jour. Sci., 4th ser., vol. 1, 1896, p. 360 (includes 

Fl.=.16).

III. Shonkinose (shonkinite) from Yogo Peak, Montana. W. F. Hillebrand. 

analyst. Weed and Pirsson, Am. Jour. Sci., 3d series, vol. 50,1895, p, 474.

IV. Montanose (shonkinite) from Shonkin Sag laccolith. W. F. Hillebra'd, 

analyst (includes S=.03; ZrO2 =.03).

V. Shonkinose (shonkinite) fromMaros Peak. Borneo. Doctor Hinden, analyst. 

C. Schmidt in Sarasin's Celebes, vol. 4, 1901, p. 23.

VI. Shonkinose (shonkinite) from MarosPeak, Borneo. Doctor Hinden, analyst.

C. Schmidt in Sarasin's Celebes, vol. 4,1901, p. 23.

VII. Kentallenose (kentallenite) from Glen Shira. Argyllshire. Scotland. W. 

Pollard, analyst. Hill and Kynaston, Quart. Jour. Q-eol. Soc., vol. 56, 

1900, p. 537..
PETJROGKAPHY OF STOCKS AND LACCOLITHS. 103

These features are found in similar rocks from other Montana localities, as has been previously pointed out. As at Square Butte, they 

are also border differentiates of a single intruded mass. Its close 

similarity to montanose, described in a following section, is seen by 

comparing the analyses (i and iv of above table). The chief difference is that the montauose is considerably higher in alkalies and 

lower in ferromagnesian oxides. A closely related rock has recently 

been described by Schmiclt from the island of Borneo under the name 

of shoiikinite. It consists of augite, biotite, olivine, orthoclase, labradorite, with accessory apatite, iron ore, nephelite, and sodalite. 

Analyses of two specimens are given under v and vi. They closely 

resemble III, and the considerable amount of alkalies shows that- they 

would fall near the subrang with the montanose, described later/*

In his work on the Monzoni rocks, Brdgger^ quoted an analysis of 

Lemburg's of a "pyroxenite" with large crystals of oi'thoclase, and 

pointed out the resemblance of these types to shonkinite. In his 

recent interesting study of the rocks of this region, Doelter c states 

that shonkinite occurs among them, but the type described is said to 

contain considerable plagioclase, which Rornberg^ calls attention to. 

The latter also states (p. 37) that shonkinite occurs on the Mulatto. 

Until more detailed studies and chemical analyses of these rocks have 

been made it will be impossible to form any definite opinion as to 

where they belong.

It is evident that while essexite, shonkinite, theralite, and alkalic 

pyroxenites all possess clear-cut and definite characters, there are 

many intermediate types whose classification in the older systems will, 

be entirely a matter of opinion. So, for example, Hill and Kynaston 

have described, under the name of kentalleuite, a rock consisting of 

essential olivine and augite with orthoclase and plagioclase in varying proportions. Its analysis is shown under vn of the table. The 

close chemical similarity to shonkinite is easily seen. In the new 

system of classification it has, however, a distinct place of its own.

Mineral composition or mode. By study of the thin sections, with 

comparison of the chemical analysis and the powders yielded by the

"A oalculatiou of the norm from analysis V gives the following results: 

Or....... 38.921 Sal. 57.55 An...... 5.2857.55 C^, ==1.4=m, salfemane.

No ...... 13.35J Order, ^=;rr|;\=.30=lendofelic=6, portugare.

Ol....... 12.36

Mt ...... 3.9440.12 . ___ _ K2Q' - 70

n........ 2.13

Ap ...... 2.69.

HoO..... 1.66

117 Bang, "caO' =-jg- =domaLkalic=2, moiichiquase.

Subrang, j^o7=j5=1.49=sodipotassic=3, shonkmose.

Total.. 99.33

& Triad. Erup. Predazzo, 1896, p. 67. 

cTschermak, Min. Mitt., vol. 21,1902, pp. 100 and 103. 

dSitzb. k. Preuss. Akad. Wiss. Berlin., Phys., Mat.,- KL, vols. 30,32,1902, pp. 675,731.
104 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

heavy liquids, it may be calculated that an average specimen of the 

Square Butte shonkinose would have this mineral composition:

Mineral composition or mode of shonkinose of Square Butte.

Alkalic feldspar ................................................ 20

Nephelite...-----..-....-.............._........................ 5

Socialite ................................................... .... 1

Augite .-.---....:.........................:................... 46

Olivine......................................................... 10

Biotite ..-........_.._....___.._.._.._____...._.._.__.____._._. 8

Iron ore ..----...._............................................ 6

Apatite........................................................ 4

Total...................:................................ 100

Classification in the new system. The calculation of the norm of 

the Square Butte rock and its position in the new system are shown 

in the following table:

Calculation of the norm of shonkinose.

Si02 . .......

ALA ------

Fe2O3 ......

FeO.........

MgO. ......

CaO -..--..

Na2O_ ......

K2 (X. ..---.

TiO2 .......

PA ------

C12 . ........

Rest ... ...

Total ..

Analysis.

46.73

10. 05'

3.53

8.20

9.27

13.22

1.81

3.76

.78

 1.51

.18

1.52

100.56

Molecular ratio.

0.779

.098

.022

.114

.232

.236

.029

.040

.010

.011

.003

........

Or.

240

' 40

40

40

Ab.

18

3

3

3

Ne.

4.4

9,9,

9,9,

22

So.

6

3

4

1

1

An.

60

30

30

30

Di.

338

,44

125

169

169

Ol.

72

38

107

72

Mt.

22

22

22

11.

10

10

10

Ap.

37

11

9,

11

Or. --....

Ab ......

An ......

Ne ---...

So

Di. ......

Ol

Mt ......

11........

Ap ......

Rest.....

22.24

1.57

8.34

6.25

07

37.91

11.36

5.10

1.52

3.70

1.52

39.37

59. 59

'~^"*'' salfemane near dofemane. 1 Fern.~59.

L 722 Order, jr =gg^g=.22=lendofelic=6, portugare.

Rang, OaO' ~ =gQ=2.3=domalkalic=2, monchiquase:

Siibrang, j^Q>=gg=1.4=sodipotassic=3, shonkinose.

P+O 49 27 Grad, -jg- = g-^g-=7.4=perpolic=l, shonkinate.

c , , (MgFe)O+CaO^ 483 ... . , bubgrad, (K Na)oO" =~o~=permirlic=l, shonkmote.

Total.. 100.48
PETROGRAPHY OF STOCKS AND LACCOLITHS. 105

In comparing the norm with the mode it is seen that in the norm 

there is about 8 per cent of anorthite, which in the mode has gone 

almost entirely into the augite, and, conversely, "about 8 per cent of 

biotite in the rock resolves itself in the norm into olivine and feldspathic minerals.

This biotite is thus a varietal mineral and produces a model variety 

of the normative rock. The texture is granular, and the type may 

th us be termed biotitic grano-shonkinose.

SHONKINOSE OF OTHER HIGHWOOD LOCALITIES.

Besides the occurrence at Square Butte, this rock is found in Palisade Butte, in the smaller laccolith in the Shonkin Sag, in the great 

stock at the head of Shonkin Creek, and in several smaller masses 

and intruded sheets, as described under the discussion of the geology. 

There is some variation among the rocks, both in texture and in the 

relative proportion of minerals. The Shonkin stock varieties are 

apt to be coarser grained and more xenomorphic than the Square 

Butte type and to have much less biotite; but aside from this the 

types do not need any special mention and are covered by the foregoing description. One exception to this, however, is the rock closely 

related to shonkinose in which leucite is an essential component. 

This is described in the following section:

LEUCITE-SHONKINOSE (LEUCITE-SHOXKINITE) OF EAST PEAK.

Introductory. The study of the material collected by Mr. Weed at 

the Shonkin stock shows some variability in the proportions of the 

salic and femic minerals. These variations are not, however, very 

great and never extreme. In the hand specimens the salic and femic 

components appear to be similar throughout the series, but under the 

microscope the salic are found to be sometimes chiefly alkalic feldspars, making the rock shonkinite (shonkinose), and sometimes 

leucite, forming missourite (grad niissourate). There are, moreover, 

transitional types between these two, the ferromagnesian minerals 

remaining the same, while the light or feldspathic ones are mixtures in 

various proportions of alkalic feldspars and leucite. Small amounts 

of nephelite, sodalite, and zeolites may also be present. In addition 

to these occurrences at the Shonkin stock, it has been found that the 

rock composing the mass of the East Peak stock at the head of Davis 

Creek is of this type and it was studied to ascertain its relations to 

the normal shonkinose.

Megascopic characters. The occurrence and geologic relations of 

this rock mass have been described in a previous chapter. In the 

specimen the rock is a dark gray of medium to fine grain, and the 

automorphic character of the augites tends to give it a somewhat porphyritic appearance. In this it differs somewhat from the coarser
106 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

and massive-looking rocks of Square Butte, Shonkiri Creek, etc. In 

addition to the augite, occasional grains of olivine are seen. The 

material forming the great cliff and the huge talus masses at its foot 

is not so fresh as the outcrops on the little ridge or foothill to the 

south at the border of the intrusion. The material analyzed came 

from this southern edge, where the rock is somewhat coarser .grained.

Microscopic, characters. In thin section the rock is seen to be made 

up of the usual minerals of shonkinose: Augite, iron ore, apatite, 

olivine, biotite, with alkalic feldspars and isotropic minerals. The 

femic minerals are of the character already described under shonkinose. The iron ore, which is not very abundant, is in small, round, 

dotted grains. The augite, in large, well-formed crystals, is like that 

described under shonkinose. There is some olivine, which' is fresh, 

and, unlike that in shonkinose, it is almost never surrounded by coronse 

of biotite. The absence of biotite in the rock is, in fact, a noticeable 

feature and is to be correlated with the occurrence of leucite. This 

mutual relation between olivine, leucite (and its feldspar equivalent, 

orthoclase), and biotite is well known, and its significance in these 

rocks has already been shown by the writer. a

The alkalic feldspars are present in small laths and larger shapeless masses which often show Carlsbad twinning. In the material 

from the cliff and talus (Nos. 757 and 758) a curious alteration of 

these feldspars was noticed. When fresh they show no zonal structure, but in these cases a process of zeolitization has gone on, so that 

sections parallel. to 010 show a marked zonal structure, alternate 

bands polarizing in yellow tones, Avhile those between are the gray 

color usual to these feldspars in the average thin section. It appears 

that some zones are much richer in soda than others, and these have 

become converted into natrolite, which has a considerably higher birefringence than the soda orthoclase which composes the intervening 

layers, thus producing the effect mentioned. The alkalic feldspars 

are also somewhat kaolinized in some of these examples.

The leucite appears abundantly in certain areas. Between crossed 

nicols these areas appear homogeneous, isotropic for the most part, 

but here and there show the faint cross-banded twinning so characteristic of larger leucites. In plain light the areas are compound and 

made up of grains, as is shown by their outlines, cracks, and the zonal 

arrangement of fine black, dotted inclusions, a well-known feature in 

leucite. It is to be noted, however, that not all of the areas show 

these inclusions or cross twinning.

There are also some areas which in plain light show all the charac-. 

ter of the leucite, but between crossed nicols break up into a fine 

mosaic of grains of low polarization. These are undoubtedly altered 

leucites, or pseudoleucites, consisting now of feldspar, nephelite, and 

zeolite granules. In the material from the cliff (757) there are no real

"Bull. Geol. Soc. America, vol. 6,1895, p. 409.
PIIISSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 107

leucites, but the mode of occurrence, arrangement of inclusions, etc., 

show that they were once present as in the rock of the south' boundary 

(760).

Chemical composition of white components. In order to investigate 

these white components a separation was made with heavy fluids and 

various crops obtained all the way from 2.60 to 2.25, a result which 

might naturally be expected in mixtures of alkalic feldspars, leucite, 

and zeolites. It was intended to analyze the material having a specific gravity of 2.45, the specific gravity of leucite, but unfortunately 

this material was lost, and the next below, at 2.38, was taken. Under 

the microscope this was found to be very pure, consisting of isotropic 

grains. About 0.2 gram was available for the analysis, which had, 

therefore, to be made with great care. The results were as follows:

Analysis of white component of leucite-shonkinose.

Si08 ........ ..................................................

A12 03 .._... ............................. . ....................

FeA -- -.-- .- ...- .-,.- - 

CaO._ ........................ ..^..... .........................

Na,0. ---..-. ------ -------- .._.-....-....-..... -..._.._

K,0 _'..... ...............................................

H2 0 ..........................................................

SOS ..........................................................

a............................ ......... .......................

MnO. ............................. ...... ...................

Total...................................................

I.

56.00

21.27

Trace.

.33

5.16

10.85

6.89

None.

None.

Trace.

100. 50

II.

0.933

.209

.005

'.084

.116

.383

By using the molecular ratios given in the last column it can be 

easily calculated that this material consists of a mixture of equal 

parts, by weight, of leucite and analcite (lc., 50.58; anc., 49.42). If 

the specific gravity of leucite be taken as 2.45 and that of analcite as 

2.30, a mixture of equal parts of the two would have a specific gravity 

of 2.38, that of the powder analyzed.

Occurrence of analcite. The presence of the analcite thus^proved 

is of interest. - It could not be detected in the ordinary way with the 

microscope on account of the leucite, both, of course, appearing as 

isotropic grains.

The question at once arises whether this analcite is primary or not. 

A study of the sections affords no direct evidence on this point, and 

the question is similar to the one raised in regard to the monchiquose 

(analcite-basalt), discussed elsewhere in this bulletin.

It should be noted, however, that while some of the material from 

this locality is considerably altered and zeolitized the specimen (760)
108 IGNEOUS ROCKS OP HIGHWOOD MOUNTAINS. [BOLL. 237.

from the southern edge under examination shows nothing of this, 

and except for the presence of analcite and some pseudoleucites, or 

what are thought to be such, the rock is fresh and unaltered. The 

femic minerals are unchanged, and in the case of the olivine .this is 

to be noted. So far as these facts afford evidence, the analcite does 

not appear to be due to secondary alteration. It is not logical to say 

that the analcite is secondary and, therefore, the rock must be 

altered, and that because the rock is altered the analcite must be 

therefore a product of alteration. The proofs of its secondary nature 

which are often advanced consist of just this reasoning in a circle. 

All that can be said in this instance is that the analcite appears like 

a primary mineral, and that the arguments for and against its primary 

nature are the same as those discussed under the head of the dikes of 

monchiquose (analcite-basalt). Some additional indirect evidence will 

be presented in a subsequent paragraph devoted to the calculation 

and discussion of the norm.

Chemical composition of the rock. An analysis of this rock (760) 

has been made under the writer's direction by Mr. E. B. Huiiburt, with 

the results shown in I of the table on the opposite page.

For purposes of comparison two analyses of typical shonkinose, the 

original one from Square Butte and the one from Yogo Peak, are 

added, and it will be seen that they are all of the same general character, but that the East Peak type has higher alkalies and somewhat 

lower bivalent oxides. This low silica with considerable alkalies and 

dominant potash has caused the formation of leucite along with the 

alkalic feldspars. Under vm is given the analysis of "missourite,' 

which has no feldspar, but whose potash is all in leucite. The silica 

in this is the same as in the "shonkinite" of Square Butte, but the 

latter has lower alkalies and therefore all feldspar and no leucite. 

Thus, from "shonkinite" through leucite-"shonkinite" to "missourite " there is a regularly graded series from potash feldspar through 

pobash feldspar and leucite to leucite alone, depending on the relative proportions of silica and potash. When it is recalled that orthoclase=K2O A12 03 6Si02 and leucite=K2 0 A12 03 4Si02 , this relation is 

easily understood. The soda in part follows the potash into the feldspar and in part, having a lesser affinity for silica, it forms lenad (feldspathoid) minerals throughout the series, which, if conditions of formation were favorable, might be hydrated, i. e., analcite.

The most nearly related type found by the writer outside of the 

High wood area is a shonkinose ("leucitophyre") from Persia, whose 

analysis, by Steinecke, is shown under iv. The chemical correspondence of these two rocks from such widely separated regions is remarkable; the only difference is that the Persian type contains a little 

less iron and a little more lime.

The correspondence between leucite-shorikinose and the nearly 

related montanose ("shonkinite") from the Shonkin Sag laccolith is
pinssoN.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 109 

Analyses of leucite-shonkinose and related rocks.

Si02 --------

AI n

FeA---------

FeO .

MgO.......l.

CaO. .........

Na2O. ........

K.,0. ------

H20+ ....

H2 0- ........

pn

TiO

p n

an

01. ...........

PT C\

n.... .....I..

MnO ------

BaO. .........

SrO ..........

O=C1 .........

Total...

I

49.59

14.51

3.51

5.53

6.17

9.04

3.52

5.60

1.95

.36

.15

.02

.13

Tr.

.49

.21

100.78 

.03

100.75

II.

46.73

10.05

3.53

8.20

9.25

13.22

1.81

3.76

1 24

.78

1.51

.18

.28

(?)

(?)

100.56 

.04

100.52

III.

48.98

12.29

2.88

5.77

9.19

9.65

2.22

4.96

.56

.26

1.44

.98

Tr.

.22

.08

.43

.08

99.99 

.10

99.89

IV.

49.65

14.39

4.21

3.46

6.27

10.12

3.21

5.46

2.37

(?)

.79

.25

(?)

(?)

100.19

100.19

V. '

47.88

12.10

3.53

4.80

8.64

9.35

2.94

5.61

1.52

.70

.12

.77

l.ll

None.

Tr.

.04

.05

.15

.46

.13

99.99 

.02

99.97

VI.

46.04

12.23

3.86

4.60

10.38

8.97

2.42

5.77

2.87

.64

1.14

Tr.

.11

Tr.

.48

.25

99.76 

.03

99.73

VII.

47.98

13.34

4.09

4.24

7.01

9.32

3.51

5.00

2.10

1.24

.58

1.03

Tr.

.21

Tr.

.50

.14

100. 29 

.07

100.22

VIII.

46.06

10.01

3.17

5.61

14.74

10.55

1.81

5.14

1.44

.73

.21

.05

.03..

Tr.

.32.

.20

99.57 

.01

99. 56

IX.

0 897

.142

-op-a

.076

154

.160

056

060

'.004

001

003

-----

I. Leucite-shonkinose (leucite-shonkinite) from East Peak, Highwood Mountains, Montana. E. B. Hurlburt, analyst.

II. Shonkinose (shonkinite) from Square Butte, Highwood Mountains, Montana. . L.V. Pirsson, analyst. Bull. Geol. Soc. America, vol. 6,1895, p. 414.

III. Shonkinose (shonkinite) from Yogo Peak, Little Belt Mountains, Montana. 

W. F. Hillebrand, analyst. Am. Jour. Sci., 3d series, vol. 50, 1895, 

p. 474.

IV. Shonkinose (leucitophyre) from near Khoi, Persia. J. Steinecke, analyst. 

Zeit. Naturw. Halle, vol. 6, 1887, p. 12.

V. Montanose (shonkinite) from Shonkin Sag laccolith, Highwood Mountains, 

Montana. W. F. Hillebrand, analyst. Am. Jour. Sci., 4th series, vol. 

12, 1901, p. 14.

VI. Biotite-cascadose (mica-basalt) from Arrow Peak dike, Highwood Mountains, Montana. H. W. Foote, analyst.

VII. Shonkinose (leucite-basalt) from Pinewood Peak flow, Highwood Mountains, Montana. H. W. Foote, analyst.

VIII. Albanose (missourite) from head of Shonkin Creek, Highwood Mountains, 

Montana. E. B. Hurlburt, analyst. Am. Jour. Sci., 4th series, vol, 2, 

1896, p. 321.

I.. Molecular proportions of No. I.
110 IGNEOUS KOCKS 0¥ HIGHWOOD MOUNTAINS. [BULL. 237.

shown by a comparison of the analyses I and v, respectively, which^ 

show how delicate is the balance which determines the formation of 

orthoclase and biotite instead of orthoclase with leucite.and olivine.

Under vi and vn are analyses of dikes and flows of the region, and 

comparison may be made not only with the leueite-shonkinose but 

also of the normal shonkinose with similar magmas of the area which 

have a different method of occurrence.

Mineral composition or mode. On account of the mingled analcite 

and leucite the mineral composition can not be accurately computed, 

but the relation of the salic to the femic minerals is expressed very 

closely in the norm, which represents also in a general way the femic 

minerals. If the leucite pseudomorph areas are reckoned as leucite, 

observation shows that the amount of leucite to feldspar is about 7 

to 3; but a considerable proportion of what is called leucite perhaps 

a third or more is really analcite.

Classification in the new system. The position of the rock in the 

new system is shown in the following computation of its analysis:

Calculation of the norm of leucite-shonkinose.

Si02 .......

Al O

Fe2O3 ------

PeO.. ......

MeO

CaO ....-.-

Na,O .......

K2 o. :......

Ti02 .......

P n

C12 .-. .---.-

Rest .......

Total.

Analysis.

49.59

14. 51

3.51

5.53

6.17

9.0.4

3.52

5.60

.36

.15

.13

2.67

100. 78

Molecular 

ratio.

0.826

.142

.022

.076

154

.161

.056

060

.005

.001

'.002

Or.

360

60

60

60

Lc. Ab.

6

1

1

1

Ne.

94

47

47

47

So.

19

6

8

9,

9,

An.

56

9,8

98

28

Di.

260

31

99

130

130

01.

36

18

55

36

Mt.

9,9,

99, '

9,9,

11.

5

5

5

Ap.

3

1

1

Or.......

Ab ...... 

An......

Ne ...... 

So.......

Di....... 

01.......

Mt...... 

11........

Ap ...... 

Rest.....

33.36

.52 

7.78 

13.35 

1.93

29.07 

5.68

5.10 

.76

.34 

2.67

56.94

40.95

Sal 56 94 

Class, p^ =£0^=1.2=111, salfemane.

Order, F =£jjgg=0.36=lendofelic=6, portugare.

Na-jO'+KjiO' 116 , ' Rang, CaO7 =~gg'=4.1=domalkalic=2, monchiquase.

KoO' 60 Subrang, j^Q7=F5=1.0=sodipotassic=3, shonkinose.

Total.. 100.56
PIRSSON.] PETKOGRAPHY OF STOCKS AND LACCOLITHS. Ill

The calculation shows the rock to be a normal shonkinose with 

orthoclase and nephelite and no leucite. The reason of this is apparent when one considers that nephelite has been calculated instead of 

analcite. The formula of nephelite is Na2 0 A12 03 2SiO2 and that of 

anal cite is Na2 0 A12 03 4S:LO2 2 H20. The formation of theoretical nephelite consequently releases enough silica to convert what would otherwise be leucite into orthoclase. This is a strong argument in favor of 

the primary nature of the analcite. If the rock had crystallized anhydrously, we should have expected the formation of the minerals 

shown in the norm. Considering our knowledge of the crystallization 

of molten, magmas and the affinities of the oxides, they could not 

have been different from these. If the analcite is secondary, the 

aqueous solutions have taken silica from orthoclase, reducing it to 

leucite and converting nephelite into analcite. In this case the leucite is also secondary. If we reject this, we must fall back on the 

view that aqueous solutions carrying soda and silica have acted on 

the rock and presumably on leucite, have removed from a part of it 

a vast amount of potash and replaced it with soda, and have not 

attacked the other minerals, especially the olivine. The action thus 

becomes an entirely selective one. It would seem simpler to suppose 

that the water vapor originally present in the magma caused the formation of analcite, and thus indirectly also of leucite.

From what has been shown it is evident that this rock has an 

abnormative mode, and since it has a granular texture it should be 

termed a grano-leucite-shonkinose.

Classification in prevailing systems. As the white components of 

this rock consist chiefly of leucite and alkalic feldspar it was at first 

provisionally termed leucite-syenite, under which name the analysis 

was first published in Bulletin 148 of the United States Geological 

Survey. Considering, however, the relative amounts of light and 

dark minerals, its chemical composition, and its regional affinities, 

the name leucite-shonkinite would be much more appropriate:

MONTANOSE (SHONKINITE) OF SHONKIN SAG LACCOLITH.

Introductory. In the rocks of the stocks and laccoliths the order 

portugare of the salfemanes in the new system, of classification is represented in both the peralkalic and the domalkalic rangs, wyomingase 

and monchiquase. In subrangs it is the sodipotassic in both, and 

under monchiquase it is shonkinose. The rocks have been fully 

described and their positions shown in the foregoing section.

Under wyomingase, where the rocks are peralkalic, subrang 3 has 

received no name, and it is here proposed to describe a Highwood type 

which occurs within it and call this subrang montanose. This is the 

material forming the dark outer zone of the Shonkin Sag laccolith,
112 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

Under prevailing systems of classification, in which regard is paid 

only to the presence of certain minerals and none to their relative 

quantities, the difference between the relative amounts of the alkalic 

minerals in this type and the shonkinose last described would not be 

recognized, and both rocks would be classed under shonkinite.

Megascopic and microscopic characters. In the hand specimen the 

rock is much like the former type, a dark-gray, medium- to coarsegrained mixture of salic and feinic minerals. There is less automorphism in the femic minerals than in the Square Butte rock.

In thin sections there appear the same minerals as in shonkinose 

iron ore, apatite, olivine, biotite, augite, alkalic feldspars, nephelite, 

sodalite, and some zeolites. The apatite and iron ore are similar; the 

fresh olivine is in the same way surrounded by biotite mantles, brown 

within and green without, where contact would be made with the 

orthoclase. The biotite is well crystallized and tends to form in long 

foils and have darker borders, as in minette; it recalls the biotite of 

theralite, but has parallel extinction. The augite in places has fine 

borders of a deeper green, from admixture of the segirite molecule. 

There is present some soda microcline with very fine albite twinning 

which is partly zeolitized. The orthoclase is like that in shonkinose. 

Considerable nephelite is present, shown by its uniaxial negative 

cross and low birefringence; it is fresh, but often incloses hexagons of 

zeolitized sodalite. The texture, fabric, etc., are as in shonkinose.

Chemical composition. The chemical composition of this rock is 

shown in a complete analysis by Dr. W. F. Hillebrand; it has been 

given under shonkinose, but is repeated on the opposite page for the 

sake of convenience.

In Washington's a tables of analyses are found two other analyses 

which come under this subrang. One is of shonkinite of the Beaver 

Creek stock in the Bearpaw Mountains; the other differs in the 

presence of much segirite as its dominant femic mineral, and one 

would naturally classify it under an entirely different grad and subgrad. It is interesting to note in connection with this analysis how 

ferrous iron plays the function usually filled by magnesia, while soda, 

in forming a femic mineral, does the same for lime.

The analysis of the Shonkin Sag montauose shows all the distinctive 

characters of the salfemic rocks of the Highwoods, and it is unnecessary to do more than point this out.

a Prof. Paper U. S. Geol. Survey No. 14,1903, p. 338.
PIKSSON.] PETKOGKAPHY OF STOCKS AND LACCOLITHS.

Analyses of montanose and related rocks.

113

SiO2 . ...................................

AlgO,- ~ ............................

Fe^O,,..... ..........................

FeO... ...........'.....................

M.gO ..................................

CaO. ..................................

Na2 0'- ............................... ..

Ka o... ........... ':....................

U2 0+. ................................

H20-. ................................

C02 . ..................................

TiO2 ........... ........................

P n

S03 .... .................................

01... .--------------_.._.-..---.,--.-..

CrA------ ----------------------------

MO...................................

MnO ...................................

BaO --------.-----._._......._.........

SrO ........-. .......................

Total.......... ..................

I.

47.88

12.10

3.53

4.80

 8.64

9.35

2.94

5.61

1.52

.70

12

.77

1.11

Trace.

.04

. .15

.46

.13

99.99

II.

50.00

9.87

3.46

5.01

11.92

8.31

2.41

5.02

1.16

.17

.31

.73

.81

.02

.08

.11

.07

Trace.

.32

.07

1 100.01

III.

48.90

 7.85

11.46

13.32

.38

1.95

7.40

3.23

1.80

.03

1.11

99.39

IV.

0.796

.119

.022

.067

. 2:1 6

.167

.047

.060

.010

.008

I. Montanose (shonkinite) from Shonkin Sag laccolith, Highwood Mountains. 

W. F. Hillebrand, analyst (includes S.=.03, Zr02 =.03, Fl.=.05, and 

V2 08 =-04).

II. Montanose (shonkinite) from Beaver Creek, Bearpaw Mountains, Montana. 

H. N. Stokes, analyst (includes Fl.=.16). Am. Jour. Sci., 4th series, vol. 

1, 1896, p. 360.

III. Montanose from Kangerdluarsuk, Greenland. C. Detlefson, analyst (includes 

ZrO2 =1.96). Rosenbusch, Elemente, p. 133,1898.

IV. Molecular proportions of No. I. . .

Bull. 237 04 8
114 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Classification in the new system. The position of the rock in the 

new system is shown in the following calculation of its norm and systematic position:

Calculation of the norm of montanose.

SiCX, ...............

A12O3 ..............

Fe203 ..............

FeO... .. .-.--.--...

MgO. ........ ...I..

CaO ...............

NaA- .............

KA --------------

TiO2 ...............

P2 05 ...............

Total ........

Analysis.

47.88

12.10

3.53

4.80

8.64

9.35

2.94

5.61

.77

1.11

3.26

99.99

Molecular ratio.

0.796

.119

.022

.067

.216

.167

.047

.060

.010

.008

Or.

360

60

60

Ne.

94

47

47

An.

94

19,

18

Di.

258

18

111

129

Ol.

61

17

105

Mt.

9,9,

9,9,

11.

10

10

Ap.

?,fi

8

Or.......

An...... 

Ne ...... 

Di. ......

01

Mt .......

n. .......

Ap...... 

Best.....

33.36 

3.34 

13.35. 

28.44 

9.08 

5.1.0 

1.52 

2.69 

3.26

50. 05 Class' Fem.-46.83^L0^m< «alfemaue. 

Order, jr=gg-yy=-0.36=lendofelic=6, portugare. 

NaoO'+KoO' 107

46.83 K.,0' 60

 P.-t-O." 37.52

Total.. 100.14 (NaK)20"

In chemical composition this rock stands very near the center of the 

subrang, and it is therefore very nearly a typical analysis. That this 

is so may be seen if we assume exact center points and then from this 

reverse the ordinary process of calculation, obtaining the theoretical 

chemical composition and the corresponding norm for the assumed 

rock.
PIBSSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS.

\ Calculation of chemical composition.

115

I.

SiO»._ ...... ..-...-.-...

AI n

FeA ------------------

FeO.. ..................

MgO. ..................

CaO -..--.............-.

Na2 0.------------------

K r\

Total ............

11.

50.8

11.6

6.5

3.0

11.4

7.9

3.5

5.4

100.1

III.

0.846

.114

.041

.041

.285

.141

.057

.057

Or.

342

57

57

57

Ab.

54

9

9

9

No.

. 9(5

48

48

48

Di.

282.

141

141

141

01.

72

144

72

Mt.

41

41

41

Norm. 

Or....... 31.7

Ab ...... 4.7

Ne ...... 13.5

Di....... 30.0

01....... 10.0

Mt ...... 9.4

Total.. 99.9

Center points. 

Class, pem =J5fl=l- 0=111, salf emano.

Order, |jr=0.37=G, londofelic.

NagO'+KoO' 

Bang, fjaTp =00=1, peralkahc.

Subrang, -" =1.0=3, sodipotassic.

The calculated chemical composition given in column n of the above 

table is seen to be, in general, very close to the actual analysis previously given.

The mode or actual mineral composition of the rock differs from 

this theoretical norm in that some of the molecules which would 

otherwise have gone to making olivine and feldspathoid minerals 

have actually united to make about 10 per cent of biotite. On account 

of this biotite and because the texture is granular the rock should be

termed "biotitic grano-montanose."

^ MISSOUROTE (MISSOURITE) OF THE SHONKIN STOCK.

Introductory. This rock has been described in a previous paper 

by the author and Mr. Weed, who collected the material and gave an 

account of its mode of occurrence in the Shonkin stock. As the type 

filled a gap in the prevailing systems of classification and was the 

first recorded instance, so far as known to the writer, of an intrusive 

rock containing unaltered leucite, it has become widely known, and 

the name was adopted in petrographic nomenclature. In the new 

classification it occupies an interesting position, as will be shown, 

and in order to facilitate the discussion the essence of the former 

description and the chemical analyses are here repeated. The locality 

and mode of occurrence have been given in the description of the 

Shonkin stock.

Megascopic characters. The rock appears dark gray, coarse
11(3 IGNEOUS BOCK$ OF HIGHWOOD MOUNTAINS. [BULL. i>37.

'grained, and resembles many basic massive rocks in appearance. 

In the specimen it is seen to be coarsely and evenly granular and to 

be composed of light and dark constituents, the proportion by bulk 

being about two of the light to three of the dark minerals. The separation by the heavy fluids shows, however, that by weight the whito 

mineral forms only one-fifth to one-quarter of the whole. The distinction in color is strongly marked and gives the rock a mottled, 

mosaic-like appearance.

Upon examination the dark constituents may be distinguished as 

chiefly a greenish-black augite in columnar masses and aggregates 

which are never automorphic, and an occasional speck of a bronzy 

brown biotite of ill-defined outline or a grain of a deep-yellow olivine. 

Filling the interspaces between these dark minerals in formless masses 

is a very pale greenish-gray substance which is leucite. The average 

size of crystal grain varies from 2 to 5 mm, so that the rock is of coarse 

granular structure, and resembles most strikingly many coarse-grained 

gabbros.

