DEPABTMENT OF THE INTEEIOE

BULLETIN

OV THE

UNITED STATES

No. 139

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1896

UNITED STATES GEOLOGICAL SURVEY

CHAELES D. WALCOTT, DIRECTOR

GEOLOGY

OF THE

CASTLE MOUNTAIN MINING DISTRICT

MONT A. 1ST A.

BY

WALTER HARVEY WEED

LOUIS VALENTINE PIRSSON

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1896

CONTENTS.

Page".

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

 Preface..................................................................... 13

CHAPTER I. Introduction ..-.......^........................................ 15

Geography.............................................................. 15

Previous exploration.................................................... 17

CHAPTER II. Topography................................................... 19

The mountain .............................-- - --- - - -- - - -- 19

Streams ................................................................ 20

CHAPTER III. Structural geology and geological history of Castle Mountain ... 22

General geological structure............................................. 22

Prominent anticlines................................................ 23

Minor folds ......................................................... 24

Geological history....................................................... 26

CHAPTER IV. Stratified rocks..........v. T ................................... 30

Algonkian.............................................................. 31

Belt beds........................................................... 32

Cambrian............................................................... 34

Flathead formation ................................................. 34

Tlathead quartzite.............................................. 34

Flathead shales. ................................................ 35

Gallatin formation .................................................. 37

Silurian ................................................................ 37

Jefferson formation ................................................. 37

Devonian............................................................... 38

Three Forks shales.................................................. 38

Carboniferous. ........................................................... 38

Madison formation.................................................. 39

Flaggy limestones............................................... 39

Massive limestones.............................................. 40

Quadrant formation................................................. 41

Juratrias............................................................... 43

Cretaceous.............................................................. 44

Kootanie ........................................................... 44

Dakota............................................................. 45

Colorado group ..................................................... 45

Benton formation............................................... 46

Niobrara formation-............................................. 46

Montana group ..................................................... 47

Pierre shales.................................................... 47

Fox Hills sandstones............................................. 47

Laramie formation.................................................. 48

Livingston formation ............................................... 49

Miocene....................!............................................ 53

Lake-bed deposits................................................... 53

* 5
6 CONTENTS.

Page.

CHAPTER V. Igneous rocks................................................... 56

Massive plutonic rocks.................................................. 58

Granite............................................................. 58

Evidences of intrusion.......................................... 59

Diorite ............................................................. 61

Contact metamorphism :...................'......................... 62

Intruded sheets and dikes of porphyry................................... 64

Dikes ............................................................... 65^

Acidic dikes .................................... ................. 65

Basic dikes ...................................................... 67

Extrusive rocks or lava flows............................................ 69

Rhyolite............................................................ 69

Basalt...................:.-.................--....-................ 71

Tuffs and breccias.................................'...................... 73

Igneous rocks not of Castle Mountain origin ............................ 76

Diabase ............................................................ 76

Interbedded ash deposits.. .......................................... 77

Crazy Mountain types. ................................................. 78

CHAPTER VI. Microscopical petrography .................................... 80

Classification ............................................................. 80

Rocks of the intruded masses........ 1................................... 82

Granite............................................................. 82

Granite-porphyry................................................... 85

Granite-porphyry with micropegmatitic groundmass................. 86

Secondary origin of micropegmatitic structure.................. 86

Included masses in the granite...................................... 87

Diorite............................................................. 89

Contact metamorphism............................................. 92

Endomorphic in the granite..................................... 92

Endomorphic in the diorite-........................ T ........... 93

Exomorphic changes.................................. ^......... 93

Rocks of the intruded sheets and dikes.................................. 95

Aplitic granite ....................................................... 95

Quartz-porphyry.... .....................^.......................... 97

Quartz-tourmaline-porphyry......................................... 99

Musselshell quartz-porphyry........................................ 102

Felsite-porphyry ..........................................i......... 103

Quartz-free porphyry............................................... 106

Black Butte porphyry.............................................. 108

Crazy Mountain rocks.............................................. 109

Lamprophyre rocks. .................................................... 110

Introductory ....................................................... 110

Augite-vogesite..................................................... Ill

Minette ............................................................ 113

Monchiquite........................................................ 114

Diabase ............................................................ 117

CHAPTER VII. Microscopical petrography of the extrusive rocks1............. 118

Rhyolite............................................................... .. 118

.Lower Fourmile rhyolite....................-.-.-.-..................... 119

Fourmile rhyolite..................... .............................. 121

Devitrification....................................................... 122

Rhyolite of Checkerboard Creek.................................... 124

Rhyolite of upper Bonanza Creek................................... 126
CONTENTS. 7

Page. 

CHAPTER VII. Microscopical petrography of the extrusive rocks Continued.

Breccias and tuffs....................................................... 127

Ehyolitic tuffs...................................................... 127

Basalts................................................................'. 129

CHAPTER VIII. General petrology of the Castle Mountain district............ 132

The dike rocks ............................................................ 141

CHAPTER IX. Glaciation of Castle Mountain................................ 143

CHAPTER X. Economic geology.............................................. 148

Coal.................................................................... 148

Checkerboard Creek................................................ 148

Warm Spring Creek................................................. 148

Polishing powder....................................................... 149

Mineral springs......................................................... 149

Precious metals......................................................... 150

Character of ore.................................................... 151

Distribution........................................................ 151

Structural conditions............................................... 152

Notes on the mines........................-..-....------------------ 152

Copper ................................................................. 156

Mining claims....................................................... 157

CHAPTERXI. Mineralogy.................................................... 158

Index....................................................................... 163

ILLUSTRATIONS.'

Page.

PLATE I. Topographic map of the Castle Mountain district, Montana..... 16 

II. Castle Mountain from the south, showing intrusive sheet in foreground...... ........................................»-....... 20

III. Geological map of the Castle Mountain district.-................ 22

IV. Geological cross sections....................................... 24

V. Panorama showing granite weathering into crags and castles .. 28

VI. Cliffs 011 Sixteeninile Creek, showing Belt formation............ 32

VII. Valley of upper Fourmile Creek ............................... 34

VIII. Falls of upper Fourmile Creek................................. 60

I.. Intruded sheets and dike on upper Fourmile Creek............. 62

X. Obsidian (rhyolite) lava flow on north flank of Castle Moiintain. 70

XI. Tuff beds on north flank of Castle Mountain................... 76

XII. Micro-drawings of Castle Mountain granite .................... 86

XIII. Moraine-covered bench south of town of Castle ................ 144

XIV. Terminal moraine on Alibaugh Creek, near town of Castle...... 146 

XV. Gorge of West Fourmile Creek ................................ 146

XVI. Cumberland mine ............................................. 152

XVII. Cumberland mine from the north .............................. 154

FIG. 1. Index map, showing location of Castle Mountain district in Montana. 16

2. Fold and fault exposed on Flagstaff Creek.......................... 24

3. Folded Laramie and Livingston beds on south side of mountain along

Musselshell Eiver ................................................ 25

4. Folded Paleozoic strata oa west side of mountain................... 25

5. Columnar section of Paleozoic rocks exposed in canyon of Checkerboard Creek ..................................................... 37

6. Columnar section of rocks exposed in canyon of North Fork of Musselshell River..................................................:.... 40

7. Diagram of tilted beds of Fourmile Creek .......................... 60

8. Orthoclase crystal from miarolitic cavity of granite................. 83

9. Diagram of Cumberland ore body................................... 153

10. Cerusite crystals ................................................... 159

11. Pyroxene crystals .................................................. 161

9

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY,

Washington, D. C., February 1, 1896.

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

report upon the geology of the Castle Mountain mining district of 

Montana, by Prof. L. V. Pirsson and myself. It has been prepared for 

publication as a bulletin of the Survey. 

Very respectfully,

WALTER HARVEY WEED,

Geologist. 

Hon. CHARLES D. WALCOTT,

Director United States Geological Survey.

11

PREFACE.

This memoir is a general study of the region described, and not a 

detailed report. The field work upon which it is based formed a part.

of the work of mapping the areal geology of the Little Belt Mountains 

sheet of the Geological Survey. The topographic map, made in 1882 

by the Northern Transcontinental Survey, is too small in scale and too 

general in character to warrant more detailed work; but as the facts 

obtained are of interest, and the region is one of economic importance, 

it has been considered worthy of a special treatment.

The cessation of mining and consequent inaccessibility of the mine 

workings prevented any underground study of the ore deposits, so that, 

with the exception of those concerning the Cumberland mine, the facts 

given have been derived from a study of the surface alone.

The field work was done during the summers of 1892 and 1893, 

Professor Pirsson accompanying the writer during the latter year and 

studying the field relations of the rocks with him. The authors wish 

to acknowledge their joint indebtedness to the Sheffield Scientific 

School of Yale University. The petrographical and chemical investigation of the igneous rocks was carried on in the mineralogical-petrographical laboratory of the Sheffield School, and the authors' cordial 

thanks are due for the facilities -and many courtesies extended to them 

during the progress of the work.

W. H. W.

13

GEOLOGY OF THE CASTLE MOUNTAIN MINING 

DISTRICT, MONTANA.

BY W. H. WEED AND L. Y. PIRSSON.

/ CHAP TEE I.

INTRODUCTION.

° GEOGRAPHY.

In the State of Montana the Yellowstone Eiver on the south and 

the encircling Missouri on the west and north inclose a part of the 

Eocky Mountain region that is formed of two main chains of mountains long known as the Big Belt and the Little Belt ranges. These 

ranges have a general north-and-south trend: the Big Belt lies to the 

westward, and is separated from the Little Belt Eange by the broad, 

intermontane valley of Smith Eiver. The area whose geology is described in the present bulletin is a portion of this region, and all parts 

of it are more than 5,000 feet above sea level. (See fig. 1, p. 16, showing location of district.) Within it lies Castle Mountain, a flat-topped 

elevation, whose crown of ruin-like crags, rising above the black pine 

forests that cover its summit, at once suggests its name. This mountain mass is a distinct geographic unit, not being directly connected 

with either of the above ranges. On the south it is separated from the 

broad valley of the Yellowstone by that imposing group of lofty peaks 

known as the Crazy Mountains; northward the dark, pine-clad plateaus 

of the Little Belt Eange rise against the sky. It thus forms a distinct 

link in the chain that defines the eastern limit of the Eocky Mountain 

cordillera, from the base of which the Great Plains sweep in unbroken 

continuity to the prairies of Dakota.

Two rivers, branches of the. Missouri, have their sources partly in 

its springs and snow banks. One of them is Smith Eiver, the two 

branches of which encompass it on the west and north. The stream 

formed by their union runs northward between the two Belt ranges, 

until, passing through a deeply cut gorge, it enters the open plains 

country and flpwg into the Missouri above Great Falls. The other

15
16 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

stream, the Musselshell, whose forks inclose the mountain on the east 

and south, flows several hundred miles through the arid wastes of the 

broken plains country, and joins the Missouri above its confluence with 

the Yellowstone.

The area represented by the accompanying map (PI. I), which 

includes Castle Mountain and parts of the adjacent valleys, is limited 

by the meridians 110° 15' arid 111° and the parallels 46° 20' and 46° 

26', and is in Meagher County, Mont.

There are few areas of equal size in the Rocky Mountain region 

which present so wide a range of rock formations. An uplift, involving the entire series of stratified rocks known in the northern Eocky 

Mountain district, has been the site of a volcano which subsequent erosion has thoroughly dissected, exposing a variety of intrusive igneous

117* US' 113' 103'

117 IDS'

FIQ. 1. Index map showing location of Castle Mountain district in Montana.

rocks with associated lava flows and fragmental deposits, together with 

the various stratified rocks that formed the foundation of the mass. As 

the mountain is thus of great geological interest, and contains, moreover, important deposits of silver-lead ores that have made its name 

familiar to the mining world during the past few years, it was made 

the object of especial study in the work of mapping the areal geology 

of the Little Belt Mountains atlas sheet of the United States Geological Survey. The small scale of the map and the general character of 

the topography did not, however, permit a closely accurate outlining 

of the geology, and this bulletin should be regarded as a general 

description rather than as a detailed study of the mining district.

By reason of its geographical position at the head of the arid Smith 

Valley, the mountain acts as a condenser for the moisture gathered 

farther westward. Snowstorms are frequent as late in the season as
U.S.GEOLOGICAL SURVEY BULLETIN N? 139 PL.I

TOPOGRAPHY BY NORTHERN TRANSCONTINENTALSURVEY 1882-83 TOPOGRAPHIC MAP OF CASTLE MOUNTAIN DISTRICT, MONTANA

CONTOUR INTERVAL 200 FEET

JULIUS BIEN a CO.N.Y.
WEEDAND-I m?nr<T?APTiv 17 PIKSSON. J (iHjOtrKAJrUi. 1 i

June, and as early as September, and the annual snowfall is heavy. 

Through the brief, hot summer that scorches the adjacent valleys the 

mountain summit is the focus of many violent thunderstorms, and in 

consequence of the moisture from them the slopes are bright with 

verdure.

The ore deposits of Castle Mountain made its name well known in 

the Northwest. Their discovery brought into a region previously 

devoted solely to stock raising the feverish activity of mining enterprise, which was succeeded, however, on the cessation of silver production, by the present period of inaction. The most important town 

within the limits of the map is White Sulphur Springs, the county 

seat of Meagher County. It is a thrifty, pleasing little city of 2,500 

inhabitants, and owes its name and location to the hot springs at the 

western base of the mountain. It is one of the oldest towns of the 

State, and is as yet without direct railroad communication, though 

numerous stage Hues radiate from it in every, direction, taking daily 

mails to Townseiid, on the Northern Pacific Railroad, 40 miles distant, 

and to Neihart, the terminus of a branch line of the Great Northern. 

The mining settlements of Castle, Robinson, and Blackhawk, situated 

on the flanks of the mountain, as shown on the map, were for a while 

very active, important places, but they were wholly dependent upon 

the mining industry, and consequently they are now nearly deserted. 

The completion of a railroad now under construction from Toston, on 

the Northern Pacific Railroad, will bring these towns in direct communication with the mining and smelting centers of the West. Good 

wagon roads also afford direct communication with Livingston to the 

south, White Sulphur Springs to the west, and Martinsdale to the east. 

The high elevation of the surrounding country and the rigorous climate 

are not favorable, however, for agriculture, and the ranches now found 

in the district are devoted to the raising of hay for winter feed of 

sheep and cattle.

PREVIOUS EXPLORATION.

The pioneer explorers of the Northwest, Lewis and Clarke, named 

upon their map both Smith River and the Musselshell, but their routes 

were north and'south of this region. In 1853 Lieutenant Mullan, of 

the Pacific Railway exploring expedition, followed up the north fork 

of the Musselshell to the head waters of Smith River, and passed down 

that stream to the westward. Captain Clift, in his exploratory march 

across Montana in 1869, also followed this route, which forms a natural 

highway from the mountain region to the plains country eastward. In 

1873 Captain Ludlow, accompanied by Messrs. G. B. Grinnell and E. 

S. Dana as geologists, followed the same route, which afterwards 

became a part of the stage route from Carroll, the head of steamboat 

navigation on the Missouri, to Helena. These geologists noted the 

salient features of the geological structure and the character of the 

Bull. 139 2
18 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

rocks along the north forks of Musselshell and Smith rivers, and 

recognized the eruptive nature of the mountain mass, whose picturesque 

crags and towers form so prominent a feature, as seen from White Sulphur Springs. These observers passed up the south fork of Smith 

Eiver on their way to the Yellowstone Park, and returning followed 

down the south branch of the Musselshell. In their report 1 they give 

brief notes of the geology along their route.

In 1880-81 the geologists of the Northern Transcontinental Survey, 

organized by the Northern Pacific Eailroad for an economic survey of 

the territory tributary to the railroad, examined the country adjacent 

to Castle Mountain in their search for coal. In the only published 

reports of this organization 2 the mountain is not mentioned, but its 

geological nature is indicated upon the accompanying maps, and Mr. 

Gr. H. Eldridge gives a section of the rocks exposed at the east end of 

the mountain.3 Mr. J. E. Wolff, then of the same survey, in his paper 

on the Crazy Mountains,4 mentions the general structure of the mountain and the eruptive character of Black Butte, an outlying elevation. 

That veteran geologist and keen observer, the late Prof. J. S. Newberry, 

described the rocks about White Sulphur Springs,5 but did not visit 

Castle Mountain itself.

It is apparent that in the hurried trips of these earlier explorers 

through the circumjacent valleys the interesting character of Castle 

Mountain was only surmised. Although the country about it was 

sparsely settled by sheep and cattle ranchmen, the mountain was 

visited only to procure necessary timber or firewood. Prospectors, 

attracted by the Copperopolis veins, may have visited its slopes and 

explored its gorges, but it was not until the discovery of the rich ore 

body of the Cumberland mine, above the town of Castle, that the mountain itself attracted any attention. An immediate rush for the tl new 

Leadville," as it was fondly called, followed this discovery. Sheep 

herders, ranchers, nearly every one from the surrounding valleys, took 

up claims and built cabins upon the mountain slopes. Eoads were cut 

through the forest and graded across steep gulches, and labor and 

money were lavishly spent while the " boom" lasted.

The future of the district is dependent upon the price of silver; 

under conditions prevailing during 1893 and 1894 the mines could not 

be profitably worked, and the mountain towns were deserted for more 

promising localities.

'Keconnoissance from Carroll, Mont., to Yellowstono Park, TJ. S. War Dept., Washington, 1876.

!Coal fields of the Northwest: Tenth Census, Vol. XV.

"Ibid., p. 740.

4Notes on the Petrography of the Crazy Mountains and Other Localities in Montana, Heidelberg, 

Germany, 1883, p. 3. r

8 Notes on the geology and botany of country adjacent to Northern Pacific Kailroad: Annala N. T. 

Acad. of Sci., Vol. Ill, No. 8, p. 242. See also Am. Jour. Sci.,-3d ser., Vol. XLI, 1891, p. 191.
CHAPTER II. 

TOPOGRAPHY.

THE MOUNTAIN.

Castle Mountain is an elevated mass rising to a height of 8,600 feet 

above sea level, its highest point being 3,600 feet above the neighboring valleys. Seen from the lower valleys, it is a broad, flat-topped 

mountain mass, its upper slopes densely wooded, with castellated

masses of granite rising above the pines. Though lacking in bold, 

jagged peaks, this mountain presents a pleasing type of scenery, while 

from its slopes the views of neighboring ranges are very impressive. 

The deep gorges that score the mountain and the upland parks and 

summit meadows are extremely picturesque.

The valley drained by the forks of the Musselshell and the Smith 

rivers sharply defines the mountain as a single mass. It consists of a 

main and higher western portion joined by a trenched plateau with an 

eastern limestone ridge, which rises abruptly from the prairie terrace 

between the forks of the Musselshell. In rude outline, as defined by 

the surrounding valleys, the whole mass has the shape of a dumb-bell.

The main summit, whose highest point is designated Elk Peak on 

the map, is a thickly timbered, nearly level area, defined by slopes 

sinking abruptly to lower mountain benches, or by the walls of sharply 

incised mountain amphitheaters, in whose snow banks the largest 

streams have their sources. Eadiating from the summit, the streams 

have cut deep mountain gorges, the ridges between being flat-topped. 

These flat summits are very characteristic of the mountain top/ Alpine 

benches,, ice-swept upland terraces, and moraine terraces are everywhere prominent.

In the eastern part of the mountain formed of the extended ridge 

previously mentioned the heavily bedded Paleozoic limestones dominate the topography. In them the streams have cut narrow gorges 

with .pinnacled walls, which contrast sharply with the gentle valley 

slopes, where the rocks are shales or Mesozoic sandstones. The larger 

topographic features of this eastern part of the mountain are of structural origin.

The whole mountain is very generally outlined by the boundary of 

the Cretaceous rocks or the soft shales of the Algonkian. Where 

the readily eroded beds of these periods occur, unaltered by local

19
20 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

metamorphism, the slopes are abrupt down to the low-lying basins 

cut in them. The details of topographic configuration are very largely 

dependent upon the nature of the rocks, whose great diversity produces 

widely differing types of scenery. The gently contoured dark slopes of 

Algonkian slate are thus in great contrast to the cliffs or towering 

ledges of light Carboniferous limestone, the great de"bris slopes of 

porphyry, or the low relief of Cretaceous shale. Upon the mountain 

flanks outward-dipping beds have in a few places been eroded into 

the revet hills, or hogback ridges, so common along the foothills 

of the Kooky Mountains. Volcanic dikes or sheets, when intruded 

into the readily eroded Cretaceous strata, are generally conspicuous, 

either as imposing walls of .rock or as sharp.comb ridges. (See PI. II.) 

West of the mountain the slopes end abruptly on the broad, very gently 

sloping surface of the valley of South Smith Eiver, once a lake bottom, 

but now an arid flat, with scanty vegetation. Copperopolis Valley, as 

seen from Volcano Butte, a prominent hill in its center, has a nearly 

level floor formed by a basaltic lava flow.

STREAMS.

A glance at the map of Castle Mountain shows that the area is drained 

by numerous streams, head-water branches of the Musselshell and 

Smith rivers. These streams, with few exceptions, have their sources 

in clear and cold springs, high up on the flanks of the mountain. When 

reenforced by the melting snows of early summer they flow down as 

impetuous torrents, sink from sight beneath the gravel beds of their 

lower courses or in the underground channels of the limestone areas, 

and often reappear as enfeebled creeks, whose murky currents, meandering across the level lower valleys, are in marked contrast to the sparkling waters of the mountain area.

The larger streams, Warm Spring Creek on the south and Willow and 

Eourmile creeks on the north, head in amphitheaters cut back to the 

highest summits of the mountain. All these flow in V-shaped gorges, 

whose varying scenery depends upon the nature of the rocks cut by 

them. The west slopes of the mountain are drained by small brooks, 

of which only Cotton wood has cut any distance into the mountain. It 

is the only creek which sends its water over the valley plain.

To the east, the sharp limestone ridge forming the end of the mountain 

is scored by small, normally dry drains debouching upon the Cretaceous 

plain.

The Musselshell heads in an amphitheater south of Elk Peak.' The 

creek is there known as Warm Spring, the constant temperature of its 

tributary springs melting the snow banks of the valley. Flowing in a 

gorge whose stream bed is 1,000 feet below the mountain benches on 

either side, .with heavily wooded, spring-wetted bottom, this stream 

passes into a broader valley cut in Cretaceous rock, and then enters a 

shallow canyon with walls of 25 to 75 feet of soft sandstone or shale."
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 139 PL u

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CASTLE MOUNTAIN, FROM THE SOUTH, SHOWING INTRUSIVE SHEET IN FOREGROUND.
STREAMS. 21

Joined by Bozeman Creek and other small streams, it turns eastward, 

and becomes the South Fork of the Musselshell. Alibaugh and its 

branches Hamilton, Hensley, and Eobinson creeks are similar in 

character, and flow over bowlder-choked channels through old glacial 

moraines to unite with the Musselshell.