Microscopic characters. The thin section under the microscope 

shows the minerals present to be apatite, iron ore, olivine, augite, biotite, leucite, and some zeolitic products.

The apatite and iron ore,' which are present rather rarely in moderate-sized grains, show nothing of especial interest beyond that they 

are found inclosed in the other minerals, and the biotite frequently 

incloses the iron ore.

The olivine is extremely fresh, unaltered in any way, and resembles 

the olivine of fresh gabbros. It contains great numbers of very fine 

glass and iron-ore inclusions. It never shows any crystal faces, but is 

in rounded, formless, auhedral grains which are frequently inclosed 

in biotite and augite.

The augite is of a pale-green color with a tone of brown; it is very 

fresh and clear, contains inclusions of ore and specks of biotite, and is 

entirely xeuomorphic, though the orientation of the ore grains is at 

times zonal, thus indicating crystal planes. It has an excellent cleavage, and twinning bands pass through it in places; it does not show 

any pleochroism.

The biotite is strongly pleochroic between a deep umber-brown and 

a pale yellow-brown; it is also entirely xenomorphic, though apt to 

surround the other minerals in bands, especially the olivine and 

iron ore. It is particularly characteristic in such cases that it passes 

from brown into an olive-green variety which has a mottled, somewhat stringy, fibrous appearance. In these cases it appears as if the 

brown variety had suffered from some magmatic process; it does not 

seem to be due to any ordinary process of weathering.

The leucite also appears in formless masses filling the interspaces 

between other minerals. It is perfectly clear and free from all inclusions, except now and then a grain of the ferromagnesian minerals. 

Between crossed nicols it shows most beautifully the cross-banded
.PIRSSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 117

twinning structure so characteristic of leucite. It is in general perfectly clear, limpid, and fresh, though in some areas, in delicate fringes 

along cracks and on the "borders of grains, a low birefraction shows 

that processes of zeolitization have commenced/'

Chemical composition. An analysis of the rock has been made by 

Mr. E. B. Huiiburt, with the following results:

Analyses of missourbte and related rocks.

Si02 .......................

ALO, -----...-.......--....

Fe2O3 ......................

FeO .......................

MsO

CaO .......................

Na2 0 ......................

K2 0.. .....................

H2 0_... ....................

Ti02 .......................

P O

MnO_. .....................

BaO .......................

SrO... .....................

SO,:.......................

Cl .........................

C1=O.. ....................

Total .......... ....

I.

46. 06

10. 01.

3.17

5.61

14.74

10.55

1.31

5.14

1.44

.73

.21

Trace.

.32

.20

.05

.03

99.57

.01

- 99.56

II.

. 47.28

11.5.6

3.52

5.71

13.17

9.20

2.73

2.17

2. 96

.88

.59

.13

(?)

(?)

."18

100. 08

.04

100. 04

in.

46. 73

10. 05

3.53

8.20

9.27

13.22

1.81

3.76

1.24

.78

1.51

.28

(?) '

(?)

None.

.18

100. 56

.04

100.52

IV.

44. 35

10.20

| 13. 50

12.31

11.47

3. 37

4 42

(?)

(?)

(?)

(?)

(?)

99. 62

la.

* 0. 767

.097

f .020

\ .078

.368

 1 DO

.021

. 054

.080

.009

I. Missourote (missourite). from head of Shonkin Creek, Highwood Mountains, 

Montana. E. B. Hnrlburt, analyst.

II. Canaptonose (leucite-absarokite). J. E. Whitfield, analyst. Hague, Am. 

Jour: Sci., 3d series, vol. 38,1889, p. 43. Iddings, Jour. Geol., vol. 3, 1895, 

p. 938.

III. Shonkinose (shonkinite) from Square Butte, Highwood Mountains. L. V. 

Pirsson, analyst. Bull. Geol. Soc. America, vol. 6, 1895, p. 414. MgO 

corrected.

IV. Leucite-basalt, from Bongsberg,by Pelrn, Eifel. . E. Hussak, analyst. Sitzb.

K. Akad Wiss. Wien, vol. 77, pt. 1, 1878. 

. la. Molecular ratios of No. I.

An analysis of it on material separated by heavy liquids gave 

A12O3 ..... ..................................................................

FeoO3 .. ........ .............................................................

CaO........................................................................

NasjO... .....................................................................

K20. ......................................................................

H»0. ............. ..........................................................

........... 54.40

........... .68

........... .10

........... .70

........... 18.80

........... 2.29

Total..................................................._..__.................. 99.33
118 IGNEOUS ROCKS OF HIGHWOOD MOUNTAINS. [DULL. 237.

This analysis brings out strongly the leading characteristics of the 

rock its very high lime, iron, and magnesia, which have compelled 

the formation of such quantities of pyroxene and olivine, and the predominance of potash over soda, which, with the low silica, has conditioned the formation of the leucite and prevented the forming of 

feldspar.

Mineral composition or mode. Taking into account the ratios 

shown by the analysis, the separations by the heavy liquid, and the 

study of the section, the rock has approximately the following mineral composition:

Mineral composition or mode of missourote.

Iron ore ................^....................................... 5

Augite .............................................^.......... 50

Olivine ---.-----.-.--......................-..-.. .-.......-.. 15

Biotite ......................................'... ......:..... 6

Leucite-...._-..._.............._.............................. 16

Analcite- ..................................................... 4

Zeolites --_---_------_----__-_-_---__.__...____-_.____:....._-.. 4

100

This composition as originally calculated must be approximately 

correct, as Prof. F. W. Clarke informs the writer that on leaching the 

rock with ammonium chloride the following percentages were obtained 

in the extract:

CaO. ........ ...... ...........................................

K2O._ --.......-......-.....--.....:....--...--------.-.-......

TSTfl O .

A.

1.73

4.09

.59

B.

1.70

3.74

.64

This gives, on calculation into mineral molecules, lencite (K Al 

Si2Oc )> 18.24; analcite (Na AL Si2 06H2 0), 4.70.

As the rock itself varies somewhat this may be considered a very 

satisfactory agreement.

Classification in prevailing systems. In the prevailing systems of 

classification this rock has a distinct place. It is the massive granular, plutonic representative of the leucite-basalts and bears the same 

relation to them that gabbro bears to common plagioclase-basalt or 

granite to rhyolite. For this reason it was given a distinct name of 

its own, missourite. The writer can not agree-with the'suggestion 

by L6winson-Lessing a that leucite-gabbro would have been appropriate, for in the usage of petrographers this would have meant a gabbro that is, a plagioclase rock with additional leucite, and this

aLowinson-Lessing, Studien ueber die Eruptivgesteine: Compte-rendu vii session Gong. Geol. 

Inter. Buss., 1897, p. 282.
PIBSSON.] PETROGRAPHY OF STOCKS AND LACCOLITHS. 119

would have been incorrect. This idea was thought of at the time and 

this name in consequence rejected.

Classification in the new system. In the new system the rock occupies an interesting position, and one which, moreover, points a moral 

in regard to all systems of rock classification based on the inherent 

nature of the things classified. Its norm is determined from the following calculation:

Calculation of the norm of missourote.

Si02 ...........

A1A ---------

Fe2 03 ---- 

FeO ...........

MgO. ..........

CaO ...........

Na-jO ............

K20... .........

TiO, ...........

PA--- -----

Rest ...........

Total ....

Analysis.

46.06

10.01

3.17

5.61

14.74

10.55

1.31

5.14

.73

21

2.04

99.57

Molecular ratio.

0.767

. .098

.,020

.078

.368

.188

.021

.054

.009

.001

Or.

3fi

R

fi

6

Lc.

192

48

48

48

Ne.

49,

21

21

21

An.

46

28

38

23

Di.

324

19

143

162

162

01.

127

30

225

127

Mt.

20

20

20

11.

9

9

9

Ap.

3

1

I

Or.......

An...... 

Lc.......

Ne ...... 

Di. ......

Ol. ......

Mt ......

n.... ....

Ap ...... 

Rest ....

3.34 

6.39 

20.93 

5.94 

35.60 

18.81 

4.64 

1.37 

.34 

2.04

nioao Sal. 36.60 ^1 TTT c.0 -\farnana ClaSS'Fem. 60.76 m, saKemane. 

36.60 Order, ^ -^ ^.-2.1-dolenic-8, bohemare.

F. 9.73 _ 

Rang, ?ra^/ tK2°'=^_3 2 -:domalkalic-2, albanase.

CaO <5o 

TT O' 'vt

Na^O 21 

60- 7(i Grad, Pi°-54- ^-9-perpollc-l, missourato.

M 6.01 

. Subgrad, ( M^F40)+CaO''_608_permirlic_limissom.ote

(Jx JNft^2^~' v
120 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

The rock is almost exactly on the dividing line between salfemane

anddofemaue, whichisi? =-==.6. If the BaO and Sr'O are taken ' Jbem 5

into account their amounts are given in the analysis on a precedingpage and they are reckoned in with the feldspars, where they clearly 

belong, the norm figures out 

Orthoclase.... 1.11

Leucite ....... 22.67

Nephelite..... 5.94

Anorthite.-... 5.28 

Hyalophane .. 1.40 

Diopside...... 36.68

Olivine........ 18.60

Magnetite ..-. 4.64 

Hmenite ...... 1.37

Apatite....... .34

Rest........... 1.52

61.63

[: g=..590=IV, dofemane. .

 P.+ O -55.28 0 n .. , , Order, M- =-g-Tg=8.9=perpohc=l, hungarare.

P 36.68 , , Section, Q=jg-gQ=1.9=2, dopyric.

CaO"+MgO+FeO 613 1 ... Rang' Na20" =~<r=1' PermlrllcTotal.... 99.55

 MgO+Feo 426 «,. ,.. Section, n'r\/i =1^7=2.5=2, domiric.

\jai\J J.D*

 , MgO 368 A n ', Subrang, -C^Q ='fg~-~^- "="i domagnesic.

 E 28.61 Qrad, p=-y-^g=3.8=5, prelenic.

, KoO'+NaoO' 75 0 Q , .. Subgrad, CaO7 =jg=3.8=1, prealkalic.

Reckoning in the^BaO and SrO, the rock just crosses the line and 

becomes a dofemane. These other divisions have not been named, and 

no name is offered for them here, since a type standing more nearly 

in the center of each should be chosen. It is then salfemane-dofemane, boheniare-hungarare, etc., and the double name should be carried, down. Since, however, the leucite is the most interesting component, and in the salfemaues the name albanase has appropriately 

been" given to the domalkalic rang of the dolenic order to recall the 

fact that in the potassic subrangs leucite will be developed and the 

well-known Italian rocks will fall here, the name missourate may be 

used for the perpolic grad and missourote for the permirlic subgrad, 

to perpetuate the idea of a leucite-augite-olivine rock that is already 

connoted with this root.

Intermediate rock types. The fact that this rock stands exactly 

on the line between the two classes shows that such transitional forms 

must occur in any system of classification which takes account of the 

relative quantities of minerals. They will be more numerous the 

more the rocks are studied and investigated. This is inherent in 

the very nature of rocks in which transitions occur in all directions, 

and is commonly looked upon by rnanj^ petrographers as a difficulty, 

and a system of classification is regarded with favor if it professes to 

do away with such difficulties. There is a disposition on the part of 

many to regard such rocks as not typical, to say they are less abundant or not so important as those which fall in the middle of a unit, 

or to regard them as varieties or facies, or anything, in fact, which 

will minimize their importance and make the difficulty of classifying 

them less. The real difficulty lies not in the rocks, but in the petrog-
PETROGRAPHY OF DIKES AND SHEETS. 121

raphers. These difficulties' will occur as long as petrographers 

endeavor to classify rocks into units which shall correspond to species 

in other domains of natural science and to force varied types to agree 

with such units. The main difficulties of each classification will disapi^ear if it is recognized that there is no natural classification of 

rocks, and that the subject-matter spreads itself in a broad uninterrupted field, which is arbitrarily divided into units for convenience in 

description and nomenclature. Rocks which lie on the border will 

then be assigned to their proper place, their importance will be felt 

to be just as great as those away from dividing lines, and their systematic position will receive due consideration. .

PETROGRAPHY OF THE DIKES AND SHEETS. 

INTRODUCTION.

As the intrusive sheets of the High woods are geologically inconspicuous when compared with the dikes, and they have no petrographic characters which distinguish them from the dikes in such a 

degree that they deserve separate mention, the dikes and sheets are 

treated together in this description of their petrography.

In the field these rocks' divide themselves roughly into light and 

dark types; in the language of the new classification, into salic and 

feniic types. The persalanes, dosalanes, and salfemanes are represented, but no types more femic than these occur. Thus they represent the rocks found in the stocks and laccoliths in granular types. 

On the other hand, theyare not much more differentiated than, those, 

and the differences are chiefly textural ones, due to different physical 

conditions in cooling and crystallizing. These facts, merely alluded 

to here, will be discussed in their bearings in the chapter dealing 

with the petrology of the Highwood region.

In tabulated form the following types of rocks are found in the 

dikes and sheets, the names in both the old and the new classification 

being given for convenience:

Pulaskose or solvsbergite-porphyry. 

Highwoodose or tinguaite of Highwood type. 

Monzonose or gauteite (monzonitic bostonite). 

Borolanose or syenite-porphyry. 

Cascadose or minette of Highwood type. 

Monchiquose or analcite-basalt (monchiqtiite). 

Monchiquose or leucite-basalt (lencite-monchiqtiite).

TRACHDPHYRO-PULASKOSE (SODALITE-SOLVSBERGITE-PORPHYRY).

Introductory. A rock of tinguoid habit is found on the divide 

between Middle and South peaks. It occurs in a narrow dike, but a 

few feet wide, which is one of the series cut off by the large intrusion 

of the Middle Peak stock whose contact edge runs along the divide.
122 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

Megascopic characters. In the center of the dike the rock is a 

medium gray with almost no tone of green. The phenocrysts of feldspar are not strongly contrasted in shape and hue with the groundmass, as in some of the other feldspathic dikes, but are more or less 

formless and resemble it in color; they are tabular and attain a breadth 

of 10 mm. Occasional tiny specks of a black, pitchy mineral are seen. 

Near the contact the rock becomes much denser, the groundmass is a 

clear dark green, and the contrast with the phenocrysts is pronounced.

Microscopic characters. In thin section the rock displays the usual 

characters of a tinguoid porphyry phenocrysts of orthoclase and 

segirite, with interior cores of segirite-augite, rather thickly scattered 

in a groundmass of segirite needles and microlites of alkali-feldspar 

arranged in marked trachytic texture. An occasional ore grain of 

apatite completes the list of the usual minerals. In addition there is 

found dark umber-brown melanite-gamet in well-defined dodecahedrons. It occurs in rather sparsely distributed, pitchy-looking specks 

on the rock surface, which may be seen with the naked eye. Its total 

amount is small. Melanite is not an uncommon constituent in this 

group of rocks, being mentioned in a number of occurrences. It has 

been found also in tinguaite (judithose) from the neighboring Judith 

Mountains. a ' "

Another not very common feature of tinguoid rocks is the presence 

Of considerable sodalite. It is seen in rather well-crystallized dodecahedrons, but in the coarse-grained rock at the center of the dike 

it is altered to zeolites, apparently natrolite. In the dense border 

facies it is clear, unaltered, and isotropic. That it is sodalite and 

not some other isotropic mineral is indicated by absence of sulphates, 

ready gelatinization, and strong reaction for chlorine.

The trachytic groundmass has in spots a flamed patchy appearance, 

which indicates the formation of zeolites, an assumption rendered

probable by the water shown in the analysis. It is possible that it 

formerly contained some nephelite as a cement, or that some is still 

present, but this can not now be proved. The smaller microlites of 

segirite in it are usually decomposed to earthy material, retaining the 

same columnar form.

Chemical composition. The chemical analysis of this type is shown 

in I of the following table. It will be seen on comparison that it 

is much like other tinguaites, but shows a great preponderance of potash over soda, like the other rocks of this part of Montana.

"Weed and Pirsson, Geology of the Judith Mountains: Eighteenth Ann. Bept. U. S. Geol. 

Survey, pt. 3,1898, p. 57L
IMUSSON.] PETEOGRAPHY OF DIKES AND SHEETS. 

Analyses of tinguoid rocks.

123

SiO2 .......................

Al O

Fe2O3 ......................

FeO .......................

MgO.. .....................

CaO .......................

Na20 ......................

K20-... ..L.... ..............

H20. ................. ......

Ti02 . ......................

MnO. ......................

BaO -.---....-..............

SrO.. ......................

P n 2U5 -...--.......-....-.-..

SO, ........................

01 .........................

0=01. ....... . ..--._-.--...

Total ................

I.

57.18

18.54

3.65

1.15

. .69

2.31

4.48

8.58

2.10

.30

Trace.

.49

Trace.

.05

.06

.77

100. 35

.17

100. 18

II.

57.63

17.53

3.46

1.18

.22

1.35

5.80

9.16

3.22

.33

Trace.

Not det.

Not det.

Trace.

.08

99.86

.03

99. 83

III.

58.90

17.70

3.94

2.37

.54

1.05

'7.37

5. 59

1.90

.40

.55

?

?

Trace.

100. 33

IV.

55. 65

20.06

3.45

1.25

.78

1.45

8.99

6.07

1.51

Trace.

99.21

V.

0.953

.181

.023

. 016

.017

.041

.073

.091

.004

.022

I. Tinguoid trachiphyro-pulaskose, .from dike on ridge between Middle and 

South peaks. H. W. Foote, analyst.

II. Judithose (tinguaite-porphyry), from dike on Cone Butte, Judith Mountains, Montana. L. V. Pirsson, analyst. Weed and Pirsson, Geol. Jtidith 

Mountains, Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, p. 569.

III. Umptekose (solvsbergite-tiiiguaite) dike, on railroad between Tjose and 

Aklungen. V. Schmelck, analyst. Brogger, Grorudit-Tinguait-Serie, 1894, 

p. 102.

IV. Laurdalose (tinguaite) dike at Asbjornsrod, Hedrum, South Norway. 

V. Schmelck, analyst. Brogger, Ganggefolge des Laurdalits, 1898, p. 377.

V. Molecular proportions .of I.

Classification in the new system. From the analysis just given it 

is easy to calculate the norm and the position of the rock in the new 

system. Without giving all the details of this in tabulated form, the 

results are shown in I of the table on the next page. The combination of high Si02 and the considerable amount of chlorine produces 

free quartz and sodalite.

The amount of chlorine in any case appears excessive and thus 

produces this anomaly. It is a good example of how carefully the 

small amount of chlorine in such rocks must be determined if it is to 

be used as a factor in determining the amount of sodalite present, and 

the same is true of SO8 and nosean. Since no quartz is present in the
124 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Calculation of the norms of tinguoid rocks.

Or..........................................

Ab. I........................................

An. ......................................

No.. . .. ..... . ....

So ........................................

Qz......... ................................

Di. ........................................

^o. ........................................

Hy

Wo ........................................

Mt.. .........................................

11... ....---..........-.---......--....

Ht.. ..-.....--.-....--.---:...-..-.---.-.

Rest .........--..-......---...-.-- .........

Total ........__...._...---...,-...

I.

50. 60

15.20

7.78

10. 65

4.86

2.81

.40

2.78

.61

1.76

2.59

100. 04

II.

50. 60

25.15

6.67

5.81

3.67

2.78

.61

1.76

2.59

99.64

III.

54.49

14.15

12.50

.99

3.42

8.78

.93

.70

.61

3.22

99.79

IV.

54.62

18.80

8.81

1.18

5.33

7.51

3.45

99. 70

I. Norm of pulaskose calculated from analysis.

II. Norm of pulaskose calculated with corrected chlorine.

III. Norm of judithose of Cone Butte.

IV. Mode of judithose of Cone Butte.

rock, we may distribute this silica so that it will raise a corresponding 

amount of sodalite to albite and diminish the chlorine accordingly. 

This gives 0.42 per cent of chlorine and the norm becomes that shown 

in II of the above table. The classification then becomes 

=10> =I> persalaneOrder. == ' =0.05 perfelic 5, canadare.

Rang, ic=2, pulaskase.

Subrang, 2 = =1.2=sodipotassic=3, pulaskose. JNa2vy »*$

 It may be remarked that No. I above would lead to the same result, 

since ^ or ^ ^ - whichever be selected.

Since the study of the rock shows it to be porphyry of trachytic 

texture with feldspar phenocrysts, it may be designated trachisalphyro-pulaskose with tinguoid habit, or, more shortly, trachiphyropulaskose. This is a very condensed expression for a rock consisting chiefly of alkali feldspars, with ferromagnesian minerals less 

than 12.5 per cent, a small amount of anorthite feldspar in which 

soda and potash are molecularly about equal, and a porphyritic 

texture, with feldspar phenocrysts and a trachytic groundmass.
rinssoN.] PETROGRAPHY OF DIKES AND SHEETS. 125

Iii this connection it is of interest to compare this rock with one of 

similar tingnoid habit from Cone Butte, in the Judith Mountains, 

previously described by the author. Its analysis is given in the table 

of analyses on page 123. It will be seen that it is similar in chemical composition, but contains less lime. and higher alkalies. Asa 

result of this it does not contain' any anorthite, but a considerable 

amount of segirite, and the larger proportion of femic minerals carries 

it into the dosalic class. This is shown on calculating its norm, Avhich 

is given in in of the preceding table. Its actual mineral composition, 

or mode, is given in iv of the preceding table, and this is so close 

to the norm that the rock has very nearly a normative mode. From 

the norm and the analysis we may classify it as follows:

Class, j5k=^4?=5.6=II, dosalane. 

Fern. 14.44

Order, ^=l|^=.18=lendofelic=6, norgare. it oo.o*

Rang, ==peralkalic=l, lanrdalase. 0

Subrang, -^0 =|§=1.3=sodipotassic=3, judithose.

JN 9*2^-' 'V

. The rock occupies a definite position in the subrangs, and for 

reason the name of the occurrence has been given to it.

Mineral composition or mode. The actual mineral composition, or 

mode, of the dike rock under description can not be determined with 

any great accuracy either from the analysis or from a study of its 

section. Combining both of these means of observation, it is reckoned 

that the mineral composition of the rock is approximately as follows :

Mineral composition or mode of traohiphyro-pulaskose.

Iron ore --..----_-..---_--..--.__-..--.--.__.._..__--..-._--. 2. 0

JEgirite-augite .....--...--..........--.......'................ 10. 0 . Orthoclase. .................................................. 50. 0

Albite ....................................................... 10.0

Anorthite. . ... ..!.... .............................. .........^ 3.0

Sodalite ...............:.......:............................. 10.0

Kaolin ...................................................... 7.0

Rest.... -...-.......-. :..... ............ ...'. ................. 8.0

Total.................................................. 100.0

In this the full amount of chlorine given by the analysis is used to 

indicate the amount of sodalite, but it is thought that this is somewhat too high, and the amount shown in the norm is believed to be 

more nearly correct. The proportion of minerals is not strikingly different from that shown in the norm, and the rock, exception being made 

of the products of alteration, has in all probability a normative mode.

Classification in prevailing systems. It is clear from the descriptions and analysis that in the prevailing systems of classification the
126 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

rock is closely related to the solvsbergites, the amount of f eldspathoid 

being relatively small for the true tinguaites. It thus lies between 

the solvsbergites and the tinguaites, and differs from the normal type 

in that nephelite is wholly or almost wholly replaced by sodalite. It 

is, therefore, a novel type and might be called a sodalite-potashsolvsbergite-porphyry. This would without doubt have a very 

attractive sound to timid souls who fear the introduction of new 

names.

TRACHIPHYRO-HIGHWOODOSE (HIGHWOOD TINGUAITE-PORPHYRY).

Introductory. The rock of the large gray dike which forms a conspicuous wall on the east side of the upper valley of Highwood Creek, 

a mile or so below the divide at Highwood, is about 12 feet wide and 

has broken through the dark, chocolate-colored breccias. In mass it 

has a somewhat platy structure, and it breaks in a splintery manner 

under the hammer. The rock has been previously described very 

briefly by Lindgren," under the designation trachyte, type (6), as 

feldspathic porphyritic with phenocrysts of augite, biotite, and sanidine in a groundrnass of the same minerals with opacite and some 

glass. He mentions especially the augite, which he considers to be 

the same throughout all of the Highwood rocks.

Megascopic characters. On a surface of fresh fracture the rock is 

a'medium greenish gray. This color is produced by great quantitie; 

of small dull-white phenocrysts very thickly sprinkled in a green 

gray groundmass of felsitic character. In size these phenocrysts run 

from 1 to 2 mm. in diameter, and have a more or less pronounced 

square outline. In addition there are a few dark specks of a ferromagnesian mineral scattered here and there, and occasionally one of 

these shows the glittering cleavage of a biotite. Examination with 

the Fens only magnifies these features; it does not bring out anything 

in addition.

Microscopic characters. In thin section there are seen to be present 

the following minerals in a compact groundmass: Iron ore, apatite, 

pyroxene, biotite, and orthoclase. . Iron ore is found only as rare, 

small grains. Apatite is seen in occasional short, stout crystals. 

Biotite is also rather infrequent and occurs in small, somewhat 

rounded tablets, which are strongly pleochroic and have dark leatherbrown and pale-yellow tones.

The pyroxene ranges in size from stout columnar crystals, 2-3 mm. 

long, down to scarcely perceptible needles. Usually they are well crystallized and bounded by the planes (100),.(HO), (010); and (111), in many 

cases (100) being largely developed. In color they are a clear pale 

green, which increases in depth from center to outside. In the largest 

crystals the center is almost colorless, and, in general, the smaller 

they are the deeper is the green color they exhibit. In the latter

a Tenth Census U. S., vol. 15, p. 726, section c.
PIRSSON.] PETROGRAPHY OF DIKES AND SHEETS. 127

case, also, they possess a faint but distinct pleochroism. The angle of 

extinction of c/\-C-is large, being 45° or over. The double refraction 

has" an index of 0.040.proximate, which is above that of pure diopside. 

All these characters .show that the pyroxene is a diopside enriched 

to some extent with the segirite molecule; it stands between diopside 

and an segirite-augite.

The feldspar phenocrysts are considerably altered to kaolin in the 

hand specimens gathered, only interior cores being still fresh and 

clear. Gooch'sffl determination of the alkalies in them shows them to 

be of orthoclase (K2 0 11.36, Na20 2.14 per cent). They occur in Carlsbad twins and in intergrown groups. They do not have good crystal 

outlines and with high powers show in the clear portions no signs of 

any micro-intergrowths with the albite molecule.

The groundinass is of the usual type found in rocks pf tinguoid 

habit. It consists of an interlaced mass of orthoclase laths woven 

through with a fine felt of ajgirite needles, dotted here and there with 

small ore grains. Biotite, which could be referred to the groundmass, 

was not seen, all the crystals noted being distinctly in the class of 

phenocrysts. This groundmass is somewhat altered, as shown by the 

turbid, clouded appearance given it by the kaolin granules, making 

its study with high power difficult. There is also more or less of an 

isotropic substance present. The character of the rock and its geologic occurrence practically preclude its being a glass; moreover, the 

rock on treatment with dilute acid gelatinizes readily, and this again 

is impossible for an alkali-alumina glass. The solution yields the 

merest trace of chlorine, which excludes sodalite, and its ready gelatinization shows that the rock can not be wholly, at any rate, made up 

of leucite. All this points toward analcite, and the abundant water 

shown in the analysis renders this still more probable. Analcite is 

known to occur in the base of rocks of the tinguoid group, and 

recently Washington 6 has described one from the vicinity of Salem, 

Mass., which consists chiefly of this mineral in the groundmass and 

which he is inclined to view as being primary^in origin. Coleman c 

also has described a tinguaite-like rock, under the name "heronite," 

whose base consists mostly of analcite which is regarded as primary.

In the present case the amount of analcite is small compared with 

the other minerals, and there is nothing to show whether it was original or not. The rock is not perfectly fresh; but, on the other hand, 

that it is somewhat altered is no proof that the analcite is secondary, 

since it is known that in all probability it may occur as a primary 

component. With the means at command at the present time there 

is no way of telling in this instance whether it is primary or not.

Chemical composition. The chemical analysis of this type is shown 

in column I of the table on the next page.

"Tenth Census U. S., vol. 15, p. 726, sec. c. 

&Am. Jour. Sci., 4th series, vol. 6,1898, p. 182. 

c Jour. Geol., vol. 7,1899, p. 431.
128 IGNEOUS ROCKS OF HIGHWOOD MOUNTAINS. [BULL. 287. 

Analyses of highwoodose and related tinguoid rocks.

Si02 . .

ALA --- 

FeA -.. 

FeO .... 

MgO.... 

CaO .... 

NaaO---. 

K,0. -

H2 0-K-. 

H2 0-... 

CO......

Ti02 ....

PA ....

SO......

Cl. ......

F. ......

MnO__..

BaO ....

SrO.....

Li2O ....

O-C1...

Total..

I.

58.04

17. 25 

2.49 

1.24 

1.79- 

8.50 

3.37 

10.06

j 1.95

.30

.22

Tr.

Tr.

Tr.

(«)

(«)

Tr.

100. 21

II.

57.18

18.54 

3.65 

1.15 

.69

2.31 

4.48

. 8.58

2.10

.30

.05

.06

.77

Tr.

.49

Tr.

Tr.

100.35

.17

100.18

III

57.46

15.40 

4.87 

.87 

1.37 

2.5.9 

5.48 

9.44

(" .82 

I .09 

.13

.60

.21

.13

.20

Tr.

Tr.

.60

.16

Tr.

100. 42

.05

100. 37

IV.

57.63

17.53 

3.46 

1.18 .22' 

1.35 

^5.80 

9.16

[ 3. 22

.33

Tr.

.08

Tr.

(°)

(a)

- Tr.

99.86

.03

99.83

V.

58.70

19.26 

3.37

' .58 

.76 

1.41 

8.55 

4.53

( 2.57 

I .07

Tr.

.10

(?)

.10

100.00

VI.

58.90

17.70 

3.94 

2.37 

.54 

1.05 

7.37 

5.59

| 1.90

.40

Tr.

.55

100.33

VII.

62.70

.16.40 

3.34 

2.35 

.79 

.95. 

7.13 

5.25

.70

.92

100.53

VIII.

55.65

20. 06 

3.45 

1.25 

.78 

1.45 

8.99 

6.07

1.51

Tr.

99.21

IX.

0.967

.167 

.015 

.017 

.045 

.062 

.054 

.107

.107

X.

0 958

.180 

.023 

.016 

.017 

.041 

.072 

090

.116

« Not determined.

I. Highwoodose (tmguaite-porphyry) from dike near head of Highwood 

Creek (No. 724). E. B. Hurlbur't, analyst.

II. Pulaskose (tinguaite-porphyry) from dike cutting rim rock of Middle Peak 

stock, ridge between Middle and South peaks (No. 746). H. W. Foote, 

analyst.

III. Judithose (Tinguaite-porphyry) from dike at head of Bear Creek, Bearpaw 

Mountains, Montana. H. N .'Stokes, analyst. Weed and Pirsson, Bearpaw 

Mountains of Montana, Am. Jour. Sci, 4th series, vol. 2, 1896, p. 192.

IV. Judithose (tinguaite-porphyry) from dike at Cone Butte, Judith Mountains, 

Montana. L. V. Pirsson, analyst. Weed and Pirsson, Geology of Judith 

Mountains, Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, p. 569.

V. Miaskose (solvsbergite) from intrusive sheet north of Shields River, Crazy 

Mountains, Montana. W. H. Melville, analyst. Wolff and Tarr. Acmitetrach.yte from the Crazy Mountains, Bull. Mus. Comp. Zool., Cambridge, 

Mass., vol. 16, No. 12,1893, p. 232.

VI. Umptekose (solvsbergite-tinguaite) from dike on railroad between Tjose and 

Aklungen, 175 kilometers from Christania, Norway. V. Schmelck, analyst. 

Brogger, Grorudit-Tinguait-Serie, 1894, p. 102.

VII. Umptekose (hornblende-solvsbergite) from Lougenthal, South Norway. 

L. Schmelck, analyst. Brogger, loc. cit., p. 80.

VIII. Laurdalose (tinguaite) dike at Asbjornsrod, Hedruni, South Norway. V. 

Schmelck, analyst. Brogger, Ganggefolge des Laurdalits, 1898, p. 377.

I.. Molecular proportions of I. 

X. Molecular proportions of II.
PIUSSON.] PETROGRAPHY OF DIKES AND SHEETS. 129

The notable features as compared with most rocks of pronounced 

tinguoid habit are the extraordinarily high potash in contrast with the 

soda and the high lime and magnesia. It is approached in this latter 

respect by the tinguaite from the Bearpaw Mountains. It will be 

seen by glancing at the column of molecular ratios (ix) that the lime 

is just sufficient to turn all the ferrous iron and magnesia into pyroxene. The considerable amount of water is to be attributed in part to 

the analcite and in part also to the kaolin. In this analysis as originally published" the amount of chlorine, 0.33 per cent, is undoubtedly 

too high. This would indicate somewhat less than 5 per cent of 

sodalite, or an amount somewhat smaller than that of chlor-apatite. 

Several very carefully conducted tests have failed to show more than 

a mere trace, which is undoubtedly from the small amount of apatite 

present. ( . Mineral composition or mode. The mineral composition can not be 

accurately computed from the analysis on account of several uncertainties and because each oxide is or may be present in two or more 

components. The analysis affords, however, a certain amount of 

data, and from it and a study of the section it can be rather closely 

reckoned that the rock consists of 20 per cent augite, including 

2 or 3 per cent of iron ore, apatite, etc., and 80 per cent feldspar and 

feldspathoids. Of the augite, about one-third to one-quarter is 

segirite and there is about 10 to 15 per cent of analcite.

Classification in prevailing systems. When one compares the 

analysis of this rock with that of typical tinguaites and solvsbergites, 

it is seen to differ from them, as before mentioned, in potash, magnesia, and lime; and this is expressed-in the minerals, since we have 

diopside and fegirite-augite instead of pegirite, orthoclase instead of 

soda orthoclase or albite, and analcite in place of uephelite. It is to 

be noted also that in silica and alumina it stands between a typical 

tinguaite and solvsbergite, as may. be seen by reference to analyses 

vn and vni. The iron is lower than in most rocks of this group. 

The rather small amount of feldspathoid present evidently places the 

rock between the solvsbergites and the tinguaites. All these facts 

show that, while by its structure and general composition it belongs in 

this group, it is evidently a distinct variety and may be called the 

Higliwood type of tinguaite.

Classification in thenew sy.stem. In the new system of classification 

the rock occupies a definite position by itself among the subrangs, 

thus filling a position otherwise vacant, and for this reason the name 

of highwoodose has been given it, by which the dopotassic subrang of 

the peralkalic rang of the perfelic order of dosalaue has been designated. This is shown in the following table of calculation of the norm.

" Bull. U. S. Geol. Survey No. 148, p. 152, Analysis B. 

Bull. 237 04 9
130 IGNEOUS ROCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Calculation of tlie norm of highwoodose.

SiO, ------

Al O

FeA- -

FeO ......

MgO. ......

CaO ......

Na,O------

K2 0. ......

TiO2 ......

PA ------

CV. ......

Rest ;.....

Total.

Analysis.

58.04

17.25

2.49

1.24

1.79

3.50

. 3.37

10.06

.30

.22

.38

1.95

100. 59

Molecular ratio.

0.967

.167

.015

.017

.045

.062

.054

.107

.004

.001

.005

Or.

642

107

107

107-

Ab.

168

28

28

28

Ne.

1?,

6

6

fi

An.

99,

11

11

11

So.

30

15

20

5

5

Di.

90

45

45

45

Wo.

3

3

3

Mt.

13

13

13

11.

4

4

4

Ap.

3

1

1

Ht.

2

2

59.49

14.67

3.06

1.70

4.86

9.72

Or.......

Ab ......

An ......

Ne ......

So.......

Di.......

Wo......

Mt...... 3.02

II........ .61

Ht....... .32

Ap ...... .34

Rest..... 1.95

 83.77

Sal. 83.77 co , ,. , . Class, pig^ > =jj^=5.8=dosalic=dosalane.

Order, p ==y-ny=0.08 perfelic=germanare.

KoO'+Na-iO' 161 Rang, ""^(jaQ/ ' =-jY=14.6=peralkalic=umptekase.

Total.. 100.08

If we consider that the rock has evident phenocrysts of feldspar 

and a microtrachytic groundmass, it may be characterized as trachisalphyro-highwoodose with tinguoid habit. The accordance between 

the norm and the mode is not complete, since analcite is developed 

at the expense of albite, and the segirite of the mode shows itself in 

iron compounds in the norm; but on the whole these constitute a 

modal variety rather than a real abnormative mode.

ROCKS OF TINGUOID HABIT (GRORUDITE-TINGUAITE SERIES).

This group of rocks was first given a distinct and coherent form by 

Brogger a in a monograph which has become one of the classics of 

petrographic literature.

The series as described by him is a group of feldspathic rocks of 

dense to porphyritic character occurring in narrow dikes and sheets 

and composed chiefly of alkali feldspars and segirite. According 

to the amount of silica present there will be free quartz (grorudite),

< Grorudit-Tinguait-Serie, 1894.
PIRSSON.] PETEOGEAPHY OF DIKES AND SHEETS. 131

little or no quartz (solvsbergite), or rocks containing nephelite (tiiiguaite) as a result of the deficiency of silica producing this molecule 

instead of albite. Rosenbusch a calls the series "rocks of tinguaitic 

habit" and refers to their green color and compact grain, which are 

indeed useful characters for megascopic determination. .

Rocks of this series have been abundantly found in central Montana in the detached outlying mountain groups, and have been 

described by Mr. Weed and the author from the Bearpaw, Judith, 

and Little Rocky mountains, and by Wolff b from the Crazy Mountains, under the name of acinite-trachyte. In these places they occur 

as narrow dikes and sheets, more rarely as marginal facies of large 

laccolithic masses of syenitic rocks (Little Rocky and Judith 

Mountains).

As appears to be generally the case in these occurrences, the rocks 

are satellites, dependent on rock complexes of alkalic habit and 

character. (

Thus in central Montana they are potash tinguaites, etc., evincing 

therefore the regional characteristic of this well-defined petrographic 

province alkalic magmas with very high potash.

In the Highwood Mountains two occurrences of rocks having this 

habit have just been described, both in dikes, and it is of interest to 

note that while the Highwood group clearly belongs to the general 

central Montana province, yet the rocks composing it show throughout certain local peculiarities which determine them as belonging to 

one clan, and the tinguoid members have certain characteristics which 

stamp them as forming a subgroup of the series the Highwood 

group since they wear, so to speak, the Highwood tartan.

The new system of classification is traversed in various directions 

by serial rock groups, according as one or more elements vary and 

there is a greater or lesser amount of a given mineral or minerals in 

the rock. The same is, of course, true in all systems of classification. 

Thus in the new system the rocks of tinguoid habit will be found 

chiefly in the persalanes and dosalanes, and in the quardofelic, perfelic, lendofelic, and lenfelic orders in each class. In the rangs they 

occur in the peralkalic and domalkalic, and in subrangs chiefly in the 

dopotassic, sodipotassic, and dosodic. This would give in theory 

about 48 subrangs in which this habit might be developed. However, 

on inspection of well-known types occurring in these subrangs, it 

becomes evident that the habit is rare or little known in the domalkalic dosalanes, and the number mentioned above will be reduced to 

about 25 or 30. Moreover, the great mass of them \yill be found clustering in the subrangs rich in soda, in the alkalic rangs in quardofelic, perfelic, and lendofelic orders.

oElerneiite der Gesteinslehre, 1898, p. 211.

6 Bull. Mus. Comp. Zool., Cambridge, Mass., vol. 16, No. 12,1893, p. 232.
132 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

TRACHIPHYRO-MONZONOSE (GAUTEITE VARIETY OF BOSTONITE).

Occurrence. In a small number of localities occur dikes and sheets 

of fine-grained, light-colored feldspathic rocks of more or less pronounced trachytic character. They are usually badly -altered and 

weathered, so much so that in some instances they are merely crumbly 

masses of soil whose form, position, and more resistant contact zones 

on the sides alone enable them to be distinguished as dikes. On digging into such masses pieces of less altered rock may be found which 

enable one to recognize the type. In a very few instances they are 

well preserved and afford fresh material for study, as in the basin at 

the head of Aspen Creek, where two dikes on spurs on the north side 

of the main creek, west of the breccia hills, cut Cretaceous sediments. 

They are a mile or so apart, and, like the High wood minette (shonkinose) dikes, they extend in the direction of the Shonkin stock. 

The eastern one is the larger, and is about 15 feet wide.

Megascopic characters. In the hand specimen the rock is a pale 

brown, rather fine grained, and compaqt, but it has a markedly rough 

trachytic feel and appearance. This surface is rather thickly sprinkled with hornblende prisms, which are black and glittering on a 

cleavage face. The prisms are 10-20 mm. in length and 1-2 mm. 

broad. In some cases they are altered and brown or rusty. With 

the lens one sees also small, clear apatites; they are not common, but 

a considerable number may be found. The crystals are of unusual 

size for a rock of this character.

Microscopic characters. Under the microscope the hornblendes 

are found to be of an olive-green and yellow pleochroic variety, allied 

to arfvedsonite and similar to the green hornblendes described in the 

border type of the rock of -the Middle Peak stock (borolanose) and 

the green variety of the syenite of Square Butte (pulakose). - The 

large apatites are well crystallized, with prisms and the common pyramid and base. . They are colorless and without notable inclusions. 

A considerable amount of iron ore in small scattered grains is also 

present.

The groundmass in which these minerals lie is composed of lathshaped feldspars, which at times tend to assume a somewhat tabular 

form. Careful study .with high powers shows that they are sometimes untwinned, sometimes singly twinned according to the Carlsbad 

law, and sometimes are albite twins. Whether twinned or-not, all 

sections whose form, edges, etc., show that they are cut in the zone 

perpendicular to 0.010 extinguish parallel with the nicol, and these 

facts in connection with the analysis prove that both alkalic and 

plagioclase feldspars are present and that the latter is an oligoclase. 

Undoubtedly the orthoclase contains considerable soda.
PIRSSON.] PETftOGftAPfiY 0$ BlKES AND SHEETS. 333

The.chlorine and sulphur trioxide shown in the analysis would 

seem to indicate that considerable socialite and nosean may be 

present. They are, however, not seen in the sections. A minute 

quantity of colorless isotropic substance is found in spots between 

the feldspars; this may be sodalite, but it may also be analcite, as 

indicated by the water. A little interstitial nephelite might also be 

present and escape detection. It is rather inferred that the chlorine 

and sulphur shown are somewhat too high, and that the chlorine 

comes for the greater part from the apatite, while a little pyrite might 

easily furnish the sulphur. It should be remembered that very small 

errors in the estimation of chlorine and sulphur within the limits of 

good analytical work will cause large variations in the amounts of 

sodalite and nosean estimated from them. a

This groundrnass is somewhat stained by yellowish ferruginous 

material, which under the microscope is seen only in spots. There 

may be some zeolitization, as indicated by the water shown in the 

analysis.

Mineral composition or mode. As nearly as this can be approximated, the rock contains about 5 per cent Of iron ore, about 1.5 per 

cent of apatite, about 10 per cent of hornblende, and the rest is made 

up of about equal proportions of oligoclase and orthoclase feldspars.

Chemical composition. An analysis of the large Aspen Creek dike, 

the eastern one of the two shown on the map, has been made by 

Doctor Foote, with the results shown in I of the table on the next page.

The moderate silica, high alumina, and alkalies show that this is 

essentially a trachytic rock, but the amount of lime, iron, and magnesia is too high for a typical rock of the class. The chlorine and sulphur have already been commented upon.

« This has accordingly been taken into account in the calculation of the norm!
134 IGNEOUS KOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Analyses of monzonose (gauteite) and related rocks.

SiO2 . ............ -----

ALA ---...---.---.-.

FeA -.--------,.--

FeO. .................

Mgo. .......:....-...

CaO

Na2 0 ................

K2O...... ,.._.._..._.

H,0+ ... ---------

H8O-. ...............

TiOt .................

P,05 .................

SO, ..................

Cl -..-.-..---.--.--..

NiO-. ........ ........

MnO. ................

BaO .................

SrO. -...:.. . ------

Cl-O. ...............

Total ...-...-..

I.

55.23

18.31

4.90

2.06

1.85

3.62

4.02

6.43

1.84

.42

.58

.23

.32

Trace.

.46

Trace.

100.27

.08

100.19

II.

54.15

18.25

3.62

2.09

2.56

4.89

4.43

6.56

3.69

Trace.

.41

-

100. 65

III.

55.52

20.05

2.52

2.40

2.10

3. 15

3.44

7.49

1.42

' .70

.51

Trace.

99.30

IV.

57.29

18. 45

4.38

1.20

2.08

3.57

4.43

5.43

2.01

.17

' 72

.46

.12

(?)

(?)

100.31

V.

58.18

18.46

2.31

3.79

1.99

3.11

3.70

6.58

.64

.68

.41

29

(?)

100. 14

VI.

56. 75

18.37

2.22

3.04

2.02

4.68

4.85

5.92

.18

1.24

Trace.

99.38

VII.

0.920

.179

.031

.029

.046

.064

.065

.068

.005

.004

I. Monzonose (gauteite) from Aspen Creek, Highwood Mountains. H. W. 

Foote, analyst. 

II. Monzonose (gauteite) from Miihlorzen, Blatt Bensen, Bohemia. R. Pfohl, 

analyst.

III. Monzonose (gauteite) from Gentungan, Maros Peak, Borneo. Doctor Hinden, analyst.

IV. Morizonose (quartz-banakite) from Stinkingwater River, Yellowstone Park. 

W. H. Melville, analyst. Iddings, Jour. Geol., vol. 3, 1895, p. 947.

V. Monzonose (odinite) from Tito, Coquimbo, Chile. A. Lindner, analyst. 

F. v. Wolff, Zeit. Deutsch. Geol. Gesell., vol. 51,1899, p. 531.

VI. Monzonose (ciminite) from L'Arso, Ischia, Italy. H. S. Washington, analyst. Arn. Jour. Sci., 4th ser., vol. 8,1899, p. 290. 

VII. Molecular ratios of I.
PETBOGKAPHY OF DIKES AND SHEETS. 135

Classification in prevailing systems. A rock of this character, containing so much plagioclase and orthoclase, and of porphyritic texture, would lie between the andesites and trachytes, and would be 

classed as a trachyandesite.. An analysis of a rock of this character 

is shown in VI. Under the heading of trachyandesitic dike rocks, 

following the system of Rosenbusch, a few years ago J-Iibsch a described 

bostonoid types from the region covered by the Bensen sheet of the 

Bohemian Mittelgebirge survey, which differed from true bostonites 

in containing considerable plagioclase. He expressly notes that on 

account of the nearly equal mixture of orthoclase and plagioclase they 

should be viewed as belonging to the monzonite family of Brogger, 

under which they represent the bostonites of the syenitic family. 

Having thus a distinct place of their own, he proposed for them the 

name gauteite, from the hamlet of Gaute, near which a group of 

these dikes occur. In mineral development they are similar to the

type just described, with hornblendes and the trachytic groundmass, 

but have in addition some augite, biotite, and numerous plagioclase 

pheuocrysts. Having thus a more pronounced porphyritic habit, they 

are less like typical bostonites than the Aspen Creek rocks. These 

themselves vary from a typical bostonoid habit in the fact that they 

carry a considerable number of phenocrysts of hornblende, the original 

types of bostonites consisting of a trachytic groundmass alone. Chemically the close correspondence between the gauteite of Aspen Creek 

and the original type of Hibsch is seen by comparing the analyses 

given in the table.

A somewhat similar rock from the Maros Peak complex in Celebes 

is also described as a gauteite b}7 Schmidt. 6 Chemically it is closely 

related, as may be seen by comparing its analysis. In mineral development the groundmass is purely orthoclase with some sodali'te, in 

which lie phenocrysts of labradorite and biotite. It lias been found 

only as an erratic, and its dike nature is surmised. At present, therefore, it can only be provisionally placed with the gauteites.

Classification in the new system. In this the type of the rock 

and the relations of orthoclase and plagioclase are shown by its falling into monzonase, where it properly belongs. This is seen in the 

following table.

o Erlaut. zur geol. Karte des Bohin Mittelgeb., Blatt Beusen, Tschermaks Min. Mitt., vol. 17, p. 84 

bSarasin's Celebes, vol. 4,1901, appendix, p. 19.
136 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BOLL. 237. 

Calculation of the norm of monzonose.

SiOj-----.-----

A12O3 .........

Fe2O, -......-..

FeO...........

MgO.. ...... .'.

CaO... .........

NajO. ........ .

K2O. ----------

TiO2 . ..........

PA ----------

Rest ..........

Total....

Analysis.

55.23

18.31

4.90

2.06

1.85

3.62

4.02

6.43

42

.58

.07

2.53

100. 02

Molecular ratio.

0.920

.179

.031

.029

.046

.064

.065

.068

.005

.004

.001

Or.

408

68

68

68

Ab.

360

60

60

60

Ne.

 10

5

5

5

An.

99

46

46

46

Di.

10

-

5

5

5

Hy.

41

41

41

Mt.

94

94

94

11.

5

5

5

Ht.

7

7

Ap.

13

4

1

4

 Or.......

An ......

"Ne ...... 

Di. ......

Hy ......

Mt ...... 

11........

- Ht.......

Ap ......

Rest.....

37.81 

31.44 

12.79 

1.42 

1.08' 

4.10 

5.57

1.12 

1.34. 

2.53

Sal. 83.46 Tr , Class. Fgjjf =fg~97 =r''-9=II, aosalane.

, Order, p=g21Q|=.017=perfelic=5.germanare. 

NaoO'+K.O' 133

K«O' 68 13.97 Subrang, J^^Q,--^ 1.0 sodipotassicTotal.. 99.96

The texture is clearly microscopic trachytic; it is porphyritic with 

hornblende phenocrysts alone; it is thus alferphyric and should be 

termed trachiphyro-hornblende-monzonose.

TRACHIPHYRO-BOROLANOSE (SYENITE-PORPHYRY).

Occurrence. In a number of places in the Highwoods occurs a dike 

rock which, by reason of its chemical, mineralogical, and textural 

peculiarities, has a definite place of its own, and in its appearance 

affords, a particular and easily recognizable type. One dike (832) 

cuts leucite-basalt and breccia near the Shonkin stock at the head of 

Shonkin Creek; another (783-), whose broken down exposure consists 

of large plate-like slabs, is found in basaltic flows and breccias in 

the spur running north from .the main ridge which terminates in 

Twin Peaks, just below the central summit on the main ridge; 

another (861) cuts a black pinnacle of basaltic breccias at the head 

of a west branch of upper Shonkin Creek, near the Arnoux stock, on 

the divide to Highwood Creek. One example (703) cuts a spur run-
PIKSSON.] PETROGRAPHY OP DIKES AND SHEETS. 137

ning up toward the mountain ridge on the west side of Highwood 

Gap; another (682) forms the crest of a little hill in the open country 

south of South Peak. The dike is about 25 feet wide and weathered 

down. The rock is platy, has rather large phenocrysts of feldspar, 

and is considerably altered and brownish from weathering. Still 

another, on the west side of Highwood Gap (697), is much weathered 

and broken down and cuts a small intrusion of another rock and also 

the basaltic dikes. These dikes have a width of from 6 to 10 feet and 

cut the basaltic dikes and breccias. The last example (745) is one of 

the dikes on the summit'ridge leading to South Peak. It is. greatly 

weathered and kaolinized. The feldspar phenocrysts are small, but 

the microscope shows that the rock belongs in this type.

Megascopic characters. In the hand specimens these rocks are of 

a general pearl-gray color. They are distinct porphyries. The 

phenocrysts consist of numerous tabular alkalic feldspars which 

attain dimensions of 12 by 2 mm. and are not of very good crystal form, small black glittering prisms of augite, and hexagonal 

tables of biotite. They are thus to be classed as alfersalphyric in 

character. These components are in general without arrangement 

in the gray ground mass, which is evidently granular and made up of 

a lesser quantity of the tiny femic grains among the salic ones. In 

two instances, however, the thin tabular feldspar phenocrysts are 

arranged in a markedly oriented manner parallel to'the walls of the 

dike and show a pronounced flow structure.

None of these occurrences affords perfectly fresh material. The 

newly fractured rock face is dead and lusterless, the feldspar cleavages lack sparkle, and the larger phenocrysts have a white porcelainlike look which shows that they are more or less altered. These 

observations do not apply to the alferric minerals, which are generally well preserved. On weathered surfaces the rock exhibits a 

light-brown crust, and in the more altered occurrences it is brown 

throughout.

Microscopic characters. In the thin sections the minerals seen 

are magnetite, apatite, diopside, biotite, and alkali feldspars. The 

iron ore and apatite present no especial peculiarities. The mica is a 

brown biotite of ordinary character. The diopside varies from very 

pale to a clear green; it rarely shows pleochroism, has a wide extinction angle, and is well crystallized in columnar forms; it is of the 

regular Highwood type described elsewhere. A few pieces of a pleochroic ajgirite-augite were observed in some sections. The feldspars 

are of the usual character of orthoclase; they undoubtedly contain, 

soda, as shown by the analysis. They are either untwinned, which is 

common among the phenocrysts, or singly twinned Carlsbads, winch 

are veiy common in the lath-like development in the groundinass. 

The phenocrysts vary much in size, not only in the different occurrences, but also in the same section, and there are formal transitions
138 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 231

from phenocryst to groundmass. The phenocrysts in all cases are 

abundant and thickty scattered. Very few feldspars that could be 

recognized as of plagioclase would be formed, and as these were in the 

more altered types the species could not be determined.

In the groundmass .the small lath-like feldspars vary considerably 

in size in the different dikes, but they all have the same form and the 

trachytic texture, and in general are interwoven-without arrangement; 

in some cases there was seen a tendency to form radial divergent 

groups, resulting in a fabric suggesting remotely the spherulitic.

The microscope shows that the ferromagnesian minerals are fresh 

and well preserved, while even in the best material the feldspars 

are considerably altered and brown with kaolin. This is especially 

true of those in the groundmass, and this brownish scattered kaolin 

dust prevents the study of the interspaces between the groundmass 

feldspars. It may be safely said, however, that there is no quartz, 

and this fact, combined with the low silica and the presence of some 

chlorine and sulphuric anhydride in the type analyzed, leads to the 

suspicion that the rocks when fresh probably contained in these 

interspaces small amounts of.lenad minerals nephelite, sodalite, and 

nosean. Some of the larger phenocrysts of feldspar contain colorless 

isotropic bays and areas of what is probably secondary analcite.

Chemical composition. For the purpose of chemical examination 

the rock of the dike cutting the Shonkin stock was selected as the one 

affording the best material. The analysis, made under the writer's 

direction by Dr. W. M. Bradley, resulted as shown in the table on the 

next page.

The features of this analysis are the low silica combined with high 

lime and alkalies and the predominance of potash over soda. The 

amount of alteration mentioned is shown by the rather considerable 

amount of water. As a whole the. analysis is typical for the general 

salic types of the Highwoods, and the analogy of this dike rock with 

the salic granular stock rocks is seen by comparing it with them. 

There is given for comparison the anatyses of the stock of Middle 

Peak and of several similar rocks from other parts of the world. 

The close similarity to the granular rocks of the stocks and the geologic occurrence tend to show that these dikes are intrusions of the 

same general stage of differentiation in the magma basin below, for 

just as the salic portions of the stocks break up through the fernic 

ones, so these salic dikes cut the femic dikes. The type above 

analyzed cuts the feinic extrusives near the Shonkin stock.
PETROGBAPHY OF DIKES AND SHEETS. 

Analyses of borolanose and related roclcs.

139

Si02 ....... ...... ......

A12O3 ....................

FeA ------------- ------

FeO--'-~----~--..--

MgO-. ............. ......

CaO ......................

.Na2O- ---------------

KA----------- ...---..-.

H30 -----,. .- 

Ti02 . ...................

MnO------ ------------

BaO ......................

SrO------------.. -- -

PA ....... -.: -..-

SO3 ....................

Cl. ........---..--- 

T.paa I)

Total ........I.....

I.

51.94

15.78

4.07

3. 17

y.48

6. 04

3.44

7.69

2.17

.39

Tr.

.42

.28

.59

.29

.08

99.83

.02

99.81

II.

52.05

15.02

2. 65

5. 52

5.39

8.14

8.17

6.10

.35

.47

Tr.

.42

.28

.21

.02

.24

100. 03

.06

99.97

III.

49.70

18.45

3.39

4.32

2.32

7.91

5.33

4.95

1.34

1.33

Tr.

.40

99.44

IV.

47.8

20.1

6.7

. .8

1.1

5.4

5. 5

7.1

2.4

.7

.8

.8

.4

99.3

V.

51.02

18.63

3.14

.84

1.02

7.89

4.13

6.08

1.10

Tr.

.59

C02 4. 53

Fl. Tr.

.16

.29

.09

.02

99. 64

VI.

0.866

. 155

.026

.044

.087

.107

.055

.082

.005

.004

.004

.002

I. Trachiphyro-borolanose (syenite-porphyry) from dike near Shonkin stock, 

Highwood Mountains. W. M. Eradley, analyst.

II. Grano-borolanose (basic syenite, syenomonzonite), main type of Middle Peak 

stock, Highwood Mountains. E. B. Hurlburt, analyst.

III. Borolanose (covite) from schoolhouse, Magnet Cove, Ark. H. S. Washington, analyst. Jour. Geol., vol. 9,1901, p. 612.

IV. Borolanose (borolanite) from Lake Borolan, Sutherland, Scotland. J. H. 

Player, analyst. Home and Teall, Trans. Royal Soc. Edinburgh, vol. 37, 

pt. 1, 1892, p. 178.

V. Bqrolanose (phonolite) from Gennersbohl, Hegau, southern Germany. G. F. 

Fohr, Inatig. Diss., Wurzburg, p. 24, 1883. (Including Cu 0.15 and trace

CrA)

VI. Molecular proportions of No. I.
140 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Mineral composition or mode. The actual quantitative mineral 

composition or mode of this rock can not be definitely reckoned from 

the analysis, since the presence of the augite and biotite introduces too 

many unknown factors. It can riot be accurately determined by the 

microscope because the condition of the feldspathic groundmass does 

not permit of accurate separation of its component parts. If, however, the groundmass is considered merely as so much feldspar, and 

no attempt is made to separate the individual microlites or to determine the amount of material in their interspaces, a fairly accurate 

computation of the other minerals as contrasted with the feldspars 

may be made by repeated measurements. The results are as follows:

Actual mineral composition of borolanose.

Feldspar .......

Biotite ---.....-

Diopside .......

Magnetite.. --.-

Apatite. --------

Total .---

Measured 

units.

............ 1,841

............ 254

--.---....-. 302

. --.-*._.---- 124

............ 68

.'........... 2,589

Volume, 

per cent.

71.1

9.9

11.7

. 4.7

2.6

100.0

Specific 

gravity.

X 2.6 -

X 3.0 =

X 3.3 =

X 5.2 =

X 3.2 -

184.86

29.90

38.61

24 44

8.32

286. 13

Weight, 

per cent.

= ' 64.6

= 10.4

= 13.5

= 8.5

= 3.0

100.0

In all 150 measurements were made, giving an average of 17 units 

on the scale for the grains, feldspars not subdivided into individual 

microlites. The last column gives the approximate percentage coinposition by weight.

Classification in prevailing systems. In the prevailing systems of 

classification this rock has no very definite position. The alkalic

feldspars and ferromagnesian minerals place it in a general way in

the syenite family, of which it would be a very basic member. Its 

chemical composition allies it with the monzonites, from which it is 

excluded by the lack of plagioclase feldspar. In spite of the high 

lime, there is molecularly enough ferrous* iron and magnesia to convert the lime into diopside and prevent the formation of plagioclase. 

On the other hand, the presence of a certain amount of the feldspathoid minerals, but not enough to carry it into the nephelitesyenite family, in conjunction with the large amount of ferromagnesian minerals, produces tendencies toward the essexites. On the 

whole, considering both the minerals and the chemical composition, 

the name syenite-porphyry would perhaps best suit it, though it is a 

divergent type of this family.
PIUSSON.] PETROGEAPHY OF DIKES AND SHEETS. 141

Classification in the new system. In the new classification the rock 

has a definite position, determined by its standard mineral composition calculated from its analysis, as may be seen by the annexed table:

Calculation of the norm of borolanose.

SiOa ....'.

AIA ....

FeA ....

FeO.....'.

MgO.-...

CaO .....

Na,0.---

TT Ci

TiO2 .....

P O

C12 . ......

S03 ......

Rest .....

Total ..

Analy-

. sis.

51.94

15.78

4.07

' 3.17

3.48

6.04

3.44

7.69

.39.

.59

.08

.29

2.87

99.83

Molecular ratio.

0.866

.155

.026

.044

.087

.107

.055

.082

.005

.004

.002

.004

-------

Or.

492

82

82

82

Ab.

108

18

18

18

An.

46

28

23

23

Ne.

46

23

23

23

So.

6

3

4

1

1

No.

12

6

10

4

2

Di.

142

9

62

71

71

01.

15

4

25

15

Mt.

26

26

26

n.

5

5

Ap.

18

4

1

4

Or... 

Ab ......

An ......

Ne ......

So. ......

No ......

Di.-... ..

01.. 

Mt ......

11........

Ap ......

Best.....

45.59

9.43

6.39

C.63

.97

2.27

15.63

2.16

6.03

.76

1.34,

2.87

71.18

25.92

Class, F^ 25193=2-3=II, dosalane.

L 977 Order, p,-=j:-j-£j=.157=lendofelio=6, norgare.

NaoO'+KoO' 137 En n ,. ., Rang, CaO' =-tjg-=5.9=cloinalkalic=2, essexase.

TT r\/ go

Subrang, j=j^-Q-,=g5=1.5=sodipotassic=3, borolanose.

Total.. 99.97

The calculation shows that the rock is very near the upper limit in 

its rang; it is thus close to the peralkalic rang laurdalase, in which 

Na20+K2O: CaO as 7:1 or greater than that. It is, therefore, very near 

judithose, the sodipotassic subrang of laurdalase, and Washington, 

in his tables of calculated analyses (p. 295), places it in that subrang, 

the omission of the small amount of sodalite giving a larger amount 

of nephelite, and this in turn less anorthite, thus causing it to just 

fall over the line in judithose.

On account of the condition of the groundmass, which prevents the 

determination of the actual amount of lenad (feldspathoid) minerals 

present, a good comparison can not be made between the norm and
142 IGNEOUS. EOCKS OF HIGHWOOD MOUNTAINS. [BUM,. 237.

the mode of this rock. As shown elsewhere in this paper, in the 

Highwood rocks olivine and lenad molecules have frequently united 

to form biotite; the presence of about 10 per cent would cause the 

rock to be a modal variety of a typical borolanose.

From all that has been said it is clear that in the new system the 

rock should be termed biotitic trachiphyro-borolanose, which is a very 

concise expression for a rock in which the feldspathic minerals are 

present in greater amount than the ferrornagnesian ones; in which 

the alkali feldspars dominate over the plagioclase ones, and both of 

these over feldspathoids and a small amount of biotite which accompanies normative diopside; in which potash and soda are approximately equal molecularly, and in which the texture is porphyritic, 

with a trachytic groundmass.

PHYRO-BIOTITE-CASCADOSE (MINETTE OF HIGHW.OOD TYPE).

Occurrence. In the Highwood Mountains are many dike rocks 

which have a dark basaltic character, with numerous prominent biotite phenocrysts. They may well be characterized by the field name 

"mica traps" or "mica-basalts." The Highwood rocks do not, as a 

rule, jdeld perfectly fresh material in their outcrops. This characteristic is in no type so marked as in the one about to be described. 

Usually these dikes are greatly weathered and broken down, and 

often their position is marked merely by heaps of gravelly debris in 

which are thickly scattered the chloritized plates of biotite forming 

the large phenocrysts, enabling one to recognize this particular type 

and distinguish it from the other basaltic dike rocks.

There are so many of these dikes in this area that it would be useless to attempt to give a list and description of all of them. They 

were found in every stage of preservation, from those just mentioned 

to a few which afford really good material and are well suited for

study and description.

One of these (736) cuts basaltic flows-and breccias on the crest of 

the divide between Lava and Arrow peaks and near the slope of the 

latter. It is about 6 feet wide and extends in the direction of Middle 

Peak. Another occurs at the eastern edge of the stock of granular 

rock on the crest of the Middle Peak-South Peak ridge, and is similar 

to the former in character. A third (678) is found as a sheet intruded 

into Cretaceous sandstones at the head of a little fork of Williams 

Creek in the open country south of the Highwood Mountains. The 

sheet is 10 to 12 feet in thickness and rests on the sandstone. 

The drainage has cut through both, forming a small, narrow gulch. 

The upper part of the sheet has weathered and is crumbly, the lower 

portion is dense, very tough, and well preserved. It has not altered 

the beds into which it is intruded. It is noticeable that both in the 

Middle Peak divide dike (744) and in this occurrence the large, flat 

micas are oriented in a flow structure, in the dike perpendicularly and
PETROGU6APHX OF DIKES AND SHEETS. 143

parallel to its vertical walls and in the sheet horizontally and parallel 

to the bedding. In the plane of these micas the rock splits rather 

readily. The weathered surface of these rocks shows a brown crust. 

They have a ver.y massive parting structure, breaking into large, 

heavy blocks, with a poorly developed spheroidal or "pillow" structure. Sometimes on the surface of the contraction planes zeolites are 

developed.

Megascopic characters. On a freshly broken surface the rock is a 

very dark stone-gray, almost black, with a tendency to conchoidal 

splintering; the grain is very fine, but not absolutely aphanitic. In 

the Williams Creek sheet the grain is somewhat coarser and the color 

somewhat lighter. Thickly scattered in this groundmass are numerous phenocrysts of biotite, whose average cross section measures about 

5 mm., but at times reaches 10 mm. They are black and glittering, 

with bronzy reflections. The prisms and pinacoid (010) are only 

moderately well developed as a rule, sometimes the clinopinacoid 

(010) is much longer in its development than the prisms (010). . The 

crystals are rather thin tabular. These large biotites and the basaltic 

character of the rock produce a strong similarity in habit'to the 

"alnoites."

In addition to these the characteristic phenocrysts of the rock 

there are greenish grains of olivine and. occasional but much rarer 

phenocrysts of a black, well-crystallized augite in moderately, stout 

prisms, which in some cases attain a length of 5 mm. These last two 

are somewhat variable in the different occurrences.

Microscopic characters. Under the microscope the minerals seen

are apatite, iron ore, diopside, biotite, olivine, feldspars, and feld-

-spathic groundmass products. Apatite is moderately abundant in a

few large irregular grains and many small prisms. Iron ore is also

abundant in small grains dotting the groundmass.