Bonanza Creek, heading in the beautiful mountain park near Blackhawk, cuts an impassable canyon through igneous rocks and white 

Paleozoic limestone, and reaching Mesozoic shales turns abruptly to 

the south and enters a broad basin, where its waters are taken for 

irrigation by the ranchmen of the valley.

The largest branch of the North Musselshell comes from the slopes 

of the Little Belt Eange north of Castle,Mountain, though Flagstaff 

Creek, whose head-water branches have cut picturesque canyons in the 

heavily bedded Paleozoic limestones of the eastern part of .the mountain, 

is an important tributary. Checkerboard Creek, heading in the springs 

near Blackhawk and the gorges cut far back toward the mountain summit, is the largest tributary of the North Musselshell. A noticeable 

peculiarity of this stream, explained later, is its abrupt bend to the 

east, cutting a deep cliff back of Oopperopolis Hill, instead of crossing 

the low, easily eroded country to the north to join Smith Eiver.

Smith Eiver is a considerable stream, whose chief tributaries are 

Fourmile and Willow creeks. The first of these is one whose waters 

join the river only in time of flood. Near its sources the streams forming the creek carry considerable water and flow impetuously over rocky 

channels, whose deep pools contain an abundance of trout. Its westerly 

branches head in copious springs issuing from the lower parts of drainage valleys devoid of stream channels since the disappearance of the 

ice sheets that once covered the mountain.

South Smith Eiver has a small and sluggish current of turbid water. 

It heads in a basin on the south flank of the mountain and meanders 

through the level valley plain to join the northern fork below White 

Sulphur Springs.
CHAPTER III.

STRUCTURAL GEOLOGY AND GEOLOGICAL HISTORY OF 

CASTLE MOUNTAIN.

. GENERAL GEOLOGICAL STRUCTURE.

Castle Mountain is a dissected volcano. Degradation has laid bare 

its structure, yet has not entirely removed the surface rocks that are 

the peculiar products of volcanic activity. The geological map (PI. Ill) 

shows clearly that the great granite mass that forms the mountain 

summit, and also the smaller body of diorite to the east, breaks through 

folded sedimentary rocks. On the northern flanks of the mountain 

remnants of lava flows and of fragmental deposits, and to the south 

morainal debris, partly obscure the Structure of the sedimentary rocks.

The mountain forms a part of the Eocky Mountain front, and its 

larger structural features belong to the general system of plications 

that characterize the outer margin of the cordillera region. The eastern part of the cordillera is in Montana formed of two main areas of 

uplift with .minor folds between them. The first or southern region 

composes the Beartooth and Snowy ranges, and extends northward, 

from Wyoming, forming the mountainous tract north of the Yellowstone Park. It is a broad anticlinal uplift, having a nucleus of Archean 

rocks, generally denuded of their former covering of Paleozoic strata 

and modified by faulting. The second area lies to the northwest of 

the first, and is formed by the broad anticlinal folds of the Belt ranges, 

whose nucleal core is made of a group of Algonkian rocks that do not 

occur in the southern ranges.

About the margins of both these areas of uplift the Paleozoic and 

Mesozoic rocks are plicated in a series of minor undulations presenting 

varying features of interest. 1 Castle Mountain occurs on the border of 

this second great area of uplift, its folds forming the geological connections between the two Belt Mountain anticlines.

In its general structural features Castle Mountain is a part of the 

great anticlinal folds of the Belt ranges. The Algonkian (Belt) area 

of its western slopes (and the vicinity of White Sulphur Springs) is 

part of the eastern end of the Big Belt anticline. The anticline that 

forms the southwestern portion of the mountain is one of the small 

sharp folds that mark the dying away of this greater fold. The northern part of the mountain is clearly part of the broad anticlinal area of

1 See Livingston folio, showing echelon folds of this vicinity: Geologic Atlas of the United States, 

folio 1, United States Geological Survey, Washington, D. C., 1894. 

22
U.S.GEOLOGICAL SURVEY BULLETIN N? 139 PL. Ill

Volcano. Butte

LEGEND

Glacial Drift

Neoceixe

Sxai&Zake beds

!v "Trr Cretaceous (?)

Cretaceous

Jurassic

C arbtmi ferous

Si Vuro -D e^'on^ an

Cambrian

ALgorQaan

Eelt formation 

IGNEOUS ROCKS

Granite arid Granite 

porphyry ofmainmass

Quartz -porphyry

(uporp hvryofJSIa t&BuUeJ

rh 

Hhvolite

[V]

RhvoliticBreccia 

"and tuffs

Basalt

Dikes and Sheets of

various acidic and

Ijasic rocks

SECTIONS

TOPOGRAPHY BY NORTHERN TRANSCOHTINEHTAISURVEY 1882-83 GEOLOGICAL MAP OF CASTLE MOUNTAIN DISTRICT, MONTANA GEOLOGY BY WALTER HARVEY WEED

CONTOUR INTERVAL 200 FEET 

______Scale______

1896

JULIUS B1EN B CO.NX
PIRSSON. J GENERAL GEOLOGICAL STRUCTURE. 23

the Little Belt Eange, laid bare by erosion, exposing the nucleal rocks 

of Belt age in Yolcano Valley, a minor transverse fold forming the 

eastern part of the mountain mass.

PROMINENT ANTICLINES.

An inspection of the geological map (PI. Ill), shows that there 

are two prominent anticlines, on the east and west sides of the mountain respectively, inclosing the main mountainous area between them. 

The axes of these anticlines have a general northwest and southeast 

direction, and pitch to the southeast, where the sharply folded Paleozoic limestones dip steeply beneath a great thickness of Livingston 

beds. Between these two dominant anticlines the sedimentary rocks 

show several minor folds, some of which are broken by the intrusive 

rocks. The anticline forming the eastern part of the mountain is 

typical of the Belt Mountain region. It is a normal fold that forms a 

lateral offshoot of the Little Belt anticline. The structure is simple, 

and by itself offers no feature of especial interest. Volcano Valley 

has been eroded in the Belt shales that form the nucleus of this fold, 

while the heavy limestone series constituting the resistant member of 

the plicated beds dips away on either side and forms the sharp plunging point of the anticlines. This fold, as seen in horizontal projection, 

is V-shaped, rapidly widening to the north, and as the tilted axis rises 

northward the fold is faulted along the foot slopes of the Little Belt 

Eange. The core of Belt shale is encircled by the Paleozoic limestones, 

that extend along the northern flanks of Castle Mountain from the 

Musselshell Canyon (Hall's) to near White Sulphur Springs. The relations of this fold to the Little Belt Eange are clearly shown in Sections 

C-D and E-F, PI. IV. The northern side (or limb) of the anticline is 

seen to be faulted at the base of the Little Belt Eange, the fault dying 

out to the southeast in the normal fold.

The second of the two larger anticlines of the district is abruptly 

cut off by the granite intrusion of Elk Peak. This fold forms the 

western flanks of Castle Mountain, and its southern extension is dissected by the branches of "Warm Spring Creek. While the Miocene 

lake beds very generally conceal the Paleozoic rocks southeast of the 

Castle Mountain area, and thus obscure the relations of this fold to 

the Big Belt Eange, it is quite clear from the studies made in the field 

that the fold is one of the lesser plications that form the southeastern 

end of the Big Belt anticline. This Warm Spring Creek anticline is 

overturned to the east. The granite intrusion that forms the mountain 

summit replaces the northern part of the fold, leaving only the extreme 

southern end intact. Its relations to the granite are clearly shown in 

Sections A-B and E-F, PI. IV. The overturned structure of the end 

of the fold is shown in the folded beds cut by Warm Spring Creek. 

North of the granite area this fold, whose continuity has been destroyed 

by volcanic agencies, is clearly continued in the Willow Creek fault.
24 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

MINOR FOLDS.

Between the two larger anticlines already noted the sedimentary 

beds are flexed in several minor folds, whose axes are in general parallel 

to the major plications. Thus a narrow synclinal basin about the forks 

of Flagstaff Creek separates the flanks of the larger fold to the east 

from a sharp and narrow anticline to the west, through which the forks 

of Flagstaff have cut narrow gorges that display excellent sections of 

the appressed fold. This plication fades out,to the northwest, passing 

into the flanks of the Yolcano Vaiiley fold before reaching the canyon 

of Checkerboard Creek. Its summit beds form the high upland lying 

between the eastern ridge and the main mountain mass of Castle Mountain, the higher limestones being flat and forming grassy plateau summits. In the stream gorges cut across this fold the dependence of the

FIG. 2. Fold and fault exposed on Flagstaff Creek.'

character of the fold upon the nature and material of the beds is strikingly shown; the contrast is especially marked between the massive 

limestones and the softer Mesozoic shales and sandstones.

.Northeast of the granite mass of Castle Mountain an area of Belt 

shales, largely metamorphosed and altered by intruded sheets of porphyry, represents the nucleus of another anticlinal fold. This fold is, 

however, largely destroyed. It is broken by the diorite intrusion along 

its eastern flanks, and its western half is entirely destroyed by the 

granite stock. Only upon the northeastern flank do the imbricated 

beds of limestone that dip away from the central core of Cambrian 

shales maintain their unaltered position and character. Between Eobinson and Castle a portion of the eastern flank of this fold still remains, 

the rocks being those in which the ore deposits of this vicinity are 

found. Below Blackhawk, Bonanza Creek cuts a small anticline, and 

exposes a core of Carboniferous limestone, with a strike fault along the 

western side, the section being shown in the accompanying diagram, 

fig. 2.
U. S. GEOLOGICAL SURVEY. BULLETIN No 139. PL.IV.

Willow Cr. Fourmile Cr. Fivoinile Cr. Smith River

s.w

N.E.

SECTION A A

South Smith River

Willow Cr.

Checkerboard Cr. j^lusselshell River

s.w. N.E.

Black Butte South Smith River 

S.W. I

SECTION B-B

Elk Peak Blackhawk: Musselshell River

N.E

SECTION C-C

GEOLOGICAL CROSS SECTIONS, CASTLE MOUNTAIN.

Scale

JULIUS'BIEN » CO.
WEED ANDT 

PIBS80N. J GENERAL GEOLOGICAL STRUCTURE. 25

The southeastern portion of the map (PL I) shows a broad valley 

basin inclosed between Castle Mountain and the slopes of the Crazy 

Mountains. This valley area is eroded in the later Cretaceous rocks 

which occupy the center of a broad synclinal 

basin. To the north the syncline forms the 

highland meadows east of Blackhawk, and 

only the earlier members of the Cretaceous 

the Bentou and Dakota remain. In the main 

syncline, however, these beds are seen only 

about the borders of the basin, the center 

being occupied by Livingston beds, showing 

numerous minor plications. The accompanying diagram (fig. 3) shows these minor foldings 

along the Musselshell from Warm Spring to 

Alibaugh creeks.

Only the extreme point of the long syncliue 

between the Crazy Mountains and the BridgerSixteenmile uplifts comes within the limits of 

the map. This is eroded into the valley in 

which the South Fork of Smith River takes 

its head, the beds forming the prow of a typical 

u canoe" fold.

Southwest of FJk Peak, the culminating 

point of the Castle Mountain mass, the lower 

slopes and valley bench-land are underlain 

by folded Paleozoic beds. The accompanying 

diagram (fig. 4) shows the section across these 

beds. The erosion of the valley of Smith 

Kiver shows another flexure of which Black 

Butte is the southwest flank.

But one other fold remains to be noted. 

This is the shallow syncline whose present 

surface shows Beuton shales, and forms a shallow basin defined by Dakota quartzite that 

lies between Fourmile and Willow creeks. 

The general trend of the axis conforms to the 

other folds, but a transverse puckering extends 

the syncline eastward at right angles.

In these folded sedimentary strata the igneous rocks occur as intrusive stocks, with apophysal sheets, and as dikes, filling fractures of 

the folds. The central stock or core of granite 

breaks across the Warm Spring fold and Willow Creek fault. It appears to have had but 

little, if any, dynamic effect, the adjacent folds preserving their original 

atitude, except on the northeast, w-here numerous intruded sheets have 

caused a local thickening and uplift. It does not seem probable that
26 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

such a large body can extend downward with unaltered dimensions, 

and it is shown in the cross sections as a laccolite-shaped mass.

Section C-D (PL IV) cuts this granite mass transversely and shows 

what is believed to be its relations to the rocks through which it breaks 

and its connection with the surrounding sheets of porphyry.

The occurrence of the igneous rocks is so exhaustively treated in a 

subsequent chapter that any further mention here of their relations is 

unnecessary.

GEOLOGICAL HISTORY.

The geological history of the Castle Mountain area embraces an 

enormous period of time, extending from Algonkian to the present. 

The history of Castle Mountain itself is much briefer, beginning with 

the uplifts of the front ranges at the close of the Laramie epoch. The 

history is therefore properly divisible into two great periods. The first 

is the period during which the sedimentary rocks were formed; the second, that in which these rocks were folded, faulted, eroded by streams 

and atmospheric agencies, broken through by volcanic intrusions and 

covered by fragmental volcanic rocks and tuffs, and later the whole 

complex carved into its present form.

.The history of the sedimentary rocks is that of the northern Eocky 

Mountain area, of which the Castle Mountain region forms so small a 

part. The earliest rocks, constituting the Belt formation, are shales, 

impure limestones, and sandstones. They are all shallow-water or shore 

deposits. The sandstones are sometimes ripple-marked and are of 

littoral origin. The contrast in character of the sediments of the Belt 

and all succeeding formations, together with the transgression of the 

sea at the beginning of Middle Cambrian time, shows a period of rest 

between the Belt and Flathead epochs.

At the close of the Belt period, which was one of slow subsidence 

and correlative deposition, the sea bottom rapidly sank and the ocean

waters transgressed the low-lying lands. This depression is indicated 

by the Flathead quartzite, which is formed of Archean materials. 

Together with the succeeding micaceous shale, the rocks indicate offshore deposits, entombmgan the ocean floor the forms of marine life 

that then nourished and forming thin beds of limestone.

The conglomerates that occur in the Upper Cambrian sediments show 

shore conditions; the pebbles are of limestone, differing from any of 

the beds of the Belt formation, but quite like those of the Flathead. 

It must be inferred, therefore, that they were derived from uplifted and 

exposed beds of Cambrian age.

The limestones representing the period of time from later Cambrian 

to the beginning of Carboniferous time show that this unstable condition of land and sea continued. Succeeding the quiet limestonemaking period, represented by the Jefferson, the arenaceous limestones, 

carrying corals, show shoaling waters, in which the argillaceous limestone beds of the Devonian were laid down.
GEOLOGICAL HISTORY. 27

With the earlier Carboniferous a period of regular marine sedimentation began, which went on uninterruptedly through a relatively long 

period of limestone deposition, while the shoreline retreated inland; 

this was followed by elevation of the sea bottom during a period represented by the Quadrant formation, whose alternations of red gypsiferous sands and argillaceous shales, with occasional lenticular beds of 

pure limestone, show shallow reaches and inclosed basins along the 

oceanic border. The fossils, indeed, indicate a fauna living in a sea 

not favorable to its full growth and development.

Succeeding the Carboniferous, there was a gentle uplifting of parts 

of the ocean floor. The total absence of Upper Carboniferous sediments, as shown by the St. Louis facies of the fossils of the highest 

Carboniferous beds found, together with the character of the rock 

forming the upper part of the terrane, shows a period of nondeposition 

at the close of Lower Carboniferous time. The conglomeratic character 

of the base of the Jurassic and the comminuted shells clearly show erosive processes, a conclusion confirmed by observations in neighboring 

regions.

The lowest Cretaceous beds are sandstones and clays, which, with 

the coal seam so constantly found at this horizon, indicate low-lying 

marshy shores and extensive mud flats. The strata afford no record of 

the long period of Lower Cretaceous time that intervened between the 

deposition of the Kootanie and the Dakota beds. The succeeding 

Dakota sandstones show the littoral deposits formed in the first part 

of a continued subsidence during which depression and deposition 

were nearly equal. The well-defined formations into which the Cretaceous is divisible in the plains country are here difficult to recognize. 

The sediments were deposited near the Cretaceous shore-line, and the 

epoch throughout was one of very slow and gradual subsidence in which 

the deposition of sediments seems to have nearly kept pace with the 

sinking of the sea bottom. The oscillations of level produced rapidly 

alternating strata, whose history can be correctly read only when compared with that of the sections farther from the shore. Gradually 

shallowing seas followed, and at the beginning of the Laramie, sandstones and shales were laid down in estuarine areas. The lignite seams 

of this formation, together with the occurrence of unios and gasteropods, 

show fresh-water conditions.

At the close of the Laramie, the relatively quiet conditions prevailing 

since early Paleozoic time came to an abrupt close. The Castle Mountain area was then raised above the sea, and the sedimentary series 

were folded in low and gentle undulations that were at once attacked 

by erosive forces. At this time volcanic activity began at other points 

on a large scale, furnishing the greater part; of the material laid down 

in the waters that laved the shores of the Eocky Mountain land. The 

faunal evidence shows estuarine conditions still prevailing along the 

margin of the newly made land, followed by a recession of the sea and
28 THE CASTLE MOUNTAIN MINING DISTKICT. [Buii.139.

the prevalence of fresh-water conditions. The period of time represented by the Livingston formations must have been a long one, as 

measured by the erosion of the folded Paleozoic rocks, but in this particular area the record is obscure. During the Livingston period 

mountain-building progressed, and the cordillera area was fully developed. This orogenic movement was general and continued through 

the succeeding Eocene period, resulting in the great folds and flexures 

of the mountain ranges, upon whose flanks the lately deposited Livingston (and early Eocene i. e., Fort Union) beds were upturned and 

folded. At this time the volcanic forces breaking through the great 

thickness of Livingston and Fort Union beds formed the intrusive 

bosses of the Crazy Mountains, sending out a multitude of dikes and 

sheets of igneous rock that penetrated the strata in every direction. A 

few of these reached northward as far as the southern part of the 

Castle Mountain area. This period was in general, however, one of prolonged erosion, resulting in the carving out of the great iutermontane 

valley between the Belt ranges and the erosion of the Castle folds into 

a rugged, broken country. These rocks, together with the sedimentsin which they are intruded, were flexed and upturned by the Eocene 

crumpling.

At the close of the Eocene, differential uplift ponded back the waters 

of the Smith River, 1 which had previously flowed eastward down 

Shields River to the Yellowstone, and formed Smith Lake. At this 

time volcanic activity began at Castle Mountain. Huge masses of 

igneous matter were forced upward in the folded rocks, and the diorite 

boss of Blackhawk and Robinson was formed.

Granitic magmas, forcing their way upward, injected sheets between 

the more readily split vShales and filled fractures in the harder rocks, 

finally breaking through the anticlinal folds of the western part of the 

area and forming a great boss or stock. This cuts the folds with so 

little disturbance that it is evident no great amount of folding can be 

ascribed to the intrusion itself, though its uprise may have been preceded by an uplifting and fracturing of the beds above it. The intrusion is represented upon the geological cross sections (PI. IV) as a 

laccolitic mass intruded in the folded Algonkian shales. This shape 

and the location of the conduit are purely theoretical, but 'this theory 

does not require the replacement of a great block of sedimentary beds, 

but their simple uplifting. That no absorption of sedimentary rocks 

took place is clearly evident, as the lack of endomorphic alteration 

at the contact shows. It seems certain that violent explosive action 

subsequently took place at this point and a large part of the sedimentary cap was blown off, the adjacent country being thus covered 

with the fine debris of stratified rocks, mixed with volcanic ash a^u.

1 Smith Kiver, named in 1845 by Lewis and Clarke for the Secretary of the Interior under Jefferson, 

is the proper name for -what is sometimes called Deep Creek. The latter is so common a name in the 

West that it lias no significance. Indeed, the stage road from Castle to the railroad follows down a 

"Deep Creek," entering the Missouri near Townaeud.
U. S. Geological Survey Bulletin No. 139 Pi. V

* t~- ^ ^^f^~f '^^Jfii Vi < v

PANORAMA OF GRANITE CRAGS WEATHERING INTO CASTLES.
GEOLOGICAL HISTORY. . 29

dust. The extent of country covered by this mantle of debris is not 

known, as it has been largely removed by erosion. It has been found 

as far north as the low divide between Sheep and Newland creeks in 

tlie Little Belt Range, 16 miles north of Castle Mountain. Upon the 

immediate mountain slopes this material must have been of considerable thickness. Remnants of it 400 feet thick still remain, but these 

light and readily transported materials were rapidly washed down 

from the mountain slopes and spurs and carried to Lake Smith, to form 

the sediments that now fill the valley.1 Subsequent to this explosive 

action the eruptions were quieter in character, consisting of great 

overflows of lava from fissures in the mountain sides. These lava 

flows streaming down the slopes filled up the hollows and cooled as 

rhyolite and obsidian.

The last phase of volcanic activity of which there is positive proof is 

that which resulted in the formation of the two small volcanoes north 

of the mountain. Of these, Volcano Butte is a most perfect example, 

its lava flows of basalt filling the valley bottom. The desiccation of 

the Miocene lake by the downcutting of Smith River followed these 

volcanic eruptions, since which time erosive forces have been unceasingly at work. A brief episode in the long period of time from later 

Miocene to the present was the formation of glaciers upon the summit 

of Castle Mountain, aiding somewhat in the degradation of the region 

and scattering their bowlder drift over a considerable part of the 

mountain.
CHAPTER IV.

 *>

STRATIFIED ROCKS.

The sedimentary rocks cover the greater part of the Castle Mountain 

area, as will be seen by reference to the geological map (PI. Ill), on 

which their distribution is indicated. In the region shown by the 

map the limestones, shales, and sandstones composing the sedimentary 

series are warped and folded so that the beds are rarely horizontal. 

Faulting has caused further modification, and the intrusion of the 

igneous rocks of the old volcano has shattered the beds and produced 

still further warping and disturbance; sheets and dikes have been 

injected between the beds or in fissures across them, and have greatly 

modified the physical character of the rocks themselves.

While the stratified series does not extend to the actual base of the 

known sedimentary series, it includes a wide variety of sediments 

representing the vast interval of time from Algonkian to Middle Miocene. Few localities in the northern Kocky Mountains show so wide a 

range of time represented by fossiliferous rocks.

The stratified rocks are divided into the usual systems, whose fossil 

forms have been recognizable in this or the adjacent regions. Following the general usage, these systems are subdivided into groups, to 

which local names are given for a particular geologic province. Thus 

in this paper the names used for the subdivisions of the various systems are those which have been adopted for the geological maps and 

reports on this part of Montana and for the Yellowstone National Park. 

The following table shows these subdivisions:

Table of sedimentary formations of the Castle Mountain mining district.

MIOCENE.

Smith Lake beds. 

CRETACEOUS.

Livingston group. 

Laramie group. 

Montana group.

Fox Hills formation. 

Pierre formation. 

Colorado group.

Niobrara shaly limestones. 

Benton shales. 

Dakota. 

Kootanie. 

30
 ON.D] " ALGONKIAN BOCKS. 31

Feet. 

JURATRIAS...................... ............ .............. 90

Ellis limestone. 

CARBONIFEROUS .............................................. 1,800

Quadrant group.

Madison limestone. 

DEVONIAN...... .............................................. 145

Three Forks shales. 

SILURIAN..................................................... 715

Jefferson limestone. 

CAMBRIAN ...................................... ; ............. 2,475

Gallatin limestone.

Flathead. 

ALGONKIAN................................................... 3,000+

Belt group.

Erosion and corrasion acting upon the diverse rocks of the sedimentary series have produced widely ranging topographic forms characteristic of the rocks themselves, and resulted in strongly contrasted 

scenery, emphasized by the vegetation, which reflects the various differences of soil. Into large areas of soft shale broad, open valleys are 

cut, resistant quartzite forms sharp-crested wooded ridges, and the 

heavy-bedded limestones make bold mountain ridges inclosing the shale 

valleys, or are cut by streams into deep and narrow gorges.

ALGONKIAN.

The Paleozoic rocks of the Castle Mountain area show two distinct 

and quite different series. The lowest of these is a group of conformable strata, mainly argillaceous shales or slates, called the Belt formation, from its great development in the ranges of that name. This 

group, of which a thickness of 8,000 feet or more is known in the 

mountains adjacent to Castle, is clearly of sedimentary origin, and 

includes sandstones and argillaceous limestone, but thus far the search 

for fossil remains has not been successful. We believe it to be of 

Algonkian age.

Above the Belt group the Paleozoic is of quite different character; 

the sediments are more modern in appearance, not metamorphosed or 

altered except locally by igneous rocks, and carry fossils at various 

horizons.

As is clearly indicated upon the accompanying geological map (PI. 

Ill), these Algoukian rocks cover the greater part of the area represented, and, as will be seen by an examination of the contours, the Belt 

slates are largely confined to the valleys and footslopes, while the 

overlying beds of this age define the higher area on the north, and, 

together with the general mass of Paleozoic limestones, constitute the 

main mountain mass.
32 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

BELT BEDS.

The Belt group has been recognized in the Bridger Eange and in the 

vicinity of the three forks of the Missouri, where it has been provisionally called Algonkian.1 Both the character of the sediments and 

their position beneath the beds of Middle Cambrian age indicate their 

similarity to the Bow Eiver beds of the Canadian geologists, in which 

Lower Cambrian fossils are found. It has, however, been decided to 

class the Belt beds as Algonkian.

The strata belonging to the Belt formation, though generally argillaceous in character, include quite pure, dark-colored limestones and 

beds of sandstone. The shales which form the predominant part of 

the series and are its most prominent feature are generally gray in 

color, rather well indurated, and sometimes break into slaty plates, so 

that they are very generally called slates by the miners. PL VI shows 

the cliffs of this series found in the canyon of Sixteenmile Creek, south 

of the Castle district, and illustrates the usual character of the exposures of the Belt beds throughout the district.

Within the Castle Mountain area the base of the Belt is not exposed, 

and the sandstones and conglomerates which compose the lower horizons are not seen. The series presents no definite, well-marked lithological horizons, but there is a general sequence as follows:

Bed shales and slates, forming top of formation.

Sandy shales, with thin bed's of ripple-marked sandstone.

Pearl-gray sericitic shales. . ...

Dark-gray, laminated, thin-bedded limestone.

Alternating shales and sandy beds. s

The rocks show a slight general metamorphism in their induration 

and inineralogic nature, and in this respect differ in a marked degree 

from the shales of the overlying Flathead formation.

The Castle Mountain district, as shown on the geological map, has

large areas of Belt rocks. The largest is the broad, shallow valley to 

the north, lying between Castle Mountain and the Little Belt Eange a 

valley cut entirely in this formation. The broad valley of Smith Eiver 

is also eroded in these soft shales, its extension west of Castle Mountain showing Belt beds largely concealed by the silts of the Miocene 

lake.

The great granite intrusion, which is the leading feature of the geology of Castle Mountain, breaks up through the Belt beds, which about 

its borders are so much altered as to be scarcely recognizable as the 

soft, readily eroded beds that compose the formation elsewhere. The 

conformable relations of the formation to the overlying fossiliferous 

rocks of the Cambrian are apparent at many localities. Along Willow 

Creek, howeverj a fault has brought the Belt shales in contact with the

'Geologic Atlas of the United States, folio 1, Livingston, Mont., Washington, D. C., 1894; also 

Bull. TJ. S. Geol. Survey No. 110, 1893, pp. 16 and 49.
U. 8. GEOLOGICAL SURVEY

CLIFFS ON SIXTEENMILE CREEK, SHOWING BELT FOR
ALGONKIAN ROCKS. 33

Mesozoic strata, and west of Castle Mountain the Smith Lake beds 

. conceal the shales in which the valley has been carved. The area covered by the unaltered rocks of this formation is in strong contrast with 

that occupied by the more recent rocks. About White Sulphur Springs 

and on the north slopes of the mountain that fall away to the basin 

drained by the head waters of the northern branches of the Musselshell and Smith rivers the Belt beds form an open country, with rounded 

hills and gently modeled slopes, bare of trees and but scantily grassed. 

The streams cut readily into the shales and fissile limestones of the 

formation, and the rains wear down the slopes rapidly.

In Castle Mountain the rocks are mostly gray in color, save near the 

top of the formation, where buff and yellow tones predominate. As 

already stated, the base of the formation is not shown at Castle Mountain. The limestones form the lowest horizon exposed, and they are 

seen well developed in the canyon of the Musselshell Biver, near the 

mouth of Checkerboard Creek. The overlying, pearl-gray, sericitic 

shales, whose silvery, silky sheen is so characteristic a feature, occur 

near White Sulphur Springs and in. the valley about Copperopolis. 

The overlying arenaceous strata in which the thin beds of sandstone 

weather out as low and rarely conspicuous reefs, whose lines can be 

seen ribbing the smooth, grassy slopes, can be observed on the northern 

flank of the mountain and in the low hills west of Volcano Butte. The 

red shales form a prominent feature along the road from White Sulphur 

Springs to Castle and Martinsdale, and their relation to the bold reef 

of Cambrian quartzite is everywhere apparent.

In the mountain proper the Belt beds are extensively altered by contact metamorphisin, the rocks being hornstdnes and .related rocks, 

forming bold exposures with mural faces and masonry-like structure, 

and breaking into angular blocks with sharp edges, smooth faces, and 

sonorous ring under the hammer. They have also lost their gray or 

red color and are darker and often green or purple in tint. Fine 

exposures occur in ail the gorges cut back to the main summit, notably 

the heads of Fourmile, Willow, Cotton wood, and Warm Spring creeks. 

The illustration (PI. VII) shows the walls of Fourmile Creek near the 

prospects located on the intrusive rocks that cut the altered shales.

The intrusive rocks penetrate the Belt formation very freely, the easy 

fracture and parting of the rocks lending itself readily to the injection 

of eruptive masses. For this reason the beds are nowhere seen free 

from intrusive sheets on the east side of the mountain, but are everywhere intruded and altered and upturned by these rocks. The west 

side of the mountain, as already stated, is faulted, bringing up much 

lower beds, and recent intrusions are absent.

On the western flank of the mountain, at the head of Cottonwood

and Warm Spring creeks, an intrusive sheet of diabase occurs, the

dense, dark, heavy rock resembling iron ore having deceived many

prospectors. The heavy pine forest and castellated crags of the

Bull. 139 3 '
34 THE CASTLE MOUNTAIN MINING DISTEICT. [BULL. 139.

granite area are readily distinguished from the areas of altered Belt 

shale upon the highest slopes of Castle Mountain. The absence of 

fossils and of definite lithological horizons makes it necessary to map 

the formation as a whole.

The Belt rocks are splendidly exposed along the course of Deep 

Creek, on the road from Castle to Townsend, where they form the 

mountains of the Big Belt Bange, west of the area -shown on the map.

In general, the copper veins of the region are found in the Belt formation. At Sixteenmile Creek, Copperopolis, and Spring Creek the 

copper ores are confined to these rocks. The ore bodies of Castle 

Mountain proper, thus far found in the Belt rocks, are not especially 

promising, and those which have been seen by us do not warrant 

development.

CAMBRIAN. 

FLATHEAD FORMATION.

The fossiliferous rocks begin with.the beds which immediately overlie 

those just described. These are unlike the latter both in character of 

sedimentation and in the occurrence of organic remains. That part of 

the overlying Paleozoic that is of Cambrian age is divisible, on both 

lithological and paleontologicalgrounds, into two formations, the Flathead and the Gallatin, the former corresponding to the sandstone and 

shale series and the latter to the limestones. Overlying the series of 

Belt strata just described there is a bed of quartzite or sandstone 

which, throughout this region, everywhere defines the base of the fossiliferous rocks. It corresponds to the "basal" quartzite of the early 

geological reports,1 which was referred to as Potsdam, and which, in 

southern Montana and Wyoming, rests directly upon the Archean 

rocks.

This sandstone, together with the overlying shales and interbedded 

limestones, constitutes theFlathead formation, which has a total thickness at Castle Mountain of 765 feet, and is characterized by fossils of 

Middle Cambrian types. The name is adopted from the well-known 

Flathead Pass across the Bridger Eange.

The formation consists of the following members:

Feet. 

Upper Flathead shales.......................................... 260

Flathead limestones............................................ 50

Lower Flathead shales.......................................... 400

Flathoad quartzite............................................. 75-100

Flathead quartzite. The quartzite, the lowest bed of this formation, 

is the most conspicuous member of the series, and is readily recognizable by its lithological character, its topographic development, 

 and its association with the underlying and overlying shales. The 

bed is from 50 to 150 feet thick, and varies somewhat in hardness at

1 Annual Report U. S. Geol. Survey of the Territories, 1872, pp. 16-25.
U. 8. GEOLOGICAL SURVEY 

VALLEY OF UPPER FOURMILE CREEK.
CAMBRIAN BOCK& 35

different horizons. The rock is pink or gray in color, usually weathering with a reddish-buff or rusty tint, and consists of firmly cemented, 

rounded grains of clear quartz and some milky feldspar. No fossils 

have been discovered in this bed, and it is not ore-bearing. At Castle 

Mountain this quartzite often forms a conspicuous, wooded reef rising 

abruptly above the gentle, grassy slopes of Belt shale and separated 

from the limestone series by a depression cut in the, soft overlying 

shales. The bed forms a prominent feature of upper Warm Spring 

Creek, where it may be seen as a bold reef extending down the slope 

west of the creek and crossing the stream below its upper forks, where 

it forms a rugged, dark-brown wall On the road from White Sulphur 

Springs to Blackhawk and Castle this bed is not so conspicuous, but it 

may be seen close to the road from the Fourmile divide to Blackhawk. 

Upon the northern flanks of the mountain it is, however, quite well 

developed, its wooded outcrop everywhere defining the boundary of 

mountain and valley from the Smith Eiver gorge, northeast of White 

Sulphur Springs, eastward to Halls, where the. ledge crosses the 

Musselshell and curves about the flanks of the Little Belt Eauge.

Flatliead shales. These shales are generally laminated, soft-green, purple, or red in color, are frequently micaceous, and they crumble readily. 

Thin-bedded limestones also occur, and in the middle of the shale belt 

they are quite well developed. These limestones are dense, gray rocks, 

carrying trilobite remains, are frequently spotted with green glauconite grains, and have a pitted, uneven surface where in contact with 

the shale. The beds are seldom more than a few inches thick, but 

often weather out as low bands showing above the shale slopes. They 

vary from dense limestones, having trilobites and other fossils on the 

upper surface of the layers, to conglomerates formed of limestone 

pebbles, often green or buff, and unlike the gray matrix. These pebbles are generally flat, but occur in the rock without arrangement. 

Together the limestones form a set of beds, with shale belts above 

and below, and are hence called Flathead limestones. Frequently 

layers of limestone oolite occur in which trilobite fragments are found, 

the rock being gray, mottled with buff, and carrying considerable 

glauconite. These thin-bedded limestones sometimes weather out in 

plates or flags of considerable size. The fossils from these limestones 

are of Middle Cambrian types.

In the western and southern slopes of Castle Mountain these shales 

and sandstones are rather argillaceous. To the north a second bed of 

quartzite is found, separated from the basal member by 400 feet of 

shale. This is seen on the hill back of Copperopolis and in the canyon 

cut through it by Checkerboard Creek. The section on page 36 gives 

 in detail the character and succession of the Flathead beds of this 

locality.
36 THE CASTLE MOUNTAIN MINING DISTRICT. .[BULL. 139.

Section of Paleozoic strata exposed in gorge of Checkerboard Creek at north base of Castle

Mountain.

No. Cambrian series.

26 Limestone; rather massive in outcrop; light-gray with buff 

tinge and pinkish spots; dense and without crystalline structure ............................................................ 80

25 Limestones, thinly bedded, bitt with massive outcrop, weathering with rough surface and faint-brown tint; hackly, guttered 

surface ......................................................... 130

24 Limestone, bluff; dark-gray...................................... 25

23 Limestone; dark-chocolate and gray.............................. 100

22 Limestone, forming bluff wall; first big cliff ledge of canyon 

walls; dark-stained and occasional reddish wall of gray limestone ........................................................... 40

21 Limestones; thinly bedded, gray................................. 550

20 Limestones; dark-chocolate colored, dense rocks, weathering

gray or white................................................... 7

19 Limestones (?); not exposed...................................... 25

18 Limestone, forming persistent ledge or cliff; dark-gray and

chocolate colored; no conglomerate; carrying trilobite ......... 55

17 Limestone, conglomerate at base and in places, passing into 

rough weathering, dark-gray limestones above; fauna of 

this horizon is Cambro-Silurian elsewhere.......................

16 Limestone; poorly exposed; splintery, gray rocks................ 75

15 Limestone; rather thinly bedded; dark-gray, mottled............ 90

14 Shale with limestone beds of one-half inch to 3 inches in thickness ............................................................ 315

13 Limestone; gray, weathering buff; carrying Cambrian fossils.... 28

12 Shale; thinly laminated, crumbly, purple-gray................... 30

11 Limestone...........................................:............ l

10 Shale; gray, crumbly............................................. 4

9 Limestone; thinly bedded, dark-gray, mottled wi th buff......... 10

Shale in one-half inch layers, carrying interbedded layers of 

fissile, impure sandstones and limestones....................... 200

7 Shale; gray and purple, thinly laminated......................... 00

6 Sandstone; stained black with'organic material................... l

5 Shale; dark-green, crumbly....................................... 5

4 Quartzite; resembling that at base of section..................... 25

3 Limestone; thinly bedded, gray limestone, weathering buff,

and carrying fossils............................................. 20

2 Shales; thinly laminated, crumbly, micaceous, green and gray,

red and purple;- not well exposed............................... 400

1 Quartzite; dark-red, buff, or gray; conglomerate at base......... 150

... I Belt shales.....................................................

Feet.

This section differs from sections of the Cambrian farther south in 

having a much greater development of the Flathead shales, which, 

with the interbedded limestones, have here a total thickness of 945 feet. 

The overlying limestones do not show the lithological differences seen 

at other localities, and as a systematic search for fossils was not'made, 

the horizons of this part of the section are only tentative. The dip is 

westward and varies from 45° for the quartzite, the lowest bed exposed, 

to 25° for the heavily bedded, light-colored Carboniferous limestones
WEED AND"! 

PIKSSOH. J

CAMBRIAN ROCKS. 37

\.Thln-bedded limeitoue.

I. I.

JLZI

i i

i . i

that form the upper end of the gorge. In topographic relief the Flathead shales form a ravine or grassy interval between the wooded 

quartzite area, and the rugged limestone outcrops above. Exposures 

are rare and must be sought along the banks of the streams or steep 

divides, as they readily weather and dis- Feet 

integrate into fertile soils, which favor 

vegetation and effectively conceal the 

beds.

GALLATIN LIMESTONES. 1

The upper part of the Cambrian beds, 

consisting mainly of quite pure limestones and characterized by fossils of an 

Upper Cambrian type, constitute the 

Gallatin formation, the rocks being typically developed in the Grallatin Eange. 

The lowest bed is a rather massive, 

heavy-bedded limestone, which has generally a chocolate color mottled with 

yellow or buff. This bed often forms a 

bold bluff wall, rising abruptly above 

the slopes formed by the more readily 

weathered beds beneath.

This limestone bed is succeeded by 

a thin belt of shales, barren of fossils 

and seldom exposed, overlain by rather 

thinly bedded limestones and conglom- 

 erate. These limestones, which are generally pebbly, constitute an important 

horizon. Beds of limestone 4 to 6 inches 

thick alternate with more crumbly, argillaceous beds 3 to 5 inches thick. The 

fossils are Upper Cambrian species, but 

include many that pass into Silurian 

rocks, and it is questionable whether 

these beds do not belong to that period. 

These conglomerates are not distinctive 

of the Gallatin, however, as those found 

in the underlying Flathead formation 

are very similar in appearance.

Fossils: Crepicephalus (Loganellus) montanensis Whitf., and Arionellus tripunctatus Whitf., from beds in valley near wagon road, west 

side of Castle Mountain.

SILURIAN. 

JEFFERSON FORMATION.

The limestones which farther south are classed as Silurian and form 

a prominent part of the Paleozoic series near the Jefferson River are 

recognizable at Castle Mountain only by their lithological character 

and their strati graphic position.

150

Shales with limestone beds.

Shales and limestones.

Qunrtzite. 

Limestone.

.Shales.

Quartzite

FIG. 5. Columnar section of Paleozoic 

beds seen iu canyon of Checkerboard 

Creek.
38 THE CASTLE MOUNTAIN MINING -DISTRICT. [BULL. 139.

The rocks are all limestones, generally massively bedded and gray 

in color at the base, grading above into dark-brown and blue-black 

saccharoidal limestones with fetid odor and a considerable proportion, 

of arenaceous material. These beds are often characterized by an 

abundance of small calcite geodes, and rarely carry indistinct remains 

of corals. The formation is distinguished with difficulty from the limestones above and below, forming with them the great limestone series 

that collectively make the mountain-building rocks'of the region. The 

black limestones occur on the northwest si'de of the mountain at the 

Grasshopper mine, where they show considerable endomorphic alteration', due to the proximity of the granite. They are also seen on the 

divide above the Cumberland mine in the rock cut beside the road 

from Robinson to Castle.

A splintery, dark-gray limestone, having a thickness of 550 feet, 

overlain by 165 feet of chocolate and dark colored fetid limestones, is, 

in the absence of any paleontological evidence, assigned to this horizon.

DEVONIAN. 

THREE FORKS SHALES.

The series of shales and thinly bedded limestones which carries a 

well-marked Devonian fauna at the Three Forks of the Missouri is not 

readily recognizable in the Castle Mountain area, as the beds are fissile 

limestones not easily separable from the overlying Carboniferous, and 

no fossil remains have been obtained. Its presence to the north and 

south, however, makes it probable that beds of this age are included, 

in the great series of Paleozoic limestones.

A total thickness of 145 feet is assigned to this period in the section in Checkerboard Canyon. The rocks are thinly bedded, though 

together they form massive exposures. They are generally gray, 

mottled or blotched with pink or buff, and often weather with marked 

red color. There is an abrupt change in the character of the beds at 

the base of the Carboniferous, so that the line of separation is made 

in the field more readily, than it can be described.

Exposures may be found in Checkerboard Canyon, of which a section 

has been already given, and along the northern and southwestern 

flanks of the mountain. Near the Cumberland mine and at Eobinson 

and Blackhawk contact metamorphism, produced by the large bodies 

of intruded igneous rock, obscures the normal character of the limestones so that they can not be distinguished from the other Paleozoic 

strata.

CARBONIFEROUS.

The beds of this age form the most prominent part of the great series 

of mountain limestones. As will be seen from the geological map, they 

cover a considerable portion of the Castle Mountain area, and to them 

is due some of its boldest and most picturesque scenery. They are,
CARBONIFEROUS ROCKS. 39

moreover, important from an economic standpoint, as they appear to 

be peculiarly fitted for ore carriers, and in them have been found most 

of the important ore bodies.

The beds of this epoch are chiefly limestone, thinly bedded, and dark 

in color at the base, and massive and white above. This series is overlain by shaly and arenaceous beds, with interbedded massive limestones 

characterized by Lower Carboniferous fossils.

In the field the rocks are naturally divisible into three series, but the 

fossil remains are all of Lower Carboniferous types and the fauna of 

the Coal Measures of the eastern United States has not been found. 

But two divisions are therefore made; the limestones are grouped 

together as the Madison limestones, this terrane being well developed 

along the river of that name, while the overlying earthy and sandy 

series is the equivalent of the Quadrant formation of the Yellowstone 

Park. The two divisions have an aggregate thickness of 1,800 feet, 

distributed as follows:

Feet.

Quadrant formation............................................... 600

Madison formation:

Massive limestone .............................................. 400 8

Thin-bedded limestone,..............__...___................. 800

The Madison limestones in this region are the most prominent part 

of the Paleozoic series. The sharp folds into which they have been 

compressed have been largely uncovered, and the ledges stand up white 

and glaring above the gentle slopes formed by the overlying beds. 

They are essentially the mountain limestones, and generally in this part 

of the Rocky Mountains sharply define the mountains from the plains 

or valleys of soft Cretaceous rocks. This peculiarity is seen at Castle 

Mountain in the anticlinal fold that forms its eastern part, and to the 

southwest of Castle, where the abrupt transition from gentle Cretaceous slopes to limestone plateaus is most striking. The upper strata 

being massive and heavily bedded, this transition is more strongly 

marked than if they were of the lower thinly bedded limestone. Upon 

the western slopes of the mountain, as on the eastern, the heavy beds 

of limestone form a line of white scalloping outcrops resting upon the 

mountain flanks.

MADISON FORMATION.