The pyroxene is a colorless to pale-green diopside without any 

noticeable pleochroism, of a wide extinction angle, and generally 

rather long columnar in development. It is well crystallized, with 

sharp angles and clean faces. It does not contain many inclusions, 

but some of the very largest crystals are somewhat spongy with inclusions of a brown glass. The small prisms of the groundmass are similar to the larger phenocrysts; various transitions in size between the 

two occur.

The large biotite phenocrysts are a pale yellowish brown; their 

pleochroism is not intense. They do not show very good crystal form 

and are apt to be embayed and ragged on the edges. They also have 

a narrow mantle around "the outer edge, of a much darker color and 

more intense pleochroism. In these characters they recall the micas 

of many "minettes," as, for example, those figured by the author"

( /Petrography of the Little Belt Mountains: Twentieth Ann. Bept. U. S. Geol. Survey, pt. 3, 

Jf 00, pi. 76, p. 528.
144 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

in such rocks from upper Sheep Creek in the Little Belt Mountains 

of Montana. Small biotites in thin tablets with sharp outlines are 

abundant in the groundmass; they have the deep-brown color and 

intense pleochroism of the border zones of the large phenocrysts.

From what has been said it seems clear that the large micas are of 

intratellnric origin; that they came up in the magmas into the dike 

fissures and were oriented parallel to its walls; that during this time 

they were in part resorbed, but grew again during the final stage of 

crystallization, adding material of the same character as the smaller 

crystals forming in the groundmass.

The olivine is somewhat variable; it is most abundant in the Middle 

Peak divide dike, but is not common; it is less abundant in the 

Arrow Peak dike, and is almost wholly wanting in the Williams Creek 

sheet. The few scattered crystals are not large nor particularly well 

crystallized.

All of these ferromagnesian minerals, both femic and alferric, are 

fresh and unaltered, except that cracks in the olivine show small lines 

of serpentinization, and the largest augites in the Williams Creek 

sheet show some calcite.

The groundmass of these rocks is, relatively to the total amount of 

phenocrysts, in minor amount, and is somewhat variable in character. It consists chiefly of orthoclase, or, perhaps better, alkali feldspars, in lath-like forms. In the Williams Creek occurrence this is 

mixed, however, with a subordinate amount of andesiue, and in the 

other two occurrences there is more or less cloudy, feebly polarizing 

material of indefinite character. As this base is somewhat altered and 

brown from kaolinization, the exact character of these substances 

can not be made out. In the Arrow Peak dike it was noticed, 

however, that some of these areas were outlined by iron-ore grains, 

bits of pyroxene, mica, etc., into circular forms, and that these were 

more or less isotropic; and in one of these, in spite of the somewhat 

kaolinized condition, a cross twinning, exactly like that of leucite, 

could be made out, especially by the aid of the sensitive tint. In other 

places plumose bundles of fibers resembling spherulites, and no doubt 

of zeolitic nature, are seen.

In short, the base, while chiefly alkali feldspar, is accompanied 

sometimes by small amounts of andesine, sometimes of leucite, sometimes of nephelite, or mixtures of these, somewhat zeolitized and 

kaolinized.

Chemical composition. For the purpose of studying the chemical 

character of these rocks the Arrow Peak dike was selected as the 

freshest and best. In the next table is given the analysis of this 

rock, by Dr. H. W. Foote, as well as anatyses of related rocks. From 

these it will be seen that these dikes are of the same general magma
riRSSON.] PETROGRAPHY OF DIKES AND SHEETS. 

Analyses of biotite-cascadose and related rocks.

145

SKV... ...... ...............

A1.A .....................

FeA -----------.:.----

FeO .......................

MgO. ......................

CaO .......................

Na,O ......................

K,0.--- ..................

H20.- .....................

TiOa . ......................

MnO.... ...-.....--...--...

BaO ...............

Si-O... .....................

PA ----------------

SO,. .......................

Cl .........................

O-C1. .....................

Total ...............I.

I.

46.04

12.23

3.86

4.60

10.38

8.97

2.42

5.77

2.87

.64

Trace.

.48

.25

1.14

.11

09. 76

.03

99.73

II.

47.88

12.10

3.53

4.80

8.64

9.35

2.94

5.61

2 22

.77

.15

.46

.13

1.11

None.

Trace.

99.69

X.30

" 99. 99

III.

48. 95

12.98

3.63

4.68

11.73

7.66

2.31

3.96

3.16

.49

.13

.67

100. 35

IV.

48.36

12.42

5.25

2.48

9.36

8.65

1.46

3.97

5.54

1.18

.13

.29

.84

99.93

V.

4.6. 73

10.05

3.53

8.20

Q 97

13.22

1.81

3.76

1 24

.78

OQ

?

?

1.51

.18

100.56

.04

100..52

VI.

0/767

.120

.024

.064

OiXQ

.161

.039

.062

.008

.008

.004

I. Phyro-biotite-cascadose (minette or mica-basalt) from dike near Arrow Peak. 

H. W. Foote, analyst.

II. Montanose (shonkinite) from Shonkin Sag laccolith. W. F. Hillebrand, 

analyst. (X=C0.2) 0.12; ZrO2 , 0.03; F, 0.05; S, 0.025; Cr2O3 , 0.035; VA, 

0.04=0.80.)

III. Lamarose (absarokite) from dike at head of Lamar River, Yellowstone Park. 

L. G. Eakins, analyst. (ladings, Jour. Geol., vol. 3, pp. 938, 943, 947.)

IV. Absarakose (absarokite) from dike south of Clark Fork, Absaroka Range, 

Wyoming. L. G. Eakins, analyst. LOG. cit.

V. Shonkinose (shonkimte) from Square Butte. L. V. Pirsson, analyst. Bull. 

Geol. Soc. America, vol. 6, 1895, p. 414.

VI. Molecular proportions of I.

Bull. 237 04 -10
146 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

as the shonkinose (shonkinite) and montanose forming the granular 

massives of the region, and that their texture and, to a considerable 

extent, their actual mineral composition have been modified by their 

physical environment. They are not, therefore, of magmas very different from the stocks which they accompany, but in reality are 

lamprophyres or differentiated femic dikes, as may be seen by comparison with the salic ones (aplitic or leucocratic forms), thus agreeing with the differentiation seen in the laccoliths and stocks or the 

massives themselves.

Under the name "absarokite," Iddings," in 1895, described a number of rocks which occur in d ikes and flows and which are of basaltic 

habit, with abundant phenocrysts of olivine and augite in a groundmass consisting of alkali-alumina minerals, chiefly alkali feldspars, 

with small amounts of plagioclase or leucite or altered feldspathoids, 

and a second generation of the phenocrystic minerals, sometimes 

accompanied by biotite. By reference to the foregoing table it will 

be seen that there is a close general resemblance between absarokites 

and the cascadose or minette under description, but they differ in 

minerals and in habit, as the cascadose contains a large amount of 

biotite in large and abundant phenocrysts. In the new system of 

classification absarokite appears in a subra.ng of its own under the 

order gallare.

Mineral composition or mode. The quantitative mineral composition differs somewhat from the calculated norm, and the rock can not 

be said to have a normative mode. This is due to the fact that the 

greater part of the olivine, some of the magnetite, and some of the 

feldspathic molecules have united to form biotite. The microscopic 

analysis of the rock by Rosiwal's method gives the following results:

Mode of phyro-biotite-cascadose.

Feldspar ...........

Biotite -.--..-.._._

Olivine...... ...

Diopside ..........

Magnetite ...... .

Apatite _..........

Total .......

Measured 

units.

......... 645

226

......... 98

.......... 379

......... 54

.._.;.... 18

......... 1,420

Volume, 

per cent.

45.4

15.8

6.9

26.7

4.0

1 2

100.0

X

v

v

v

V

v

Specific 

gravity.

2.6 =

3.0 -

3.3 =

3.3 -

5.2 =

3.2 -

118.0

47.4

22.8

88.1

20.8

3.8

300.9

Weight, 

per cent.

= 39.2

- 15.8

= 7.6

- 29. 3

= 6.9

- 1.2

100.0

In the feldspar item there is included not only the alkalic feldspars which make up the bulk of the feldspathic base, but the lenad 

(feldspathoid) minerals as well. Owing to the conditions of their

Jour. Geol., vol. 3,1895, p. 935,
PIHSSON.] PETROGRAPHY OF DIKES AND SHEETS. 147

admixture and the clouding by kaolinization it is impossible to measure them separately with, any degree of certainty, though taken 

together they can be measured as a whole with ease and accuracy. 

The results given in the above table are of interest; they appear at 

first sight to differ considerably from the norm given later, but when 

examined it will be found that the only important divergence is in 

the presence of about 16 per cent of biotite wjrich has been formed at 

the expense of some of the olivine and feldspathic minerals in the 

norm. If this is true, the weight of the feldspathic minerals plus 

the olivine in the norm should equal the feldspathic minerals plus 

the biotite plus the olivine of the mode. This results as follows:

F. 0.1 F. Bi 01

48.76 + 13.00=61.76=39.2+15.8+7.6 = 62.6

. The result is as close as could be expected. Bearing this fact in 

mind, it will be seen that on the whole the agreement between the 

calculation from the chemical analysis and the result of microscopic 

analysis is reasonably close. About 6 per cent of olivine and about 

10 per cent of feldspathic molecules have been united to form about" 

16 per cent of biotite. In composition biotites vary considerably, as 

may be seen by reference to tables of analyses of them; the one from 

the " rnonclliquite" of Horberig, Oberbergen has about equal parts of 

olivine and lenad molecules. Splitting the biotite up in the proportions mentioned above, we have the adjoined comparison between the 

norm obtained from the analysis and from the corrected mode:

Comparison of norm ivitli mode of cascaclose.

T? ' 

Ol ............................................................

Di ...................'..........._.............................

M. ...........................................................

An

Norm.

48.76

13.00

25.04

6.79

2.69

Mode.

49.2

13.4

29.3

6.9

1.2

Had the feldspathic base been fresh and clear, no doubt equally 

good results could have been obtained from it.

Classification in prevailing systems. In the prevailing systems 

these rocks should probably be classed as minettes; they are lamprophyric dikes consisting chiefly of alkali feldspars, biotite, and 

augite, with abundant phenocrysts of biotite and none of the feldspars. 

The minettes are not a well-defined group chemically, as may be seen 

by reference to the table of anatyses given by Zirkel," but these types

«Lelarta\icli der Petrographie, vol. 2,1894, p. 349,
148 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

differ in a considerable degree from the common ones, and the lenad 

(feldspathoid) minerals found in the base accentuate this point. 

Their most marked feature is the large biotite phenociysts, which give 

them a strong alnoid habit. They might be called the High wood type 

of minettes.

Classification in the new system. In the new classification these 

rocks belong in the sodipotassic subrang of kamerunase, as may be 

seen by the calculation of the analysis into its standard mineral composition, shown in the annexed table. The norm derived from this 

and the classification are given below.

Calculation of the norm of biotite-cascadose. -

SiOa ........

AIA ..:....

FeA -------

FeO ........

MffO

CaO ........

Na2O .......

K.O.--- ....

TiO

P o

C12 . .........

Best........

Total .

Analysis.

46.04

12.23

3.86

4.60

10.38

8.97

2.42

5.77

.64

1.14

.11'

3.60

99.76

Molecular 

ratio.

0.767

.120

.024

.064

.259

.161

.039

.062

.008

,008

.002

Or.

258

43

43

43

Lc.

76

19

19

19

Ne.

69

31

31

31

So.

19

6

 8

9

9

An.

49

91

91

91

Di.

228

13

101

114

114

01.

88

19

158

88

Mt.

94

94

94

11.

8

8

8

Ap.

?6

8

8

Or. ......

An ...... 

Lc...^...

Ne.......

So 

Di.......

Ol. 

Mt.......

11........

Ap ......

Rest.....

Total..

23.911

5.84 

8. 28 148.76

8.80

l.OsJ

25.041

13.00

5. 57 147. 42

1 22

2.69J 

3.60

99.88

Class Sal " - 48- TO 10 III salfeinane

vyActoOj "PQYTI d7 A9 ~~ J-AA, isci»ii-c;iAi«iiJ<3.

 NaoO+K2O 101 ., .,.. .

K«O 62 Subrang, -^^0=^=1.6 sodipotassic=3, cascadose.

In class it stands at the center point; in order it is not far from 

the line, and is therefore near portugare; in rang it is near the top, 

not very far from the peralkalic line, and thus close to subrang 

3 in malignase. It differs from montanose in containing smaller 

amounts of silica and lime, but the main cause of difference in
nnssoN.] PETKOGBAPHY OF DIKES AND SHEETS. 149

classification lies in the difference in the relative amount of alkalies and alumina available in felic (feldspathic) and lenic (feldspathoid) molecules, which produces nearly twice as much anorthite in 

this rock, sufficient to carry it into, a domalkalic rang.

Texture and name. From what has b.een said it is clear that these 

rocks are holocrystalline granular, with a finely granular to aphanitic 

base. In fabric they are porphyritic, and since the dominant phenocryst is biotite they are alferphyric.

In the ne.w classification this subrang under the salfemanes was not 

named, and in Washington's" tables of analyses it was represented 

by this rock alone. It is therefore proposed to call the subrang cascadose, from Cascade County, Mont., where the type described occurs.

Since the mode is abnormative, and biotite, which is the critical 

mineral, is alferric and appears largely in phenocrysts, the rock may 

be designated-an alf erphyro biotite-cascadose, or more concisely phyrobiotite-cascadose. This is a very concise and definite expression to 

indicate a rock which is made up of about equal parts of salic and 

fenaic minerals, in which the feldspathic dominate over the lenad 

(feldspathoid) molecules and both together over the anorthite molecule; in which potash and soda are present in approximately the 

same quantity; in which both femic and salic mineral molecules have 

united to form a good deal of biotite, and which is megascopicaUy 

porphyritic with phenocrysts of biotite. The name surely has decided 

advantages over the somewhat vague term of minette of the older 

systems.

MONCHIQTJOSE (ANALCITE-BASALT).

Introductory. There exists in the Highwoods a vast number of 

dikes; indeed, no geologic feature is more characteristic of the group, 

as has been already explained and shown on the geologic map. Of 

these dikes a very great number belong to the type about to be 

described; their field occurrence and characters have been described 

on pages 31-36.

The rocks of this type were thoroughly investigated by Lindgren, & 

who showed that they contain analcite as a prominent component 

and under such conditions and with such associations that he Avas 

compelled to regard it as a primary mineral. The author, in the subsequent study of these rocks and of closely related types from other 

localities in Montana, agreed with Lindgren and published a paper, 6' 

in which Lindgren's ideas wrere brought out and supported by additional evidence, but it should be remembered that Lindgren first recognized analcite as a possible primary component of igneous rocks, gave 

the proofs which demonstrate it, and offered the explanation of its 

mode of formation under such circumstances.

"Chemical analyses of igneous rocks: Prof. Paper U. S. Geol. Survey No. 14,1903, p. 347. 

& Eruptive rocks from Montana; Proc. California Acad. Sci., ser. 2, vol. 3,1890, p, 51. 

o The monchiquites or analcite group of igneous rocks; Jour. Geol., vol. 4,1896, p. 679.
150 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Since Lindgren's. original paper, which, was brief and wanting in 

some details, is inaccessible to many petrographers, and the study of 

more abundant and widely distributed material from the High woods 

has brought out new facts, a full description of this type, based partly 

on Lindgren's studies and partly on those of the author, is given, 

together with an analysis and a discussion of its position in the new 

system of classification.

Megascopic characters. On a freshly fractured surface these rocks 

are generally a dark greenish black, varying into olive or grayish, 

tones. They are at times thickly dotted with phenocrysts of a stout, 

well-developed black or greenish-black augite which attains a length, 

of 10 mm. They are also abundantly spotted with round white phenocrysts attaining a diameter of 1 to 2 mm. Rarely small flecks of biotite are visible and sometimes greenish grains of olivine. Sometimes 

the augite is very conspicuous as a phenocryst and the white crystals 

are rare; sometimes the reverse is true, the latter are extremely 

abundant, and the rock assumes a dark-gray color. In the majority 

of cases both are abundant. Thus in the variance of these two phenocrysts two extremes connected by all degrees of transition are seen. 

In the geologic description of these rocks in a previous chapter it was 

stated that the dikes of the region could be divided into three types, 

one containing prominent phenocrysts of biotite, to which the field 

name Highwood minette or mica trap could be given and which 

.have been described petrographically as biotite-cascadose; one with 

phenocrysts of augite (augite-porphyry or augitophyre), and one with 

phenocrysts of analcite (analcite-basalt). These last two are the 

extremes of the type under consideration, and although megascopically 

there is considerable difference between them, petrographically they 

are the same, their appearance depending on the relative proportions 

of the augite and white mineral (analcite) which develop as phenocrysts 

or remain in the groundmass.

Microscopic characters. In thin section the following minerals are 

seen: Iron ore, apatite, biotite, olivine, augite, and analcite. The 

iron ore and apatite offer nothing deserving mention. "The olivine 

is usually sharp edged, clear, and fresh, sometimes surrounded by a 

narrow border of biotite. When decomposing, a yellowish brown ferruginous serpentine results. A mineral of the spinel group is 

observed as an inclusion in the olivine. " a

The augite is of the usual Highwood type, of a pale-green, nonpleochroic color, well crystallized. It often becomes darker green toward 

the border, from increase, no doubt, of segirite. The extinction angle 

is very wide, up to 45° or more, the cleavage good. Forms observed 

commonly (110), (Oil), (100), and (111). Twinning on (100) rather 

common. There is great variation in size from the largest crystals

"Lindgren, op. cit., p. 53.
PETROGKAPHY OF DIKES AND SHEETS.

-noted to the small inicrolites of the gvonndmass. It frequently contains inclusions of glass and magnetite. 

In regard to the analcite Lindgren states:

It appears embedded in the groundmass in hexagonal, seldom octagonal most 

frequently simply rounded sections. In size they do not exceed one millimeter, 

and are frequently much smaller. Most of the crystals are perfectly isotroftic, 

but not quite clear, being clouded somewhat by minute interpositions, which, 

under large magnifying power, prove to be largely gas, in part also glass inclusions. The former have often a very irregular form. Irregular spots showing 

"a faint double refraction are sometimes noted, more in some sections than in 

others. Under favorable circumstances an imperfect cleavage in two directions 

crossing each other perpendicularly may also be noticed. Minute fragments from 

an exceptionally large crystal melt rather easily and quietly before the blowpipe 

to a white enamel. In one thin section a large crystal showing irregular octagonal 

form with very distinct cleavage was selected for experiment. It was uncommonly clear and perfectly isotropic. Hydrochloric acid dissolves ib easily iipon 

very slight heating under abundant formation of chloride of sodium. Ignition 

does not make it opaque and does not produce double refraction. No microscope 

reaction for Cl or S03 could be obtained.

The gronndmass of these rocks, in which the above minerals are 

embedded, consists of minute dark prisms of angite, grains of iron 

ore, and small analcite crystals, which form a colorless background 

upon which everything else is displayed. Lindgren remarks with 

truth that glass is probably not present. Under the conditions in 

whicli rocks of such chemical composition have formed it does not 

seem that glass could be present, any more than in a typical granite.

Discussion of analcite. That the isotropic mineral described above 

is actually analcite was shown by Lindgren by the separation of it by 

heavy fluids and by the analyses of the product obtained. This was 

performed on two samples, with the results given in I and II of the 

appended table.

Analyses of analcite.

SiO. ......................................

ALA ........................................

Fe,03 -------------------------------------

CaO ................ ........................

MgO .........................................

Na2O. ..:................... ................

K.O. ...........................................

H2O. ---------------------------------------

Cl .......... ......-.....................'......

SO............................................

Total ............ ^.....................

I.

54.90

23.30

Trace.

1.90

.70

10. 40

1.60

7.50

None.

None.

100. 30

II.

49.87

22.55

1.51

2.62

1.28

10.92

2.66

11.05

Trace.

None.

102.46

III.

52.38

22.92

.75

2.26

.99

10.66

2.13

7.50

99.59

IV.

0.873

.225

.005

.041

.025

.173

.022

.417
152 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Since these analyses were made on very small quantities the ordinary analytical errors become appreciable, and therefore the average 

of the two would undoubtedly be more correct than either alone, as 

the errors would tend to balance one another. The determination of 

water in n was made on only 0.1 gram and is evidently too high, and 

may be excluded. The average is shoAvn in ill and its molecular 

ratios in iv. The lime, iron, and magnesia are due to admixed microlites. of pyroxene, and deducting sufficient silica to turn them into a 

silicate of the general formula RO.Si02 the remaining ratios have the 

following relation:

SiOj--'--------..--..'.-.--..-----------.. 0.802 = 4.00 = 4

A1A-------------------,- - ---- --------- -225 = 1.12 = 1

K2O+Na,O............I.---..-...-......-.. .195. = .97 = 1

H2O..._..................................... .417 = 2.08 = 2

This gives the analcite formula Na2O, A12 03 , 4Si02 , 2H2O, some of 

the soda being replaced by a little potash, as often happens.

It is thus shown that the products obtained by Lindgren and analyzed by Melville were undoubtedly analcite; but the author has been 

interested to discover, if possible,' whether leucite might also be 

present. As mentioned by Lindgren, it is clear that the material is 

not sodalite or nosean, and its form precludes an alteration from 

nephelite. On the other hand, it has been shown on a previous page, 

under the heading "Leucite-shon'dnose," that in the fresh rock of the 

East Peak stock both leucite and analcite occur simultaneously, and 

reasons are given for considering the latter a primary mineral. 

Under the microscope leucite might be present and not distinguishable from analcite.

For this purpose a specimen from a dike on Arrow Peak was 

selected which corresponds very closely with the foregoing description 

of the type. The isotropic phenocrysts are of a very pale-brownish 

tone, and some of them have a few tiny inclusions of iron ore arranged 

in one or two concentric circles, suggesting the arrangement seen in 

leucite. Under crossed nieols occasional spots of faint double refraction, were seen. These suggestions of leucite caused the selection of 

the sample. All the other minerals are perfectly fresh and normal.

The rock was crushed, sifted, the dust washed out by suspension 

in distilled water, and after a large amount of magnetite had been 

extracted with the magnet the powder was placed in the potassium 

mercuric iodide solution. The specific gravity of the solution was 

then regularly lowered and determined, and the resulting precipitates 

collected and examined. At first, of course, all the pure augite and 

 remaining magnetite fell, and after this the same minerals containing
PIKSSON.] PETROGRAPHY OF DIKES AND SHEETS. ' 153

some white material were precipitated. The succeeding precipitates 

were as follows:

No. 1. Specific gravity, 2.56; very little, impure.

No. 2. Specific gravity, 2.52; very little, impure.

No. 3. Specific gravity, 2.45; very little, impure.

No. 4. Specific gravity, 2.40; some, lighter in color.

No. 5. Specific gravity, 2.36: more, purer.

No. 6. Specific gravity, 2.32; large amount, quite pure.

No. 7. Specific gravity, 2.28; nearly all the remainder, very pure.

The first four portions were very small in amount and were contaminated with particles of the dark femic minerals, chiefly augite. 

These became less, but not until 2.36 was reached did they practically 

disappear. It was thought that if at 2.45 anything came down in 

noticeable amount it could be analyzed and the question whether it 

was leucite be determined. From 2.56 to 2.36, however, the portions 

obtained were entirely too small and impure for analysis; they evidently consisted of compound grains of substances above 2.56 and 

below 2.36, which were mingled in varying proportions and gradually 

fell as the specific gravity of the liquid was lowered. Below 2.32 a 

pure product was left which contained by far the greater part of the 

white salic component of the rock. The analyses in Lindgren's work 

render it unnecessary to make a new one of this material; it can be 

nothing else than analcite, and as qualitative tests showed water, 

soda, and alumina as its main constituents, this point may be regarded 

as settled.

As the result of this examination, then, it can be said with confidence that in the sample selected analcite is certainly the dominant 

mineral, and leucite can be present in only very minute amounts, i f 

at all; therefore, Lindgren's conclusion that in dikes in the Highwoods there is a rock type composed of augite, iron ore, olivine, and 

analcite, with these minerals all fresh and all crystallized in their 

own characteristic forms, is confirmed.

In the author's opinion it would not be correct to conclude that 

necessarily all the rocks which show round white phenocrysts, and. 

which are so abundant in the Highwoods, are pure analcite rocks. 

 The analysis of a specimen from High wood Gap shows a considerable 

amount of potash, and no mineral which might contain potash except 

leucite or its alteration products. The same is true for the basaltic 

surface lavas described later. Furthermore, the sheets about Square 

Butte and the Shonkin Sag laccolith, which megascopically and 

microscopically are similar to the rocks under discussion, are very 

evidently intruded portions of the shonkinite magmas of those occurrences whose textural development has been determined by the physical conditions attending their crystallization and solidification.
154 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 1-3?.

Their white phenocryste should be regarded in large part as pseudoleucites. Regarding this rock, Lindgren says: °

In some specimens of the rock in question the larger part of the colorless mineral is faintly doubly refracting, showing bluish gray colors between crossed 

nicols. The crystals are then not so well defined and often take the form of 

rounded spots separated. by the groundmass and small porphyritic augites and 

olivines. These rounded spots between crossed nicols divide into irregular, sometimes also regular, triangular fields. I regarded this (see Vol. XV< Tenth Census 

U. S.) as double refracting analcite. When isolated it has the specific gravity of 

analcite and, according to an analysis of impure material, a similar composition, 

although the percentage of silica is too low. No chlorine or sulphur. Specific 

gravity, 2.24.

It should be noticed that the rocks in which this variety occurs are perfectly 

fresh, even more so than those containing the isotropic mineral; the olivine and 

augite show no trace of decomposition.

In the analcite-basalts, as described here, there is no evidence of decomposition, 

except that the olivine is occasionally converted into yellowish brown serpentine. 

In other specimens, however, it is seen that the analcite offers but slight resistance 

to decomposition. Needles of a zeolite with vivid colors of interference, probably 

stilbite, penetrate the analcite in all directions, and soon every crystal is transformed into an aggregate of zeolites. Large stilbite crystals are found in the 

decomposing rock. The augite is much more resistant and frequently remains 

intact when all the other constituents have been entirely decomposed.

The author is inclined to believe that these doubly refracting areas 

are in part pure analcite, in part pseudoleucites, and the ze®litic 

alteration proceeds in both, the zeolite being perhaps in part stilbite, 

but also in large part natrolite and possibly other zeolites as well.

In summation the author believes that some of these rocks, which 

appear so much alike in hand specimens, are pure primary analcitebasalts; some are primary analcite- and leucite-basalts, and in some 

these minerals have suffered secondary zeolitic and pseudomorphic 

alterations. He sees no reason why primary analcite and leucite 

should noc be present in the same rock. It is difficult to imagine how 

part of the leucite should have changed to analcite and part not, if all 

the minerals are otherwise fresh, and the crystal form and occurrence 

of the analcite negatives the view that it is secondary after any other 

mineral (see p. 111).

The primary nature of analcite as a rock-forming mineral, brought 

out by Lindgren and the author, has been recognized by many petrographers, 5 and its occurrence and mode of formation are so thoroughly

« Proc. California Acad. Sci., 2d ser., vol. 3, p. 56.

b Cross, Analcite-basalt from Colorado: Jour. GeoL, vol. 5,1897, p. C84.

Washington, Am. Jour. Sci., 4th series, vol. 6,1898, p. 182.

Brogger, Ganggefolge des Laurdalits, 1897, p. 103.

Card and Mingaye, Eecords Geol. Surv. New South Wales, 1902, vol. 7, pt. 2, p. 93.

Evans, Quart. Jour. Geol. Soc., vol. 57,1901, p. 38.

Ogilvie, Jour. Geol., vol. 10,1902, p. 500.

Lacroix, Roches alcalines d'Ampasindava: Nouv. Arch, du Museum, 4th series, vol. 1,1902, 

p. 197.

Pirsson, Am. Jour. Sci., 4th series, vol. 13,1902, p. 161.
PIRSSON.] PETROGRAPHY OF BIKES AND SHEETS'. 155

discussed that further reference to this point is unnecessary. The 

writer would only remark, in passing, that this view must not be carried too far, and that while some analcite seems undoubtedly primary, 

a great deal must undoubtedly be secondary. There is presumably a 

stage in rock crystallization where, as the earlier components are separated, there gradually results a mother liquor composed of silica, 

alumina, alkalies, and water, which will crystallize into feldspars, 

feldspars and quartz, or feldspathoids and analcite, with partial or 

complete exclusion of the water vapor, according as proportions and 

conditions vary. The transition is not sharp between this stage and 

that where the attacking of the earlier components by the excluded 

vapors with formation of new minerals (pneuraatolytic stage) probably 

begins. The view as to whether certain compounds are primary or 

secondary must in many cases be largely a subjective one. The general view is that secondary minerals are those formed from previously 

existent ones, but how shall we classify those formed from a glass by 

devitrification proceeding from aqueous vapors? a

Cliemical composition. To determine the chemical composition of 

these rocks and their relation to the other magmas of the region, a fresh 

example of average type from a dike on the east side of the very summit of the divide in Highwood Gap was analyzed by Doctor Foote, 

with the results given in I. Two analyses of similar rocks from the 

neighboring Little Belt Mountains are given for comparison, and it 

will be seen that chemically they are very much like this rock. Some 

other analyses from, other petrographic areas also show a strong 

general resemblance. The analysis of the dark rock of the Shonkin 

Sag laccolith is presented for comparison of the shonkinoid magmas 

of the Highwood area. They are much alike except in the relations 

of the alkalies.

" This inquiry becomes more pertinent since Morozewicz, in a very complete and able investigation of some monchiquoid rocks (Ganggesteine des Bezirk's von Taganrog: Mem. du Com. 

Geol. St.Petersburg, vol. 8,1903,p. 45),.suggests that the analcite which they contain is produced 

by the weathering of a glass which happened to have the composition of analcite, minus the 

water. This interesting paper reached the writer after this bulletin had been handed in for 

publication, and he regrets that he is able only to notice it by this note in the revision.
156 IGNEOUS ROCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Analyses of monchiquose (analcite-basalt) and related rocks.

Si02 _. .............

ALA -------------

Fe2 0, .............

FeO... ............

MgO.. ............

CaO ..............

Na2O.... ..........

K2O.-1-. ...........

H2 0+...._ .......

H2 0-.... .........

CO2 . ....... .......

TiO2 ..............

P2O5 ..............

S03 ...............

Cl -. .............

CrA .............

NiO.. .............

MnO.. ............

BaO ..............

SrO.... ...........

Fl ................

Zr68 ..............

O=F

Total........

I.

47.82

13. 56

 4.73

4.54

7.49

8.91

4.37

3.23

3.37

.67

1.10

Trace.

.04

Trace.

.16

21

100. 20

II.

48.39

11.64

4.09

3.57

12.55

7.64

4.14

3.24

2.56

.28

None.

.73

.45

.08

Trace.

.07

None.

Trace.

.32

.15

99.90

III.

48.35

13.27

4.38

3.23

8.36

9.94

3.35

3.01

2.89

.90

.30

.52

.40

Trace.

. .04

.19

.54

.09

.25

100.01

.11

99.90

IV.

45.59

12.98

4.97

4.70

8.36

11.09

. 4.53

1.04

3.40

.51

1.32

.91

.05

.14

.13

.12

..03

99.87

V.

41.10

14.82

2.35

10.38

9.43

10.56

3. 94

1.28

2.31

.39

.26

3.20

.19

.09

Trace.

Trace.

.14

.06

100. 50

VI.

43.39'

16.67

3.47'

8.80

7.30

8.79

3.30

2.17

2.67

.29

.39

2.20

.41

.19

.02

Trace.

Trace.

.19

.02

Trace.

100.28

VII.

47.83

12. 10

3.53

4.80

-8.64

9.35

2.94

5.61

1.52

.70

.12

.77

1.11

None.

Trace.

.04

Trace.

.15

.46

.13

.05

.03

99.99

VIII.

0.797

.131

.030

.063

.187

.159

.070

.034

.187

.038

.008

I. Monchiquose (analcite-basalt) from dike on east side Highwood Gap. H. W. 

Foote, analyst.

II. Analcite-basalt from Bandbox Mountain, Little Belt Mountains, Montana. 

W. F. Hillebrand, analyst. Twentieth Ann. Rept. U. S. Geol. Surv., pt. 

3, 1900, p. 545.

III. Monchiquose (analcite-basalt) from Big Baldy Mountain, Little Belt Mountains, Montana. W. F. Hillebrand, analyst. Loc. cit., p. 548.

IV. Monchiquose (analcite-basalt) from The Basin, Cripple Creek, Colorado. 

W. F. Hillebrand, analyst. W. Cross, Jour. Geol., vol. 5, 1897, p. 689.

V. Analcite-basalt from Fernhill dike, Sydney, New South Wales. H. P. 

White, analyst. Records Geol. Surv. New South Wales, 1902, vol. 7, 

- p. 93. .

VI. Analcite-basalt from Bondi boss, Sydney, New South Wales. .J. C. H. 

Mingaye, analyst. Loc. cit.

VII. Montanose (shonkinite) from Shonkin Sag laccolith, Highwood Mountains.

W. F. Hillebrand, analyst. 

VIII. Molecular proportions of I.
FIKSSON.J PETROGRAPHY OF DIKES AND SHEETS. 157

Mineral composition or mode. The mineral composition can be 

found by calculation from the analysis. It is as follows:

 Mineral composition or mode of monchiquose.

Leucite..-'..-.....--...-.-..---.......-..-...--.--..----..... 14.8

Analcite..._._-...._....'.___..____._._..____..____.__________ 30.0

Augite ._-.----__---.---------------_..---.---.-----.--_...-.. 35.2

divine....---.--.--.-------------------.----------..------. 8.0

. Iron ore ..._._--.___-___.._..__ ____.._______........_._...___. 8.2

Apatite-......-.---....-...-..--.--.----...................... 2.6

Rest.....-..--..-....-.-..-..-.--..--..--.--..-...-....-.-... 1.2

Total.................................................. 100.0

Classification in prevailing systems. In these, if one follows the 

law of priority, Lindgren's name should stand and this rock would be 

termed analcite-basalt, or if leucite is also present, as in the above, 

analcite-leucite-basalt. In Rosenbusch's system its occurrence in 

lamprophyric dikes must be taken into account, and the rock falls 

into the camptonite-inonchiquite-alnoite series and would be an 

analcite-leucite-inonchiqnite.

Classification in the new system. In this its systematic position is 

seen in the following computation of its norm and position:

Calculation of the norm of monchiquoxe.

SiOa . .......

AI n

Fe203 ------

FeO. .......

MgO. ......

CaO. .......

Na20.------

K,0.. ......

TiO2 _.:...._

p n

C12 . ........

Rest ----...

Analysis.

47.82

13.56

4.73

4.54

7.49

8.91

4.37

3.23

.67

1.10

.04

3.74

Molecular ratio.

0.797

.131

.030

.063

.187

.159

.070

.034

.008

-.008

.001

Or.

204

34

34

34

Ab.

204

34

34

34

Ne.

68

34

34

34

So.

8

1

9,

1

1

An.

56

28

98

28

Di.

210

12

93

105

105

01.

53

13

94

53

Mt.

30

30

30

11.

8

8

8

Ap.

?.«

8

8

Or

Ab ......

An ......

Ne ......

So

Di. ......

01

Mt ......

n. .......

Ap ......

Best.....

18.9

17.8

7.8

9.7

K

 ,

23.4

7.9

7.0

1.2

2.6

3.7

54.7

42.1

Sal 54.7 Class, fi^Jf =J2"i=l-3=ni, salfemane.

Order, p=g:g=0.23=lendofelic=6, portugare.

_ jj Rang, 

Subrang.

104 

= 2g~=3. 7=domalkalic=2, monchiquase.

34 / yQ=0.49=dosodic=4, nionchiquose.

Total.. 100.5
.158 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

The texture is eminently porphyritic; the mode is abnofmative, 

with analcite as the critical mineral; the rock may thus be- termed 

phyro-analcite-monchiquose. If it is thought desirable to indicate that 

both salic and femic phenocrysfcs are present the rock may be termed 

salfemphyro-analcite-monchiquose. This is a very concise expression 

to indicate a rock which consists of about equal proportions of light 

and dark minerals, which has a certain general chemical composition, 

in which the ordinary silico-aluminous alkalic minerals are replaced 

by analcite, and which has a porphyritic texture with phenocrysts of 

analcite and augite.

PETROGRAPHY OF EXTRUSIVE FLOWS, BRECCIAS, AND TUFFS. 

GENERAL PETEO GRAPHIC DESCRIPTION.

The extrusive rocks occurring in the High woods are readily divided 

into two groups the light-colored feldspathic salic rocks, which for 

field purposes have been roughly classed as andesites, and the dark 

aiigitic rocks or basalts.

Feldspathic lavas and tuffs. Those of the first group are dense rocks 

of a very light color, usually a pale brown inclining to gray, sometimes 

a light gray. Close examination shows that they are very apt to contain streaky portions or broken angular fragments whose form is distinguished by a slightly different color or texture from the inclosing 

matrix. These are clearly flow breccias portions of the lava which 

after having solidified have been broken again by movements of the 

Adscous mass and kneaded through it without remelting. Close 

inspection of them also shows in many cases small black shining 

specks, minute crystals of an iron-bearing mineral.

When these rocks have been greatly weathered and perhaps subjected to heat and steam from later effusions of lava, they assume 

darker colors with reddish tones, owing to the oxidation of the iron 

mineral, and in some such cases as, for instance, at the head of Little 

Belt Creek and on upper Briar Creek they are of a strong, almost 

brick, red color.

Where ash beds or fine tuffs of these feldspathic lavas occur, as on 

the Middle Peak divide, they have the same light-gray color, but are 

even and dense and are indurated to very dense compact rock masses, 

often with a platy structure; in these cases they are not easily distinguished from fine-grained, light-colored sandstones.

Basaltic lavas. Basaltic lavas occur in a much greater variety of 

forms than the preceding. They appeai^as dense and compact, either 

with or without embedded porphyritic minerals, as amygdaloids, as 

scoriaceous lava, and as conglomerates or breccias.

The compact basalts are in places black or greenish-black rocks, as 

on the Pinewood Peak saddle; at other places they are dark stone 

gray and sometimes platy, as on Arrow and Lava peaks. As they
PIRSSON.] EXTRUSIVE FLOWS, BRECCIAS, AND TUFFS. 159

become lighter in color the embedded minerals or phenocrysts make 

their appearance. The rocks are sometimes thickly stippled with 

tiny white specks of what is generally an altered leucite, or pseudoleucite. They have a pin-pricked appearance, resembling certain 

well-known leucite-bearing rocks' of the Eifel district in Germany. In 

other examples these psendoleucites are much farther apart and much 

larger, having the size of medium-coarse shot. They are regularly 

sprinkled through- the rock and many dark-green prisms of angite 

accompany them. A gray-colored example of this type collected on 

the upper slopes of the basin west of the Arnoux stock, whose waters 

flow into Shonkin Creek opposite North Peak, so exactly resembles 

specimens of the leucitophyre of Rieden by the Laacher See in the 

Eifel district of Germany that one can scarcely be distinguished from 

the other.

Amygdaloidal basaltic lavas. The amygdaloidal lavas vary in color 

from medium to dark gray; in some cases the amygdules are as large 

as hazelnuts, but generally they are about the size of peas and are 

rather thickly scattered through the rock. They are solid and composed of fibrous zeolites, usually natrolite and stilbite. Sometimes 

they are flattened ovoids arranged in layers and lines showing movement which occurred in the viscous mass before the formation of the 

zeolites and which drew out and extended the steam pores in which 

the zeolites were subsequently deposited. This type of lava is common in the masses composing Lava and Arrow peaks.

In another type the amygdules are about the size of small shot and 

thickly scattered through the rock. They are white and so closely 

resemble the pseudoleucite phenocrysts that except on very minute 

inspection it is difficult to tell them apart. The lens shows that they 

are gray with a faint pink tone and fibrous, while the pseudoleucites 

are greenish and. granular. This type is found abundantly on Pinewood Peak.

In all of these amygdaloidal lavas the rock is more or less altered; 

if olivine occurs it is converted into a dark-red opaque substance; 

often the augites are changed, but they appear to have withstood the 

action of the heated waters and vapors and weathering better than 

any of the other original minerals.

Scoriaceous basaltic lavas. These are closely related to the foregoing, differing only in that the steam cavities are not filled with 

zeolites. The slopes of Arrow and Lava peaks are composed largely 

of a debris of fragments of scoriaceous and at times almost pumiceous 

vesicular lava, varying in color from dark stone gray to deep mahogany red. The only definite recognizable mineral to be seen in this 

material is the augite in black glittering prisms.

While part of it may be the upper surface of flows, a large- portion 

of it is projected matter, varying from ash and lapilli to bombs of 

considerable size.
160 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Basaltic tuffs and breccias. These are dark-colored rocks, sometimes dark gray or dark brown, but commonly of a chocolate color. 

The tuffs or ash beds are fine in texture and compact, showing considerable induration; they have a dull, hackly fracture, on whose 

surfaces are many glimmering points of light reflected from minute 

fragments of crystals. The breccias are made up of the same material, 

but embedded in them are innumerable bits of rock, some angular, 

others somewhat rounded. Among these the most common are pieces 

of baked, reddened, and hardened shale, and of the leucitic lavas previously mentioned. The included chunks are of all sizes up to a foot or 

so in diameter. In some cases, as in material collected on upper 

Highwood Creek about a mile north of the Highwood Gap divide and 

near the large wall dike, the inclosed pieces are rounded sufficiently 

to produce the appearance of a volcanic conglomerate.

The ash beds and -breccias described are found all over the eastern 

and northern parts of the mountains and as isolated or prolonged 

patches among the sedimentary foothills, as on upper Davis and 

Aspen creeks.

For the purpose of detailed investigation of the petrographic character of the rocks composing the effusive masses, types of the feldspathic and basaltic lavas described above have been selected for 

study-and analysis in each case the freshest and best material that 

could be found. The greater part of the ejected Highwood matter is 

unsuited by various processes of alteration for petrographic investigation, but in a considerable number of instances reasonably fresh 

material of both kinds was found. The study of the thin sections 

has shown that, from the standpoint of advanced classification, practically only two types are present, as already roughly determined in 

the field and shown on the geologic map.

The sections show, it is true, a slight variation in the specimens 

collected from different parts of the field, as will be mentioned later, 

but nothing that would warrant further separation. Under prevailing systems of classification the types would be latite (trachyandesite) 

and leucite-analcite-basalt; under the new system they are adamellose 

and shonkinose.

TRACHIPHYRO-HORNBLENDE-ADAMELLOSE (LATITE OR TRACHY

ANDESITE).

Occurrence. The best appearing material of the lighter colored 

feldspathic lavas was found on the north side of the upper reaches of 

North Willow Creek as it issues from the mountains. As may be . seen by reference to the geologic map, the feldspathic lavas are 

prolonged in this direction outward from the mountain mass and 

form bold projecting masses and crags. On these exposed surfaces 

the effusive character of the material is clearly shown by flowage 

lines which dip away from the mountains and have the wavy, twist-
I-IRSSON.] EXTKUSIVE FLOWS, BRECCIAS, AND TUFFS. 161

ing nature so often seen in lavas. At the same time the rock has a 

clearly brecciated character, angular chunks of various sizes being 

brought out in the weathering. The appearance indicates clearly 

that it is a flow breccia, the first cooled and solidified crust having 

been broken up and rolled in angular fragments through the still viscous moving mass.

Megascopic characters. On a broken surface the rock is a palepurplish brown or light-chocolate color spotted with angular fragments of a slightly different tone of the body color, but in some cases 

of a dull gray. The contrast and the brecciated character are not 

marked. The fragments .on the average are of about the size of a 

hazelnut. The rock is dotted with small dark specks of a ferromagnesian mineral which often shows a lustrous cleavage surface.

Microscopic characters. In thin section the microscope discloses 

the following minerals: Iron ore, apatite, hornblende, biotite, labradorite,*7 alkali feldspar, quartz (?), and glass.

Iron ore is not common. It occurs in the usual small grains and is 

more abundant in some brecciated areas than in others. The apatite 

is seen in the common small prismatic crystals and shows no especial 

features.

The hornblende is very abundant, though it occurs in greater quantity in some areas than in others. It occurs in short, stout prisms, 

bounded, when well crystallized, by 110, 010, and 111; long slender 

ones are rare. The cleavage, as usual, is marked. It has a rather 

striking and remarkable color and pleochroisin.

c =deep orange-red, inclining to brownish in some specimens.

ft=orange.

a=pale to lemon-yellow.

The inclination to brownish tones is somewhat variable, not only in 

the different specimens but according to the strength of the illumination. It is to be noted that these colors are deepest in those specimens which, by reason of a ferruginous pigment distributed through 

them, have megascopically strong reddish toues. Since the mineral 

appears uniform and homogeneous, it is difficult to think that the 

color is the result of alteration or staining; on the other hand, the 

hornblende is of the common green variety in specimens collected at 

the little canyon of Highwood Creek above the Cretaceous part of its 

valley, near the Shonkin stock, and at the head of Little Belt Creek 

on the south slopes of Pinewood Peak. At the last-named locality 

the green crystals contain occasional spots of the red type, so that 

from the evidence at hand it does not appear certain whether the 

more common.red kind is an alteration of the green or not. In this 

connection it is of interest to note that where the hornblende is deep 

red the biotite, which is a rare constituent occurring in small phenocrysts like the hornblende, also shows so nearly the same colors in its

Bull. 237 04 11
162 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

pleochroism that it is distinguished from the hornblende with difficulty. In the red variety the angle c on JC is' nearly zero and the 

double refraction is not very strong.

The labradorite is the next most striking mineral. It does not 

occur as a phenocryst, but is confined to the groundmass, where it 

appears as short laths which vary in size, ranging from some that are 

almost as large as small phenocrysts to very minute ones. Carlsbad 

and albite twinning are common and afford excellent sections for 

determining the ratios of An to Ab. A number of measurements 

showed that Ab3An4 is the labradorite present; tbere were one or two 

which gave Ab7An8 . These laths are in some cases bent in the viscous 

flow structure.

The alkali-feldspar appears also in small laths and in fragments distinguished from the labradorite by lack of twinning, lower birefringence, and parallel extinction. The sections are too small for detailed 

observations. *

This mesh of interwoven minerals is surrounded by films of a low 

or nonpolarizing substance. In some cases, especially abound the 

alkali-feldspar sections, high powers show "pepper and salt" mantles, which might have become micrographic or micropoikilitic. areas 

if the scale on which they crystallized had been larger; these areas 

resemble devitrified acid (quarfelic) glasses in appearance. In other 

places these mantles are not seen, and the whole arrangement suggests 

a final remnant of glass in the quickly crystallizing flow which has 

since then partially devitrified in places, giving rise to quartz and 

alkali feldspar, though the scale is too small for direct proof.

To make sure that a feldspathoid was not present instead of glass, 

the rock was treated with nitric acid, but no gelatinous silica could be 

obtained.

Varieties of the type. In the different specimens collected are some 

variations which are worth noting. Those in the hornblende have 

been described above: In the material from the head of Little Belt 

Creek was observed some quartz in scattered angular pieces; the 

labradorite appears more distinctly as a phenocryst, and the groundmass is more largely composed of the "pepper and salt" aggregate of 

minute quartz and alkali feldspar. In some specimens from Briar 

Creek a few crystals of sporadic augite well crystallized and of a brownish tone were noted; augite occurs also in the acidic (perfelic) extrusive material at the debouchment of High wood Creek previously mentioned, and the groundmass of this rock is more felt-like than in the 

one last described, though it is still exceedingly minute. The material 

from the Shonkin stock has rather more feldspar and less hornblende.

Mineral composition or mode. The mineral composition was ascertained by Rosiwal's method. The fine feldspars could not, however, 

be measured separately, and all of the feldspar was therefore lumped 

into areas and the glass or residual material was included. Two 

traverses, a and b, give the following results. It will be noticed
EXTRUSIVE FLOWS, BRECCIAS, AND TUFFS. 163

that biotite was observed only once, and therefore constitutes an 

unimportant constituent. The very fine needles and tiny granules of 

apatite could not well be measured and were disregarded. The 

analysis proves that 0.7 per cent must be present. The chief value 

of the determination is that it shows the relation between the hornblende and the feldspars.

Table showing the calculation of the mode of adamellose.

mm.

, mm.

o, 100 meas.... ......................

1 T, mm< a+b, 10Q meas ......................

a, No. meas. ..-..---.....-.--._----...

b, No. meas- ......-.--..--..-----.....

a+b, No. meas ......-.-......-..--...

b, per cent. ........_.-..-----..--..-..

a-\-b, per cent ..........-..-.-..-..-..

Iron ore.

21

Oft

47

15

17

32

1.7

1,9

1.8

Hornblende.

1^iQ

 228

387

43

54

97

13.1'

16.9

15.1

Feldspar, 

etc.

1,024

1,088

2,112

50

56

106

84.6

81

82.7

Biotite.

6

0

6

1

0

6

0.5

0

0.3

Total.

1,210

1,342

2,552

109

127

236

99.9

99.8

99.9

It is impossible to determine the relative amounts of plagioclase, 

orthoclase, and quartz or glass, and these must be taken collectively.

Chemical composition. This is shown in I of the following table of 

analyses. It is like many andesoid rocks, but the considerable amount 

of potash is to be noted. Since biotite and other potash-bearing silicates are absent, except orthoclase, there must be about 25 per cent 

of that mineral present, or at least capable of forming. Compared 

with similar rocks of other regions, the only one that closely resembles 

it with regard to adamellose is the one from Thibet described by 

Backstrom. The type that seems to be most like it chemically is 

one on the border between monzonose and adamellose, shown in in. 

Under the name "vulsinite" Washington described Italian lavas 

which contained abundant labradorite in addition to the alkali feldspars of trachytes, and which mineralogically resemble the present 

rock. An analysis of one of these is shown in iv. Like the Willow 

Creek rock, the r.ock contains considerable lime and potash. Ransome 

proposed the term "latite" for all effusive rocks "standing chemically about midway between the andesites and trachytes; " a this definition would include the Willow Creek rock. An analysis of a 

typical latite is shown in v, and it has an essential resemblance to 

the other analyses.

If this effusive is compared with any of the granular deep-seated

"Loc. cit. infra., p. 375.
164 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

masses of the district, the one to which it has the nearest resemblance 

is the shoshonose or monzonite of Highwood Peak. The latter is a 

less siliceous rock and contains more lime, iron, and magnesia. The 

material around Pinewood Peak unquestionably belongs to the Highwood Peak center of eruption, and this type thus stands as intermediate between the shoshonose or monzonite and the pulaskose or 

syenite, and represents a less differentiated stage than either of them. 

This would tend to show that it is also old*er, and when one considers 

the relative geologic and topographic positions of the rock masses 

this idea appears to be confirmed. .

Analyses of adamellose or trachyandesite and related rocks.

SiO2 ..............

Al O

Fe203 .............

FeO -........--..

MgO. ............

CaO .............

Na20 ............

K20._ ............

H2 0+.. ..........

H2 0-.. ..........

TiO2 .............

P2O5 -------------

SO...............

Cl ................

MnO-... ---...-..

BaO .............

SrO.. ............

Total ......

I.

59.24

13.84

5.46

1.36

4.79

5.60

3.13

4.22

2.02

.22

.34

.08

.04

Trace.

Trace.

100. 34

n.

61.45

14.36

2.75

4.61

2.73

4.34

3.98

3.75

.87

1.37

100.21

III.

' 57.04

13.66

4.96

1.77

4.43

6.23

3.08

4.95

1.10

1.11

.94

.63

.17

.22

Trace.

NiO .07

100.36

rv.

5*5. 69

19.08

4.07

3.26

3.41

6.87

2.89

4.41

.17

Trace.

99.85

V.

56.78

16.86

3.56

2.93

3.41

6.57

3.19

3.48

1.21

.15

. 1.15

.42

C02 .18

99.89

VI.

51.00

17. 2l

2.41

4.23

6.19

9.15

2.88

4.93

.63

.13

.33

.14

Trace.

Trace.

.34

.14

99.60

VII.

0.987

.134

.034

.019

.120

.100

 .050

.044

112

.003

.002

I. Adamellose (trachyandesite) from North Willow Creek. Highwood Mountains, Montana. E. B. Hnrlburt and B. Barnes, analysts.

II. Adamellose (bronzite-andesite) from Thibet. H. Backstrom, analyst. (Pet. 

Mitt., Erg. Hft. No. 131, p. 2.)

III. Monzonose-adamellose, (mica-basalt), from Santa Maria basin, Ariz. 

W. F. Hillebrand, analyst. Iddings, Bull. Philos. Soc. Wash. v vol. 12, 

1892, p. 212.

IV. Shoshonose (biptite-vulsinite) from Monte Santa Croce, Eocca, Monfina, 

Italy. H. S. Washington, analyst. Jour. Geol., vol. 5,1897, p. 252.

V. Shoshonose (augite-latite) from Clover Meadow, Tuolumne County, Cal. 

Gr. Steiger, analyst. Ransome, Am. Jour. Sci., 4th series, vol. 5, 1898, p. 

363.

VI. Shoshonose (monzonite) from Highwood Peak. E. B. Hurlburt, analyst.

VII. Molecular proportions of I. ."
PIRSSON.] EXTRUSIVE FLOWS, BRECCIAS, AND TUFFS. 165

Texture. This has been partly indicated in the foregoing. Megascopically these rocks are firm, compact, and between the trachytic 

and felsitic in character, and they are porphyritic only on close inspection. They are actually microtrachytic and almost microporphyritic.

Classification in the new system. The position of this type in the 

new classification is found in the following calculation of its norm:

Calculation of the norm of adamellose.

SiO,. .......

ALA ------

FeA --

FeO.. ..:...

MgO. ......

CaO ......

NajO. ------

K2 0. .......

Ti02 ........

PA .......

HAetc....

Total..

59.24

13.84

5.4G

1.36

4.79

5.60

3.13

4.22

.22

.34

2.14

100. 34

0.987

.134

.034

.019

.120

.100

.050

.044

.003

.002

........

Or.

264

44

44

44

Ab.

300

50

50

50

An.

80

40

40

40

Di.

106

53

53

53

Hy.

67

67

67

Mt.

16

16

16

11.

3

3

3

fit.

c

18

18

Ap.

7

?,

2

Qtz.

170

170

Or....... 24.5

Ab ...... 26.2

An ...... 11.1

Qz....... 10.2,

Di....... 11.4

Hy ...... 6.7

Mt ...... 3.C

II........ .5

Ht.......

Ap ......

Rest.....

2.9

.7 

2.1

72.0

25.8

Class, Sal 72 0

2~rn;=2 - 8=11, closalano.

Q 

--

10 2 Order, jj,-=jT£-g=0.17=quardofelic=4, austrare. 

Rang, ' ' Oaf)' ' ==r2. 3=donialkalic=2, dacase,

Subraug, j^Q7=j-Q=0.88=sodipotassic=8, adamellose.

Total.. 99.9

The comparison of the norm with the mode shows that the diopside 

and hypersthene of the norm, and perhaps also some of the iron ore 

and soda (i. e., albite), are replaced in the mode by hornblende. The 

amounts do not exactly correspond, but this is probably due to the 

fact that in diopside a much larger amount of lime is needed than in 

hornblende, and in the mode this lime appears as anorthite. The 

alumina needed for anorthite might be supplied from albite, the soda 

of the latter going into the hornblende with iron. On account of the 

15 per cent of the alferric mineral hornblende, the mode is an abnormative one.

The textural characters have been already given; they are expressed
-166 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

by the term trachiphyro. In the new system the rock should therefore be termed trachiphyro-hornblende-adamellose.

Classification in prevailing systems. Casual study of this rock 

under the microscope alone would determine it as a hornblendeandesite. When the analysis is considered and it is perceived how 

much of the orthoclase molecule must be present, it can not be considered a typical andesite, but must assume an intermediate position 

between the andesites and trachytes, and is therefore a trachyandesite, 

or, if Ransome's proposed term be employed, a hornblende-latite.

PHYRO-SHONKINOSE (ANALCITE-LEUCITE-BASALT).

Occurrence. Perfectly fresh unchanged material of the basaltic 

extrusives has not been found in the area. The difficulty in some 

cases of distinguishing between the outcrops of dikes and the effusives 

which they cut has been previously alluded to. The dikes are not 

far from the original surface, and there was no marked difference in 

physical conditions, consequently in minerals and texture they closely 

resemble the flow rocks, to which no doubt they gave origin. There is 

any amount of vesicular, scoriaceous, and amygdaloida! lava, all more 

or less altered, but for investigation it was desired to have a compact 

and, as far as might be, fresh lava, and at the same time to avoid 

material concerning which there might be some doubt as to its dike 

nature. For this purpose" the basalt flow on the saddle between Highwood and Pinewood peaks was selected. It is by no means ideal 

material, as will be seen, but it is as good as the district affords, and 

serves a satisfactory purpose for the study of this effusive magma.

Megascopic characters. In the hand specimen the rock has fche 

ordinary gray-black compact basaltic appearance. Close inspection 

shows numerous small, dark, augite prisms as phenocrysts. The fracture is rough and hackly. In places small threads and irregular spots 

of a white substance are seen, indicating some secondary material 

deposited in it. It is mostly calcite.

Microscopic characters. In thin section the rock is seen to consist 

of phenocrysts of augite, altered olivine, and- leucite (and analcite) in 

a base consisting of a smaller generation- of leucite, iron ore, and a 

greenish glass. The augite is of the usual greenish High wood type, 

full of glass inclusions; it is fairly well crystallized, unaltered, and 

attains a length of 2 mm. The leucite shows the ordinary outlines of 

this mineral; it is entirely free from the usual inclusions and does 

not show the multiple twinning structure, due probably to its rather 

small size, as the crystals do not exceed 0.4 mm. in diameter. Its 

fractures are filled with calcite and a greenish material of a chloritic 

nature, substances which have wandered in from the groundmass. 

Where this greenish substance is present there is a feeble aggregate
PIBSSON.] EXTRUSIVE FLOWS, BRECCIAS, AND TUFFS. 167

polarization between crossed nieols; elsewhere the mineral is completely isotropic. There are irregular and often elongated areas of a 

colorless, limpid, isotropic substance of low refractive index and distinct cubic cleavage; these are undoubtedly analcite. It is observed 

that some of the polygonal-shaped sections also have a cubic cleavage, 

and these may be leucites altered to analcite, but this seems doubtful. 

The chemical analysis and the microscopic study prove clearly that 

both of these minerals are present, but since neither shows any 

marked individuality, it is impossible to distinguish clearly between 

them.

Olivine is not a common mineral; it is not present as unchanged 

material, since it is completely altered to a black substance and 

recalls certain resorption pseudomorphs. It is readily recognized by 

the characteristic sections and rough cross cleavage; the cracks are 

filled with greenish chloritic substance.

The glassy groundmass is in part idiochromatic; in part colorless 

of itself, but it is completely filled with the greenish chloritic dust, 

which colors it pale green. It is also filled with the dots and rods of 

black iron ore so commonly seen in partially crystallized femic lavas. 

Under low power this combines into an aggregate which appears as 

a greenish-brown base.

Chemical composition. An analysis of this type, by Doctor Foote, 

is given under I in the table on the next page.

The analysis shows the characteristics of the Highwood magmas, as 

may be seen by reference to n, one of the granular rocks of the stocks, 

and in, the same kind of magma in dike form; the latter, however, 

shows a different relation of the alkalies, a fact discussed under the 

description of that type. The type which in some other regions seems 

to bear the closest resemblance to this rock is a leucite-bearing effusive 

from Khoi, Persia, whose analysis has been previously quoted and 

is here repeated. For the sake of comparison, analyses of two other 

leucitic rocks are given, and it will be noted that they have a much 

higher ratio of soda to potash than the one under discussion. This 

is generally true, and in the present case is due to the considerable 

quantity of analcite present.
168 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Analyses of phyro-shonkinose (leucite-basalt) and related rocks.

SiO....... ............

A1A ................

FeA ...............'.

FeO........ .... ------

MgO... ...............

CaO ...........'.......

NajO.. ...............

K2O... ...............

H2 0+-- --------

H2O-. ........... .---

C02 . ......... .........

TiO2 ......._.._......'

PA -----------------

S03 ..................

Cl ...................

CuO. ......... ......

NiO - -.... 

MnO.' .............

BaO _....-.._...._....

SrO ............. ..'....

Cl-O.... ........'....

Total ._-._-_--.'

I.

47.98

13.34

4.09

4.24

7.01

9.32

3.51

5.00

2.10

1.24

.58

1.03

Trace.

.21

Trace.

.50

.14

100.29 

.07

100 22

II.

49.59

14.51

3.51

5.53

6.17

. 9.04

3.52

5.60

1.95

.36

.15

.02

.13

Trace.

.49

.21

100.78 

.03

100. 75

III.

47.82

13.56

4.73

4.54-

7.49

8.91

4.37

3.23

3.37

.67

1.10

Trace.

.04

Trace.

.16

.21

100. 20 

.01

.100.19

IV.

49.65

14.39

4.21

3.48

6.27

10.12

3.21

5.46

2.37

.79

.25

 100. 19

V.

46.51

11.86

. 7.59

4.39

4.73

- 7.41

2.39

8.71

2.45

1.10

.83

.80

Trace.

.04

Trace.

.04

.22

.50

.16

99.73 

.01

99.72

VI.

46.06

10.01

3.17

5.61

14.74

10.55

1.81

5.14

1.44

.73

.21

.05

.03

Trace.

.32

.20

99. 57 

.01

99.56

VH.

0.798

.130

.026

.058

.175

.166

.056

.053

.113

.008

.007

.006

------

I. Shonkinose (leucite-basalt) from saddle between Highwood and Pinewood 

peaks. H. W. Foote, analyst.

II. Leucite-shonkinose (leucite-shonkinite) from East Peak stock, Highwood 

Mountains. E. B. Htirlburt, analyst.

III. Monchiquose (analcite-basalt) from dike in Highwood Gap. H. W. Foote, 

analyst.

IV. Shonkinose (leucitophyre) from near Khoi, Persia. J. Steinecke, analyst. 

Zeit. Naturw. Halle, vol. 6,1887, p. 12.

V. Chotose (leucitite) from Bearpaw Peak, Bearpaw Mountains, Montana. 

H. N. Stokes, analyst. Weed and Pirsson, Am. Jour. Sci., 4th series, vol. 

2,1896, p. 147.

VI. Missourote (missourite) from Shonkin stock, Shonkin Creek, Highwood 

Mountains. E. B. Hurlburt, analyst.

VII. Molecular proportions of No. 1.
riRSSON.] EXTRUSIVE FLOWS, BRECCIAS, AND TUFFS. 169

Mineral composition or mode. The mineral composition can not 

be determined with exactness because of the impossibility of distinguishing between small leucites and analcites, ahd in certain areas 

between these and the isotropic base. The latter is also specked with 

iron ore and other minerals in form too minute for measurement. 

Making due allowance for these things, the following table shows a 

measurement determination made with as careful estimations as 

possible:

Mode of shonkinose.

Minerals.

Apatite . . -_..-------_.-.--

Calcite ......................

Olivine -...---- -.---._-- _...

Augite _.._-_----.--_---._ .

Leucite ....................

Analcite ..--..-...--.-...-.

Base .......................

Total ................

Units 

measure.

....... 55

.1..... 19

....... 26

....... 122

....... 225

........ 176

---.... 129

....... 608

....... 1,360

Volume, Specific 

per cent, gravity.

4.0 X 5.2 =

1.8 X 3.2 =

1.9 X 2.7 =

9.0 X 3.4 =

16.5 X 3.3 =

12.9 X 2.4 =

9.5 X 2.2 =

44.7 X 2.3 =

99.8

20.80

4.16

5.13

30.60

54.45

30.96

20.90

102. 81

269. 81

Weight, 

per cent.

= 4.17

= 1.57

= 2.00

= 11.81

= 20. 98

= 11.92

= 8.07

= 39.69

100. 21

The ratios of the f emic minerals to one another and to the salic ones 

plus the base are the most nearly correct of this determination, which, 

on the whole, gives a fair idea of the relative quantities of the minerals 

present.

Classification in prevailing systems. In these the rock is of course 

a leucite-basalt in which part of the leucite is replaced by analcite, 

which appears to be of secondary origin. It resembles very strikingly 

the leucite-basalts of the Bearpaw Mountains, described some years 

since by the writer. The fact that the olivine is not present as such 

by reason of its resorptive alteration does not change the classification and throw it into the leucitites; its chemical composition shows 

that it belongs in the former group.

Classification in the new system. The position of this type in the 

new system is shown in the following calculation, which gives its 

norm and place:
170 IGNEOUS BOOKS OF HIGHWOOB MOUNTAINS. [BOLL. 237. 

i

Calculation of the norm of shonkinose.

sio2 ...._.:......

ALA ------------

Fe2 03 ------------

FeO. .............

MgO. ............

CaO .............

NaA----- -----

K2O-_ ------ ------

Ti02 ..... . 

p n

a

- Rest _-...---.----

Total ......

Analysis.

47.98

13.34

4.09

4.24

7.01

9.32

3.51

5.00

.58

1.03

.21

4.06

100. 29

Molecular ratio.

0.798

.130

.026

.058

.175

.166

.056

.053

.008

.007

.003

Or.

318

53

53

53

Ab.

66

11

11

11

Ne.

90

45

45

45

An.

49

91

91

91

Di.

" 244

15

107

122

122

Ol.

38

9

68

38

'Mt.

26

26

26

n.

8

8

8

Ap.

23

7

3

7

Or....

Ab ...

An ...

Ne....

Di....

01....

Mt....

n.....

Ap ...

Rest..

... 29.47

... 5.76

... 5.84

... 12.78

... 26.83

... 5.68

... 6.03

1 33

... 3.35 

... 4.06

Class, 

53.85 

Order

Bang, 

42- U Subra

Sal 53.85 Fem~42.11~1 - 2=I"n-' salfemane.

, -p ^ 07 0. 31 lendofelic=6, portugare. 

Na20'+KoO' 109 CaO' ~ gj' o.l doiualkalic=2, uiuuoliiquase.

Total.. 100.02

TMs shows that the rock is a shonkinose, as its chemical composition at once indicates. It is interesting to observe that the norm 

shows no leucite whatever, but in its place orthoclase associated with 

nephelite. This arises from the fact that leucite and analcite replace 

them in bhe mode, and water plays its role. These relations are easily 

seen in the following equation:

Orthoclase. Nephelite. Water. Leucite. Analcite. 

K,Al;!Sic0lc -|-Na2Al2Si208+2H2 0=K2Al2SiA2+Na2Al2Si4012 .2H2 0.

That is, as water vapor was present analcite was formed and 

reduced what otherwise would have been orthoclase to leucite. This 

relation has been previously discussed in the description of the shonkinose of East Peak, and its bearing on the primary origin of the analcite has been pointed out. In the present case, however, there is 

evidence which was lacking in that rock. In this rock the leucites 

have their own definite crystal form and are evidently primary. They 

did not crystallize as orthoclase, to be reduced later on; they are not 

pseudomorphs; they are original. But if they are original, is not the 

analcite original also? It is the same question as in the East Peak case.