Flaggy limestones. rlmmediately overlying the yellowish shaly limestone forming the top of the series referred to the Devonian there is a 

series of rather dark-colored, compact, fine-grained limestone, splitting 

readily along the bedding planes into slabs or flags. The total thickness of these thinly bedded limestones included between the Devonian 

and the base of the massive limestone is 800 feet. Throughout this 

thickness there is a considerable variety in color, from dark-gray or 

blue-gray to brown and buff, and from compact amorphous limestones 

to those that are coarsely crystalline. The rocks frequently carry
40 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

'Sandstones and red earths.

chert, but it is irregularly distributed, of varying color, and forms 

singular masses.

Fossils occur in the lowest beds of this series and are abundant 

throughout. The most noticeable feature is the great abundance of 

crinoid disks which appear in the lowest layer. In the Devonian rocks 

they are small, scarce, and inconspicuous; here the rock is largely

composed of them. Neither 

fossils nor lithologic differences 

offer any grounds for dividing

shales, red and wack. these beds into different horizons.

Massive limestone. Above 

the series of thinly bedded 

limestones just described there

jg^S^^ ^Thinly bedded limestones, alternating with red jg a thickhCSS Of 400 feet Of

clay and shales.

white, massive limestone. The 

rock is always light-colored, 

generally dense, but sometimes 

crystalline, with irregularly 

distributed chert rarely forming layers through it. As a 

whole itisreadily distinguished 

from the darker, thinly bedded 

limestone beneath, both from 

the character of the rock and 

the great difference in weathering. The beds are thick and 

division planes seldom noticeable, the rough, cavernous, 

irregular weathering and uneven fracture being quite constant characters. Where cut 

through by streams it forms 

narrow gorges with precipitous 

walls showing cavernous recesses. It frequently holds 

caves, and abandoned waterways arenotuncoinmon. Where 

the streams penetrate it. to the

softer rocks it forms the gateways, so common in this mountain region, 

from the upper valley or gorges to the open lowland.

The rocks may be seen well exposed at many localities. The ore 

body of the Cumberland mine lies in this bed, and the slopes about the 

mine show good exposures, though the rock is slightly metamorphosed. 

The gorge of Warm Spring Creek and the adjacent mountain benches 

show this horizon where it has been somewhat carefully prospected.

'Jurassic sandstone.

 Massive limestone.

'Red and purple earthy limestones.

 Limestone carrying carboniferous fossils.

Green shale 

beds, and

eries, carrying fossils in calcareous 

olitic limestones.

Thinly bedded limestones.

Madison limestones.

JTia. 6. Columnar section of beds exposed in canyon 

of North Fork of Musselshell Kiver.
CARBONIFEROUS ROCKS. 41

When sharply folded the limestone is often brecciated along incipient 

fault lines and iu such places is often colored by iron. This has led 

many prospectors to look for minerals at such places.

The road from White Sulphur to Castle passes up the gorge of Fourmile Creek, cut in Carboniferous rocks, this horizon being prominent, 

and it is also seen where the various forks of Flagstaff Creek cut their 

way to join the Musselshell. It is a most important element in the 

scenery of the region and a key to the position of the adjacent beds 

and to the geological structure.

QUADRANT FORMATION.

This consists of a variable series of sandstones and shales, with pure 

and impure limestones, that overlies the great limestone series of the region. In the southern part of the State the formation consists of quite 

pure white sandstones witli occasional intercalated limestones, but argillaceous materials appear farther to the northward, and at Castle Mountain form an essential part of the formation. A characteristic feature 

is the occurrence of impure earthy beds of bright-red color immediately 

above the massive limestones last described. Higher in the series beds 

of bright-green shale, usually alternating with sandstones or impure 

limestones, form a conspicuous and readily recognizable horizon.

The total thickness of the formation averages 600 feet, the upper 

limit being the granular, buffi, or pink sandstone of the Juratrias. The 

following section illustrates the character of the beds. It is taken at 

the beginning of the Musselshell Canyon, at the northeast base of the 

mountain.

Section of Carboniferous and Juratrias beds exposed at east end of Musselshell. Canyon.

Mesozoic: Feot. 

Black shales.................................................. .......... 500 .

Red earths............................................................. ......

Sandstone, "buff-colored, granular and crumbly.......................... 20

Red sandstone ........ 1................................................ 5

Sandstone, indurated and tough, gray to buff........................... 10

Sandstone, a shaly gray rock ........................................... 5

Interval............................................................... 10

Sandstone, supposed Dakota, gray at base................. .............. 10

No exposure ........................................................... 20

Limestone, light-brown, rusty colored................................... 2

Not exposed ........................................................... 30

Shaly sandstone, a soft light-gray rock, weathering down readily and

probably passing iuto impure limestone, with red, nodular streakings. 12 

Sandstone, shaly at top, soft-red to buff, and massive at base. Probably

represents base of Juratrias .......................................... 28

 Paleozoic:

Interval............................................................... 30

Massive limestone, gr.ay, blotched with.pink, crystalline and granular,

resembles sandstone.................................................. 15-20

Interval, showing red, shaly earths ..................................... 15
42 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

Paleozoic Continued. Feet. 

Limestone, gray massive bed ........................................... 5

Eed earths, magnesian or impure limestone; red clays with limestone balls. 8 

Limestone, dense, pure, gray, structureless .............................. 3

Purple magnesian limestone, weathering to red earths holding nodular

limy masses a few inches in diameter................................. 15

Limestone bed, upper part gray in color and quite massive, with abundant

crinoid stems and Lower Carboniferous fossils. Lower 10 feet thinly

bedded (1 to 2 feet) and red stained................................... 25

Black shale, not well exposed........................................... 20

Limestone, dense and massive, not platy ................................ 5

Shale, dark-colored..................................................... 3

Limestone, carrying fossil remains ..................................... 1

Shale, black........................................................... 1£

Limestone, carrying brachiopods and other Carboniferous fossils ........ £

Shale, reddish at base .................................................. 27

Shale, probably green, poorly exposed................................... 15

Limestone, dense, dove-colored, platy . fracturing, without crystalline

structure or fossils................................................... 1

Shale, bright-green .................................................... 20

Limestone, impure, green............................................... 1

Shale, gray ............................................................ 10

Shale, green........................................................... 18

Shale, weathering to black or blue-black clays.......................... 15

Limestone, a dense, platy, buff-colored, splintery fracturing, oolitic rock. 1 

Shale, dark-green, weathering to black or rusty colored earths .......... 8

Shale, light-green, tinged with purple at top, where it passes into a . .

darker shale above................................................... 5

Shale, light-green, neither soft nor laminated, carries Fenestella and crinoid

stems, etc............'................................................ 3

Earthy bed, limy and argillaceous, weathers to red clay with green, limy

masses in it.......................................................... 15

Limestone, dense, finely crystalline, crackled............................ 5

Limestone, impure and shaly, gray, weathering buff..................... 5

Limestone, oolitic, platy................................................ 5

Limestone, gray, fissile and splintery, dove-colored to light-gray......... 15

This section shows clearly the variable nature of the deposits of the 

closing part of Carboniferous time. The limestones associated with 

the green shales are frequently oolitic in character, and like the thick 

beds higher in the series carry typical Lower Carboniferous fossils.

The green shales form a marked persistent horizon throughout the 

Castle Mountain area, rarely showing exposures, but weathering as 

gray earth. These green shales are exposed just below the Cumberland mine above Castle, where a cutting for a new road has left an 

excellent exposure. They are also seen on the hills east of White Sulphur Springs, and on the slopes west of the Grasshopper mine. At 

this latter place a section shows essentially the same sequence and 

thickness as that at Musselshell Canyon. Above Checkerboard Canyon, back of Copperopolis, these shales and the associated beds are well 

exposed, and their relation to the limestones is clearly seen. Another 

excellent exposure is found in the canyon of Flagstaff Creek, where 

that stream cuts through the Flagstaff anticline.
JURATRIAS ROCKS. ° 43

Just above the Fourmile Canyon, through which the Castle road is 

built, the bright-green shales are exposed on the slopes and have been 

laid bare in prospecting. They are cut here by the big dike elsewhere 

mentioned.

The limestone which forms the top of the Quadrant formation is a 

massive bed weathering as a rough white ledge, and carrying abundant Carboniferous fossils. It is quite like parts of the massive limestone of the Madison.

JURATRIAS.

The Ellis limestones take their name from the old military post of 

Fort Ellis, about 50 miles southwest of Castle Mountain, and are 

prominently developed in the .mountain of that name. In the Castle 

Mountain district they are distinguished with difficulty, and must be 

sought with care. Their position beneath the Dakota quartzite and 

above the massive white limestone that caps the Quadrant formation 

^defines their limits, but fossils are not always found.

The beds assigned to this age include a sandstone, usually granular, 

buff in color, but weathering red or pink, overlain by a dense white 

limestone resembling that immediately beneath it, but carrying Jurassic fossils. This passes into a sandstone above, and is capped by red 

earthy shales. The beds included between the fossiliferous Carboniferous and the quartzite assumed to be the base of the Cretaceous 

are 90 feet. There is a total absence of the drab-colored argillaceous 

limestones which form the bulk of the formation farther south, and 

which contain an abundance of fossils, yet the period is represented 

by the purer limestone that here carries the same species found in the 

argillaceous beds.

Exposures were recognized in the canyon of Flagstaff Creek, and 

the following fossils, identified by Mr. Stanton, were collected:

Ostrea sp.

Camptonectes extenuatus M. and H.

Gervillia montanenais Meek.

Astarte packardi White.

Trigonia montanensis Meek.

Pinna kingi Meek.

Pkolodomya kingi Meek.

In the section made at the mouth of the Musselshell Canyon no 

fossils were found, but the stratigraphic succession is the same.

The Triassic period, if represented at all, forms the sandstone at the 

base of the Ellis. The extensive Bed Beds of Wyoming have, so far 

as we have observed, no development in Montana, and beds simulating them are Carboniferous in age, as indicated in discussing the 

Quadrant formation.

The Ellis is usually defined by an upper sandstone separated from 

the Cretaceous sandstone (or conglomerate) above by red shales and 

fissile sandy beds.
44 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

The abrupt change from Carboniferous to Juratrias is seen in exposures a few miles east of this section at Musselshell Canyon., where a 

dense pinkish-gray limestone full of Lower Carboniferous fossils is 

succeeded by a sandstone with a foot or two of conglomerate at the 

base, the pebbles being of chert and limestone. Both the conglomerate and the sandstone into which it grades carry abundant Jurassic 

.fossils, mainly Ostrea glabra.

CRETACEOUS. 

KOOTANIE.

The strata whose outcrops have been included with the other Cretaceous terranes, and which are shown by one color upon the accompanying geologic map (PI. Ill), certainly include at the base a series of 

sandstone and shales equivalent to the Great Falls beds farther north, 

known to be of Kootanie age. Although .there is no definite paleontologic evidence establishing the existence of the Dakota group in 

Montana, the series of beds lying above the fossiliferous beds of Jurassic 

age and beneath the black shales of the Benton formation have been 

heretofore provisionally called Dakota, and so mapped. The characteristic conglomerate which in the southern part of the State occurs near 

the base of the series of beds thus delimited corresponds in lithologic 

character and habit with the beds in Colorado described as Dakota, 

but this is rather weak evidence on which to correlate a formation. In 

the neighborhood of the Great Falls of the Missouri, where the Jurassic 

has been recognized 1 and the Beuton reaches its typical development, 

the interval between the two formations includes a thickness of several 

hundred feet of arenaceous and argillaceous sediments, together with 

an important coal seam. Fossil plant remains gathered from these 

intervening beds are of Kootanie or Lower Cretaceous types. No 

unconformity is recognizable bet ween this and the Benton shales (fossiliferous), but sediments of similar character extend to the top of the 

series. The only paleontologic evidence consists of fresh-water shells, 

which alone furnish no evidence for discrimination. It is thus fairly 

open to question whether the rocks called Dakota in this region are 

really of that age.

In the Castle Mountain area the Kootanie is believed to be present j 

at least the coal seam mined at Checkerboard Creek, together with the 

adjacent beds, is believed to be the equivalent of that found along the 

flanks of the Little Belt Eauge farther north. The overlying beds 

resemble in character those generally described as Dakota in the northern Eocky Mountain region. The beds supposed to be of Kootanie 

age consist of thinly bedded sandstones and red earthy shales, with a 

coal seam near the top, beneath a heavy qnartzite bed that later is arbitrarily assumed as the base of the Dakota. The total thickness of these 

beds does not exceed 300 feet.

1 W. H. Weed, Two Montana coal fields: Bull. Geol. Soc. America, Vol. Ill, p. 309.
TmssoN.D] CRETACEOUS ROCKS. 45

DAKOTA.

The overlying strata, supposed to represent the Dakota, consist of 

a heavy sandstone bed at the base, overlain by varying succession of 

shaly sandstones and earthy shales. The section given in fig. 6 shows 

the sequence and thickness of the beds at the inouth of the Musselshell 

Canyon.

The basal bed is generally sandstone, often a conglomerate at the 

base, the pebbles being of black and white chert or quartzite. The 

sandstone is often indurated to a quartzite, is gray in color, but weathers 

with a rusty ferruginous crust. It breaks generally into massive blocks, 

and by its resistance to weathering occupies conspicuous positions 

wherever it occurs. Its outcrops are generally covered by a growth of 

pines. The relation of this bed to the fossiliferous Jura is well seen in 

the synclinal trough east of Blackhawk. A similar relation is seen in 

the canyon of Flagstaff Creek. The quartzite, which, is so readily 

recognizable, may be used as a ready reference plane, denning the Carboniferous from the shaly Cretaceous beds in which prospecting for ore 

deposits is useless. Above Checkerboard Canyon the coal seam lies just 

beneath the quartzite, and in prospecting for a further extension of the 

seain, that is the bed to follow. Near Castle this bed is conglomeratic, 

and is well exposed on the hillsides above the town. Its distribution 

is shown upon the map, and it is so general along the outer margins of 

the limestone area as to make a mention of specific localities unnecessary. No fossils have been found in the quartzite, but the red clays 

above it have yielded fragmentary fossil remains from the faulted area 

southeast of Elk Peak, near the quarry where the sandstone is taken 

out for building purposes for use at White Sulphur Springs. These 

fossils have been examined by Mr. T. W. Stanton, who reports that 

they are fragments of a unio and of a bone, possibly of a turtle.

A peculiar feature of the series of beds included under the name 

Dakota is the presence of a bed of dense lilac or purple rock, which 

closely resembles in field appearance ^a dense volcanic rock. Under 

the microscope this is seen to be a volcanic ash, thus proving the 

occurrence of volcanic activity during the Dakotan epoch. This is in 

perfect accord with the observations of Dr. Dawson upon the Kootanie 

and Dakota rocks of Crows $Test Pass in the Canadian Rocky Mountains. This purple bed is a persistent horizon throughout the district 

and has been recognized by one of the writers throughout the region 

as far south of Castle Mountain as the Teton Eange of Wyoming.

COLORADO GROUP.

Within the Castle Mountain area the distinction between the Colorado and Montana groups of the Cretaceous system is an arbitrary 

one, as no satisfactory paleontologic evidence has been procured upon 

which to base a division of the groups.
46 THE CASTLE MOUNTAIN MINING DISTEICT. [BULL. 139.

The two divisions of the Colorado the Benton and Mobrara formations are not distinguished on the map, the line of division being 

too indefinite to permit separation.

Benton formation. This is a series of black carbonaceous shales, 

with occasional thin beds of sandstone and arenaceous shales. It is 

essentially an argillaceous formation, whose dark-gray or black color 

permits its ready distinction from the beds below. Concretionary clay 

ironstones occurring in the black shales frequently carry fossils, the 

shales being generally barren. The sandstones are usually impure 

and argillaceous and occur most prominently in the upper part of the 

Benton.

Good exposures occur on the east side of Warm Spring Creek below 

the road crossing. The black shale is here fossiliferous, the following 

species identified by Mr. T. W. Stanton having been collected: Inoceramus umbonatus, M. & H.; Baculites asper, Morton.

Niobrara formation. This formation constitutes the upper part of 

the Colorado group, and consists in this region of rather lighter colored, 

gray, arenaceous shales and impure sandstones, passing gradually into 

the black shales beneath and differing from the lead-color and gray 

clays of the Pierre above. The sandy shales, when sharply upturned, 

frequently weather out in irregular reefs projecting a few inches above 

the slopes of gray shale and resembling long lines of tombstones.

The upper limit of the Mobrara is assumed to be the conglomerate, 

formed of black and white quartz pebbles, that is a readily distinguishable horizon in the vicinity of Castle Mountain.

In the valley of Alibaugh Creek, a few miles west of the town of 

Castle, the Colorado shales are highly indurated and metamorphosed 

the result of the intrusion of the granite core by which the shales are 

here abruptly cut. The process of alteration has been so complete that 

the formation has lost all its ordinary characteristics, and here closely 

resembles the similarly altered Algonkiau beds so commonly found 

upon the flanks of the granite massif. The rocks consist of dense, 

flinty hornstones and vitreous quartzite, and light-colored, porcellanous, adinole-like rocks. The original bedding is only recognizable upon 

careful examination, the rocks being jointed and breaking readily into 

small cubical or angular pieces.

The total thickness of the group' is estimated at 2,200 feet. Good 

exposures are rather uncommon, though the formations cover extensive 

areas of the Castle Mountain region. The town of Castle is built upon 

these and the overlying shales of the Montana group, and good exposures are seen in the bluffs of Warm Spring Creek below the road. 

Bonanza Creek has cut a narrow valley through them on its way to the 

broad open country of Coyote Basin, and the black shales are seen in 

the synclinal trough east of Willow Creek near White Sulphur Springs.

The sections of the beds exposed at the south base of Castle Mountain given on subsequent pages, show the character of the Colorado 

group as developed in this region.
CRETACEOUS ROCKS. 47

MONTANA GROUP.

This embraces the two formations, Pierre shales and Fox Hills 

sandstones, which are not readily distinguishable at this locality, and 

are indicated by one color upon the map.

Pierre shales. This formation consists chiefly of a thickness of 2,000 

feet of leaden-gray clays or clay shales, carrying rounded, limy concretions, and having occasional thin beds of sandstone in the series. 

These shales form, a very sticky soil, becoming dry and cracked during 

the summer and weathering into clays that most effectively conceal the 

beds themselves. No definite line between this and the overlying Fox 

Hills can be drawn in this region, as the beds become more and more 

arenaceous and grade into the sandy shales of the latter.

Fox Hills sandstones. This formation consists in this region of a body 

of gray sandy shales of varying texture and appearance, passing into 

a buff-colored sandstone at the top.

' Exposures of the Montana group occur upon the southern and eastern sides of Castle Mountain, but the actual outcrops are rarely seen. 

A section made along the ridge between Alibaugh Creek and the town 

of Castle is given herewith. The section already noted gives the 

sequence and thickness of the beds as far as known. Along the course 

of lower Warm Spring Creek and in the hills south of the road about 

Woodman Creek the beds may be seen fairly exposed.

Section near the town of Castle.

Foot.

Alternating shales, sandstones, and arenaceous beds belonging to 

the Benton formation ......................................... 1,000

Quartzite, varying to sandstone, forming prominent outcrop and 

probably of Dakota age..................;.................... 20

Sandy shales, breaking into fine d6bris and seldom exposed...... 110

Sandy shales, carrying indurated sandstone layers near the base. 

The series is stained black with carbonaceous material and 

resembles the Dakota shales of the Yellowstone Park section... 300 

Hard, dense, black limestone, weathering dark-gray. The rock is 

tough, carries chert, and is not crystalline..................... 20

Impure sandy shales ............................................ 10

Quartzite, the central part of the bed being somewhat shaly..... 36

Impure argillaceous limestones.................................. 110

Intruded sheet of porphyry ...................................... 12

Limestone, dense but not crystalline, gray in color, and breaking 

readily into fine de"bris........................................ 100

Olive-colored, dense sandstone .-...........................;.... 10

Light-colored limestone, breaking readily into fine debris. Upon 

fresh fracture the rock appears of granular texture and of darkgray color, carrying streakings of lighter material............. 90

Dark-gray limestone, carrying shell remains which appear to be 

Carboniferous .. ....^......................................... 40

White limestone, much decomposed, but well exposed in prospect

pit ........................................................... 10

Lead-colored limestones, cut by transversely trending dike....... 25
48 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

Feet.

Shaly beds, which, are baked and hardened, resembling impure limestones ........................................................ 105

Intruded sheet of porphyry.. ..................................... 25

Dark-purple and brown shales, which are much baked and form 

dense and hard hornstones.................................... 20

Intruded sheet of porphyry...................................... 15

Metamorphosed black shales .................................... 15

Altered green shales and calcareous beds........................ 25

Flaggy limestone, much decomposed and having porous, spongy 

layers, but no fossils.......................................... 5

Debris of light-colored limestone................................ 30

Chert-bearing, dove colored, dense limestone..................... 2

Altered shales and intercalated limestones....................... 60

Limestone bed forming ledge, and exposed white line crossing 

slope ......................................................... 7

Altered limestones or shale, white, dense in structure and not crystalline, with some flinty, baked shale that is black and resembles 

the Algonkian ................. -~.............................. 75

Intruded porphyry sheet........................................ 15

Green Carboniferous shales, which are baked to a gray, splintery 

limestone-like rock............................................ 125

Limestone, much altered, breaking into small angular fragments. 

In structure the rock is finely saccharoidal, often brecciated, 

much decomposed, and varies from a pink to a gray color....... 50

Leaden-gray spotted limestone.................................. 3

Intruded porphyry sheet........................................ 6

Limestone, brown-gray in color, thinly bedded, and evidently not 

belonging to the massive1 series of Carboniferous limestone. 

These beds extend to the contact with the granite stock. The 

contact is marked by considerable alteration, the granite being 

the usual fine-grained porphyritic forms, with a zone not over 

50 feet wide of rotted, decomposed rock, showing baked crystalline limestone and porphyry decomposed to a light-yellow rock, 

quite different from the adjacent altered limestones, which are 

. brown-gray, and not perceptibly whitened by contact metamorphism, although finely granular in structure ................... 150

THE LAEAMIE.

This formation, which is so generally coal-bearing along the front of 

the Rocky Mountains, is not well exposed in the Castle Mountain area. 

It is essentially a series of light-colored arenaceous beds, in strong 

contrast with the dark-colored argillaceous shales beneath and the 

somber-colored beds of the Livingston above.

The strata consist of missive light-colored sandstones with intercalations of gray shale and seams of coal. The rocks are well cemented, 

and the sandstones formed of well-rounded grains of quartz with, little 

feldspar and mica No fossils have been collected from these beds at 

Castle Mountain. Good exposures occur at the mouth of Warm Spring 

Creek, where the beds are sharply upturned, and the ledges can be 

traced along the slopes to the south, outlining'a minor anticlinal fold 

into which the sediments have been flexed. A few openings have been
CRETACEOUS ROCKS. .49

made on the coal seams in this vicinity and show the best fuels yet discovered near the mines. The seam at the forks of the creek overlies 

250 feet of gray Laramie sandstone, and is overlain by about the same 

thickness of similar rocks. The strata are vertical at this place.