The texture of the rock is porphyritic, and it is therefore a phyroshonkinose.
CHAPTER VII. 

GENERAL PETROLOGY OF THE HIGHWOOD REGION.

INTRODUCTION.

As a group the Highwood rocks show certain features which serve 

to bring them into a definite family and to distinguish them from 

rock groups of other regions. The fact that a complex of igneous 

rocks from a given region may possess what may be termed "clan" 

characters is now so well known to all petrographers that it is unnecessary to dwell upon the fact. Service to the science at present is best 

rendered by bringing to light so large a number of such clans that 

sufficient data may be obtained to enable petrologists to reduce this 

relation of igneous rocks to definite order and understanding.

In some groups this clan character is best expressed in the relations 

of the minerals to one another, or in the texture or production of a 

certain mineral or minerals through the .series. In other cases it is 

best shown by chemical characters which persist through a number 

of magmas. In the Highwood rocks it is exhibited in part by the 

mineral and in parb by the chemical composition. It is, of course, 

understood that the mineral pecularities are dependent on the chemical composition combined with the physical conditions under which 

the magmas cooled and crystallized. For this reason the general 

chemical character of the Highwood magmas will be considered first.

CHEMICAL CHARACTERS OF HIGHWOOD MAGMAS.

For consideration of this subject twenty analyses are available. 

They are given in the tables following. Of these analyses, vi is calculated from the microscopic mineral analysis, confirmed as previously shown by-a partial chemical analysis; xin is a partial analysis 

m which TiO2 and P2 05 have been weighed with alumina; the rest 

are accurate modern chemical analyses, made according to the latest 

approved methods. By them practically every considerable rock 

mass and kind of rock in the district is represented.

171
172 IGNEOUS BOOKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Analyses of Highwood rocks.

SiO,-...

A1A -

Fe2 03 ..

FeO....

MgO...

CaO ...

Na2O...

K,O....

H2 0-K.

H2 0-..

CO2 ---.

Ti02 ...

PA 

SO,....

01 .....

MnO...

BaO ...

SrO....

O=C1..

Total.

I.

65.54

17, 81

.74

1.15

.98

1.92

5.55

5.58

.54

.11

Trace

99. 92

99.92

II.

57.18

18.54

3.65

1.15

.69

2.31

4.48

8.58

2.10

.30

.05

.06

.77

Trace.

.49

Trace.

100.35

.17

100. 18

III.

56. 45

-20.08

1.31

4.39

.6-3

2.14

5.61

7.13

1.51

26

.29

.13

.43

.09

100. 45

.10

100.35

IV.

59.24

13.84

5.46

1.36

4.79

5.60

3.13

4.22

2 02

.22

.34

.08

.04

Trace.

100. 34

.01

100. 33

.V.

58. 04

17.25

2.49

1.24

1.79

3.50

3.37

10.06

1.95

.30

.22

Trace.

Trace.

Trace.

100.21

100. 21

VI.

53.47.

12.43

6.19

3.73

3.07

7.23

3.40

7.59

.40

1.19

.84

.62

100.16

100. 16

VIl.

55.23

18.31

4.90

2.06

1.85

3.63

4.02

6.43

1.84

.42

.58

.23

.32

Trace.

.46

Trace.

100. 28

.08

100.20

VIII.

51.00

17.21

2.41

4.23

6.19

9.15

2.88

4.93

.63

.13

.33

.03

Trace

Trace

.34

.14

99.60

99.60

IX.

51.75

14.52

5.08

3.58

4.55

7.04

2.93

7.61

2 25

.23

.18

T^TO/^O

.05

Trace.

.30

.07

100. 14

.01

100. 13

X.

52.05

15.02

2.65

5. 52

5.39

8.14

3.17

6.10

.35

.47

.21

no

24

Trace.

.42

.28

100.03

.06

99.97

I. Pulaskose (syenite) from Highwood Peak. L. V. Pirsson and W. L. 

Mitchell, analysts. (712.)"«

II. Pulaskose (tinguaite-porphyry) from dike at edge of Middle Peak stock. 

H. W. Foote, analyst. (746.)

III. Pulaskose (sodalite-syenite) from Square Butte. W. H. Melville, analyst. (767.)

IV. Adamellose (trachyandesite) from flow North Willow Creek. E. B. Hurlburt, analyst. (787.)

V. Highwoodose (tinguaite-porphyry) of Highwood type from great dike 

below Highwood Gap. E. B. Hurlburt, analyst. (724.)

VI. Highwoodose (nosean-syenite) from south side of Highwood Gap. Microscopically calculated, L. V. Pirsson, analyst. (683.)

VII. Monzonose (gauteite) from dike on upper Aspen Creek. H. W. Foote,

analyst. (854.) 

VIII. Shoshonose (monzonite) from Highwood Peak. E. B. Hurlburt, analyst. 

(715.)

IX. Fergusose (fergusite) from Arnoux stock at head of Shonkin Creek. E. B. 

Hurlburt, analyst. (827.)

X. Borolanose (basic syenite) from Middle Peak stock. E. B. Hurlburt, analyst. (739.)

a The numbers are those of the original collection.
PIBSSON.] CHEMICAL CHARACTERS OF HIGHWOOD MAGMAS. 173

Analyses of Higlnvood rocks.

Si02 -.-.-

A12 03 -.-- 

Fe2 03 .... 

FeO ---..

MgO 

CaO -----

Na2O .... 

K,0 

H2 0+.... 

H2 0-.-_.

C02 .

TiOs ..

PA--...

80.. 

Cl .......

Cr2 03 

NiO-----.

MnO--...

BaO -.-.-

SrO---...

O=C1-...

Total ..

XI.

51.94

15.78 

4.07 

3.17

3.48

6.04

3.44 

7.69

2.17

.39

.59

.29

.'08

Trace

42

.28

99.83 

.02

99.81

XII.

50.11

17.13 

3.73 

3.28

2.47

5.09

3.72

7.47

4.47

.82

.67

.08

.07

Trace.

.63

.35

100. 09 

.02

100. 07

XIII.

50.00

19.36

3.87 

2.67

2.18

4.96

3.63

8.52

3.53 

.46

(a)

(«)

99.18

99.18

XIV.

47.88

12.10 

3.53

4.80

8.64

9.35

2.94 

5.61

1.52 

.70

12

.77

1.11

None.

Trace

.03

Trace

.15

.46

.13

99.99

99.99

'XV.

49.59

14.51 

3.51 

5.53

6.17

Q 04

3.52 

5.60

1.95

.36

.15

.02

.13

Trace.

.49

.21

100. 78 

.03

100.75

XVI.

47.98

13.34 

4.09 

4 24

7.01

9.32

3.51 

5.00

2.10

1.24

.58

1.03

Trace.

.21

Trace.

.50

.14

100. 29 

.05

100. 24

XVII.

46.73

10.05 

3.53

8.20

9.27

13 22

1.81 

3.76

1.24

.78

1.51

.18

.28

(?)

(?)

100.56 

.04

100. 52

XVIII.

47.82

13.56 

4.73

4.54

7.49

8.91

4.37 

3.23

3.37

.67

1.10

.04

Trace.

.16

.21

100. 20 

.01

100.19

XIX.

46.04

12.23. 

3.86 

4.60

10.38

8.97

2.42

5.77

2.87

.64

1.14

Trace

.11

Trace

.48

.25

99.76 

.03

99.73

XX.

46.06

10.01 

3.17 

5.61

14.74

10.55

1.31

5.14

1.44

.73

.21

.05

.03

Trace.

.32

.20

99.57 

.01

99.56

a In A12O3 .

XI. Borolanose (syenite-porphyry) from dike at edge of Shonkin stock. 

W. M. Bradley, analyst. (832.)

XII. Borolanose (basic syenite) from Palisade Butte. H. W. Foote, analyst. 

(772.) ' . '

XIII. Borolanose (basic syenite) from Shonkin Sag laccolith. W. F. Hillebrand, analyst (partial anal.) (1006.)

XIV. Montanose (shonkinite) from Shonkin Sag laccolith. W. F. Hillebrand, 

analyst (including ZrO2 =0.03; F=0.05; S=0.03; Cr2O3 =0.03; V2O3= 

0.04). (1004.)

XV. Leucite-shonkinose (leucite-shonkinite) from East Peak stock. E. B. 

Hurlburt, analyst. (760.)

XVI. Shonkinose (leucite-basalt) from flow on saddle between Highwood and 

Pinewood peaks. H. W. Foote, analyst. (799.)

XVII. Shonkinose (shonkinite) from Square Butte. L. V. Pirsson, analyst. 

(765.)

XVIII. Monchiquose (analcite-basalt) from dike on east side Highwood Gap. 

H. W. Foote, analyst. (687.)

XIX. Biotite-cascadose (Highwood minette) from dike on Arrow Peak. H. W.

Foote, analyst. (736.)

XX. Missourote (albanose) or missourite from Shonkin stock, head of Shonkin 

Creek. E. B. Hurlburt, analyst. (824.)
174 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

\

A study of these analyses shows at once that the magmas are generally low in silica, which ranges from 65 to 46 per cent, but in the 

majority of types is about 50 per cent. Alumina is present in about 

average amount, and runs from 20 to 10 per cent. With diminishing 

silica and alumina the lime, iron, and magnesia rise. The most interesting and important relations are to be seen in the alkalies. There 

is some variation in the ratios of soda to potash, but in general the 

potash predominates. In some cases, especially in the siliceous and 

in the most femic or basic magmas, this predominance of potash is 

marked. It is to be noted also that high alkalies occur with high 

lime for example, in one case K2 0 7.5, Na2 0 3.0, and CaO 7.0 per 

cent, respectively. This combination, the alkalic character with 

high lime and potash, is the distinctive feature of the Highwood 

magmas, which runs through the whole family and gives it a distinctive petrographic stamp. »

That this is an uncommon feature in rock magmas is clearly seen 

in Washington's tables of analyses, where perpotassic and dopotassic 

rocks are rare. In speaking of this, Washington remarks 01 that of 

the whole number of analyses the perpotassic and dopotassic taken 

together form only 5.1 per cent, the perpotassic only 0.5 per cent. 

The Highwood analyses contribute a considerable proportion of these 

magmas. Outside of the central Montana province the only regions 

in which rocks so high in potash are found, so far as the writer knows, 

is in Italy and in the Leucite Hills of Wyoming.

In the table of analyses there is one exception, xvm, the monchiquose dike, in which soda predominates. This may have been caused 

by a secondary enrichment in soda.

The parent magma of the High woods the source from which these 

types sprung must have possessed the same general characters as the 

types themselves. It was thus of a very definite nature and its probable composition will be discussed in a later portion of this chapter.

a Prof. Paper U. S. Geol. Survey No. 14, p. 104.
PIKSSON.] CHEMICAL CHARACTERS OF HIGHWOOD MAGMAS, 

NORMS OF HIGHWOOD ROCKS.

The chemical character of the rocks is, of course, seen also in the 

following tables of their calculated norms. The order in which these 

norms are given is the same as in the preceding table of analyses. 

On examining this table the feature that first attracts attention is 

the general dominance of orthoclase over albite a dominance that 

increases toward the femic end of the series, and at this end there is 

only one marked exception. Toward this end also as the silica falls 

the orthoclase is eventually replaced by leucite. These facts show 

the strongly potassic nature of these magmas. Like the increase 111 

nephelite, augite, olivine, and apatite toward the femic end, they are 

the result of expressing in mineral form the series of chemical analyses 

previously given, and show, of course, similar distinctive features. 

The manner in which the iron ores range through the group with 

small variations in percentage is also an interesting feature.

Taken as a whole the series shows also how gradually rocks merge 

into one another chemically and mineralogically, and how arbitrary 

must be lines of distinction based on these properties. The group 

affords also in several ways an excellent illustration of the new system 

of classification. These features are much more clearly seen by comparing the norms than by comparing the analyses. Thus the first three 

are of pulaskose; alkali feldspars are the chief components; small 

quantities of other minerals occur, but play very subordinate roles. 

In adamellose alkali feldspars are the most important, but are associated with considerable quantities of quartz, anorthite, and diopside. 

In highwoodose alkali feldspars with some diopside are again the chief 

minerals, all the rest being in subordinate amounts, and the notable 

feature is the very great preponderance of orthoclase over albite. In 

monzonose the albite and orthoclase are equal, and the notable increase 

in anorthite is the chief feature. In shoshonose the series is becoming 

distinctly more femic, the alkali feldspars are balanced by an equal 

amount of anorthite and diopside, while nephelite and olivine are not 

insignificant. Next comes fergusose, a new type of remarkable texture, consisting of orthoclase, nephelite, and diopside; the great preponderance of the orthoclase is, as in highwoodose, the distinctive 

feature, and one that separates it from the norms of rocks formerly 

classed as nephelite-syenites, since the latter contain quantities of soda 

which expresses itself in large amounts of nephelite, albite, or both.
176 IGNEOUS KOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237. 

Calculated norms of Highwood rocks.

Orthoclase .... 

Albite ........

Quartz ........

Leucite ... ..

Nephelite .....

Sodalite -...,..

Diopside -.-...

Oli vine.. ------

Hvpersthene ..

Wollastonite ..

Hematite ------

Hyalophane . . .

I.

32.7 

46.7

7.0

7.3

2.0

2 7

1 2

2

II.

50.6 

25 2

6.7

5.8

3.7

2.8-

.6

1.8

III.

42.3 

28.3

9.7

3.4

6.0

5.9

1.9

.6

.3

IV.

24.5 

26 °

11.1

10.2

11 4

6.7

3.6

.5

2.9

.7

V.

59.5 

14.7

3.1

1.7

4.9

9.7

.4

3.0

.6

.3

.3

VI.

45.0 

13.1

1.4

5.7

17.4

1.9

7.7

2.3

2.0

2.8

VII:

37.8 

31.4

12.8

1.4

1.1

4.1

5.6

'.8

1.1

1.3

VIII.

28.9 

10.0

19.5

8.0

19.4

8.3

3.5

.2

.7

IX.

45.0 

3.7

3 9

11.4

23.75

1.5

7.4

.5

.3

X.

36.1

7.3

9.2

7.4

2.9

24.5

6.2

3.9

.9

.3

I. Pulaskose from Highwood Peak.

II. Pulaskose from dike at Middle Peak.

III. Pulaskose from Square Butte.

IV. Adamellose from flow on North Willow Creek.

V. Highwoodose from dike north of Highwood Gap.

VI. Highwoodose from south of Highwood Gap.

VII. Monzonose from dike on upper Aspen Creek. 

VIII. Shoshonose from Highwood Peak.

IX. Fergusose from Arnoux stock, Shonkin Creek.

X. Borolanose from Middle Peak stock.
PIKSSON.] CHEMICAL CHARACTERS OF HIGHWOOD MAGMAS. 177 

Calculated norms of Highioood rocks.

Orthoclase .... 

Albite ........

Anorthite

Quartz _ ...____

Leucite .......

Noselite .......

Diopside .--.-_

Olivine. -------

Hypersthene ..

Magnetite..... 

ITmenite. ._..__

Hematite ......

Apatite .._....

Acmite...-.-..

Hyalophane . - -

XI.

45.0 

9.4

6.1

6.0

1.0

' 2.8

16.7

1.8

5.8

.7

1.0

XII.

44.5 

9.4

8.1

11.1

.7

10.2

2.2

5.3 

1.5

1.7

XIII.

49.5

8.9

.9

16.8

10.8

.8

5.6 

.9

.7

XIV.

33.4

3.3

13.4

28.4

9.1

5.1 

1.5

2.7

XV.

33.4 

.5

7.8

13.4

1.9

29.1

5.7

5.1

.8

.3

XVI.

29.5

5.8

5.8

12.8

26.8

5.7

6.0

1.2

2.4

XVII.

22.2 

1.6

8.3

6.3

1.0

37.9

11.4

5.1

1.5

3.7

XVIII

18.9 

17.8

7.8

9.7

.5

23.4

7.9

7.0 

1.2

2.6

XIX.

23.9

5.8

8.3

8.8

1.9

25.0

13.0

5.6' 

1.2

2.7

XX.

1.1

5.3

22 7

5.9

36.7

18.6

4.6 

1.4

.3

1.4

XI. Borolanose-from dike, Shonkin stock.

XII. Borolanose from Palisade Butte.

XIII. Borolanose from Shonkin Sag laccolith.

XIV. Montanose from Shonkin Sag laccolith.

XV. Shonkinose from East Peak stock.

XVI. Shonkinose from flow, Pinewood Peak saddle.

XVII. Shonkinose from Square Butte. 

XVIII. Monchiquose from dike, High wood Gap.

XIX. Cascadose from dike, Arrow Peak.

XX. Missourote (albanose) from Shonkin stock.

Bull. 237 04 -12
178 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. U37.

This is followed by borolanose, of which there are four examples, x 

to xni; orthoclase is still the chief ingredient, but there is a considerable increase in the soda minerals, albite and nephelite, and also 

in anorthite. In montanose there is a great increase in augite and 

olivine, but there is only 3 per cent of anorthite. The rock is thus 

composed of alkalic feldspars and lenads, potash strongly dominating, 

with f errornagnesian minerals; it is peralkalic. This lesser amount' 

of anorthite distinguishes montanose from the next type, shonkinose, 

which contains enough anorthite to carry it over the line into the 

domalkalic rang monchiquase, but, as the norms show, the montanose and shonkinose are very close together. In xvm, the 

exceptional type of the region, the preponderance of the sodic minerals carries the rock into the dosodic subrang mouchiquose and 

differentiates it from shonkinose. In the next norm, that of cascadose, the large proportion of lenad minerals, as compared with 

feldspar, carries us into a different order, the lenfelic one under salfemane, kamerunare; leucite begins to appear, and this finds an 

important position in albanose (missourote). The latter, it will be 

remembered, on account of the predominance of augite and olivine, 

stands on the very border line of the next class, dofeinane.

GEOLOGIC OCCURRENCE OF THE DIFFERENT MAGMAS.

The geologic occurrence of the magmas is by no means" the' same as 

the geologic occurrence of the rocks, for the rocks are the result of 

not only the chemical properties but the physical' environment of 

the magma. Thus, as is well known, the same magma in different 

modes of occurrence, produces rocks which differ in minerals and texture. This is illustrated in a general way in the High woods. Thus, 

for instance, the intrusive stock of East Peak, the dike of Highwood 

minette (cascadose) of Arrow Peak, and the basaltic effusive (shonkinose) of Pinewood Peak saddle are from practically the same magma, 

with slight variations, as may be seen in the following comparison:

Comparison of rocks different in geologic occurrence but similar in chemical

composition.

SiO2 ---- -----.------ ....... .........

ALA . ,. . . --... 

Fe2 03 ---._--.-__--_-.-_--___--- -- _. -

FeO..---- -----_- ---. .----

MeO

CaO -------- - --- - -- -- -. -

Na,0----- -----. - -- -------- ---

K,O - -. - -------.---.---..

East ' 

Peak 

stock.

49.6

14.5

3.5

5.5

6.2

9.0

3.5

5.6

Arrow 

Peak 

dike.

46.1

12.2

8.7

4.6

.10.3

9.0

2.4

5.8

Pinewood 

Peak 

flow.

48.0

13. 3

4.1

4.2

7.0

9.3

3.5

5.0
FIRS SON.] GEOLOGIC OCCURKENCE OF THE MAGMAS. 179

The intrusive 'rock (fergusite) of the Arnoux stock and the dike 

(syenite-porphyry or borolanose) at the edge of the Shonkin stock 

have practically the same magma.

Comparison of chemical composition of stock and dike.

Si02 ..._ ..................................................... -

A1A ------------------------------------------------------- -

FeA- --------------------------------------------------- ----

FeO - -..-;...-.- .. -.-..- .-. -..

MgO ...........::............................................

CaO... ..................................................... .-

Na.,0....................... .......... ........ ...... I.......

K20..... ...._...__.._.................--. --.-- .-----"

Stock.

51.8

1.4.5

5.1

3.6

4.6

7.0

2.9

7.6

Dike.

51.9

15.8

4.1

3.2

3.5

6.0

3.4

7.7

Other instances might be adduced, but these are sufficient to show 

that there is no peculiar relation in this area between the chemical 

composition of the magmas and their geologic mode of occurrence. 

Whatever be their origin below, the same material has been forced 

upward into both the stocks and the dikes and has appeared in effusive 

form on the surface. The dikes therefore do not appear to be diaschistic in the sense of Brogger a that is, they are not complementary; 

they are rather aschistic. It appears to the writer, however, that the 

stocks themselves show diaschistic features and that in the sense of 

Brogger both stocks and dikes, and the laccoliths as well, belong to 

the same category and represent "laccolithic differentiation." This 

is treated later in this work. -

It may then be stated that in the Highwoods neither the dikes nor the 

flows represent any more highly differentiated forms of magmas than 

do the stocks which they accompany. Special cases may be selected 

in which types on the one hand more salic, and on the other more 

femic, than some particular stock would appear, but this does not 

affect the general truth of the statement. On the contrary, the most 

highly differentiated types which the district affords the pulaskose 

(syenite) of Highwood Peak and the albanose (missourite) of the 

Shonkin stock are both granular rocks occurring in the stocks themselves.

STOCKS AND LACCOLITHS.

Without discussing at this point the general question of the origin 

of igneous rocks, it may be pointed out that the occurrence of two 

distinct types, such as the syenite (pulaskose) arid monzonite (shoshonose) of Highwood Peak in the canal of such an evident volcanic

aGrorudit-Tinguait-Serie, 1894, p. 153.
180 IGNEOUS EOCKS OF HIGHWOOD MOUNTAINS. [BOM,. 237.

center can not fail to be of great interest. In the same way the 

occurrence of shonkinite with kernels of syenite in the laccoliths 

has an important bearing on the question of differentiation in igneous 

magmas, a bearing which has already been discussed by various 

authors. The general advance which has been made in solving problems of this character during the last ten years has led the writer to 

modify in some degree his earlier views in regard to the laccoliths in 

the Highwoods. These modifications have been in part given under 

the geologic description of Square Butte and will be mentioned in the 

following discussions.

RELATIVE VOLUMES OF THE DIFFERENT MAGMAS.

In a general way the magmas may be roughly divided into two main 

groups salic and femic, or acid and basic, or basaltic and trachytic, 

to give a choice of terms. The areal mapping and field study shows 

that in this area the amount of the salic magmas compared with the 

femic is small. This is seen in a variety of ways. It is shown by the 

dikes, as it is safe to say that for one salic dike there are twenty 

femic of the same size. It is also seen in the effusive materials the 

flows and breccias the total volume of the femic far exceeding that 

of the salic, in spite of the fact that the latter, being overlain by the 

femic, have been saved from erosion, while the femic have in considerable volume disappeared. The same fact is shown in the stocks, the 

volume of Highwood syenite being smaller than the monzonite, yet it 

is one of the largest masses of salic rock in the district. The other 

stocks do not show any strictly salic types, being salfemic in composition. But the most striking evidence is found in the laccoliths, 

which are worth some attention, as they afford interesting data in this 

connection which may be used in several directions and in the elucidation of their structure.

SHONKIN SAG LACCOLITH.

If it is assumed that this laccolith is a short section of a cylinder 

with a diameter of a mile and an average height of 150 feet, it contains very nearly 3,000,000,000 cubic feet of rock. If it is assumed 

that the mass of augite-syenite (borolanose) which forms the kernel 

has a diameter of 2,000 feet and an average thickness of 50 feet, it 

would have a volume somewhat over 150,000,000 cubic feet, in round 

numbers; that is, the syenite constitutes about one-twentieth of the 

whole mass. These figures are, of course, approximate (see p. 47). 

They can not, however, be very far from the truth, and for the purpose they are just as valuable as if they were exact. The point is 

that there is in the laccolith about 19 times as much shonkinite (inontanose) as syenite (borolanose). The bearing of these proportions 

will be brought out in a later paragraph.
'RELATIVE VOLUMES OF THE MAGMAS. 181

SQUARE BUTTE.

By reference to the cross section of this laccolith given on page 54 

it will be seen that it can not be considered as having an exactly 

cylindrical shape. A considerable portion of the shonkinite has been 

lost by erosion, but if it is assumed that the laccolith has the form 

and cross section given, the relative volumes generated by such figures of revolution around a common vertical axis may be computed. 

This has been kindly done for me by Prof. J. Barrell, who obtained 

the volume by multiplying the area on one aide of the axis by the distance passed over by its center of gravity in the revolution about the 

axis, both analytic and graphic methods being used as checks; The 

result of this is as follows:

Cubic mile.

Total volume of the laccolith.----.--._.--__-.__-__--_---..-.-. 0.69 

Inner volume of syenite..-..-..--...-.------------------------ -13

Outer volume of shonkinite .-...........^......^............... .56

That is to say, the syenite comprises 18.8 per cent of the laccolith 

and the shonkinite 81.2 per cent, thus showing, as previously stated, 

that the femic magmas are in large excess.

PALISADE BUTTE.

At Palisade Butte there is no such sharp line of demarcation 

between the light and dark portions. Moreover, the amount of erosion has been so great that the laccolith can not be restored beyond 

imagining that it had a form similar to that of Square Butte. Enough 

remains, however, to show clearly that the volume of the dark shonkinite is (and was) much greater than that of the lighter-colored rock.

DIFFERENTIATION IN LACCOLITHS.

From the proportions just given in the case of Square Butte and 

the Shonkin Sag laccolith some interesting and important relations 

may be deduced. If one believes that these three laccoliths, with 

their varied and peculiar structures, present good examples of the 

results of differentiation, it would be also natural to think, since they 

stand close together in a group, that they had been formed from a 

common magma and in toto should be similar in composition. Since 

the chemical composition of the parts and their relative volumes are 

known the composition of the original magma is easily computed, with 

the results given in the next table.
182 IGNEOUS BOOKS OF HIGHWOOD MOUNTAIN'S. [BOIL. 237. 

Computation of original magmas of laccoliths.

SiO2 ._.. .............................

A12 0S ..... ..............................

Fe2O3 ----------------------------------

FeO-.-....: -_--..-.-.___---'-----.. --.-

MgO ..............-........;. ...........

CaO.... ...... ............ ..:...........

Na,2O... .................................

K2O_--- --------------------------------

I.

50.0

19.4

3.9

2.7

2.2

5.0

3.6

8.5

II.

47.9

12 1

3.5

4.8

8.6

9.4

3.0

5.6

III.

'48.0

12 4

3.5

4.7

8.3

9.2

3.0

5.8

IV.

56.5

20.1

1.3

4.4

0.6

2 1

5.6

7.1

V.

46.7

10.1

3.5

8.2

9.7

13.2

1.8

3.8

VI.

48.5

12.0

3.0

7.4

7.8

11.1

2.5

4.4

I. Syenite (borolanose) from Shonkin Sag laccolith. 

II; Shonkinite (montanose) from Shonkin Sag laccolith. 

III. Mixture, 1 part syenite and 19 parts shonkinite, of I and II. 

.IV. Syenite (pulaskose) from Square Butte.

V. Shonkinite (shonkinose) from Square Butte.

VI. Mixture, 13 parts syenite and 56 shonkinite, of IV and V.

When one considers that the relative volumes are approximations 

and that the composition of small pieces is held to represent the composition of large masses, it must be confessed that in and vi are 

very much alike; they represent the same magma and differ mostly 

in ferrous iron and lime a result which is due to the fact that the 

specimen from which analysis v was made probably contains rather 

more augite than the average of the whole mass.

It is clear, then, that the laccoliths could have been formed by the 

intrusion of .bodies of a shonkinitic magma from a source below, which 

furnished a uniform material, and that the syenite masses in that 

case must have been produced by the local interior concentration of 

a very small part of the feldspathic elements of these magmas.

So far as the writer can see, the arrangement of the varied parts in 

the laccoliths could have occurred only in one of three ways: First, 

there might have been by some process an injection of shonkinite, and 

afterward one of syenite into its center. Against this the mechanical 

difficulties seem insurmountable; there is no explanation of the lack 

of contact phenomena between the two kinds of rock; it is difficult 

to understand why the process occurred in two laccoliths while in the 

third there should be a gradual transition from shonkinite to syenite, 

and why separate upward movements of syenite magma should occur 

in this region only in the centers of the laccoliths. In view of these 

facts this theory may be confidently dismissed.

OSMOTIC THEORY.

The structure a of the laccoliths has been explained by the assumption that the magma was really of a syenite character, and that by

a Johnston-Lavis, H. J., High wood Mountains of Montana and magma tic differentiation, a criticism: Brit. Assn. Adv. Sci., Rept. Liverpool Meeting, 1896, p. 792.
pinssoN.] DIFFERENTIATION IN LACCOLITHS. 183

absorption of sedimentary material it became changed to shonkinite 

lime, iron, and magnesia being the oxides absorbed. This is the 

osmotic theory. The writer has already pointed out" that this view is 

untenable for several reasons. There are no sediments in this region 

which have the proper composition to effect such a change, which must 

have taken place after the intrusion. There is nothing wanting in 

the sediments, and one can plainly see in the Shonkin Sag laccolith 

that the beds have simply been lifted and that nothing has been 

absorbed. Moreover, the intrusion has occurred in sandstones, not 

in calcareous, inagnesian, iron-bearing beds. In addition, it is not a 

question of a narrow basic mantle, for, as just pointed out, the greater 

part of these masses consists of shonkinitic magma, and therefore, in 

the case of Square Butte, for example, the amount of material absorbed 

would have to be sufficient to reduce the 12.8 per cent alkalies of the 

SyeilitiC rock to the 5.6 per cent of the shonkinitic, or to increase the 

magnesia from 0.6 to 9.7 for four-fifths of the entire IliaSS. This 

would require the melting up and absorption of an incredible amount 

of country rock when one considers the amount of magma. Again, 

one may pertinently ask, If this occurred in the laccoliths why did it 

not occur at Highwood Peak, where the syenitic rock is found in 

direct contact with similar sediments without any basic border intervening? Whatever may be the merits of the osmotic theory in explaining the phenomena seen in other regions and in regard to them the 

writer offers no opinion it is certain that in the localities in Montana 

studied by him, at Castle Mountain, at Togo Peak, and in the Highwoods, where basic outer rock mantles occur, the hypothesis has not 

a leg to stand upon, and it would never be offered by any petrographer 

who had studied the occurrences in these districts.

The facts disclosed compel us to fall back upon the third method 

of explanation that the laccoliths have been formed in the place 

where they now are by processes which took place in a body of magma 

that was originally homogeneous.

THEORIES OP DIFFERENTIATION.

The term differentiation has been consistently used by the writer for 

a number of years to express the idea that different rocks are 

formed from a parent body of homogeneous, magma without reference to the processes which have caused this result. This is considered to be demonstrated through the repeated observations of 

a great number of careful and competent students of petrologic 

phenomena. In the opinion of the author this term should be used 

in a geologic sense, as in a geologic way it has been demonstrated. 

Many theories have been advanced in explanation of this process 

along the lines of physical chemistry, but no one of them has been 

admitted by all to be a competent cause in every case. This in no

a Shonkin Sag laccolith: Am. Jour. Sci., 4tli series, vol. 12,1901, p. 13.
184 IGNEOUS ROOKS OF HIGHWOOD MOUNTAINS. [BULL. 23?.

wise invalidates the actuality of the-process. The geologist has 

pointed out the fact; it remains for the physical chemist to explain it 

if he can. Nothing is to be gained by denying it.

The many discussions of this subject which have been presented by 

workers in petrology are so well known that the writer has no intention of giving them in resume in this place; the more important ones 

have been recently well summed up in a paper by Schweig a on the 

differentiation of igneous magmas, who adds an important suggestion 

of his own, which will be considered later. The theories may be 

classified into three groups those which depend on the force of 

crystallization, those depending on some other form of molecular flow, 

and those which appeal to some force whose methods of operation are 

unknown, such as electricity.

Becker b has shown that pure molecular flow or diffusion is an agency 

acting with extreme slowness, and therefore as an explanation for 

such relatively small bodies of magma as the High wood laccoliths 

intruded into the upper crust and cooling with relative rapidity, the 

writer is not inclined to regard it as competent or probable. Its 

rapidity, however, supposing that it-can take place, must depend in 

large measure on the fluidity of the magma, and this is of necessity 

more or less unknown. It must not be forgotten, however, that so 

long as crystallization is able to take place the viscosity of a magma 

is never too great to prevent molecular flow, since it is through this 

that the molecules are able to arrange themselves in crystal form. 

The molecular oxides may not move over a great distance, but they 

are able to move.

Schweig c has offered the important suggestion that differentiation 

may be caused by the crystallizing out of definite minerals through fall 

of temperature or increase of pressure and the separation of such 

crystals from the mother liquor through specific gravity. If this 

takes place under high pressure, then by the removal of such pressure the crystals would become melted again and furnish chemically 

different magmas.

It seems rather difficult to accept this view in the simple form thus 

stated, as experience does not seem to show that there is the settling 

out of crystals through a greater specific gravity as postulated. 

Nothing is more common than to observe great vertical thicknesses 

of igneous rocks exposed by erosion, often for several thousand feet, 

and find that they are of uniform character throughout from top to 

bottom. This the author has seen repeatedly, and many instances 

will occur, no doubt, to.all petrographers. Indeed, the author has 

never seen any instance where a direct proof of such settling could be 

observed, though a few are claimed in the literature. On the other

a Differentiation der Magmen: Neues Jahrb. fur Min., Beil. Bd. 17,1903, p. 516. 