Exposures of the Laramie also occur east of Eobinson Creek, a couple 

of miles below Castle. The total thickness probably averages 500 feet. 

The formation is readily located by its relation to the brown beds of the 

Livingston formation.

Near Castle the ridge of volcanic rock south of the schoblhouse marks 

the base of the Livingston formation.

LIVINGSTON FORMATION.

Upon the south and southeast sides of Castle Mountain there is a 

great thickness of dark-colored sandstones, grits, and conglomerates, 

with interbedded shales, constituting the Livingston formation. 1 The 

rocks of this age, together with a higher series, form the great mass of 

the Crazy Mountains, and the wide valley between that group and the 

Bridger range and Sixteenmile mountains is cut in these strata.

The beds constituting the formation are generally dark-colored, of 

various shades of brown or rarely green. They are usually coarse 

sandstones, composed almost wholly of andesitic volcanic material that 

shows rapid deposition and but little sorting by water. About 200 feet 

above the base of the series occurs a light-colored bed that is a pure 

white sandstone, presenting a strong contrast to the adjacent ledges. 

This is a horizon by which it is possible to trace out the intricate folding that prevails along the course of the South Musselshell. The beds 

are very generally intruded by sheets of igneous rock of Crazy Mountain types, whose upturned edges weather as bold combs, rising above 

the slopes formed of Livingston rocks.

An excellent section of these .rocks, and one showing their foldings 

conformable with the underlying Mesozoic series, is seen where the road 

'from Livingston and Bozeman crosses the divide between South Musselshell and Smith rivers.

Section of beds exposed at head of Smith River.

Feet. 

Brown sandstone or tuff bed, well indurated and breaking into

small, angular fragments of one-fourth to three-fourths inch 

across. This ledge forms foot-slope ridges and buttresses rising 

abruptly from the walls of the valley south of Castle Mountain.. 25

Fine-grained tuffaceous strata, often brown and carrying lignitic 

shale, but inconstant in character and variable horizontally... 105

Light-gray sandstone formed of the sorted grains of volcanic 

material; dips eastward at 45°. This bed forms the valley riin. 25

Tuffaceous sandstone with concentric, spherical, shelly weathering ............................................/............... 5

Tuffaceous sandstone, brown in color and varying to green. Generally crumbly and carrying round concretions of all sizes, up to 

2 feet, which resemble cannon balls............................ 40

i W. IT. Weed, Lnramio and overlying Liviugstou formation: Bull. U. S. Geol. Survey No. 105.

Bull. 139 4
50 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

Feet. 

Fine-grained tuffaceous sandstone, green or grayish-green in color,

crumbling readily to a dicy, shelly ddbris and carrying occasional spherical concretions from 2 to 3 feet in diameter ....... 15

Fine-grained, shaly, tuffaceous sandstone........................ 36

Tuffaceous beds, generally concealed by other ddbris............. 353

Conglomerate composed largely of pebbles of volcanic material.. 5 

Soft tuffaceous sandstone........................................ 90

Dark-green and rusty-brown tuffaceous beds, varying considerably 

in texture and coarseness of composition. The beds change 

rapidly in character horizontally..... A. ....................... 35

Green, fine-grained tuffaceous sandstone, crumbling readily to a 

coarse, sandy ddbris and much resembling the tuffaceous beds 

of the fossil forests in Yellowstone Park........................ 5

Fine-grained greenish sandstone, weathering brown.............. 7

Limestone; gray on fresh fracture and weathering with a sealbrown surface................................................. 2

Umber-colored tuffaceous beds. The series thus far given is of the 

general brown or umber tint, and differs in this respect from the 

underlying beds, which are generally green in color............. 10

Granular sandstone, breaking readily into green, splintery ddbris, 

whose fracture is at right angles to bedding, forming small 

angular bits an inch in diameter............................... 5

Bed of gritty sandstone of a bluish-green color .................. 3

Black tuffaceous shale, breaking into fine, angular ddbris......... 15

Limestone bed .................................................. J

Greenish-gray sandstone, weathering into roundish ledges, but 

breaking into angular bits. This rock is unlike a true volcanic 

breccia and quite distinct in character from the Laramie beds... 10 

Fine-grained tuffaceous beds formed of volcanic material and 

weathering readily, so that exposures are seldom seen. These 

rocks form the small inclosed valley at the head of the stream^. 300 

Light-brown, fine-grained, steel-gray sandstone, dipping south at 

60°. The rock weathers in smooth, round forms and is a very 

dense sandstone resembling limestone in appearance ........... 6

.Sandstone; light-gray in color; composed of angular bits of material that are not markedly volcanic. It is separated from a 

ledge above by 2 feet of shale................................. 5 ,

Fine-grained, buff-brown, tuffaceous beds, seldom well exposed, 

but showing crumbly, black pieces, forming a 'sandy ddbris 

covering the exposures........................................ 250

Very fine-grained, tuffaceous rocks, olive-green in color.......... 10

Shaly beds, crumbling readily to fine, sandy ddbris and black 

earth ......................................................... 125

Conglomerate and volcanic breccia; contains well-rounded pebbles up to 6 inches in diameter, and also a well-cemented, angu- . 

lar breccia and grit. The ledge dips south at 60° and forms a 

very rough and jagged comb that stands out above the gentler 

slopes. The upper part of this bed may be classed as a true volcanic breccia, as it carries angular bits of andesite up to an inch 

in diameter, scattered through a cementing material of finegrained tuff................................................... 100

Soft, readily weathering, tuffaceous beds of an earthy-brown color. 200 

Sandstones, white in color and forming long lines of tombstonelike flags, rising above the grassy slopes....................... 10

Dark, rusty-brown and black shale, seldom exposed.............. 300
CRETACEOUS ROCKS. 51

Feet.

Sandstone, resembling volcanic grits, but fine-grained and of doubtful character .................................................. 50

Shale, light-brown in color and weathering readily to soft, earthy 

d6bris ........................................................ 20

Brown shale, not well exposed.................................... 135

Sandstone, gray, composed of the sorted grains of quartzose and 

feldspathic material, rock being of Laramie type.............. 5

Shales; soft, shaly beds seldom showing their character either in 

d6bris or in soil, and not exposed in this section. No traces of 

coal were seen, although this must be Laramie ................. 1,950

Olive-green sandstone, dipping 55° south ....................... 5

Impure sandstone, breaking into laminae one-half inch thick and 

showing fucoid markings...................................... 45

Shales; weathered down and showing dark-colored, clayey debris. 280

Gritty sandstone that becomes a conglomerate in places. It resembles the Dakota, but is finer grained, and the quartzite pebbles 

are of many varieties.......................................... 5

Shales, which are not exposed ................................... 220

Sandstone, well indurated, hard, yellowish on fresh fracture and 

weathering gray. Forms very massive outcrop on mountain 

slope, where it breaks into flags whose lines extend along the 

slope. The rock is formed of well-sorted and fine-grained granitic material.................................................. 20

Shaly beds and sandstones, not exposed.......................... 55

Sandstone that is somewhat calcareous; contains impressions of 

fossils; dip, 65°................................................ 8

Interval in which beds are not exposed.......................... 120

Sandstone, hard and vitreous, passing into quartzite; dip, 53°... 15 

Shales, not well exposed except near the underlying beds, where 

the rocks are gray and Argillaceous...... ...................... 112

Sandstone, fine-grained, well-bedded, dense, and of a brownishgray color.................................................... 5

Shales and impure limestones and sandstone beds, whose character 

is seen in the gopher heapings upon the slope.................. 430

Black shale, very soft, laminated, weathering to soft, black earths. 

The slope formed by this shale is strewn with blocks of quartzite 

from some higher ledge........................................ 200

Red earths, formed by the weathering down of beds not exposed. 210 

No exposure .................................................... 165

Sandstone, gray, and breaking into flaggy masses................ 6

Red earths...................................................... 12

Sandstone, quite massive, dipping at 65°; probably the source of 

the quartzite blocks seen below............................... 5

Sandstone, generally soft and weathering down, so that good exposures are not often seen..................................... 200

Quartzite, changing to sandstone, forming prominent ridge near 

the summit of the slope. It forms a pine-crested cone that is 

everywhere prominent. The rock is massive and crumbles under 

weathering, and is in general indurated enough to be called a 

quartzite ................................ ..................... 15

Above this sandstone there is an interval in which the beds are 

not exposed, and then the massive Carboniferous limestones are 

seen. It is probable that a fault occurs between this sandstone 

and the white Carboniferous limestones, as the well-marked, green 

shales of the Carboniferous are not seen.
52 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

The foregoing section is given to indicate the variable nature of the 

series. The rocks called sandstones do not, however, resemble the 

sandstones of other formations. The grains are angular, formed of 

fragments of volcanic rock or a variety of comminuted minerals, and 

in some places grade into fine breccias. They are really water-laid 

tuffs; the finer material forming the shales. Associated with these, 

tuffaceous sandstones there are, however, beds of conglomerate whose 

pebbles are well worn and rounded, formed largely of volcanic rocks, 

"diorite-porphyries," but including a few pebbles of sedimentary rocks. 

The relations of the Livingston are well known, the formation having 

been studied over a very considerable area in Montana. It is quite 

certain that the rocks represent a period of sedimentation following a 

Laramie uplift, and record a long period of erosion.

The most interesting feature of the formation as developed in the 

area under discussion is the occurrence of a bed composed very largely 

of oyster shells specifically identified by Mr. T. W. Stauton as Ostrea 

subtrigonalis a widely distributed species not heretofore known to 

occur above the Laramie formation.

The relative position of this bed is seen in the following section of 

the beds exposed between Hensley and Hamilton creeks:

Section of Livingston teds exposed on ridge between Hensley and Hamilton creeks, near

Castle Mountain: ^

Feet.

Green tuffaceous shale .......................................... 200

Fine-grained tuffaceous beds, seldom exposed v .................. 100

Dark-brown grit................................................ 15

Mottled, impure, limy sandstone................................. 5

Shale, greenish-gray in color and of tuffaceous material.......... 15

Oyster bed ....................................I.................. 2

Alternating beds of brownish grits or sandstones (coarse tuffs)

and line tuffs forming shales ...................*............... 36

Gray tuffaceous shale ....................................""....... 30

Coarse green and gray tuffaceous sandstones..................... 25

White hornstone rock ........................................... 5

Alternating hard and less hard purple and gray tuffaceous grits.. 200

Tuffaceous grits, weathering readily............................. 75

Dark-purple tuffaceous earths aud crumbly sands................ 60

Brown tuffaceous grits, breaking into small angular bits ......... 30

Volcanic tuff or breccia, weathering brown...................... 25

Tuffaceous sandstone, crumbling to brown, sandy d6bris ......... 135

Gray limestone, weathering brown.............................. 2

Crumbly, brown, tuffaceous sandstones.......................... 105

Limestone, dark-gray, weathering brown........................ 1

Tuffaceous sandstones, crumbling to brown sand................. 62

Dark-blue, compact, cryptocrystalline limestone, weathering

brown ........................................................ I

Tuffaceous sandstones, brown, crumbly.......................... 140

Light-gray sandstone ........................................... 10

Limestone, blue, granular....................................... 5

Tuft'aceous sandstone.......-..............,,,,,,,,,..,,,,,.,.,,. §0
MIOCENE ROCKS. 53

Feet. 

Limestone, dark-blue, weathering brown........................ 2

Tuffaceous sandstones with bed of grit in center................. 200

Limestone ...................................................... 1

Tuffaceous.sandstones of varying texture........................ 237

Coal seam....................................................... 5

This section shows a number of thin beds of limestone in the lower 

part of the series, which were carefully examined for fossils, but without success. '

The Livingston formation is not a coal-bearing horizon, but its beds 

are of use as indicating the proximity of the coal-bearing Larainie 

sandstones.

The overlying Fort Union beds, into which the Livingston passes, 

also carry coal, but they are lignites, soft and crumbly,-and of no value 

when the Laramie coals can be had. This overlying series consists, of 

drab clays and loosely cemented,' cross-bedded sandstones. It does 

not occur in the Castle Mountain district.

MIOCENE. 

LAKE-BED DEPOSITS (DEEP RIVER BEDS).

The sediments deposited in the waters of Lake Smith cover a considerable portion of the Castle Mountain area, filling the valley bottom 

of the South Fork of Smith River and extending up the main stream 

to the mouth of Fourmile Creek. Their exact extent at the last locality 

and in the foothill country about White Sulphur Springs is difficult to 

define, as the beds pass gradually into the fine, volcanic tuffs that cover 

the lower slopes of the mountain. These lake beds are a part of the 

sediments which cover the Smith Eiver Valley for many miles northwest of Castle Mountain. They were first described by Grinnell and 

Dana 1 in 1875, and collections made from the valley were described by 

Cope 2 in 1879 and later. The beds of this horizon were thoroughly 

explored by the Princeton expedition of 1891, under the direction of 

Prof. W. B. Scott,3 who has published a memoir in which the abundant 

fossil fauna of the lake beds is fully described.

Within the Castle Mountain area the lake beds are not so well developed as those found far northward in the valley. Below the mouth of 

Newland Creek two horizons are readily distinguished by their lithological character, and they differ even more markedly in the fossils 

which they contain, and, according to Professor Scott, they are separated by a probable unconformity. The lower beds consist of a series 

of well-indurated, cream-colored clays, which attain a thickness of 

some 200 feet. The line of separation between this underlying formation of clays and the overlying less-indurated sandstones and conglomerates is in some places very 'well marked. The upper beds consist of

1 On the ne\v Tertiary lake basin: Am. Jour. Sci., 3d ser., Vol. XI.

"Bull.!!. S.G-.anclG. S., Vol. V, 1879. Am. Naturalist, Vol. XX, p. 369, and Vol. XXI, pp. 456,457.

»Tran9. Am. Pliilos. Soc., Vol. XVIII, 1895, pp. 55-185.
54 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

rather incoherent strata, much cross-bedded and greatly resembling 

the Pliocene lake beds so common in the mountain valleys of Montana. 

They are composed of more or less waterworn material, varying from 

fine siliceous marls to large pebbles, and forming beds of silt and conglomerate. Within the limits of the area described in this bulletin, 

this upper series of beds is the one which is shown upon the map, and , 

the lower horizon has not been differentiated. The strata are of 

heterogenous nature and vary rapidly in kind and composition. 

Loosely compacted sands and layers of pulverulent volcanic dust, composed mainly of fine glass fragments, alternate with conglomerates and 

marly, tuffaceous limestones. The coarser sediments show crossbedding, and the conglomerates are often merely lenses of coarser 

material occurring in the sandstones. The prevailing colors are 

white, light-gray, or 'faint-chocolate, and the exposures are always 

light and conspicuous in tint. The beds being porous, the areas 

covered by them are generally arid wastes on which sage-brush and a 

scanty growth of grass form the only vegetation.

Exposures are seen only along the streams and valley bluffs, the surface being concealed by soil and the recent wash from the neighboring 

slopes. The greater part of the area south of White Sulphur Springs 

is a gentle slope of flat bench-land but little elevated above the marshy 

meadows of the South Fork of Smith River, and so nearly horizontal as 

to suggest the dry bed of the old lake itself. It is an area that is dry 

and barren, but capable of cultivation if irrigated. In general character the deposits of Lake Smith greatly resemble the lower part of 

the Tertiary of the Great Plains area, and the lake beds rest upon the 

eroded edges of all earlier formations, from the Belt shales of the Algonkian to the volcanic grits of Livingston age. In the vicinity of Castle 

Mountain the finer-grained beds are very largely formed of particles 

of volcanic glass. They are in part due to ash showers whose fine 

dust fell upon the waters of the lake during the activity of the Castle 

Mountain volcano, and are, no doubt, mostly formed of the same loose, 

incoherent material washed down and carried by the streams into the 

waters of Lake Smith.

Within the immediate Castle Mountain region the beds have not 

proved fossiliferous, although Grinnell and Dana found rhinoceros 

bones 2 miles south of Moss Agate Springs, a locality somewhere near 

White Sulphur Springs. The collections of vertebrate remains obtained 

by Professor Scott are of especial interest in this connection, as they 

furnish undoubted evidence as to the precise age of the beds in which 

they occur, and as these beds were formed of the material furnished 

by the Castle Mountain volcano, the fossils may be accepted as evidence of the exact age of the period of volcanic activity. Of the two 

horizons differentiated by Professor Scott, the lower is characterized by fossils which are of Lower Miocene [John Day] types, and the 

upper by a grouping of remains which place it at the base of the 

Loup Pork epoch of the Upper Miocene, of which they form a well-
MIOCENE ROCKS. , 55

marked subdivision, a subdivision which is not known in other regions. 

The European equivalent of the upper series of beds appears to be the 

Upper Miocene of Sansan and Simoore. The following species of mammals were found by Professor Scott in the lower or John. Day series of 

beds:

Cynodesmus tliooides Scott.

Steneofiber montanua Scott.

CiBnopus ep.

Miohippns annectens? Marsh.

M. anceps? Marsh.

M. (Anchitherium) equiceps? Cope.

Mesoreodon chelonyx Scott.

M. intermedius Scott.

Pcebrotherium sp.

Hypertragulus calcaratus Cope.

Tfie upper beds (Loup Fork) present a very different assemblage of 

species. The list given by Professor Scott, which combines the collections made by the Princeton party and those previously gathered by 

Professor Cope's assistant, contains the following species:

Canis? anceps Scott.

Chalicotherium? sp.

Aphelops sp.

Mioliippus sp.

AncMtherium equinum Scott.

Desraatippus crenidens Scott.

Protohippus sejunctus Cope.

Protohippus (Merychippns) insignia Leidy.

Merychyus (Ticholeptns) zygoniaticus Cope.

M. pariogonus Cope.

Merycochrerus montanus Cope.

Cyclopidius sinus Cope.

C. emydinus Cope.

C.incisivus Scott.

Pithecistes brevifacies Cope.

P. decedens Cope.

P. heterodoii Cope.

 Protolabis ep.

Procamelus sp.

Blastomeryx borealis Cope.

B. antelopinus Scott.

Mastodon proavus Cope.

Professor Scott, in speaking of these remains, says that "the lack of 

small animals in the collections is very noticeable. The lower beds 

have yielded but one rodent, and the upper none at all. Only two 

carnivores, both dogs, have been found. None of the Insectivora or 

Chiroptera have been detected. The fauna, so far as known, consists, 

therefore, almost entirely of medium and large sized ungulates," including Protohippus, Miohippus, and Mesohippus, with a new genus of 

equines which fills the gap between the two earlier forms, several rhinoceroses, camels, a mastodon and other Probocidae, together with 

many ancestors of our present species of ungulates.
CHAPTER V. 

IGNEOUS BOCKS.

In this chapter it is proposed to give an account of the mode of 

occurrence of the igneous rocks and their relations to each other and 

to the sedimentary rocks with which they occur, together with a more 

detailed description of^such localities as are of especial interest.

The petrographic and chemical investigation of the varied rock 

types is deferred to the following chapter, while the general discussion 

of the petrologic history of the district, founded upon the facts observed 

in the field and those brought out by research in the laboratory, will be 

given in a still subsequent one.

In pursuance of this plan it has been deemed best to divide the 

rocks into such natural groups as are found by observation to occur in 

the field a classification dependent partly on chemical differences in the 

magmas from which the rocks originated, and partly on their mode of 

geological occurrence, the last factor having determined, in a general 

way, the physical forces conditioning their crystallization and textures 

and thereby their independence as rock types. As has already been 

stated, in the Castle Mountain mass we have to deal with a greatly 

eroded and degraded center of volcanic activity. It is, in fact, to use 

the apt term for such occurrences, a "dissected?' volcano. As a natural sequence of this fact, there are presented all the different types of 

crystallization and structure possible for an igneous magma to assume 

under the most varied conditions of cooling and pressure. Unlike, 

however, the great inassives of the Crazy Mountains immediately to 

the southward, there is no such complexity arid variability in the chemical and mineral nature of the rocks, except in a comparatively few 

instances to be noted later. In general, the rocks have been derived 

from a highly siliceous magma, rich in alumina and alkalies and poor 

in other components. Therefore the great majority of them belong to 

the granite family, which, for the reasons just given, exists in every 

type of structure and development, as granite, granite-porphyry, quartzporphyry, rhyolite, rhyolitic obsidian, and rhyolitic tuffs and breccias. 

Associated with these, but in much smaller amount, there are a peculiar augite-diorite, porphyries passing into porphyrites, lamprophyric 

rocks in dikes and small intruded bosses, and flows of basalt. All of 

these rocks belong historically to what may be termed a single geologic 

time-phase of volcanic activity, and with a few exceptions all the other 

rocks of the district are clearly of sedimentary origin.

Although Castle Mountain no longer presents the slightest remnant 

of a volcanic cone, the fact that such a cone formerly existed and that 

active igneous agencies were here at work is strikingly apparent upon 

56
IGNEOUS ROCKS. 57

an examination of the region; it is advisable, therefore, to consider 

briefly the evidence, in order that we may better understand the natural 

grouping of the rocks that follows. It should be borne in mind that 

of all the varied formations to which geologic agencies may give rise, 

none are more ephemeral, speaking in a geologic, sense, than the exterior products of volcanic activity. Thrown out and piled up in a brief 

period of time, volcanic materials, from their loose and incoherent state 

and their frequently fragile and perishable nature, are most quickly 

eroded and carried away, or metamorphosed by the various forces that 

act upon them, from the moment of appearance. The failure to recognize this fact has often prevented the recognition of their former occurrence in many districts.

Imagine that in a region of already folded and tilted sedimentary 

strata, there is formed a center of eruptive activity which passes through 

its various life phases and becomes extinct, and that the mass thus 

produced is then greatly cut down by the forces constantly striving to 

bring it to baselevel; it is evident that at the center there would be 

exposed a core of massive igneous rock representing the former conduit, surrounded by sedimentary rocks more or less altered from contact 

inetarnorphism, intruded by sheets of igneous material, and cracked 

and filled with radial dikes. The effusive and ejected matter would be 

found at a greater distance, with a more or less roughly annular disposition, lying upon the outer slopes of the general mass, and itself cut 

by erosion into varied topography. Such a theoretical disposition 

would, of course, be greatly modified by attendant circumstances, which 

need not be further enlarged upon.