&Some queries on rock differentiation: Am. Jour. Sci., 4th series, vol. 8,1897, p. 21. 

oLoc. cit., p. 663.
PIRSSON.] DIFFERENTIATION IN LACCOLITHS. 185

hand, a priori it would seem to be a very natural result, and where it 

does not happen its failure must be attributed to the viscosity of the 

mother liquor in which the crystals are formed. Yet it is evident that 

the mother liquor can not be so viscous as to prevent molecular flow, 

otherwise crystallization could not take place. These are perplexing 

questions, and our knowledge of the physical properties of molten 

magmas is as yet far too incomplete to enable us to answer them with 

certainty.

It must be admitted, however, that the cross sections of the Highwood laccoliths furnish some evidence that specific gravity has been 

a factor in their formation, for the bottom portions are composed, 

as has been shown, of a relatively great thickness of heavy femic 

rock, upon which rests a much less thickness of lighter salic rock, 

and then in the Shonkin Sag laccolith, the only one whose upper part 

is still uneroded, is a very small thickness of the femic type again. .It

Avould be interesting to know in this connection whether the association of syenitic and shonkinoid intrusive rock masses, described by 

Merrill,08 if seen in better exposures or more thoroughly studied, would 

show the femic rock below as well as above the salic type, and thus 

conform to the Highwood occurrences. More complete study and 

description woiild give a better idea of what seems to be a most interesting occurrence of differentiation in place.

DIFFERENTIATION PRODUCED BY CRYSTALLIZATION.

In recent years rock masses which show differentiation in place 

have received great attention f rom petrographers, and justly so, since 

they furnish the most precise indications yet obtained concerning the 

nature of the processes'that have caused differentiation. Especially 

is this true of those processes which show different border zones, such 

as the Highwood laccoliths. The tendency at present appears to be to 

view them as caused by some process of crystallization, and in this connection Washington 6 has pointed out that we should expect that rock 

type which appears in the largest amount to be the one formed at the 

border. This is on the principle that in the case of a solvent and solute 

it is the solvent that crystallizes out first, an idea which, as applied to 

crystallization in molten magmas, we owe to Lagorio. 0 Washington 

further-suggests that, the differentiated border zones would be found 

in laccoliths of medium composition, monzonitic and foyaitic, while 

in extreme types, such as are granitic on the one hand and gabbroid 

on the other, with the solvent in very great excess the small amount 

of solute would be mechanically caught and crystallized with it or 

forced inward and solidified as a small core at the center of the mass.

"Notes on some eruptive rocks from Montana: Proc. U. S. Nat. Mus., vol. 17,1895, pp. 643,665. 

Conf., also, Bull. U. S. Geol. Survey No. 110,1893, p. 43.

''Igneous complex of Magnet Cove: Bull. Geol. Soc. America, vol. 11,1900, p. 409. 

« Tschermaks Min. Mitt., vol. 8,1887, p. 513.
186 IGNEOUS ROCKS OF HIGHWOOD MOUNTAINS. [BOM,. 237..

This conception of Washington's with respect to the femic ("gabbroid ") type is perfectly realized in the Shonkin Sag laccolith.

Washington, however, attributes the whole process to the force of 

crystallization, and does not deem convection currents to be at all 

essential. a . He says: "It would go on by collecting along the rough 

borders in accordance with the well-known tendency of crystallizing 

bodies to grow about sharp nuclei, the solute molecules being mechanically pushed aside toward the center."

With this view, which is a clear exposition of the idea, also expressed 

by others, that the force of crystallization is alone competent to produce differentiated border zones, the writer can not agree, because, as 

it seems to him, there is a misconception involved in it, for a series 

of very small units is made to do duty as one very large unit. Let 

us suppose that we have a mass of mobile magma a mile in diameter and that the moment has arrived in the process of cooling when 

some compound can crystallize out. This begins at the outer border, 

as stated, and not at the center. Now, in the center are oxide molecules, which are to be moved to the outer border, a distance of half a 

mile.- It can scarcely be thought that the force of crystallization can 

act through an intervening distance of such magnitude with a pull 

sufficient to attract the central molecules to the outer border, for crystallization acts only through relatively short distances. It is of no aid 

to the solution of the problem to think that at any intervening dis-. 

tance the molecules would come together and form another crystal, 

for the moment that has happened the forces of crystallization would, 

so to speak, come to rest and cease to operate. The crystal, then, at 

the intervening distance is, so far as the forces within its radius of 

action are concerned, a dead body, and we must find some force 

other than that of crystallization to transport it to the outer border. 

Unless the pull is felt from border to center it is difficult to see how

crystallization can operate. It might also be imagined that the force 

does not operate directly upon the central molecule, but indirectly 

through the intervening ones, these being linked together, by their tendency to crystallize, like a train of cars. The train as a whole would 

then move in the direction of the molecules at either end, upon which 

a stronger force operates, and in this case the locomotive that moves 

it are those molecules at its outer end, which, being within the crystallizing influence of the outer border, are irresistibly drawn forward to 

the solidifying crystal. Then come the next in the train, and so on. 

The objection to this method of explanation is that, if the attraction 

between the mineral molecules were strong enough to act as a link in the 

train, they would crystallize and the continuity of the train would be 

broken. We should have to imagine the whole mass in a state of absolute equilibrium, each molecule attracted equally in all directions by

oLoc. cit., p. 410.
PIRSSON.] DIFFERENTIATION IN LACCOLITHS. 187

its fellows, the whole being drawn without a break in the continuity of 

events toward the outer border. Should there be a break anywhere 

in the equilibrium, the molecules would fly together and crystallize in 

that spot, which would set up a new center of attraction as powerful 

as any spot at the outer edge, and the molecules would, over large 

areas, move thither, and this of course would disturb the equilibrium 

elsewhere; thus the whole mass would go on crystallizing without reference to the outer border. In fact, if we appeal to crystallization 

alone in this way, convection currents are not only no aid but they 

can riot be present, since they would tend to disturb equilibrium, 

and there must be no disturbance of any kind. Thus it appears to 

the writer that the mere statement of the conditions necessary for this 

view are sufficient to refute it. It seems that if we consider crystallization alone as the operating force, in the ultimate analysis it 

resolves itself into the force acting from border to center, and when 

one considers the magnitude of the distance and the frictkmal resistance in the magma, which in any case can never be perfectly mobile, 

but must possess a certain amount of viscosity, it does not appear 

possible to appeal to this alone as a competent agent to produce such 

results. Accepting crystallization alone, it would also be difficult to 

explain why the lower layer of femic rock is so very thick and the 

upper one. so very thin.

ELECTRICITY.

Some writers have suggested that electric currents may play some 

function in the process of differentiation. But as yet we know so little 

in regard to the electric properties of molten magmas, beyond the 

fact that they appear to be similar to aqueous salt solutions, a that 

nothing of value can be advanced in this direction. When electricity 

is suggested as a possible agent the thought of the German proverb 

comes irresistibly to mind:

" Was man sich nicht erklaren kann 

Das sieht man als elektrisch an.''

COMBINED EFFECT OF CONVECTION AND CRYSTALLIZATION.

In the original paper on Square Butte the writer placed great stress 

on molecular diffusion, and was inclined to believe that crystallization had played no part in determining its differentiation, the fact that 

different femic minerals were found in the two rock varieties leading 

to this conclusion. In the light of recent work, and especially of 

Becker's proof of the slowness of molecular diffusion, this view should 

now be modified, and it is thought that a combination of convection 

currents and the tendency to crystallize first -at the outer walls of the 

laccolithic chamber may possibly be sufficient causes. It seems

"Barus and Iddings, Electrical conductivity in rock magmas: Am. Jour. Sci., 3d series, vol. 

44,1892, p. 242.
188 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BOLL. 237.

almost impossible to resist the view that in an inclosed mass of magma 

sufficiently mobile for local differentiation to take place convection 

currents due to unequal cooling would occur. On the upper surface 

and along the outer walls cooling would take place more rapidly; on 

the floor of the chamber, protected by the heated mass above and 

with heated rocks below, less rapidly. Thus there would be a tendency along the top and sides for the magma to grow heavier and to 

descend. Material from the more highly heated central part would 

tend to rise and replace this, and thus currents would be established 

in the magma, rising in the center, flowing off to the sides at the top, 

and descending along the cooler walls. This process is illustrated 

by the familiar experiment of showing the convection currents in 

a vessel of boiling water with sawdust. Such currents, once established, would continue as long as sufficient mobility remained in the 

magma to permit them.

At some period crystallization would take place, and this most naturally would begin at the outer walls. It would not begin at the top 

because the material would arrive there from below at its highest temperature. Moving off toward the sides the material begins to cool and 

descend and becomes coolest as it nears the.floor; here crystallization 

would commence. The first substance to crystallize is the solvent, 

which in this case would be the femic minerals, chiefly augite. Part 

of the material solidified would remain attached to the outer wall and 

form a gradually increasing crust, and part would be in the form of free 

crystals swimming in the liquid and carried on in the current. Probably at first, as the liquid moved inward over the floor of the laccolith 

and became reheated, these crystals would reinelt, giving rise to numerous small spots of magnia of a different composition, which would slowly 

diffuse. As time went on, however, there would be a constantly 

increasing tendency for the crystals to endure; they would be carried

greater and greater distances. But as they are solid objects and of 

greater specific gravity than the liquid, there might be a tendency for 

the crystals to drag behind and accumulate on the floor of the chamber. 

Moreover, from the heat set free at the time of their crystallization 

and from the resulting concentration of the chemically combined 

water vapor in the magma, the residual liquid would tend to have its 

mobility kept undiminished, since these would be factors which would 

tend to counteract the increase in viscosity due to cooling. In this 

manner it may be possible to understand how there would form a 

femic marginal crust and a great thickness of the femic material at 

the bottom of the laccolith. As the cooling went on the edges of the 

outer crust would rise more and more toward the top, finally spreading over it, and as a result the crust should be thinner on the top 

than elsewhere, as in the Shonkin Sag laccolith, in which the upper 

crust of femic rock is still preserved.
nussoN.] DIFFERENTIATION IN LACCOLITHS. 189

o If, notwithstanding the evidence previously mentioned, there is a 

tendency for the crystals, on account of their greater specific gravity, 

to overcome viscous resistance and sink through the fluid, this might 

greatly aid in the process just described. And it is also possible to 

imagine that if the crystals melted there might be, in spite of diffusive 

tendencies, a retaliation in some degree of the spots of more femic 

magma thus formed and their local accumulation.

In this way the solute in this case the oxides which form feldspars might gradually accumulate toward the center and eventually 

solidify there. The process might go on until a definite eutetic solution resulted, when crystallization might produce a rock very different 

from that which had been forming. This would explain the case of 

the salic rock of Square Butte, for the resulting inner mass contained 

such a proportion of the femic elements, water, vapor, etc., that these 

combined to form hornblende instead of augite. In the Shonkin Sag 

laccolith the water vapors appear to have largely concentrated along

the inner walls of the outer femic crust, producing, when the inner 

mass solidified, a mantle with pegmatitic development. Although the 

erosive dissection at Palisade Butte is much greater than at the other 

two laccoliths, the remaining portions would seem to indicate that the 

process went on much more rapidly, and consequently the separation 

into parts was much less pronounced. The explanation just offered 

is based, with modifications, essentially on the suggestion of Becker a 

as to the process by which differentiation can occur in laccoliths 

through a combination of crystallization and convection currents'.

The hypothesis tentatively offered above seems to explain those, 

the most common, cases of laccolithic differentiation in which the outer 

shell is of femic type; it does not so well explain those with a salic 

outer shell. Washington 6 has suggested that in those magmas of 

preponderant salic character the salic molecules compose the solvent, 

and as this crystallizes first it produces the salic mantle. The only 

difficulty in this view is that it would then be necessary to show that 

in the rock mass composing the salic mantle the salic minerals have 

crystallized first, and are therefore autornorphic against the femic 

ones, thus proving that the latter crystallized last. The writer does 

not know of any evidence concerning this point in the literature, and 

has never had the opportunity of studying a distinct laccolith with 

salic border, the only case where the mass was more salic at the 

margin, coming under his observation, being the Blackhawk intrusive 

stock in the Castle Mountains. 0 In this case the mass was not an 

inclosed body of magma such as would be formed in a laccolith, and

a Fractional crystallization of rocks: Am. Jour. Sci., 4th series, vol. 4,1897, p. 257. 

*>Loc. cit., p. 410.

c Weed and Pirsson, Geology of tlie Castle Mountain mining district: Bull. U. S. G-eol. Survey 

No. 139,189C, p. 89.
190 IGNEOUS EOCKS OF HIGHWOOD MOUNTAINS. [BUM,. 237.

it passes from a granular rock at the center to a porphyry at the outer 

edge, and the process appears to have been otherwise than in the 

Highwood laccoliths.

DIFFERENTIATION IN THE STOCKS.

The process of differentiation suggested above for the laccoliths 

could not apply to the stocks which form eruptive centers, at least 

not without modification of the process. This is perhaps the best 

illustrated at Highwood Peak. Here we find the same association of 

salic and femic types seen in the laccoliths, but while the two taken 

together form a single mass intruded in the sediments, there is no 

such orderly arrangement of parts, and the two kinds have a sharp 

contact against each other, the pulaskose being the later, since it 

holds angular fragments of the monzonoid rock. There have been, 

therefore, two successive upthrusts of magma, the second after the 

former had solidified; and the differentiation has not occurred in the 

place where the masses now are, but at some lower level; and it also 

could not have been by the separation of a solid from a liquid by the 

simple process suggested above in the laccoliths, since both were 

intruded in liquid form.

The suggestion that a differentiated stock might be explained by 

laccolithic differentiation below, followed by a later upward movement of the mass, has been already made by the writer in the case of 

Yogo Peak, in the Little Belt Mountains of Montana,0 and this idea 

has been recently extended by Prof. F. D. Adams to explain the 

varied rock types and their arrangement at Mount Johnson, in the 

Province of Quebec, 6 where syenite (laurvikose) is found associated 

with essexite (essexose). In this case, however, the rock varieties 

grade in to. each other; there is not a sharp contact between them, 

showing that the upward movement took place before any solidification occurred and involved both alike. This might have been the 

case at Yogo Peak, but could not have been at Highwood Peak, as 

mentioned above.

From this center of eruption there have been in all four upthrusts 

of magma, two of them shown by extrusive flows, two in the stock 

itself. They have occurred in the order and with the compositions 

shown in the subjoined table:

a Petrography of the igneous rocks of the Little Belt Mountains, Montana: Twentieth Ann. 

Eept. U. S. Geol. Survey, pt. 3,1900, p. 566. 

b The Monteregian Hills: Jour. Geol., vol. 11,1903, p. 281.
PIRSSON.] - DIFFERENTIATION IN THE STOCKS. 

Composition of Highwood Peak magmas.

191

Si02 _... ............. ........................

A12 0, .......................................

Fe2O3 ......................................

FeO. ........................................

MgO. .......................................

CaO ........................................

K2 0. ................................. .......

I.

59.2

13.8

5.5

1.4

4.5

5.6

3.1

4.2

II.

48.0

.13.3

4.1

4.2

7.0

9.3

3.5

5.6

III.

51.0

 17.2

2.4

4.2

6.2

9.1

2.9

4.9

rv.

65.5

17.8

.7

. 1.1

1.0

.9

5.5

5.6

I. Adamellose (trachyandesite) flow from North Willow Creek.

II. Shonkinose (analcite-basalt) from Pinewood saddle.

III. Shoshonose (monzonite) from Highwood Peak.

IV. Pulaskose (syenite) from Highwood Peak.

This is based on the view that the stocks are of later age than the 

extrusives surrounding them. At East Peak, for example, this is 

clearly the case, for the two, as described .elsewhere, are seen in contact, the stock cutting upward through the effusives. They are not 

seen in such clear contact at Highwood Peak, and yet the arrangement, as may be seen on the map, is such as to lead to this conclusion, 

which is, moreover, the general one in such cases.

Repeated attempts have been made to discover whether in the aboveseries of analyses any mathematical relations are present which would 

throw light on the processes of differentiation or would serve to connect them with the other magmas of the area. These attempts have 

not been so completely successful as could be wished. The great difficulty in the way of such work is that we have no exact knowledge of 

the relative volumes of magmas involved. Some general facts are, 

however, clear. Thus, if we should suppose that n has differentiated 

out of I, we can see that there is much less silica, the same alumina, 

a large increase in ferrous iron, lime, and magnesia, and a small 

increase in alkalies, especially potash. If it were true that II represents a differentiated product of I, then there should be a complementary magma to correspond. Using various proportions of I and II, 

attempts have been made to calculate this magma and see if it would 

be shown by in, iv, or some other analyzed magma of the district, 

but without much definite success. The study that has been made 

leads to the conclusion that all four represent differentiated products, 

and if they have developed from a parent magma then m i+n n=a 

and m m+n rv=b, while m'a+n/b= original magma, and none of 

these have been found and analyzed. We do not know the relative 

volumes, as we did in the laccoliths, and can not therefore assign values
192 IGNEOUS BOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

to m, n, m', and n' and discover the exact composition of the original 

magma or of its first cleavage products.

It is of interest to note, however, that if we assume the simplest 

case, that the volumes of all four are equal that is, if we take their 

simple averages, we obtain these results:

Averages and comparison of analyses.

SiO,.. ...... ........................

Al O

Fe2 03 ----------------------------

FeO. _'.___.__.___._.-_._ __-.._____...

MgO. ..............................

CaO.. ............... ...............

Na2O. ............................ .-

K,0 -- .- - --:-- -- -

A.

53.6

13.5

4.8

2.8

5.8

7.4

3.3

4.9

B.

58.2

17.5

1.6

2.6

3.6

5.0

4.2

5.3

C.

55.9

15.0

3.2

2.7

4.7

6.2

3.7

5.1

D.

52.9

15.6

3.0

4.8

5.2

8.2

3.2

4.9

E.

52.1

15.0

2 7

5.5

5.4

8.1

3.1

"6.1

A. Average of I and II from Highwood Peak center. 

B. Average of III and IV from Highwood Peak center. 

C. Ave'rage of I, II, III; and IV from Highwood Peak center. 

D. Average of monzonite analyses (Petrography Little Belt Mountains, Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, 1900, p. 478). 

E. Borolanose (basic syenite) from Middle Peak stock Highwood Mountains.

The examination of the average of these analyses will show that it is 

that of a typical monzonitic magma. In D there is given for comparison the average of a number of analyses of monzonites used in 

the description of the rocks of Yogo Peak. It has a very close resemblance to this. It is also of interest to observe that, while neither it 

nor i nor n is exactly like any Highwood magma, they have a general resemblance to many of them; for example, C and E are much 

alike. In this connection it should be recalled that the Middle Peak 

stock is much older, as shown elsewhere, than any of the products of 

igneous activity belonging to the Highwood Peak center, and that, 

with the exception of possessing a border facies, which differs from 

the main mass chiefly in a textural manner and either not at all 

or but very slightly in a chemical way, it is entirely undifferentiated. It is therefore probable that it represents the original magma 

from which I, II, in, and iv have differentiated, and it is possible 

that by combining them in the proper proportions this composition 

could be more exactly realized. The other stocks of the area are of 

the same category as that at Middle Peak, and the further discussion 

of the origin of all these is deferred until that of the dikes has been 

taken up.
PIRSSON.] DIFFERENTIATION IN THE STOCKS. 193 

COMPOSITION OF THE ORIGINAL MAGMA.

Before dismissing the stock rocks it is of interest to surmise yet 

further what might) have been the composition of the original magma 

from which they were all derived. It is understood that this can, of 

course, only be don,e by averaging those of them of which we have 

analyses. For this purpose there are available analyses of the Middle 

Peak, East Peak, and Arnoux stocks. To use the High wood stock, we 

must obtain some average of its two rock types. The monzonitic rock 

is, however, in excess over the syenitic, how much i£ not known, but 

certainly more than twice as much is present. To be on the safe side, 

we will assume it is twice as much, not more. If this is added and 

the average of all four taken, we shall haA^e the result given in 

column A of the following table. In column B is given the analysis of 

the Middle Peak stock, which, as previously mentioned, is much the 

oldest in the district, and therefore probably the least differentiated. 

Considering how rough such an approximation must be the agreement 

is really very close. It seems probable, then, that the original magma 

from which all these rocks were derived had approximately, the com- i 

position shown in column A.

Average and comparison of analyses.

sic. _...._......_......:_..._.......____..__.... _____ _. .___'_...

ALA ..............................I...............................

Fe,08 ...-.:....-_......--.._..--........_..._._.._...__.._.......

FeO.. ............. --...-- ..----.....'.. ....-.-................

MgO -..-..--.....-_.-...-.-.. -....._._._.....__._...........

CaO. ................. ....i.. .............:.............. .....

Na.,0 ......:..............:...................... ^. ............

K.,0.. ..........................................................

A.

52.2

15.3.

3.3

4.4

5.2

7.8

3.3

6.1

B.

52.0

15.0

2 7

5.5

5:4

8.1

3.2

6.1

DIFFERENTIATION AND DERIVATION OF DIKES.

Just as in the preceding section it has been shown that a lack of 

definite knowledge of the relative volumes of the magmas prevents us 

from obtaining exact results by combining them, so the same difficulty 

arises in trying to determine the oi'igin'and derivation of the dikes. 

In spite of this, however, some general relations may be shown which 

are interesting and instructive. For instance, the dikes in and 

around Highwood Gap are clearly referable to either salic or femic 

types, and taken together they are complementary, so that it seems 

probable that if combined in proper proportions they would indicate 

the parent magma from which they were derived. For this purpose 

there are four analyses, two of salic and two of fernic types, as shown

Bull. 237 04 13
194 IGNEOUS EOCKS OF HIGHWOOD MOUNTAINS. [KOLL. 237.

by I, ii, in, and iv in the subjoined, table of analyses. In the next 

four columns, v, vi, vil, and vm, are shown the averages obtained 

by combining each salic and femic magma in equal proportions.

Combination and comparison of analyses of dikes and stocks. 

SiOj......

A12 03 .... 

Fe2 03 .... 

FeO......

MgO.....

CaO---.-.

Na2O.....

K2 0--...-

I.

58.0

17.3 

2.5

1.2

1.8

3.5.

3.4

10.1

II.

57.2

18.5 

3.6 

1.2

0.7

2.3

4 5

8.6

III.

46.0

12.2 

3.9 

4.6

10.4

9.0

2 4

5.8

IV.

47.8

13.6

4.7 

4.5

7.5

8.9

4.4

3.2

V.

52; 0

14.7 

3.2 

2.9

6.1

6.3

2.9

7.9

VI.

51.1

15.3 

3.7 

2.9

5.6

3.4

7 2

VII.

52.9

15.4 

3.6 

2.9

4.7

6.2

q o

6.7

VIII.

52.5

15.5 

4.2 

2.9

4.1

5.6

4.5

K q

IX.

52.0

15.0

3.7

K K

5.. 4

8.1

3 9

6.1

X.

51.8

14.5 

5.1

q a

4.6

'7.0

2 9

7.6

XI.

PJ9 0

15. 3 

3.3

4,4

5 °'

' 7,8

q q

6,1

I. Highwoodose (tinguaite-porphyry) from dike at Highwood Gap.

II. Pulaskose (tinguaite-porphyry) from dike at Middle Peak. South Peak 

ridge.

III. Cascadose (Highwood rninette) from dike at Arrow Peak.

IV. Monchiquose (analcite-basalt) from dike at Highwood Gap,

V. Average of I and III.

VI. Average of II and III.

VII. Average of I and IV. 

VIII. Average of II and IV. 

IX. Borolanose (basic syenite) from dike at Middle Peak stock..

X. Fergnsose (fergusite) from dike at Arnonx stock, Shonkin Creek, 

XI. Average of four stocks.

X\ 

In ix and x are analyses of two of the stocks and in ix the average

of four of them, which has been obtained as explained on a previous 

page. The close general similarity of the averages obtained by this 

simplest of all methods among themselves and with the stocks and 

their average must convince every unprejudiced reader that it has 

been perfectly possible for the varied dikes to originate by the splitting up of the magmas represented > in the stocks. It is also evident that it has been done in the main by the concentration of lime, 

iron, and magnesia in one portion of the original magma and a consequent enrichment in silica, alumina, and alkalies in another. The 

main differences between the stocks arid the dike averages are in the 

relations of lime and ferrous ajid ferric irons. If we consider only 

total iron, disregarding the state of oxidation, the differences mostly 

disappear. In lime, however, the differences are over 1 per cent, 

but this is the only striking disagreement to be observed.

It will be noted that this presupposes that the volume of material 

of which the salic dikes are composed is. equal in amount to that 

of which the femic dikes are formed, and it was pointed out at the 

beginning of this chapter that in actual volume as they appear at
pinssoN.] GENEEAL DIFFEEEJSTTIATION OF IGNEOUS EOCKS. 195

the surface the salic dikes are very greatly outclassed by the femic 

ones. If we accept the view that the complementary dikes have been 

formed by the dissociative differentiation of a magma in composition 

similar to that of the Middle Peak stock, the unavoidable inference 

would be that the femic by-product had appeared at higher horizons 

in dike form much more largely than the salic, and that the greaterpart of the latter had solidified at lower levels as intrusive masses, 

to be, perhaps, exposed by later and deeper erosion. If this is so, it 

would 'be only in keeping with what the writer has previously pointed 

out as the rule in such cases/'

On the other hand, the salic dikes could have formed from a much 

more femic magma than that indicated above, just as the salic cores 

of the laccoliths have, though not necessarily by exactly the same 

processes, and this would, of course, have left much larger volumes 

of femic material to form dikes. Either this supposition or the one 

mentioned above is a perfectly reasonable one, and the evidence at 

hand is not sufficient to decide definitely between them.

GENERAL DIFFERENTIATION OF IGNEOUS ROCKS.

In a relatively small body of inclosed magma, such as we find in the 

laccoliths, it is not difficult to imagine a process by which differentiation has taken place and an outer femic mantle produced by crystallization aided by other agencies, such as convection currents. This 

involves the separation of a solid from a liquid, and evidently the 

differentiation in stocks, such as Highwood Peak and the complementary dikes where both varieties have been injected in liquid form, 

could not be explained in this way, at least not without modification. 

Still less could we explain thus the variation of the mass of one stock 

from the mass of another if we believe that they have been formed 

from some greater, deeper body of magma that was once homogeneous.

Schweig has suggested, as previously mentioned, that under sufficient pressure compounds might be forced to crystallize out, and that 

then, descending, through greater specific gravity, heterogeneity would 

be produced. If the pressure should then be relieved, they would be 

incapable of existing in solid form, would melt, and liquid masses of 

differing composition might thus be formed. In some ways this is a 

very tempting hypothesis. For instance, as has been shown, there 

occurred at Highwood Peak an eruption of salic lava (adamellose, 

trachyahdesite) filled with more or less resorbed hornblende, and this 

was followed by femic lava (shonkinose, analcite-leucite-basalt) in 

which augite completely replaces hornblende. We could imagine that 

in the lava column under great pressure hornblende was forced to 

crystallize, .especially as under such a condition the water vapor 

necessary for its formation, but not for augite, would be present.

a Am. Jour. Sci., vol. 50, 1.895, p. 116.
196 IGNEOUS EOCKS OF HIGHWOOD MOUNTAINS. I BOLL. 237.

\

These hornblendes, descending, through gravity, would enrich the 

lower portion of the column in lime, iron, and magnesia. When eruption then took place a salic lava would be ejected, and the pressure 

being removed the remaining hornblendes which had not yet had time 

to descend would start to remelt, but would not have time enough 

before the mass chilled and prevented complete refusion. Hence we 

find them partially resorbed. This would be followed by a femic 

magma, but the water having mostly escaped and the pressure been 

relieved with the establishment of the opening of the conduit to the 

surface, this lava would have iron ore, olivine, and augite in the 

place of hornblende, which had had opportunity to completely remelt.

So far the hypothesis is tenable, but in the following events we are 

confronted by the fact that at Highwood Peak, in the next eruption, 

that of the monzonitic rock of the stock, there is a less femic magma, 

which was succeeded by one the most salic in the whole district a 

quartz-bearing pulaskose. Evidently some entirely new arrangement 

in the mechanism of movement and eruption must be devised to meet 

this. The same difficulty arises in the case of some of the dikes, 

whose order of succession is given in a later paragraph.

And again, if we accept what appears to be the field evidence at 

Middle Peak, the differentiated dikes made their appearance earlier 

than the stock whose composition they unitedly represent.

These will serve as fair examples, which might be almost indefinitely 

increased from other districts, of the difficulties that stand in the 

way of a complete acceptance of this theory. To them should be 

added, as already pointed out, the homogeneous character of masses 

exposed by erosion through great vertical distances. In fairness, 

however,'it should be admitted that these difficulties are just as pertinent to many other explanations which have been offered for the 

differentiation of molten magmas as they are for this one.

As time goes on it becomes more evident that the problem is-an 

increasingly complex one; no single explanation of one occurrence will 

do for all. In what we call the differentiation of rock magmas many 

factors have worked together; in one place one factor or set of factors 

has been dominant, in other places these are less so. For a single occurrence a satisfactory explanation may be offered, but a general one 

must take a comprehensive view of. the whole field, of the varied 

phenomena to be explained, and of the difficulties which arise. 

There are varied agencies to be considered crystallization, convection 

currents, molecular diffusion under varied aspects, tendency to form 

eutectic mixtures, addition and subtraction of water vapor, increase 

and decrease of heat, increase and relief of pressure, the mechanism 

of the movements of magma produced by crustal displacements, the 

effects of these upon the relative saturation of the magmas with certain compounds, the effects of varied electrical status, and many 

others.
PIRSSON.] GENERAL DIFFERENTIATION OF IGNEOUS ROCKS. 197

1 The truth of the matter is that we are as yet not at all prepared to 

give a general explanation of the differentiation of magmas. There 

are many things to be learned before we can do so. Every suggestion which has as much merit as that of Schweig's helps, but can not 

be accepted for the whole .truth. It appears to the writer that at the 

present time a statement of the problem presented by some of the 

phenomena and of the difficulties to be met would be most useful and 

would perhaps tend to prevent the development of obviously wrong 

hypotheses. Further discussion of this subject here would transcend 

the proper limits of this work, and the writer hopes to take it up at 

another time and place.

MATHEMATICAL RELATIONS OF MAGMAS SHOWN BY GRAPHIC METHODS.

In the discussion of the origin of the rocks of the Little Belt Mountains the author showed a that their molecular ratios, as given by. the 

analyses, could be arranged in a simple linear series forming a diagram from which, if the percentage of one oxide of an element in a rock 

in the.district was known, its entire chemical composition could be 

deduced. Washington 6 has extended the same process to the complex at Magnet Cove, Arkansas. With the Highwood analyses the 

attempt to form a similar simple linear series has not succeeded, and it 

is evident that they are more complex or that the requisite data are 

not at hand. In addition to the relations which have been previously 

given in this discussion others might be shown; thus, if we should 

take one part of the syenitic rock of the Square Butte laccolith and 

combine it with one part of the shonkinoid portion of the same mass 

(see in and xvn of the table of Highwood analyses previously given), 

we should obtain the result presented in column A below, which may 

be compared with the analysis of the Middle Peak stock, given in 

column B. The general resemblance is very close.

Comparison of analyses.

SiO2 .______...........'--.-__. ...--..- ._-......_.__....-.---.

A1^OS . ............................... ..!.... ......................

FeA ------------------------------------------------------------

FeO.. ............................................................

MgO. ..._.............-...:'.........---------............. -...:..

CaO ............................ ...1...,..^............... :.......

Na20_... .........................................................

KaO

A.

51.5

15.0

2 4

6.3

5.1

7.6

3.7

5.4

B.

52.0

15.0

2.6

5.5

5.4

8.1-

3.2

6.1

« Twentieth Ann. Rept. U..S. Geol. Survey, pt. 3,1900, p. 569.

t> Foyaite-ijolite series of Magnet Cove: Jour. Geol., vol. 9,1901, p. (545.
198 IGNEOUS EOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

Although 'such relations are to be seen, the attempt to combine 

them and to place the whole group in mathematical relation has so 

far not succeeded, possibly because there is no definite relation, but 

in the belief of the writer it is because there is in this series no such 

initial starting point as was afforded by the analyses of the differentiated mass of Togo Peak.

ARRANGEMENT OF VOLCANIC CENTERS.

There is no particular plan in the arrangement of the volcanic centers in the Highwoods; they show no such disposition as would enable 

one to say that they are placed on fault lines. It is evident that there 

is no profound faulting or tectonic disturbance to be seen in the Highwood area; the facts at hand are quite to the contrary. The general 

plan of the mountains and their geologic history already given show 

that the first upward movements of the magmas began with intrusion 

of laccoliths and of one stock. There is no evidence that this was 

attended by surface outbreaks, though this may have been the case. 

These movements were attended with shattering of the strata and the 

intrusion of dikes, and it was through this mass of weakened strata 

that the later outbursts took place, with intrusions of stocks now here 

now there. The outbreaks at the Shonkin center were of great violence and attended with profound effects upon the circumjacent beds, 

as is clearly shown by the remarkable aureole of radiant dikes that 

surround it, though it is possible that these were initiated near the 

stock in part by the contraction due to later loss of heat. The volcanic centers then appear to be caused by local outbreaks, the reason 

for the original selection of this locality not being evident.

There is at the present time with some geologists a tendency to deny 

that volcanic centers of eruption are determined by fault lines and 

fissures. It is true that in many regions they do not show any direct

evidence of this, and the facts appear to indicate that they may occur 

quite independent of such lines, but it seems to the writer that it is 

not necessary to suppose they have been formed only after one 

method. The great amount of direct evidence at hand proves that 

they are sometimes located upon fault lines and sometimes they have 

been started by explosive chimneys blown through the crust, such as 

the Maaren of the Eifel district. In this case they do not have a 

definite arrangement of ground plan.

The Castle Mountain volcano to the south seems an example in this 

part of Montana of the relation between volcanoes and fault lines. a

« Weed and Pirsson, Geology of the Castle Mountain mining district: Bull. U. S. Geol. Survey 

No. 139,1896.
GENEEAL PETEOLOGT OF THE* EEGION. 199 

AGE AND ORDER OF SUCCESSION OF THE IGNEOUS ROCKS.

The absolute geologic age of the period of volcanic activity in the 

High wood group can not be any more nearly told than that the products break through or are piled upon the Montana formation of the Cretaceous S}7 stem, which are the youngest stratified rocks.of the region. 

The outbreaks, however, may date from a somewhat later period than 

the elose of the Montana, for the eastern laccoliths, which are probably 

the oldest igneous rocks, lie in the Montana itself, and the form and 

granular character of the masses indicate that these intrusions took 

place at considerable depths below the surface. There may then 

have been upon them beds younger than the Montana, which have 

since been carried away by erosion, but there is no direct evidence of 

this. It is evident, from the fact that the fragmental volcanic material lies in places practically upon the same beds as those which form 

the floor of the laccoliths, that a long period of erosion must have separated the intrusion of the latter from the outpouring of the former. 

How long this period was we have no means of judging, but one thing 

is certain, that the general geologic relations of the. district and the 

character of its rocks indicate that its igneous activities are to be 

referred to a single geologic time phase. It is to be noted also that 

no beds of later age than the Montana are to be seen under the edges 

of the eroded cover of volcanic debris. From all of these circumstances it may be inferred that the time of igneous activity in the 

Highwoods was coincident with that of the general geologic disturbances at the close of the Cretaceous and in the early Tertiary, which 

have so profoundly affected the general Rocky Mountain region.

The order of succession of the igneous rocks has, in part, been previously mentioned. The geologic facts at our command show clearly 

distinct periods in the order of succession of the upward movement 

of the molten magmas, as follows: First, intrusion of the laccoliths, 

followed by very considerable erosion; second, outbreaks of volcanic 

activity yielding feldspathic lavas, followed by some erosion; third, 

intrusion of the stocks into the masses of feldspathic and basaltic 

extrusives.

Connected with these main episodes are a number of minor ones 

Avhich deserve consideration on account of their bearing on the petrologic history of the region. The stock at Middle Peak, on the ridge 

between Highwood and South peaks, must be much older than the 

other stocks and antedate the outbreaks of extrusive material. This 

seems clear from the position of the sedimentary beds in the ridge, 

which are horizontal and in undisturbed position right up to the contact, of the intrusion. It is impossible to imagine that the intrusion 

could have occurred along the crest of a narrow ridge composed of 

horizontal beds, displacing one half of the ridge and yet not disturb-
200 IGNEOUS EOCKS OF HIGHWOOD MOUNTAINS. [BULL. 237.

ing the short, narrow strips of strata left in the other half. A great 

amount of erosion has occurred, by which the stock has been exposed 

to depths yielding moderately coarse granular rock and the surrounding strata carried away, the most resistant portion, the contact edge of 

metamorphosed sediments, standing up as a ridge. Far below this 

ridge, down in.the valleys, as at Comb Butte as well as upon it, He 

extrusive materials, and into these extrusives the other stocks are 

thrust. Hence the intrusion of Middle Peak, like that of the laccoliths, antedates by a considerable period of erosion the volcanic outbreaks, as stated above.

The ridge, however,, as shown on the map, is cut transversely by a 

great, number of dikes, of both feldspathic and basaltic types. These 

have also aided in the metamorphism and general stiffening of the 

structure and in resistance to erosion. They run directly to the contact edge of the stock and then cease. For this reason they clearly 

appear to be cut off'by it and to be older than it is. While the 

exposures of the stock on the western slopes of the ridge are much 

broken down and largely of slide rock, this general fact seems evident. The evidence in the field then shows that there was here, first, 

an intrusion of various dikes, then that of the stock cutting them off, 

then a period of erosion, and then volcanic outbreaks. The field evidence in regard to the dikes is, on the other hand, opposed by a certain petrographic fact exactly the same types of rocks found in the 

dikes are also found elsewhere in the area in other dikes cutting the 

breccias and therefore of much later date. This iy especially notable, 

for example, in those basaltic types' full of large biotite phenocrysts 

(Highwood minettes), called in the new classification " phyro-biotitle 

shonkinose." One of these occurs on the Middle Peak ridge and also 

cutting the extrusives of Lava Peak. It is, of course, in nowise impossible that this should occur, yet at the same time it is surprising to.

find such a peculiar and distinctive type of rock produced at two dif^ 

ferent periods separated by a long interval of time. The field evidence, however, is so much the stronger that we must accept this as 

a fact, if the previous interpretation is correct. That it is is also 

indicated by the recurrence of certain peculiar leucitic types in the 

laccoliths and again later in and around the Shonkin core.

In regard to the relative age of the feldspathic and basaltic dikes 

in the Middle Peak ridge, the only evidence was found on the slopes 

leading down to High-wood Gap, where the basaltic type was found in 

one case clearly cutting the feldspathic.

In regard to the later system of dikes, all that can be said is that the 

basaltic ones cut the basaltic flows and breccias and that the feldspathic ones cutting the- Shonkin stock are, so far as we have evidence, the latest rocks of the region.
PIRSSON.] OEDEE OF SUCCESSION. . 201

Summing up, then, all the evidence at hand as previously given, we 

have as the full order of succession of the igneous rocks:

	Intrusions of laccoliths (salfemic magmas splitting into dosalic and salfemic).

a Intrusion of dikes, feldspathic (dosalic).

6 Intrusion of dikes, basaltic (salfemic).

c Intrusion of Middle Peak stock (dosalie).

2 Erosion interval, followed by High wood volcano.

a Outbreak of feldspathic (dosalic) lavas.

& Outbreak of basaltic (salfemic) lavas.

c Intrusion of High wood monzonite (dosalic).

d Intrusion of Higliwood syenite (persalic).

4 Short erosion interval, followed by Shonkin volcano.

a Outbreak of basaltic (salfemic) lavas.

b Intrusion of stocks, Shonkin, East, and Arnoux (salfemic).

c Intrusion of dikes, basaltic (salfemic).

d Intrusion of dikes, feldspathic (dosalic and persalic?).

INDEX.

A.

Page.

Absarokite. See Lam arose; Absarokose. 

Absarokose, analysis of.................... 145

Adaraellose, analyses of.............. 164,172,191

occurrence of........................... 37

See also Trachiphyro-hornblende-adamellose. 

Adams, F. D., cited on differentiation of

stocks.......................... 190

Age of igneous rocks ..................... 199-201

Akerite. See Toscanose; Phlegrose.

Albanose, analyses of ..................... 86,109

See also Missourote.

Amygdaloidal basaltic lavas, character of.. 159 

Analcite, analysis of, from pulaskose (sodalite-syenite) of Square Butte ... 68 

in leucite-shonkinose (leucite-shonkinite) of East Peak............ 107-108

in monchiquose (analcite-basalt).... 151-155

Analcite-basalt, analyses of.......... 156,168,173

See also Monchiquose. 

Analcite-leucite-basalt. See Phyro-shonkinose. 

Arkansose (leucitite), analysis of.......... 86

Arnoux stock, fergusose (fergusite) of,

chemical composition of. 74,85-87,172 

fergusose (fergusite) of, classification

of............................... 87-89

microscopic characters of........... 84-85

mineral composition of............ 88,176

occurrence and megascopic characters of .......................... 83

plate showing...................... 82

texture of .......................... 87

geology of.............................. 28-29

Arrow Peak, altitude of.................... 16

phyro-biotite-cascadose from, analysis

of......................... 109,145,173

mineral composition of............. 177

Aspen Creek, monzonose from, analysis

of............................. 134,172

Augite-latite, analysis of................... 164

See also Shoshonose. 

Augite-syenite, occurrence of.............. 48

B. ,

Biickstrom, H., cited on adamellose........ 163

Barkevikite, analysis and molecular ratio 

of, from pulaskose (socialitesyenite) of Square Butte ....... 68

Barus, Carl, and Iddings, J. P., cited on 

electrical conductivity in molten magmas.................... 187

Basalt, analysis of.................... 109,168,173

See also Shonkinose. 

Basaltic dikes, occurrence of....:.......... 35

Basaltic extrusives, occurrence of.......... 38-39

Basaltic lavas, character of .............. 158-159

Basaltic tuffs and breccias, character of.... 160

Bayley, W. S., cited on hornblende-bearing

rockfromRed Hill............. 70

Becker, G. F., cited on differentiation... 184,189 

Bibliography............................... 15

Biotite-cascadose. See Phyro-biotitc-cascadose. 

Biotite-vulsinite, analysis of................ 164

See also Shoshonose. 

Borolanite, analysis of.................... 92,139

See also Borolanose. 

Borolanose, analyses of.......... 74,79,92,139,173

of Palisade Butte, petrography of...... 95-96

of Shonkin Sag laccolith, petrography

of............................... 96-97

used in computation of original magma

of Shonkin Sag laccolith ....... 182

Scealso Grano-borolanose; Trachiphyroborolanose. 

Bostonite var. gauteite. See Trachiphyromonzonose.

Breccias, petrography of tuffs and flows.. 158-170 

Breccias and flows, volcanic, plate showing. 16

 See also Extrusive flows and breccias. 

Breccias and tuffs, basaltic, character of... 160 

Brogger, W. C., cited on grorudite-tinguaite

series........................... 139

cited on Monzoni rocks ................ 103

cited on monzonite of Monte Mulatto.. 77 

Bronzite-andesite, analysis of .............. 164

See also Adamellose. 

Byrnes Creek, dike on..............'........ 32

C.

Camptonose (leucite-absarokite), analysis

of............................... 117

Cascade formation, occurrence and character of......................... 55

Cascadose. See Phyro-biotite-cascadose.

Chotose (leucitite) analysis of............. 86,168

Ciminite, analysis of ....................... 134

See also Monzonose.

Classification of igneous rocks, table showing ............................. 59-60

Coleman, A. P., cited on heronite........... 127

Colorado formation, occurrence and character of......................... 55-56

Contact facies of Shoshonose (monzonite)

of SouthPeak.................. 83

203
204 INDEX.

Page. 

Contact phenomena of Highwood Peak

stock........................... 22-23

of Middle Peak stock................... 23-24

Convection, production of differentiation 

in laccoliths by crystallization 

and........................... 187-190

Coues, Elliott, cited on Highwood Mountains ........................... 16

Covite, analysis of,........................ 92,139

See also Borolanose. 

Cross, 'Whitman, cited on pyroxenes of

Cripple Creek district.......... 72

Crystallization, production of differentiation-in laccoliths by.......... 185-187

production of differentiation in laccoliths by convection and ...... 187-190

D.

Dakota formation, occurrence and characterof............................ 65

Dana, E. S., cited on pyroxene............. 61

Davis. W. M., cited on dikes ............... 31

cited on geology of Highwood Mountains. ............................. 18

Davis Creek, view on....................... 16

Differentiation in dikes.................. 193-195

in igneous rocks...................... 195-198

in laccoliths, osmotic theory of ...... 182-183

production, of, by convection and

crystallization................ 187-190

production of, by crystallization.. 185-187 

production of, by electricity ....... 187

theories of........................ 183-185

in stocks, discussion of............... 190-193

Dikelets in grano-shoshonose (monzonite)

of Highwood Peak ............. 82-83

Dikes, character of.......................... 32-34

differentiation and derivation of..... 193-195

radial disposition of.................... 31-32

relative ages of......................... 35-36

rocks composing ....................... 34

See also Feldspathic dikes; Basaltic

dikes. 

Dikes and sheets, petrography of........ 121-170

Diopside, composition of, from Highwood

Peak............................ 61

Doelter, C., cited on shonkinite of Monzoni. 103 

Drainage, direction and character of...... 16

E.

Eagle formation, occurrence and character

of............................... 56

East Peak, leucite-shonkinose (leuciteshonkinite) of, analcite in.... 107-108

leucite-shonkinose (leucite-shonkinite)

of, chemical composition of.. 108-110,

168,173

classification of................... 110-111

megascopic characters of......... 105-106

 microscopic characters of .......... 106

mineral composition of........... 110,177

East Peak stock, geology of................ 24-26

Electricity, production of differentiation in

laccoliths by.................... 187

Endomorphic contact phenomena at Middle Peak stock................... 24

Page. 

Erosion monoliths, view of................. 48

Extrusive flows and breccias, distribution of 36 

Extrusive flows, breccias, and tuffs, petrography of...................... 158-170

Extrusive rocks, sources of.............\... 39-41

Extrusives. See also Feldspathic extrusives; Basaltic extrusives.

Feldspathic dikes, occurrence of........... 34

Feldspathic extrusives, occurrence of...... 37-38

Feldspathic lavas and tuffs, character of... 158 

Femic dikes, occurrence of................ 35

Fergusite. See Fergusose.

Fergusose (fergusite) of Arnoux stock,

chemical composition of....... 74,

85-87,172 

of Arnoux stock, classification of...... 87-89

microscopic characters of.......... 84-85

'mineral composition of............ 88,176

occurrence and megascopic characters of.......................... 83

plate showing...................... 82

texture of .......................... 87

Flows and breccias, volcanic, plateshowing. 16

See also Extrusive flows and breccias. 

Flows, breccias, and-tuffs, petrography of. 158-170

G.

Gauteite, analyses of..................... 134,172

See also Trachiphyro-monzonose. 

Geography and topography, discussion of.. 15-16 

Gooch, F. A., cited on feldspar in trachiphyro-highwoodose............ 127

Graeff, F., cited on fergusose from Brazil... 84 

Grano-borolanose (basic syenite) of Middle

Peak, border facies of.......... 94-95

of Middle Peak, chemical composition

of....'................ 74,91-93,139,172

classification of..................... 93-94

megascopic character of........... 89,176

microscopic characters of.......... 89-91

mineral composition of......... 92-93,176

Grano-pulaskose (syenite var. pulaskite) of 

Highwood Peak, chemical composition of................. 63,172,191

of Highwood Peak, classification of.... 64-65

microscopic characters of.......... 61-62

mineral composition of............ 64,176

occurrence and megascopic characters of .......................... 60

Grano-shoshonose (monzonite) of Highwood Peak, chemical composition of ...... 78-80, 92,164,172,191,194

of Highwood Peak, classification of.... 81-82

contact facies of.................... 83

dikeletsin. ........^................ 82-83

microscopic characters of.......... 77-78

mineral composition and texture

of.............................. 80,176

occurrence and megascopic character of........................... 76

Grorudite-tinguaite series, petrography

of............................. 130-131
INDEX. 205

H.

Page.

Hackmann, V., cited on nosean............ 76

Hawes, G. W., cited on hornblende-syenite

from Columbia, N. H........... 62

Hayden, F. V., cited on geology of Highwood Mountains............:.. 17

Hibsch, J. E., cited on bostonoid rocks .... 135

High wood Creek, dike near................ 34

Highwood Gap, dikes at, map showing..... 36

h i g h w o o d o s e (tinguaite-porphyry)

from, analysis of.......... 128,172,194

monchiquose (analcite-basalt) from,

analysis of.................... 150,168

view north from........................ 16

Highwood minette. See Phyro-biotite-cascadose. 

Highwood Mountains, geologic map of.... 20

location of.............................. 14-15

petrology of..........................171-201

topographic map of..................... 14

topography and geology of............. 15-16

Highwood Peak, altitude of................ 16

diopside from, composition of.......... 61

grano-pulaskose (syenite var. pulaskite)

of, analysis of............. 63,172,191

classification of..................... 64-65

microscopic characters of.......... 61-62

mineral composition of........... 64,176

occurrence and megascopic character of........................... 60

grano-shoshonose (monzonite)of, chemical composition of............ 78-80,

92,164,172,191,194 

classification of..................... 81,82

contact facies of. '. .................. 83

dikeletsin.......................... 82-83

microscopic character of............ 77-78

mineral composition and texture

of.............................. 80,176

occurrence and megascopic character of........................... 76

magmas at, composition of........ ^.. 191,192

rocks at ................................ 20-23

Highwood Peak stock, contact phenomena

at............................... 22-23

geology of.....................:........ 20-23

Highwood rocks, analyses of............. 172-173

norms of.............................. 175-178

Highwood tinguai te-porphyry. See Trachiphyro-highwoodose. 

Highwoodose. See Trachiphyro-highwoodose. 

Hornblende, analysis of, from sodalitesyenite of Square Butte ........ 67

Hornblende-adarnellose. See Trachiphyrohornblende-adamellose. 

Hornblende-solvsbergite. See Umptekose.

I. 

Iddings, J. P., cited on absarokite.......... 14G

Iddings, J. P., and Barus, Carl, cited on 

electrical conductivity in molten magmas.................... 187

Igneous rocks, age and succession of..... 199-201

classification of.........................' 59-60

differentiation of..................... 195-198

geology of ............................... 20-29

Intrusive sheets, geology of................ 30-31

J.

Page. 

Janeirose (leucitophyre), analysis of....... 86

Johnston-Lavis, H..J., cited on structure of

laccoliths...................... 52,182

Jointing at East Peak stock................ 25

Judithose (tinguaite-porphyry), analysis

of............................. 123,128

K.

Kentallenite, analysis of ................... 102

Kentallenose, analyses of.. '. .............. 79,102

L.

Laccolith. See Shonkin Sag laccolith. 

Laccoliths, differentiation in............. 181-190

geology of.............................. 42-54

magmas of, computation of ............ 182

petrography of stocks and............. 60-121

Lagorio, A., cited on differentiation by crystallization ...................... 185

Lamarose (absarokite), analysis of......... 145

Latite. See Trachiphyro-hornblende-adamellose: 

Laurdalose (tinguaite), analysis of......... 123

Lavas, basaltic, character of............. 158-159

Lavas and tuffs, feldspathic, character of.. 158 

Leucite, analyses of......................... 86

Leucite-absarokite. See Camptonose. 

Leucite-basalt, analysis of ....... 109,117,168,173

occurrence of........................... 45

See also Shonkinose.

Leucite-shonkinite. See Leucite-shonkinose. 

Leucite-shonkinose (leucite-shonkinite) of

East Peak, analcite in........ 107-108

of East Peak, chemical compo'sition

of..................... 108-110,168,173

classification of.................. 110-111

megascopic characters of ........ 105-106

microscopic, characters of.......... 106

mineral composition of............. 110

occurrence of...................... 25-26

Leucitite. See Chotose; Albanose; Arkansose. 

Leucitophyre. See Shonkinose; Janeirose. 

Lindgren, Waldemar, cited on analcite.... 35

cited on analcite-basalts...... 149,150,151,154

cited on dikes .......................... 31

cited on geology of Highwood Mountains. ........................:.. 18

cited on Highwood Mountains......... 57

cited on sodalite-syenite......:......... 76

cited on trachiphyro-highwoodose ..... 126

Lindgren, Waldemar, and Melville, W. H.,

cited on Square Butte.......... 49

Liparose, analyses of........................ 63

Lowinson-Lessing, F., cited on missourite.. 118 

 Ludlow, W., cited on Highwood Mountains. 17

M.

Magmas, chemical characters of......... 171-178

computation of........................ 182

geologic occurrence of...:............ -178-180

mathematical relations of............ 197-198

relative volumes of................... 180-181

Mann, dike near ........................... 32

Melville, W. H., work of ................... 18

Melville, W. H., and Lindgren, Waldemar,

cited 011 Square Butte.......... 49
206 INDEX.

Page. 

Merrill, G. P., cited on eruptive rocks of

Montana ........................ 185

Miaskose (solvsbergite), analysis'of........ 128

Mica-basalt, analyses of.......... 109,145,164,173

See also Shonkinose; Phyro-biotite-cascadose. 

Micromonzonite, analysis of ............... 79

Middle Peak, grano-borolanose (basic syenite of, border facies of ........ 94-95

grano-borolanose (basic syenite) of,

chemical composition of ....... 74,

91-93,139,172 

classification of..................... 93-94

megascopic characters of........... 89

microscopic characters of .......... 89-91

mineral composition of......... 92-93,176

Middle Peak stock, geology of.............. 23-24

pulaskose (tinguaite-porphyry) from,

analysis of...................... 128

Minette, occurrence of .................. 30-31,35

of High-wood type. See Phyro-biotitecascadose. 

Missourite, analysis of......... 86,109,117,168,173

occurrence of........................... 27

map showing....................... 28

See also Missourote. 

Missourote (missourite) of Shonkin stock,

chemical composition of ....... 86,

109,117,168,173 

of Shonkin stock, classification of.... 118-120

megascopic characters of......... 115-116

microscopic characters of .......... 116

mineral composition of .......... 118,177

Monchiquose (analcitc-basalt), analcite

in ............................ 151-155

chemical composition of ..... 155-156,168,173

classification of....................... 157-158

megascopic characters of............... 150

microscopic characters of....:....... 150-151

mineral composition of .....'......... 157,177

Monoliths, occurrence of................... 50-51

Montana formation, occurrence and character of......................... 56

Montanose (shonkinite) of Shonkin Sag

laccolith, characters of....... .111-121

of Shonkin Sag laccolith, chemical composition of...................... 92,

102,109,112-113,145,156,173 

classification of................... 114-115

mineral composition of............. 177

used in computation of original magma

of Shonkin Sag laccolith....... 182

Monzonite, analyses of..................... 79

occurrence of........................... 21

See also Shoshonose; Grano-shoshonose;

Kentallenose; Monzonose. 

Monzonose, analyses of............... 79,134,172

See also Trachiphyro-monzonose. 

Monzonose-adamellose (mica-basalt), arialj ysis of ........................... 164

Mullins, John, reference to ................ 17

North Willow Creek, adamellose (trachyandesite) from, analyses of.. 164,172,191 

Nosean-syenite. See Tracho-highwoodose.

0.

Page.

Odinite, analysis of......................... 134

Orendose (wyomingite), analysis of........ 86

Osann, A., cited on nosean................. 73,76

Osmotic theory of differentiation in laccoliths, discussion of............ 182-183

P.

Palisade Butte, borolanose (syenite) from,

analysis of...................... 92

borolanose (syenite) from, petrogiaphy of.......................... 95-96

dikes near.............................. 33

general description of.................. 48

laccolithic character of................ 49

rock variation in .............:......... 48

shonkinose columns near, view of ..... 48

syenite and shonkinite in, relative volumes of......................... 181

Parting, platy, at Square Butte, origin of.. 52-53 

Petrography, discussion of ................ 57-170

Phlegrose (syenite), analyses of............ 63

Phonolite. See Borolanose. 

Phyro-biotite-cascadose (minette of Highwood type), chemical composition of................ 109,144-146,173

classification of....................... 147-149

megascopic characters of............... 143

microscopic characters of............. 143-144

mineral composition of............... 146-147

occurrence of............. v ........... 1 142

texture and name of................... 149

Phyro-shonkinose (analcite-leucite-basalt),

chemical composition of ..... 167-168

classification of....................... 169-170

microscopic characters of............ 166-167

mineral composition of................. 169

occurrence and megascopic characters

of...................'............. 166

Piuewood Peak flow, shonkinose (basalt)

from, analysis of................ 109

Pinewood saddle, shonkinose (analcitebasalt) from, analysis of........ 191

Pirsson, L. V., cited on analcite............ 35

cited on analcite of Little Belt Mountains............................ 72

cited on analcite-basalts ............... 149

cited on differentiation in stocks....... 190

cited on fergusose from Montana ...... 85

cited on minettes of Little Belt Mountains. ........................... 143

cited'on monzonite of Yogo Peak...... 77

cited on rocks of Little Belt Mountains. 197 

Pirsson, L. V., and Weed, W. H., cited on

Castle Mountain volcano....... 198

cited on Castle Mountains.............. 189

cited on hornstone in Castle Mountains. ........................... '24

cited on tinguaite (judithose).......... 122

Platy parting at Square Butte, origin of ... 52-53

Porphyritic akerite. See Phlegrose.

Pulaskite, analyses of...................... 63

See also Pulaskose; Grauo-pulaskose. 

Pulaskose, analyses of.............. 63,68,128,172

Pulaskose (sodalite - syenite) of Square 

Butte, analysis and m'olecular 

ratio of ......................... 68
207

Page. 

Pulaskose (sodali te-syenite) of Square Butte,

classification of................. 69-70

of Square Butte, comparison with related types ..................... 70

computation of original magma of. 182 

megascopic characters and texture

of............................... 66

mineral composition .............. 67,176

plate showing...................... 54

See also Grano-pulaskose; Trachiphyropulaskose.

Pyroxene, analysis of, f rom Edenville, N. Y. 61 

analysis of, from shonkinose (shonkinite) of Square Butte................. 98

Q.

Quartz-banakite, analysis of................ 134

R.

Rammclsberg, C., cited on biotite of Monzoni ..............'.....'.........

Ramsay, W., cited on platy parting...... 62

Raynolds, W. P., cited on Highwood Mountains. .........................'.. 17

Rosenbusch, H., cited on calcite between

feldspars........................ - 62

cited on leucitophyre ....I............. 86

cited on nionzonite..................... 82

cited on rocks of Montana.............. 57

cited on rocks of tinguoid habit........ 131

cited on surface lavas .................. 87

S.

Salic dikes, occurrence of.................. 34

Schmidt, C., cited on gauteite ............. 135

Schweig, M., cited on differentiation....... 184

Scorinceous basaltic lavas, character of..... 159

Sedimentary rocks, occurrence and character of......................... 55-56

Sheets and dikes, petrography of......... 121-170

Shonkin Creek, albanose (missourite) from,

analysis of......... 86,109,117,168,173

fergusose (fergusite) from, analysis of.. 74,

86,172

missourite from, analysis of. 86,109,117,168,173 

trachiphyro-borolanose from, analysis

of.............................. 74,139

Shonkin Sag, major laccolith of ............ 43-48

minor laccolith of...................... 43

Shonkin Sag laccolith, borolanose (syenite)

of, analysis of................... 92

borolanose (syenite) of, petrography of. 96-97 

dissection of, cause of................... 46.

end of, description of.................... 44

general features of..................... 43-44

' interior of.............................. 45

montanose (shonkinite) of, analysis of. 92,

102,109,113,145,156,173.

characters of..................... 111-112

classification of................... 114-115

computation of original magma of. 182 

mineral composition of ............ 177

montanose and borolanose in, relative

volumes of...................... 181

Page. 

Shonkin Sag laccolith, plan of............. 44

rock composing ........................ 44

sections of.............................. 47

stereogram of........................... 46

structure of ............................ 46-48

syenite of, computation of original

magma of ...................... 182

view of.............................'.... 54

Shonkin stock, geology of.................. 26-28

missourote (missourite) of, chemical

composition of .... 86,109,117,168,173

classification of................... 118-120

megascopic characters of......... 115-116

microscopic characters of .......... 116

mineral composition of .......... 118,177

Shonkinite, analysis of ................... 92,

102,109,113,117,145,156,173 

occurrence of..................... 43,45,46,48

See also Montanose; Shonkinose. 

Shonkinose (shonkinite), columns of, view

Of............................... 48

of Square Butte, chemical composition

of............. 102-103,109,117,145,173

classification of................... 104-105

megascopic characters of........... 97

microscopic characters of ......... 98-101

mineral composition of...... 103-104,177

plate showing...................... 100

used in computation of original magma

of SquareButte................. - 182

See also Phyro-shonkinose. 

Shoshonose. See Grano-shoshonose. 

Slopes, character of........................ 16

Sodalite, analysis of, from pulaskose (sodalite-syenite) of Square Butte ... 68 

Sodalite-solvsbergite-porphyry. See Trachiphyro-pulaskose. 

Sodalite-syenite. See Pulaskose. 

Solvsbergite. See Miaskose. 

Solvsbergite-tinguaite. See Umptekose. 

South Peak, borolanose (basic syenite)

from, analysis of................ 74

tracho-highwoodose (nosean-syenite) 

from near, chemical composition of...................... 72-74,172

microscopic characters of.......... 71-72

occurrence and megascopic characters of........................ 71

Square Butte, diagrammatic section at.... 53-54

erosion monoliths at, view of .......... 48

general description of.................. 49-50

laccolithic origin of.................... 50

monoliths at............................. 50-51

platy parting at, origin of.............. 52-53

pulaskose (sodalite-syenite) of, analysis

and molecular ratio of........ 68,172

pulaskose (sodalite-syenite) from, classi-

. fication of...................... 69-70

comparison with related types of... 70 

computation of original magma of. 182 

microscopic characters of .......... 66-67

mineral composition of............ 67,176

pulaskose and shonkinose at, plate

showing........................ 54

shonkinose (shonkinite) of, chemical

composition of. 102-103,109,117,145,173
208 INDEX.

Page. 

Square Butte, shonkinose (shonkinite) of,

classification of............... 104-105

shonkinose (shonkinite) of, megascopic

characters of................... 97

microscopic characters of ......... 98-101

mineral composition of....... 103-104,177

plate showing...................... 100

used in computation of original

magma of...................... 182

syenite and shonkinite in, relative volumes of. .......'................. 181

white rock at........................... 51-52

Stevens, Isaac, cited on Highwood Mountains. ........................... 17

Stocks, differentiation in, discussion of .. 190-193 

geology of.............................. 20-29

original magma of, composition of..... 193

Stocks and laccoliths, petrography of..... 60-121

Streams, directions and character of....... 16

Syenite, analyses of ......................... 63,172

occurrence of..................... 21-22,45-4G

See also Borolauose; Pulaskose; Phlegrose; Toscanose; Liparose. 

Syenite, basic. See Grano:borolanose; Borblanose. 

Syenite-porphyry. See Trachiphyro-borolanose.

Syenite var. pulaskite. See Grano-pulaskose.

T.

Thornton ranch, intrusive sheet near...... 30

Tinguaite. See Laurdalose.

Tinguaite dike, analysis of.................. 128

Tinguaite-porphyry, analyses of ..... 123, 128, 172

See also Pulaskose; Judithose; Trachiphyro-highwoodose. 

Tinguoid habit, rocks of (grorudite-tinguaite series), petrography of.... 130-131

Topography and geography, discussion of.. 15-16 

Toscanose (syenite), analyses of............ 63

Trachiphyro-borolanose (syenite-porphyry),

chemical composition of........ 74,

138-139,173 

classification of....................... 140-142

megascopic characters of............... 137

microscopic characters of.............. 137-138

mineral composition of........:........ 140

occurrence of......................... 136-137

Trachiphyro-highwoodose (Highwood tinguaite-porphyry), chemical 

composition of ....... 127-129,172,194.

classification of..................:.... 129-130

mineral composition of................. 129

megascopic and microscopic characters

of............................. 126-127

Trachiphyro-hornbiende-adainellose (latite 

or trachyandesite), chemical 

composition of ....... 163-164,172,191

classification of....................... 165-166

megascopic characters of .............. 161

microscopic characters of............ 161-162

mineral composition of............... 162-163

occurrence of........................... 160

texture of .............................. 165

varieties of............................. 162

Page.

Trachiphyro-monzonose (gauteite var. of 

bostonite), chemical composi- 

. tion of..."................. 133-134,172

classification of....................... 135-136

megascopic and microscopic characters

of............................. 132-133

mineral composition of................. 133

occurrence of.....................:..... 132

Trachiphyro - pulaskose (tinguaite - porphyry) , analysis of....... 123,128,172

Trachiphyro-pulaskose (sodalite-solvsbergite-porphyry), chemical composition of..........'.. 122-123,128,172

classification of............'........... 123-125

mineral composition of................. 125

occurrence and character of ......... 121-122

Tracho-highwoodose (noseaii-syenite), from 

near South Peak, chemical composition of.................. 72-74,172

from near South Peak, microscopic

characters of ................... 71-72

occurrence and megascopic characters of .......................... 71

Trachyandesite (adamellose), analysisof. 164,172 

occurrence of........................... 37

See also Trachiphyro-hornblende-adamellose. 

Trachytes, types of......................... . £8

Tuft's and breccias, basaltic, character of... 160 

Tuffs and lavas, feldspathic, character of... 158 

Tuffs, flows, and breccias, petrography of. 158-170

U.

Umptekose (solvsbergite-tinguaite) analysis of ........................ 123,128

V. 

Volcanic centers, arrangement of.......... 198

Volcanic d6bris, slopes of, plate showing... 16 

Volcanic flows and breccias, plate showing. 16

Washington, H. S., acknowledgments to... 13 

cited on analcite from ting-uoid rocks.. 127 

cited on differentiation by crystallization ............................ 185

cited on differentiation of salic rocks.. 189 

Weed, W. H., acknowledgments to ......... 13,26

reference to ...................... 31,32,42,49

work of................................. 18

Weed, W. H., and Pirsson, L. V., cited on.

Castle Mountain volcano....... 198

cited on Castle Mountains ............. 189

cited on hornstone in Castle Mountains. ............................ 24

cited on tinguaite (Judithose).......... 122

White, C. A., reference to .................. 49

work of................................. 18

Williams, J. F., cited on fergusose from

Arkansas....................... 84

Williams Creek, intrusive sheet near....... 30

Wolff, J. E., cited on hornstone in Crazy

Mountains...................... 24

Wyorningite. See Orendose.

Zirkel, Ferdinand, cited on minettes....... 147

o