Just such an arrangement of rocks and material is presented in .a 

general way by the Castle Mountain mass, as maybe seen by reference 

to the map (PI. III). There is a central core of granite surrounded by 

upturned sedimentary rocks which are as a rule more or less highly 

metamorphosed and injected with numerous sheets of porphyry, and 

in places cracked and filled with dikes. Further from the center and 

forming the foothills in considerable part, though chiefly to the east 

and north, are masses of rhyolite and rhyolitic obsidian, filling at times 

hollows in older eroded valleys and resting uiicoinforrnably upon the 

strata, while other exposures are those of breccias and rhyolitic tuffs, 

the breccias themselves being overlain in places by rhyolite and cut by 

dikes.

Such, briefly presented, are the evidences of the former volcanic 

nature of the Castle Mountain mass, and the resultant rocks fall naturally into the following groups, according to which they will be treated:

I. Massive plutouic rocks' of the central mass.

II. Porphyritic rocks of the intruded sheets and dikes. 

III. Extrusive rocks in lava flows. 

IY. Tuffs and breccias.

Y. Igneous rocks of the district not belonging to the Castle 

Mountain center.
58 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

I. MASSIVE PLUTONIC ROCKS.

The massive plutonic rocks of this seat of igneous activity are represented by granite and a peculiar augite-diorite, and they occur in two 

distinct and separate areas. ,

GRANITE.

This rock composes the main portion of Castle Mountain, and, as may 

be seen by reference to the map, its exposure forms a rudely elliptical 

area about 8 miles long in its greatest diameter and about 4£ miles 

broad. It forms the solid central core of the mountain, and, as has 

been shown by the description of the topographic form, it is weathered 

to a low dome-shaped mass, presenting none of those sharply serrated 

peaks and spires which are generally so characteristic of the erosion 

of granitic masses in the younger mountain chains. In part, at least, 

' this is due to its texture, which is, in general, for a granitic rock, rather 

loose, and filled with miarolitic cavities, thus leading to easy weathering and decay. Nowhere, indeed, does it present that solid, heavy, 

massive, coarse-grained texture which is generally characteristic of 

the older Paleozoic and pre-Paleozoic granites a fact which is in part 

due to the erosion which has cut these older Plutonic rocks down to 

deeper levels, where the material has solidified more slowly and under 

greater pressure, and, in part, to the action of the great dynamic forces 

of the orogenic processes which have compacted them, closing at the 

same time all of their miarolitic cavities. The Castle Mountain granite 

might be compared to the lighter, somewhat spongy form that certain 

metals assume when merely cast from the melting pot, while these 

older granites are more like the denser, tougher form the same metal 

assumes when hammered and rolled."

The term granite is applied to the whole of this great intruded stock, 

partly because it is composed mostly of granite, the magma in solidifying having commonly assumed the granitic texture and minerals, and 

partly for the sake of convenience,, because the mass is a geologic unit. 

Nevertheless, within comparatively short distances there are passages 

into finer-grained porphyritic forms, which, in accordance with the prevailing German usage, would be termed granite-porphyry. The whole 

mass shows, indeed, in all places a strong tendency toward a porphyritic structure a fact which will be more fully discussed later.

In general, the granite area presents a smooth, gently contoured surface, diversified by open, grassy meadows, alternating with groves of 

pines- and with few prominent oultcrops. About the margin, however, 

the rock is harder, tougher, and more compact, and breaks into great 

cubes and blocks, which strew the surface. It is especially around the 

margin that the craggy castellated piles occur, which, as previously 

mentioned, have given the mountain its name. In places these crags 

rise sharply from the otherwise evjen slopes and project above the forest
. ' GRANITE. 59

growth like towers. They do, in'fact, often most strongly resemble 

ruined castles, the regular jointing of the mass giving an effect of 

broken walls of heavy masonry. They are shown in Plate V, which, 

gives an excellent panoramic view of the margin of the granite area, 

presenting the general character and jointing of the rock, but which 

does not show the castellated character of the crags as well as it is 

sometimes seen.

The height to which these crags rise above the slopes on which they 

are situated varies considerably, but in general it is not more than 40 

to 50 feet. When occurring on the very rim of the mass they often 

descend sharply on the outer side to much greater distances, commonly 

into a,ravine. Areas on the mountain are also covered with rock piles 

formed from the natural falling apart of the jointed mass.

The jointing of the granite varies considerably, being in some places 

in broad, thin plates, and in others in large, rude parallelepipedons or 

sphenoids, often with incurved faces, the size of the blocks thus formed 

varying within a wide range from 10 feet long to as many inches. A 

feature of the granite, particularly in those areas whe're it passes into 

a granite-porphyry, is the presence within it of spherical masses, which, 

on exposed surfaces, by a difference in grain, and also at times in. color, 

are sharply differentiated from the general mass and become noticeable 

to the eye. They vary greatly in size, from many feet to a few inches 

in diameter. They are more fully described in the petrographical portion of this work.

As-is common in all such granitic masses, it is cut by many white 

veins or dikes of fine-grained aplite, which are generally but a few 

inches in width.

It can not be supposed that this huge mass as a whole represents 

merely the filled up and consolidated canal of an old volcanic throat. 

It is not only too large to lend color to such a supposition, but the effusive masses which lie outside of it are too small in amount to represent 

what would in such a case have been a volcanic center of enormous 

size. It is much more reasonable to conceive that it was originally a 

huge intruded mass, and that at some point, breaking through the sedimentary cover which has since been removed by erosion, there was 

then started an outlet for the effusive and projected material.

We have been unable to locate this point precisely, but from the fact 

that the flows of rhyolite are confined almost entirely to the eastern 

half of the district, it seems reasonable to suppose that it was situated 

somewhere along the eastern portion of the mass. The amount of dissection, or, in other words, of denudation, has been so great that it is 

impossible to reconstruct many of the finer details, such as others have 

found it possible to reconstruct in similar districts.

Evidences of intrusion. Evidences of the intrusive nature of the 

granite are everywhere apparent. Except upon the western slopes, 

where the beds dip steeply away from the central core, the granite
60 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 130.

abruptly cuts off the sedimentary strata and abuts against beds of all 

ages. At a number of localities sheets of porphyry intruded into the 

stratified rocks are traceable to and into the main granite mass. Moreover, the granite area is everywhere surrounded by a ring of altered, 

metamorphosed sedimentary rocks, whose alteration is greatest near the 

granite and decreases with distance from it.

The abrupt cutting off of the sedimentary beds is well shown about 

the head waters of Alibaugh Creek and also near the town of Castle. 

At the latter place the workings of the Cumberland mine show that the 

plane of contact has here an inclination of 55° away from the center of 

the granite mass. One of the best exposures where the intruded sheets 

of porphyry may be traced into the granite is seen in the section on 

upper Fourmile Creek shown in the diagram, fig. 7. The valley is deep 

and narrow, and in places thickly timbered with small pines (see PL 

VII). The steep slopes are in other places covered with slide rock. 

Through the upper part of the valley, which has a rapid downward 

slope, the creek has cut a small, narrow canyon in the valley floor, the 

canyon being in places 30 feet or so in depth (see PI. VIII). Through

PIG. 7. Diagram showing tilting of the sedimentary beds by the intrusion of sheets of porphyry 

from the granite stock, seen on upper Fonrmilo Creek.

this the stream has a rapid, torrential course, carrying down bowlders, 

logs, and other drift material. Thus the walls are in general scraped 

bare and afford good exposures of the rocks. Where the stream 

crosses the contact with the Belt shales, which dip downstream at an 

angle of '20°, the latter are split by intruded sheets which project from 

the granite. Two such, each several feet in thickness, are beautifully 

exposed at the contact, while others alternate with the Belt formation 

in ascending the stream to this point. The granite, which is porphyritic 

and is in fact near the contact a coarse-grained granite-porphyry, rapidly grows denser in grain as it passes out into the sheets, where it 

eventually assumes the texture and structure of a quartz-porphyry. 1

To further add to the interest at this point, a photograph of which 

is given in PI. IX, both the Belt shales and the intruded sheets of 

porphyry are cut across at right angles to the course of the stream by 

a lamprophyre dike of augite-vogesite some 5J feet in width. The

] The terms porphyry and porphyrite are used in this work to designate, without reference to their 

age, such dense holocrystalline porphyritic rocks as are found in intrusions.
U. S GEOLOGICAL SURVEY 

FALLS OF UPPER FOURMILE CRE.E.K.
WEED ANDT

rmssoN. J D1OKI1JL. mnRTTTT

nature of the exposure is such that the passage of the granite out into 

the intruded sheets and the torn and opened ends of the sedimentary 

beds can be seen in one continuous exposure.

Other examples of the connection' of the intruded sheets with the 

granite mass may be seen in various localities, though in none are they 

so. well exposed as in that just mentioned. While the abundance of 

these intrusive sheets at Castle Mountain forms a striking feature 

of the general geology, the paucity of dikes is noticeable, and few dikelike apophyses of the granite cutting the adjacent rocks have been 

observed.

DIOEITE.

This stock of intruded rock, as shown on the map (PI. Ill), is a rudely 

circular mass about a mile in diameter, with an apophysis thrust out to 

the northeastward, through the middle of which the upper waters of 

Bonanza Creek have cut a deep ravine. The mass possesses the coarse 

grain and solid texture typical of an abyssal igneous rock which has 

solidified under great pressure. Nevertheless, at its outer edge, and 

especially toward the western border, as typically shown iii the exposures on the hill above Blackhawk, west of the Legal Tender mine, it 

passes into a denser form, and becomes porphyritic. It then weathers 

out into crags, and has a jointing in all respects like the granite, possesses similar spheroidal masses with a circular parting and denser 

grain, and in every way shows its consanguinity and genetic relationship to the neighboring granite mass.

The coarse-grained central portion of the intrusion seems to weather 

away most easily. Indeed, as shown in a prospector's trench on the 

open meadow crossed by the wagon road from Blackhawk to Kobinson, it is in places thoroughly rotted by superficial decay to a soft 

crumbly mass, easily disintegrated between the fingers. Somewhat 

deeper, however, it is beautifully fresh and well preserved, as shown by 

the material thrown out from a prospect shaft in this meadow to the 

southeast of the present road. The material exposed by the cutting 

made for the road just above Kobinson is also very fresh and unaltered.

The character of this rock changes toward the outer edge, becoming 

steadily more siliceous until at the boundary it assumes the composition of a quartz-diorite-porphyrite, though rather low in quartz. It 

is abundantly cut by aplite and quartz-porphyry dikes which have the 

composition of the granite, as will be shown later. Some of these 

dikes may be seen as low, small ridges crossing the open meadow and 

the wagon road just mentioned, with an east-and-west trend, which if 

prolonged would carry them, into the granite mass. A fine example of 

the aplite dikes may be seen among the material thrown out at the 

prospect shaft" mentioned above, the pale-yellow color and fine grain 

of the narrow dike contrasting strongly with the dark-gray, coarsegrained diorite.
62 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

The exact relation between this intruded stock and the great granite 

mass is not absolutely certain, since they nowhere come into contact, 

being separated by a large block of upturned Belt shales full of 

intruded sheets of porphyries, as-shown on the map. Since, however, 

it is cut by numerous dikes which trend toward the granite and have 

its exact chemical and mineral nature, while the granite is not cut by 

dikes of rock of the nature of the syenite, as well as for other reasons 

which will be discussed later, it is assumed to be the older of the two. 

This assumption seems also to accord with recent chemical work in 

general petrology, and while no further evidence can be adduced in 

favor of it, it serves as a working hypothesis for the explanation of the 

geologic history of the district, and best accords with all the facts 

observed.

CONTACT METAMORPHISM.

The intrusive masses of Castle Mountain have strongly marked their 

entrance into the sedimentary beds by the amount of metamorphism 

which they have effected The Belt shales near the contact are baked 

into tough, hornstone-like rocks with an extremely dense grain and 

conchoidal or splintery fracture like that of jasper. They have lost 

their fissile character and have a coarse jointing, so that under .the 

blow of the hammer they break up into rhombohedra of irregular 

shapes. Instead of the black or dark-gray color so frequently shown, 

they are chiefly reddish or greenish in tone, with a banded appearance, 

which is all that remains to distinguish- the former bedding planes. 

They resemble strongly certain varieties of adinole formed by the contact action of intruded diabases on the slates of the Hartz Mountains 

in Germany.

Nowhere in the metamorphic zone has there been observed that kind 

of contact metamorphism which produces in clay-slates a regular spotting of the metamorphosed rock due to the formation of small nodules 

made up by collections of minerals, and which is a common phenomenon to be observed in the metamorphism of clay-slates.

The metamorphism is most clearly seen, of course, in the immediate 

contact with the igneous rock. The changes become less perceptible 

in passing outward from it, and so gradually that no exact lines can be 

drawn between the altered and unaltered areas. The width of the 

metamorphic zone varies somewhat in different localities, according to 

circumstances. It is, however, nowhere very great, a quarter of a mile 

fully sufficing to carry one into the area of unaltered shale or slate. 

Considering the great size of the granite intrusion, this is a rather 

narrow zone. The reason for this is perhaps to be found in the fact 

that, as already shown, the granite area does not represent in its entire 

mass the filling up of a previous volcanic conduit, but is largely a 

single intrusion. Around volcanic throats, metamorphism is apt to be 

intense from the continued supply of heated material from below, while 

in a single intrusion the heat is only that initially furnished. Thus in
U. 8. GEOLOGICAL SURVEY 

INTRUDED SHEETS AND DIKE ON UPPER FOURMILE C
CONTACT METAMORPHTSM. 63

the latter case the amount of metamorphism may be small, and indeed, 

as has been shown in the case of some laccolites, may be nearly wanting. 

The result is also dependent upon other factors, such as the mineralizing vapors of water, fluorine, etc., whose presence in greater or less 

amount would naturally influence the result.

The valley of the main upper fork of Fourmile Creek (see PL VII), 

which has been previously mentioned as evidencing the connection 

between the granite and intruded sheets of porphyry, is one of the best 

localities showing the metamorphosed character of the Belt formation 

(see PI. VIII). The heat and vapors from the great number of intruded 

sheets has, without doubt, also aided this action. The steep slopes of 

this valley, in places often thickly covered by dense growths of small 

pines, and in others open and covered with slide rock, show numerous 

exposures of the sedimentary strata. These outcrops are formed of 

hard, dense, and jagged rocks,of the character already described, and 

differ most strikingly from those of other areas shown on the map, 

where the Belt shales are unacted upon by contact metamorphism. At 

such places smooth, rounded slopes prevail and actual outcrops are 

difficult to find, only the soil, filled with fine fragments of disintegrated shales, showing the country rock beneath. Outcrops and consequent topography of a kind similar to that on Fourmile Creek are 

also found around the upper head valleys of Cottonwood and Willow 

creeks; descending from the smooth upper slopes of the granite, the 

streams immediately cut deep ravines in the metamorphosed sedimentary zone, and these widen out into smooth open valleys as they enter 

the softer, unaltered shales of the Belt formation.

The action of the intruded masses upon the limestones with which 

they come in contact is also marked, though that produced by the 

diorite area is much the more so. Thus, at the divide at the head of 

the eastern upper fork of Willow Creek, in the neighborhood of the 

Grasshopper mine, the Silurian ("?) limestones are brought against the 

granite, and within a few rods they show considerable alteration in 

color and crystallization. In other localities, as at the Cumberland 

mine above Castje, the limestones are greatly altered, partly, also, 

from intruded sheets and from hot mineralizing solutions which have 

deposited the ores within them.

The most striking effects produced upon the limestones by the igneous intrusions are to be found in the neighborhood of Robinson and of 

Blackhawk, where the metamorphism is due to the diorite mass. The 

limestone hills around both of these localities have been bleached and 

whitened into marble, which is, in places, as in the road-cutting opposite 

Blackhawk post-office, very coarsely granular, yielding broad cleavage plates of calcite an inch or more across. In places, also, various 

silicate minerals, such as garnet, phlogopite, vesuvianite, and pyroxene 

are to be found, due to the metamorphic action upon the silica and 

alumina of the former sand and clay mixed with the impure limestone,
64 THE CASTLE MOUNTAIN MINING DISTRICT. [DULL. 139.

containing liine, magnesia, and iron. Excellent examples of this are 

found in the material thrown out of the prospect shafts on upper Bonanza 

Creek, a little below the point where the present wagon road from Blackhawk to Bobinson leaves the stream.

It is in the areas of altered limestone that the deposits of ore which 

have made the Castle Mountain region so well known in Montana are 

found. This phase of the subject will be more fully treated in the 

chapter on the mining resources of the district. The fact, however, is 

prominently brought to one's attention in viewing the country below 

from any of the high spurs of the mountain which give an outlook to 

the eastward. That the contact rock is the place to seek ore has been 

well known to the miners and prospectors, and this fact has caused 

these areas to be thoroughly explored. Looking across the country 

the contact line on every ridge may be seen, marked by the white heaps 

of impure marble thrown put from the prospectors' trenches.

II. INTRUDED SHEETS AND DIKES OF PORPHYRY.

In parts of the Castle Mountain area intruded sheets occur in such 

great number that it has been found irirpossible to depict them all upon 

a inap of so small a scale, and hence, in some portions of the district, 

the sheets shown on the map should be understood as representing a 

much greater number. Especially is this true of the upturned block 

of Belt shales which separates the granite and diorite areas. The open 

hill between the heads of Fourmile and Checkerboard creeks is crowded 

with these sheets, only a few of which can be shown on the map. They 

range in thickness from a foot to 3 or 4 feet. Their connection with the 

granite in the Fourmile Valley below the hill has already been mentioned. The mountain spurs above Castle are full of intruded sheets, 

as are also the hills above Blackhawk in the metamorphosed limestone 

and Belt- slates. Sheets of porphyry derived from the granite mass 

also occur in considerable numbers in the Cretaceous beds below the 

town of Castle. At this point the strata are steeply upturned, and the 

greater resisting power of the igneous rocks to weathering processes 

causes them to stand up in combs crossing the ridges. As a rule the 

intruded sheets of the region do not extend far out from the central 

core, but are confined to the immediate vicinity of the granite mass. 

The absence of intrusive sheets in the upturned rocks along the western flank of the mountain is to be explained partly by their attitude. 

These sheets all show little if any folding subsequent to their injection.

As previously mentioned, segregated masses occur in the granite and 

diorite areas, which differ from the inclosing rocks in composition and 

texture. From certain observed facts it seems evident that similar 

masses exist also in the intruded sheets and in extrusive lavas. In 

these, however, they do not possess the spherical form, but appear to 

be drawn out into flat, lenticular, or spindle-shaped bodies,, as if, 

having formed while the molten masses were still in a very liquid
WEED AND"] mm^C! ' fi ^

PIHSSON. J UlKH.b. DO

condition, they had been stretched, when the intrusions and flows 

occurred, with a more stiffly viscous material. Thus when the outcrops 

of flows or the edges of upturned sheets occur, at first sight these segregated masses appear like dikes cutting them, and their real nature 

is to be seen only after a more detailed study of the exposures. This 

simulation of a dike is often aided by a difference in the weathering 

and erosion of the varying rock masses, which causes either the main 

rock or the inclusion to be prominent.

DIKES.

Comparatively speaking, there are not many dikes to be found in the 

Castle Mountain area, and consequently those that occur play but an 

insignificant role in its structural geology and in the formation of its 

topography. There is thus a marked difference between this region 

and that of the neighboring Crazy Mountains, whose vast and complicated system of dikes forms so marked and distinguishing a feature of 

its geologic structure. The explanation of this fact is probably to be 

found in the circumstance that eruptive activity began to manifest itself 

in the Castle Mountain district after the sedimentary strata had been 

already upturned and eroded, as previously pointed out. Thus areas 

and lines of weakness already established became the seat of intrusions, 

which therefore took place without extreme shattering of the strata 

and consequent formation of dikes. It will be noticed that those which 

do occur have a general radial disposition with respect to the central 

mass, a phenomenon often observed in other regions where they are 

more numerous around eruptive masses.

Those which are found, however,illustrate very well several points of 

interest which render them worthy of a detailed description. They may 

be briefly divided into two classes dikes of acidic and of basic rocks, 

the former rich in silica, alumina, and alkalis; the latter poor in silica 

and distinguished by large amounts of lime, magnesia, and iron oxides. 

Thus the molten masses forming the dikes of the first class have crystallized into rocks composed chiefly of feldspar, often with accessory 

quartz and with small amounts of mica or hornblende.

Acidic dikes. The rocks of these dikes are light-colored porphyries 

with large phenocrysts of feldspar. According to the different physical conditions under which they have cooled and crystallized, they 

have assumed various textures, and are to be distinguished as micro 

granites, granite-porphyries, quartz-porphyries, and feldspar-porphyries, often grading into porphyrites -by an increase in the amount of 

soda-lime feldspar. Examples of the microgranites are found in the 

narrow dikes cutting the diorite, the rocks having the composition of 

the main granite mass. Types of the granite-porphyry are found in 

the big dike, 12 to 14 feet wide, which cuts the tilted Belt shales on the 

mountain side on the main head of Fourmile Creek, and which forms 

so conspicuous a feature of the view looking up this valley from the 

Bull. 139 5
66 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

point where the present road from White Sulphur Springs to Castle 

leaves Fourmile Creek as it makes its abrupt turn to the southward 

(see PL III, geologic map, and PL VII). The Belt shales, which are here 

indurated and altered, have been eroded so much faster than the dike 

that the latter rises for a long distance above the steep slope in a great 

wall 20 to 30 feet high and 12 feet broad.

Another example is found in a somewhat similar dike which has also 

been brought into prominence by erosion, and forms a wall seen upon 

the hillside north of the present road from White Sulphur Springs to 

Castle, a short distance above Reed's ranch on Fourmile Creek. This 

dike has cut through the Cambrian limestones, which, by the action of its 

contact, have been for some distance whitened into marble. The rock 

itself is distinguished by the large porphyritic crystals of orthoclase 

feldspar which it carries and which become smaller and nearly disappear at the contact. A dike of exactly the same kind of rock, with 

similar large phenocrysts, appears on the hill to the northeast of the 

one just mentioned, and it may, indeed, be its continuation, though 

positive proof on this point is lacking.

A dike of porphyry, 30 feet wide, is seen cutting the green Carboniferous shales a short distance below Reed's ranch on Fourmile Creek. 

The rock belongs to the acid series of dikes and has produced but little 

alteration in the adjacent shales.

Dikes of porphyry, inclining to quartz-porphyry, also cut the diorite 

mass, as shown on the map. (PL III.) They appear as low ridges 

cutting across the grassy slopes of the upland meadows, through which 

the present road from Blackhawk to Eobinson passes. The rocks of 

which they are composed are greatly weathered, so that their exact 

nature is uncertain.

Three parallel dikes of porphyry are to be found cutting through the 

upturned Cambrian limestones which form part of the western limb of 

the synclinal fold to the northeast of Blackhawk. They are reddish 

rocks, and at a distance appear clearly as low walls about 5 to 6 feet 

wide, crossing the light-colored limestone hill. They trend to the 

northeast and eventually die out in the Dakota sandstones and shales. 

In composition they are feldspar-porphyries passing into porphyrites.

Porphyry dikes are also found cutting the altered sedimentary rocks 

near the. granite contact northwest of Castle, between that town and 

Alibaugh Creek. Another dike of similar character occurs on the slope 

west of the stream.

In passing over the hill, the road from Blackhawk to the mouth of 

Checkerboard Creek follows the'.most southern of these dikes in 

descending to the level Cretaceous flat of the trough of the syncline. 

It is noticed that these dikes have effected almost no contact inetarnorphism in the adjacent limestones.

Dikes of somewhat similar rocks are also found in the upturned Cre-' 

taceous beds below the town of Castle. It is also probable that the
WEED AND-] mmrq R7

PIHSSON; J UlK&b. O 4

tilted masses of Belt shales which rest against the granite on the nortneast side between the granite and diorite and to the north of the latter 

are cut by acid dikes as well as filled with intruded sheets. These 

areas have been most extensively prospected by the miners, and the 

mines of Robinson and Blackhawk are mostly to be found in them. 

In some of the prospect shafts contacts of igneous rocks with the sedimentary strata are to be seen, which would indicate dikes both of acid 

and basic varieties, but the exposures at the surface are either covered 

or of such a nature that they do not afford positive evidence.

Basic (lilies. Dikes1 of the second or basic class are rarer. They form 

heavy trap-like rocks of a black, dark-gray, or greenish color, are 

usually dense in texture, and carry porphyritic crystals of mica, augite, 

hornblende, or olivine. They belong to that class of rocks to which the 

name " lamprophyre" has been given, and which are to be found in close 

connection with areas of eruptive volcanic activity. Usually they are

found to be the very latest phenomena of intrusion and cut the other 

volcanic rocks with which they come in contact. This is also the case 

in the Castle Mountain district, though, as they are few in number and 

extremely subject to subaerial decay, only a few such instances have 

been observed. For this reason, as pointed out by Iddiugs, 1 when these 

rocks have reached the surface and formed lavas they are among the 

first of the volcanic products to be carried away by denudation. As 

erosion has been so extensive at Castle Mountain, such extrusive 

masses, if they formerly existed, have been mostly carried away; yet 

this has not been invariably the case, and we will endeavor to show 

later that in some cases, at least, the extrusive equivalents of these 

lamprophyres still exist.

Vogt2 has recently pointed out that such basic rocks are liable to 

carry sulphide ores as masses segregated from the molten magma. 

Instances of this have also been observed at Castle Mountain, as in the 

case of the Little May prospect shaft, where chalcopyrite occurs in such 

a dark-greenish decomposed rock. The prospectors, indeed, seem to 

have a practical knowledge of this, since it is observed that wherever 

these rocks occur they are apt to be extensively prospected, especially 

if somewhat decayed, since they are then more easily worked.

It is well known that under atmospheric and aqueous agencies these 

rocks are liable to two kinds of alteration. In the first they become 

rich in carbonates; in the second kaolin is formed, often to such an 

extent that of the dike only a mass of clay is finally left. This latter 

process seems especially true of that variety of lamprophyre rocks 

which consist chiefly of orthoclase with much biotite, and which are 

known as ininettes or mica-traps. These are not found in the Castle 

Mountain district, and the alteration of such lamprophyre dikes as do

1 Origin of igneous rocks: Bull. Philos. Soc. Washington, Vol. XII, 1892, p. 168.

2 Zeitsclirift f. prak. Geol., Vol. I, 1893, p. 4 et seq..
68 THE CASTLE MOUNTAIN MINING DISTRICT. [BOLL. 139.

occur is of the first class the alteration into carbonates, especially 

calcite.

One example of these dike rocks is found on the west side of Willow 

Creek, where the dike has a northerly trend and cuts somewhat altered 

Belt shales not far from the margin of the granite mass. It occurs in a 

little saddle below an irrigation ditch which carries water from Willow 

Creek down a dry gulch to the plain to the westward. It has been 

prospected, and some fresh material has thus been thrown out. The 

rock is black and heavy and contains crystals of yellow oli vine; it consists chiefly of augite, and is related to the mou'chiquites. On upper 

Fourmile Creek, a view of which is shown in PL IX, is another fine 

exposure of a lamprophyre dike. This view shows the east wall of the 

little gorge cut by the creek. A sheet of quartz-porphyry, 30 feet 

thick, is underlain by 20 feet of dark-gray and purple, much altered 

Belt shale, dipping 20° downstream, and resting upon a porphyry 

sheet that is an extension of the granite mass. These rocks are cut by 

a dark-colored dike 5£ feet wide.

Another dike of basic character is found crossing Fivemile Creek 

above the limestone canyon. It cuts the rhyolitic breccias which form 

the stream bluffs and the limestones that form the slopes to the east. 

The rock is so greatly altered and decayed, however, that its precise 

petrographic nature, beyond the fact that it belongs in this class, can 

not be determined.

Another dike of a similar nature is found in the Cambrian strata on 

the hill north of Fourmile Creek, at the point where the stream turns 

to the westward. Its trend is such that it must intersect the large dike 

with big feldspar crystals previously mentioned, but the intersection is 

not exposed, and their relative ages can not be determined.

That the peculiar basic magmas which form 'the rocks of this class 

need not necessarily occur only in dike form, as has been urged by some 

authors, is well shown by a massive intrusion in the shape of a small 

stock just below the mouth of Fourmile Creek. ' Smith River has cut 

through the mass and laid bare excellent exposures. This intrusion 

has occurred in Cambrian limestones, but the contact is clearly defined 

only on the north side; elsewhere the limestones have been covered 

with rhyolitic tuffs and the alluvium or lake beds formed by their wash. 

Where the contact is exposed the mass is seen to have locally altered 

the adjacent limestones by contact metamorphism. The rock is darkgray in color, fine-grained in texture, and carries phenocrysts of biotite. 

It consists chiefly of biotite and orthoclase and is most nearly allied 

to those rocks of this class which have been called minettes.

Besides these dikes which have been mentioned and which all belong 

to the Castle Mountain center of eruptive activity, there occur in the 

Cretaceous strata in the southeastern portion of the district a number 

of dikes whose genetic character is quite distinct from them. Some of 

them belong to the Crazy Mountain period of activity, and by their
WEED ANDT PTIVnT TTP fiQ riBSSON. J KHYOLUE. Oy

mineral and lithologic character may be readily referred to Crazy 

Mountain types. One such has been covered, after former erosion, by 

the great rhyolite flow to the southeast of the diorite area. Others, 

owing partly to their mineralogic character and partly to their decayed 

condition, are of uncertain origin. Such, for instance, is one whose 

outcrop branches and winds in a most curious manner, cutting the 

Cretaceous strata and intruded sheets of the theralite south of Bonanza 

Creek, about two miles above the Musselshell Eiver.

III. EXTRUSIVE ROCKS OR LAVA FLOWS. 

RHYOLITE.

These occur only in the eastern half of the district, as may be seen 

by reference to the map (PI. III). They consist of two varieties of rock 

rhyolite, the effusive equivalent of the granite, and basalt. It is probable that the rhyolite flows had originally a much greater extension 

than they now possess, having been cut away by erosion. The masses 

that remain are generally of great thickness in proportion to their size, 

and are remnants filling old erosion hollows. They grade from holocrystalline rocks, practically the same in structure, texture, and mineral 

composition as the intrusive quartz-porphyries, through semicrystalliue 

forms having spherulites, flow structures, etc., to glassy pitchstones. 

These variations occur at times in the same mass. In appearance they 

are reddish to gray colored, at times nearly white, usually very dense, 

with conchoidal fracture. Occasional small crystals of quartz and 

feldspar may be seen as phenocrysts. They are often extensively 

jointed so that the talus slopes resulting from their denudation are 

composed of small angular fragments.

The greatest mass of rhyolite which has been found in any one place 

is that which occurs northeast of Fourmile Creek, opposite that portion of the stream where several branches from the south flow into it. 

This mass has great thickness. Near the stream it rests upon rhyolitic 

breccias which are perhaps 75 feet thick, and from this point it rises to 

the summit of the hill, 1,000 feet above the stream. The breccias 

upon which it rests are greatly indurated and have a massive appearance and jointing. The rhyolite itself varies from place to place in its 

texture and crystallization. Half way up the hill the rock has a beautiful, pronounced flow structure, dark-gray or black lines in a lavendercolored rock, showing that the plane of flowage dipped at 60° away 

from the mountain. Near the top of the hill the rock becomes dense 

and massive and no appearance of flow structure is to be seen. The 

mass extends down the slopes toward the north until its outcrops are 

lost in glacial drift and the debris of rhyolitic breccias. In places masses of a more coarsely crystalline texture occur, and these are apparently of a similar nature to those found in the granite and intruded 

sheets, as already mentioned. The edges of the southeastern portion
70 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

of the mass show that it rests unconformably upon upturned sedimentary rocks.

Another very similar mass of rhyolite, 250 feet thick, which is seen 

to occupy an old erosion hollow, is found about 3 miles north and 

slightly east of the above, not far from where the road from White Sulphur Springs to Martinsdale crosses Fourmile Creek. The rock is 

very dense and microcrystalline; lines of liowage have not been 

observed in it. It forms the crest of a prominent hill and rests on 

upturned and eroded Cambrian strata.

Another locality for rhyolitic rocks occurs on the north side of Smith 

River, about a mile below the point where Fourmile empties into it. It 

is here left as a series of caps on little points which form part of the 

lower slopes of the limestone masses that rise in the hills on this side 

of Smith River. These little caps have a parallel east-and west trend, 

but appear to be mere remnants of a much larger mass. The rock is 

very white and dense, with small phenocrysts of quartz.

A great flow of rhyolite also occurs to the south of Bonanza Creek 

in that portion of the foothills included by its sudden turn to the south. 

The mass lies on the southeast slopes of these hills, as shown on the 

map. In the stream hollows to the east it has considerable thickness, 

and it thins out toward the top of the ridge, being cut off by erosion. 

The rock has a platy structure, dipping down hill, and where it is cut 

by a fork of Bonanza Creek the exposure shows columnar parting. 

This rhyolite flow rests upon upturned and eroded Cretaceous sediments, largely Dakota sandstones, together with shales penetrated by 

dikes of Crazy. Mountain types of rocks. Nowhere were breccias 

observed beneath the flow. This indicates either that the earlier ash 

and breccia showers were completely removed from this area by erosive 

agencies before the outpouring of the rhyolite, or that no breccias were 

thrown out upon this side of the volcano. The entire absence of breccias from the surface of this part of the mountain may, however, be due 

to the very complete dissection and erosion that have taken place since 

the cessation of volcanic activity. Moreover, while the outbursts of 

fragmental material were undoubtedly the earliest manifestations of 

volcanic energy in the district, yet this particular area may not have 

been so covered. The rock itself differs somewhat in its appearance 

from the others previously mentioned. It has a buff-colored, felsitic 

grouudmass with abundant included crystals of sanidine, white in 

color, which give it a strongly marked porphyritic appearance. The 

rock found in place is somewhat friable and decayed. Great quantities 

of it appear in the form, of glacial pebbles in the-stream valleys below, 

and this material is often fresher, with a dense, tough, felsitic groundmass of a greenish-gray to yellowish-brown color, with small included 

crystals of feldspar and quartz.

Rhyolitic flows are found resting on rhyolitic tuffs and breccias above 

the canyon that Checkerboard Creek has cut in the heavy-bedded
0. S. GEOLOGICAL SUftvEV 

t

OBSIDIAN IRHYOLITE) LAVA FLOW ON NORTH FLANK OF 
WEED AND"! , Tl A C A T T 71 PIKSSON. J BAbALT. (1

Carboniferous limestones. The several forks of the creek which unite 

just before- entering the canyon have cut down through these masses, 

giving excellent exposures. The most northerly of them is. a very dark 

rock, microcrystalline, with megascopic quartz phenocrysts.

The other two masses are mostly glassy and are very clearly flows or 

portions of one original flow. In the accompanying plate (PI. X) a view 

is given of the western wall of the most southerly of these two masses. 

The exposure is about 40 feet thick, and the rock is dark-gray in outcrop, weathering in great blocks with rude columnar parting, which is 

shown in the view, made from a photograph. The rock composing this 

flow is peculiar; in the main it is a grayish, lithoidal rhyolite, somewhat porous in texture, consisting mostly of glass. In this are embedded 

great quantities of a black glass in small angular fragments, which is 

a pitchstone, since it contains 4£ per cent of water. These fragments 

vary in size from a few inches down. In appearance they strikingly 

resemble pieces of the Cretaceous coal found in the region. As this coal 

occurs a short distance to the southward, where it has been worked, 

drift fragments of it and the pitchstone are found, which at first glance 

can not be distinguished. The rock is also much cracked and is filled 

with lamination planes, which complete the resemblance. Besides these 

fragments of pitchstone, the rhyolite flow contains also occasional small 

fragments of limestone arid shale, which appear to be very slightly metamorphosed. All of these included materials are arranged in a very 

evident flow structure, easily seen on the face of the outcrops, but too 

large in scale to show well in hand specimens. There are also planes of 

lamination in the mass, which cause it to break much more readily in 

these directions.

The rhyolite is therefore a flow breccia. Such an arrangement in a 

lava, in which it consists of mingled portions, differing in colors and 

appearance, but chemically alike, is called the eutaxitic structure, and 

in the present case its origin can be easily explained by supposing that, 

as the rhyolite lava was extruded, a certain layer of it cooled and solidified as pitchstone, while-the remainder, as a stiffly viscous mass rolling 

slowly onward over itself, crushed the solidified layer and kneaded it 

through its mass. Although fluid enough to move, it was not hot 

enough to refuse the pitchstone fragments, which therefore retain their 

original shape.

BASALT.

Flows of basalt occur in four distinct areas in the Castle Mountain 

region, as shown on the map. One of these extends in a long mass 

sloping downward to the southeast from the hills to the west of Lake 

Creek and between it and the road to Neihart. The source and geologic relationships of this mass are uncertain, beyond the fact that it 

is an extensive lava flow resting on Belt shales.

By far the most considerable body of this rock that occurs in this 

district, however, occupies a central position in the level, open valley of
72 THE CASTLE MOUNTAIN MINING DJSTKICT. [BULL. 139.

the upper part of the north fork of Smith Eiver. It is a large flow, 

covering about G square miles In general its surface is quite uniform; 

it is thickest toward the northeast and tapers to a very thin edge to 

the west and south. The contour line shows that it has been cut out 

slightly along the middle in a north-and-south direction, following the 

drainage of the branch of Smith Eiver that crosses it.

While the exact source and geologic history of the remnants of basaltic flows are so often doubtful, this is not the case with the one under 

discussion. Its origin a'nd the fact that it is the very latest manifestation of volcanic activity in the Castle Mountain area are evidenced in 

several very unmistakable wayis. The latter point is shown by the 

very shallow canyon which Smith Eiver has cut in the mass. Looking 

across the plain, one sees no evidence of the river whatever, only a level 

stretch of table-land; but ou crossing it the river is suddenly found 

running in a narrow box canyon from 10 to 20 feet deep, the walls of 

which are often perpendicular. Evidently the river has but recently 

commenced this work, and has not yet succeeded, although it has a 

rapid and nearly torrential course, in cutting its gorge through the 

basalt. This is the more marked when the configuration here is compared with the comparatively deep and wide valley cut in limestone 

strata below, before the river debouches on the plain near White Sulphur Springs.

Another evidence is afforded by the basalt itself, which in several 

places, as near the mouth of Fivemile Creek, for example, is seen to 

have rested upon the alluvial clay deposits formed by the washing and 

degradation of the rhyolitic tuffs and breccias previously shown to form 

so large a proportion of the Tertiary lake beds. Where the thinned-out 

edge of the basalt flow has come upon these clays they are reddened, 

toughened, and have a brick-like appearance.

That the extent of the flow, as delimited upon the map, corresponds

closely to its original boundaries is indicated by a more vesicular condition of the rock, by its thinning out near the margin, and by the 

occurrence of masses of tachylite or basaltic glass seen near the mouth 

of Lake Creek.

The greatest point of interest in connection with this flow is its place 

of origin, which was Yolcauo Butte. This mass rises as an isolated, 

conical butte about 500 feet above the nearly level, gently contoured, 

open valley, and forms a dominant and striking feature in the landscape. It is the eroded remnant of a former volcanic cone, as is evident from its position, topography, and relationship to the flow which 

extends from its foot, as well as from its internal structure It consists 

in the main of highly pumiceous, brecciated material, mingled with 

small "bombs" of scoria in great quantities, alternating with denser 

solid flows, which pitch sharply down toward the plain. The scoriaceous material is filled with small angular fragments of Belt shales, reddeued and baked, while these rarely or never occur in the denser flow
TUFFS AND BRECCIAS. 73

rock. Thus it would seem that in breaking through the upturned 

shales and forming a conduit the blown-up, brecciated, scoriaceous 

material, which indicates explosive action, became filled with fragments 

of the soft shales, and the conduit having once formed, the material 

which followed, forming the flows, had a freer exit, and therefore does 

not contain them. The former presence of the vapors which so generally accompany volcanic activity is well shown by the altered and 

highly arnygdaloidal condition of the scoriaceous masses Whose pores 

are filled with calcite and zeolitic materials.

Evidences of eruptive activity in extremely basic lavas having been 

found at this point, careful search was made for basic breccias of this 

material in the vicinity, which resulted in the discovery of a few small 

exposures on the hill immediately south of the mouth of Fivernile Creek. 

These consist mostly of small, round, vesicular, basaltic bombs, mixed 

with a basic cement, the breccia resting on the upturned and eroded 

Cambrian limestones. The exposures are very small and thin and are 

only the vanishing remnant of former masses. They are in close connection with remnants of rhyolitic breccias and flows, and, as far as the 

outcrops would warrant an inference, were judged to be resting upon 

them. They are of great value, however, in strengthening the evidence 

deduced at Volcano Butte as to its former-explosive nature. The whole 

evidence collected goes to show, however, that the life of this small 

subsidiary volcano, which broke out on the edge of the great Castle 

Mountain mass long after it had become extinct as an active volcano, 

was comparatively short, and that after building its cone and ejecting 

upon the surrounding country a limited amount of material, it exhausted 

its activity in the great lava flow which broke through its western side 

and spread over the floor of Smith Kiver valley.

Another small basaltic cone rises above the level valley floor some 

miles east of Volcano Butte. It resembles the latter very closely, and 

a lava flow from it covers the eastern part of Volcano valley as far as 

Spring Creek; the flow rests on tine rhyolitic tuff and is well exposed 

about the head of the North Musselshell and the forks of Bichinond 

Creek.

It may be further stated that the basaltic rocks mentioned in the 

foregoing occurrences are of the usual black or very dark gray, dense, 

heavy type, in places somewhat vesicular, and usually much jointed. 

Fine columnar parting, so often observed in basalts, was nowhere found.

IV. TUFFS AND BRECCIAS.

In no way does an extinct volcano leave more unmistakable proofs of 

its former activity than by accumulations of tuffs and breccias. Since 

these, however, as a rule are the most recent deposits of a country and 

mantle all the other formations, they are also the first to be carried 

away by subsequent erosion. At Castle Mountain, as has been, shown,
74 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

the amount of erosion since the volcanic period has been very great, 

and hence these ejected materials have been largely removed.

It is probable that a very considerable cone must have at one time 

existed here; if so, it has nearly entirely disappeared and we can only 

surmise its size and position. The dissection by denudation has indeed 

been complete, and, speaking from a geologic standpoint, only the rapidly vanishing remnants of the former masses of breccias and tuffs are 

left. Fortunately, these remnants remain as invaluable documents 

to show the former history of the volcano, but even they, in a short 

geologic cycle will have utterly disappeared.

Consisting of light, rather incoherent particles, these rocks have 

been readily removed by surface waters, and it is largely due to this 

that the old lakes were rapidly filled up. The lake beds, indeed, consist largely and in places almost wholly of this material, as does also, 

to a very considerable extent, the alluvium of the present stream channels in those areas where tuffs and breccias abound. As has been 

pointed out in the description of the Miocene lake beds, it is difficult, 

indeed, at times to tell whether one is dealing with this material in its 

original position, where it fell, or with sedimentary deposits formed by 

its removal, the smooth, rounded surfaces without outcrops increasing 

the difficulty. Nor does microscopic examination aid in discrimination, 

for undoubted tuffs and lake beds are often found to consist of the 

same material, minute fragments of glass mingled with equally minute 

fragments of comminuted shales and other sedimentary rocks. In 

several localities, therefore, where these deposits border each other 

the exact boundaries must remain somewhat uncertain.

The area where these tuffs and breccias occur is shown on the map, 

and it will there be seen that it is chiefly to the north and east of the 

great granite mass. One interesting locality is on upper Fourmile 

Creek? on the north side of the stream, near the point to where several 

tributaries run in from the south. It has already beeu mentioned that 

the upper part of this hill is composed of a great thickness of rhyolite. 

In the canyon cut here by the creek this rhyolite is seen resting on 

heavily bedded rhyolitic breccias. The breccia is from 75 to 100 feet 

in thickness and has been greatly indurated, so that it breaks, joints, 

and erodes similar to a massive rock. The cement is of a greenish 

color, and this is filled with fragments of shales, limestones, and at 

times of igneous rocks. These fragments have been more or less 

altered; the igneous rocks, which appear to have been of porphyritic 

nature, are usually greenish in tone, and recall fragments of porphyritic rocks which have been altered by aqueous vapors at high temperatures.

Another excellent exposure is found near one of the upper heads of 

Checkerboard Creek, the breccias forming a small hill whose lightgreen color makes it a prominent feature of the landscape. On the 

northwest side this hill has been cut down by a little drainage into a
. TUFFS AND BRECCIAS. 75

cliff about 150 feet high. The breccias rest on upturned and eroded 

Dakota sandstones (quartzites 1?). The rock formed by this breccia is 

quite typical for the whole area over which the breccias have been 

found, excepting the occurrence on Fourmile Greek noted above. 

The cement is of a light-buff or pale-brown color, firm, but somewhat 

open and porous in texture, and this material, which is clearly a compacted ashy is thickly crowded with angular fragments of sedimentary 

rocks, chiefly of shales, but with mingled masses of limestone. These 

included pieces are generally small not above an inch or two across 

decreasing to microscopic dimensions. They do not appear to have 

suffered any rnetamorphism.

In general, the breccias remain as thin caps on the hills as, for 

example, those through which Fivemile Creek cuts its way. These 

hills and the open country between Fourmile and Fivemile creeks 

attract attention by their peculiar appearance. Eock exposures are

rare, and it is only along the bluff banks of the streams and on the 

steeper hillsides that the geologic structure is seen. The fragments 

forming the breccias fell upon a greatly eroded country, and the knobs 

and bowlder-like masses of quartzite and limestone that are occasionally seen are parts of this prevolcanic topography emerging through 

denudation from the cover of volcanic d6bris. In other places, where 

the breccias have filled old hollows, they are better preserved and are 

often of considerable thickness, as in Fivemile Canyon, where the 

stream has cut through them and they are seen penetrated by basic 

dikes. The farthest point to the southeast at which they have been 

seen is on lower Flagstaff Creek, where a few very small patches were 

found on the sides of the hills of Cambrian limestone.

The enormous amount and finely comminuted character of the fragments of the sedimentary rocks that they carry are of great interest, 

as they prove that violent explosions must have occurred to blow out 

so great a quantity of the stratified rocks which formed the foundation 

of the old volcano, and through which the former conduit of the lavas 

which succeeded these breccias, and upon which they have nearly 

everywhere been found to rest, must have passed. It should be also 

stated that these explosions appear to have been the first phase of 

external igneous activity, since the breccias, wherever found, are resting directly upon the old eroded land surface of sedimentary rocks, 

while the lavas rest upon the breccias. Nor have the breccias ever 

been found succeeding each other with any other material between; 

they belong exclusively to one period of eruption, and may, indeed, 

represent but one great and violent explosive outburst which cleared 

the way for the lava flows that succeeded, recalling in this respect 

some of those which have given rise to the Maaren in the Eifel district 

in eastern Germany.

The .tuft' represents the lighter, finer, volcanic ash which succeeded 

the rain of heavier, coarser material which formed the breccias. The
76 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

latter naturally grade into it, and as it is composed of light, incoherent 

particles it has suffered so greatly from degradation that but little of it 

remains, and actual outcrops are difficult to find. For reasons already 

given, it also grades into the lake beds and alluvium. In general, in a 

washed-down and transported condition, it is found in quantities as the 

soil, more or less mingled with other material and lying upon the other 

formations. This is shown by microscopic examination, which proves 

the material to be largely composed of fine, angular fragments of volcanic glass. Much of the area which is delimited upon the map is thus 

composed of mingled tuff and breccia in a degraded condition. Careful 

search, however, revealed the existence of a bed of tuff some 40 feet in 

thickness in an undisturbed condition, as it svas originally piled up, the 

exposure being situated on the slopes west of the canyon of Checkerboard Creek. The outcrop shows an excellent exposure to the southwest, and a view of it is given in the accompanying illustration (PI. XI). 

The face of the outcrop has smooth, rounded surfaces, which may be 

seen in the view, and these bear abundant evidences of rain-washing 

in little rill channels and pendulous portions and globules formed by 

the drying of drops of dirt-laden water. The consistency of the tuff is 

about that of a tough clay. A lens from 1 to 3 inches thick, holding 

quartzite, Mesozoic sandstone, and pebbles of volcanic rock, was noted 

in the exposure, but with this exception no pebbles were seen, and the 

character of the rocks is quite constant. It is of a pale-brown, earthy 

color, and microscopic examination shows that it is formed mostly of 

small glass shards, together with broken crystals and tiny bits of shale. 

The chemical analysis also, as shown in Chapter VI, allies it with 

rhyolite in chemical composition. By gradation in the deposits the 

mass shows that character of rude bedding commonly seen in accumulations of ash showers, and the beds dip at 5° to the northeast, and 

therefore away from Castle Mountain. The material rests upon 

breccias, and has itself been protected from erosion by glacial drift.

V. IGNEOUS ROCKS NOT OF CASTLE MOUNTAIN ORIGIN.

Besides those rock masses which by reason of their geologic position 

or their structural and chemical peculiarities can be referred without 

doubt to the Castle Mountain period of eruptive activity, there are, as 

previously mentioned, a few others whose origin is clearly different 

or in doubt. Since, however, they throw light on the geologic history 

of the district, they are worth a brief description at this i>oiut.

DIABASE.

This rock occurs on the southwest side of the great granite mass as 

a thick sheet intruded in Belt shales, and with them upturned against 

the mountain and folded up at that point. Wherever it has been seen 

it presents the same appearance and mode of occurrence, that of a
TUFF BEDS ON NORTH FLANK OF CASTLE MOUN
ASH DEPOSITS. 77

heavy, black intruded sheet of trap, some 20 feet in thickness, lying 

conformably in the beds of the Belt formation and like them tilted and 

folded. Its conformability, its granularity, microstructure, and the 

alteration it has produced in the shales, all prove that it is an intrusive and not a surface flow. From these facts it is also evident that 

it is the oldest igneous rock in the district, antedating all others by a 

long period of geologic time. Its- conformability and the intricate 

manner in which it is folded, at once negative the idea that it could 

have been contemporaneous with the Castle Mountain outbreak. Moreover, iu other localities to the east and southeast, where the Belt formation is exposed over large areas, similar sheets of trap rock, consisting of a precisely similar diabase, are found intruded in them; they 

have been found near the canyon of Sixteenmile Creek, some 20 miles 

to the southeast.

It is therefore evident that while these beds were horizontal, and 

previous to the Mesozoic disturbances which folded them, there were 

extensive injections of trap sheets, on a much smaller scale, but iu a 

similar manner to the intrusion of the trap in the Juratrias areas 

along the Atlantic border. Indeed, so strongly do the rocks themselves resemble the eastern Mesozoic diabases in their outward appearance, microstructure, and mineral composition, that specimens from 

different localities could not be distinguished. The rock possesses, 

wherever seen, a rhomboidal or platy parting, and a poor columnar 

structure was also observed in places. Weathered surfaces and 

those of the joint planes are of the rich brown color so often characteristic of this rock.

INTEEBEDDED ASH DEPOSITS.

At a certain horizon in the series of Dakota sandstones, quartzites, 

and shales is found a rock of a very fine grain, conchoidal fracture, and 

of a dark-purple to a lavender color. Lithologically it differs entirely 

from any of the strata in which it occurs, but it is conformably interbedded in them wherever seen. Outcrops have been found in several 

widely separated localities: in the Cretaceous syncline area to the 

northeast of Blackhawk, where it is cut by one of the dikes previously described, iu the Cretaceous shales in the basin of the creek 

east of Lower Bonanza, and in similar shales on the east branch of 

Willow Creek. In the field it was supposed that this was a fine-grained, 

intrusive rock, but a microscopic examination of thin sections showed 

at once that it was composed of clastic material, chiefly angular fragments of quartz and feldspar mixed in a cement composed largely of 

fragments of volcanic glass. These latter show the cusp-shaped forms 

so characteristic of the shards ejected in volcanic eruptions. The rock 

represents, therefore, a volcanic ash bed, and we thus have evidence of 

igneous activity in neighboring districts while these Mesozoic strata 

were being deposited. The conformability of the ash deposit points tq
78 THE CASTLE MOUNTAIN MINING DISTECT. [BOLL. 139.

its having been laid down in water in a manner similar to the shales 

and sandstones themselves, the latter beds both above and below being 

of precisely similar lithologic nature.

CRAZY MOUNTAIN TYPES OE ROCKS.

The wide, open valley of the South Branch of the Musselshell, cut in 

the soft Cretaceous strata, is everywhere penetrated and seamed by 

intruded sheets of igneous rocks accompanied by occasional dikes 

which are easily recognized in the field to be of Crazy Mountains types 

and origin. A number of these are shown on the map. Although 

topographically the Musselshell divides the Crazy Mountains area 

from that of Castle Mountain, geologically the separation is not perfect, 

since the same varied and extensive network of peculiar intrusive rocks 

which are characteristic of the Crazy Mountains is carried over into 

the foothills of Castle Mountain on the south. They consist of porphyrites, acmite trachytes, and the " theralite" of Eosenbusch, and 

have been described by Wolff. 1 Since a full description of these peculiar rocks, together with an account of the modes of occurrence, will 

appear in another publication of the Survey, on the Geology of the Crazy 

Mountains, the reader is there referred for a detailed account of them. 

That these rocks antedate the Castle Mountain period of eruption is 

clearly shown by a number of facts, one of the most evident being that 

a dike of acmite-trachyte, which has evidently been eroded, is covered 

by the great flow of rhyolite which extends down the slopes inclosed 

by the bend of Bonanza Creek. These sheets certainly occur to the 

north of Bonanza Creek in the Dakota sandstones which lie in the 

bottom of the synclinal trough, and this marks the greatest distance 

to which, in this direction, the Crazy Mountain intrusives have penetrated. These rocks have been kindly examined by Professor Wolif, 

who confirms the authors in their supposition as to their distinctive 

Crazy Mountain habit and mineral nature.

Besides those which are clearly types of Crazy Mountain rocks, there 

occur others near Bonanza Creek and between it and Flagstaff, which, 

partly from their mineral nature, but more on account of their decayed 

condition, must remain uncertain, whether they are of Crazy Mountain 

or of Castle Mountain origin.

In addition to those intruded sheets or masses whose direct connection with the main center can be either clearly seen or indirectly proved, 

there are several smaller masses of igneous rock occurring within the 

limits of the map. It is probable that to a certain degree they are 

contemporaneous in origin, and, although they occur in Paleozoic or 

pre-Paleozoic strata, were injected during the Tertiary disturbances

'Neues Jahrb. Min. 1885, Vol. I, p. 69; 1890, Vol. I, p. 192. Eosenbusch, Mass. Great., 1887, p. 247. 

Petrog. Crazy Mountains, Montana: Northern Transcontinental Survey, 1885. Geol. Crazy Mountains : Bull. Geol. Soc. Amer., Vol. Ill, 1892, pp. 445-452. Acmite-trachyte Crazy Mountains, Wolff and 

Tarr: Bull. Mus. Comp. Zool. Harvard, Coll., Vol. XVI, No. 12,1893.
CRAZY MOUNTAIN TYPES. 79

which closed the long eras of subsidence in this region. By their mineral and chemical composition they are so closely related to the igneous 

rocks of Castle Mountain that no doubt can exist that the same 

deep-seated liquid reservoir furnished the material for both. One of 

these masses is that which forms the main portion and crest of Black 

Butte. The intrusion has occurred in Cambrian sandstones. Besides 

this mass there are several intruded sheets of porphyry of a similar 

nature in the immediate vicinity, as shown on the map. It is certain 

that there are several sheets at this point, and possibly they have been 

connected with a laccolitic intrusion of which Black Butte now presents 

an eroded remnant.

Similarly intruded masses occur in the Belt shales near the junction of 

Checkerboard Creek with the Musselshell. They are dense, tough, lightcolored rocks quartz-porphyries and resist erosion well, so that where 

they occur they form hills, though it is probable that the topography 

depends also in part upon geological structure at these points. These 

masses have produced a contact metaniorphisin in the slates and argillaceous limestones of the Belt formation, but it is very local in character 

and confined to the immediate neighborhood of the igneous rock. 

Through one of these hills the Musselshell has cut down a very narrow 

canyon, giving excellent exposures of the igneous and sedimentary 

rocks, as noted by Messrs. Dana and G-rinnell. 1

'Report of ii Reconnaissance from Carroll, Moiit., to Yellowstone Park. TJ. S. War Dept., Washington, 1876, p. 113.
CHAPTER VI. 

MICROSCOPICAL PETROGRAPHY.

With but very few exceptions, the igneous rocks of Castle Mountain 

present only well-known and well characterized types. They are confined mainly to varieties of the granitic group, and from a structural 

point of view they offer all the different forms of crystallization that a 

highly siliceous magma can assume under the most varied physical conditions; they range from.a medium-grained granite on the one hand to 

glassy pitchstone and tuffs on the other. In the idea of volume conveyed by the superficial areas of the rock masses these acid forms far 

outrank all of the other varieties in. the district combined.

Associated with them, however, as has been pointed out in the preceding chapter, are basic rocks in much smaller amount, while rocks of 

intermediate character play but a subordinate role. The basic rocks, 

are basalt and a few lamprophyric dikes, while those of intermediate 

composition are confined to a diorite approaching syenite and to porphyries at times inclining to porphyrites.

The petrographic study of the rocks of the district is therefore chiefly 

that of granite, granite-porphyry, quartz-porphyry, rhyolite, and rhyolitic pitchstories and tuffs.

CLASSIFICATION.

In .the present unfortunate state of confusion regarding the classification of igneous rocks and the use of petrographic terminology, it is 

generally necessary for a writer to define exactly the sense in which 

his terms are used. It will probably be a matter of great difficulty to 

obtain in the future any international concordance on this subject, but 

it would seem as if the times were ripe overripe, in fact for entirely 

dropping at least distinctions in classification based on the geologic age 

of igneous rocks. A method which is of very little practical use and 

which can not fail to produce wrong impressions should certainly be 

dispensed with. It is and ought to be the privilege of science to free 

itself from the errors of the past rather than to drag them forward as 

an increasing load to its own impediment.

We notice with regret that Professor Zirkel, in the recent edition of 

his great work, 1 has been unable to free himself from tradition in this 

respect, and that, admitting the uselessness of this distinction for 

coarse-grained rocks, he still retains it for finer-grained porphyritic 

ones, a proceeding which, it seems to us, is lacking in logic. We can

1 Lehrbucli tier Petrographie, Leipzig, 1893, p. 838 et scq. 

80
WpFnisso.N D] MICROSCOPICAL PETROGRAPHY. 81

not agree with him when he claims that it should be retained on account 

of the useful purpose it serves in geology. If a distinction which was 

formerly based on erroneous conceptions should be retained for geologic 

purposes, why, we may ask, should the cut be made arbitrarily at the 

beginning of the Tertiary, or, indeed, why should we have but two sets 

of names for the same objects why not three, four, or one for each 

geologic era, as proposed by I^brayl? As in England, the distinction 

has never held sway in this country, owing largely to the writings of 

Dana, and it may now be held as certain that it never will, though at 

times one may notice a sporadic usage of terms in conformity with this 

idea. As Iddings has already indicated, 1 the original use of many of 

the most important terms that are used for paleo-igneous rocks, such 

as porphyry and porphyrite, was clearly a structural one, the idea of 

age being fastened upon them later. As all of the rocks in the Castle 

Mountain area, with but one exception, are Tertiary, it is evident that 

all terms in this work are used without reference to geologic age. So 

we have:

Granite, so coarsely granular that the component particles are readily 

seen by the eye alone. It is often porphyritic, and occurs in the central 

stock.

Quartz-porphyry, with groundmass so finely granular that the component particles can not be distinguished by the eye; occurs chiefly in 

the intruded sheets, and passes into

Rhyolite, which contains the partly crystallized glassy forms or those 

dense ones which only under the microscope reveal themselves to be 

holocrystalline, and forms almost exclusively the flow masses. The term 

granite-porphyry is also used for the transitional form between granite 

and quartz-porphyry, to designate the finely grained facies of the 

granite, which are often full of porphyritic feldspars.

In the same way the terms porphyry and porphyrite are used to 

designate porphyritic rocks of a holocrystalline, fine-grained texture, 

which in this district are found in intruded sheets and dikes, the former 

consisting mainly of alkali feldspars, the latter chiefly of lime-soda 

ones.

In thus using these terms without reference to geologic age, as has 

been previously done by several petrographers, and in confining them 

to certain structural forms generally, though not necessarily, occurring 

in sheets, dikes, and intrusions, we believe that a great advantage to 

petrographic terminology is gained; for if, for example, the terms 

rhyolite and quartz-porphyry are used only with strict reference to 

the geologic age of certain rock masses, we have then no intermediate 

term between granite on the one hand and glassy semilithoidal material 

on the other, since the terms quartz-porphyry and rhyolite are both 

required to cover all intermediate structural forms. While this is true 

in theory, yet, since among the older rocks the glassy to lithoidal forms

1 The eruptive rocks of Electric Peak: Twelfth Ann. Eept. U. S. Gool. Survey, p. 582.

Bull. 139 G
82 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

have been largely swept away by denudation or so changed by secondary processes of devitrification, etc., that they are largely indistinguishable from originally holocrystalline granular ones, it is a fact that 

practically these terms have been used also in a structural sense. It 

would therefore seem logical to drop the conception based on geologic 

age and let practice and theory combine. The term granite-porphyry 

has been used to partly cover this deficiency, but many writers, following JR-osenbusch, are inclined to restrict its usage to a definite rngde of 

geologic occurrence, and even if this restriction should be disregarded 

the structural sense in which it has been used would not cover the 

necessary ground.

As pointed out by Prof. J. D. Dana, 1 it is unfortunate that the word 

porphyry has been perverted from its original use as a purely structural 

term designating all kinds of porphyritic rocks, to become the name of 

a particular kind of rock. In the same way the use of the term granulite, which might so well be employed to designate all granular rocks 

which are not porphyritic, is prevented by its being the name of a rock 

of comparatively limited occurrence. Two such general terms, it would 

seem, are greatly needed in petrographic terminology, and it would be 

especially a great benefit if the term porphyry could be detached from 

its specific use and confined entirely to a general structural one.

ROCKS OF THE INTRUDED MASSES. 

G-RANITE.

This rock is found best and most typically developed in the southern 

portion of the great mountain mass, as at Elk Peak and above the town 

of Castle, and rnorainal bowlders of it fill the deep ravines of the various streams issuing from this side of the mountain. A hand specimen 

collected at Elk Peak appears at first glance to be an extremely coarsegrained rock of a mottled grayish-white color. A closer examination 

sllOWS, however, that this coarse-grained appearance is due to the 

great number of feldspar crystals, about 1 centimeter long by about 

a half centimeter broad, which lie thickly crowded together in a much 

finer grained groundmass, whose grains average about 1 millimeter 

in diameter, and which would thus form by themselves a rather finegrained granite. The large feldspar phenocrysts are seen to be Carlsbad twins. The ferro-magnesian minerals are biotite and hornblende. 

They form a considerable part of the finer-grained portion, giving 

it its grayish tone. They never attain such proportions as to rank, 

like the large feldspars, as phenocrysts in contradistinction to the 

medium-grained mass. From this type all gradations may be found 

to forms whose groundmass sinks to grains of microscopic dimensions, giving transitional varieties into granite-porphyry. These varieties, and that which comes under the head of granite-porphyry proper,

1 Manual of Mineralogy and Petrography, 1887, p. 441.
WEED AND"1 n r> A XTTTT? 8 3

pmssoN. J - GRANIIE. 00

form probably the larger part of the mountain mass and lie chiefly to 

the north and west. It is impossible to draw any lines between them 

either structurally or upon the map, and hence for convenience the 

term granite has been employed in a general geologic sense in the other 

portions of this bulletin,- for the whole intruded stock.

The granite, and in general the whole rock mass throughout, is very 

miarolitic; that is, full of cavities of various sizes which have been 

produced by the contraction in volume incident to the cooling and 

crystallizing of a liquid mass. These cavities, while abundant, rarely 

reach great size, the largest observed being an inch or two long. 

They are thickly studded with free, projecting, outgrown crystals of 

quartz and feldspar. The latter, which at times reach a length of 1 

centimeter, show the common habit of a combination of the prism 

with the positive and negative unit rhombohedrons (1011 and Olll). 

The feldspars, which are much smaller, are white or pink in color, dull 

in luster as a rule, and developed tabular on the clinopinacoid. They 

are almost universally Carlsbad twins, presenting the simple forms c 

(001), b (010), m (110), and x (T01). The plane x _______ 

(T01) having nearly the same inclination to the 

vertical axis as the base, the two in the twinned 

crystal lie nearly in one plane, as shown in the 

accompanying fig. 8.

The plane x (T01) is much rougher than the 

basal plane, and the prism is extremely short. 

As the planes of the crystals are nearly always $w- s.-orthociaso crystal 

dull except the clinopiuacoid, which is much stri- ^iarolitic cavity °f 

ated by a vicinal prism face, no good measurements could be obtained on the goniometer. On one crystal the prism 

faces were measured as follows: l

in A m (110 A 110) meas. 61° 45', calc. 61° 13'.

Not enough material for an analysis of these feldspars is at hand, 

but from a consideration of the bulk analysis of the rock, given later, it 

is probable that they are anorthoclase containing to some extent the 

albite molecule.

These miarolitic cavities are thus similar to those found in the wellknown granite at Baveno, on the Lago Maggiore and in the granite 

porphyry of the Mourne Mountains in Ireland, though on a much 

smaller scale. Ho secondary minerals, such as zeolites or calcite, have 

been found on them, nor any of the ferro-magnesian group, though these 

are abundant in the rock itself. It seems quite probable that the wellcrystallized condition of the minerals in the cavities is due to the mineralizing influence of water vapors at a high temperature, which, escaping from the cooling magma in which they had been held chemically 

combined under great pressure, would naturally collect in such cavities,

1 Similar crystals have boon described and figured by Kokscharow, Conf. Min. Kuss., Vol. 'V, p. 

125; Atlas Plate LXXIII, fig. 8.
84 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

thus producing those conditions which Daubree has so successfully 

imitated in his well-known experiments. In this respect they are 

also similar to the conditions existing in lithophysai, which contains 

similar minerals, as pointed out and described by Iddings and Penfield.1

Microscopically, in thin sections, the following minerals are disclosed : 

Iron ore, apatite, titanite, zircon, biotite, hornblende, oligoclase, orthoclase, and quartz, the constituents of a normal hornblende-biotitegranite. Their freshness or state of preservation varies, of course, 

with the. individual specimens collected from different parts of the 

mass. The biotite has a very intense pleochoism between a pale ocher- < 

yellow and a deep umber-brown; the, hornblende is greeii with a tone 

of brown, also pleochroic, arid with a medium. angle of extinction. 

Both of these minerals are always idioinorphic; they are not abundant 

in the sections and have not been observed in intergrowths or in such 

relations that their relative ages could be determined. The oligoclase 

is generally quite fresh and striated in very fine lamellae: the nearly 

parallel extinction to the twinning plane in all cases indicates an acid 

oligoclase, as might naturally be expected from the composition of the 

rock. The orthoclase or nonstriated feldspar is never fresh with the 

sanidine habit, but cloudy from incipient kaolinization, like those of 

the older granular rocks. Microcline has never been observed in any 

of the sections; a natural result of the unpressed condition of .the 

rock. The quartz shows no undulatory extinction and is clear, but 

contains many inclusions, both liquid and gaseous. Glass inclusions 

are also common, often of dihexahedral form and having a fixed bubble. 

The apatite, titanite, and zircon are not common and present nothing 

of especial interest.

An analysis of the freshest material which could be obtained at Elk 

Peak was made, with the following results:

Analysis of granite from Elk Peak. 

[By L. V. Pirssoii.]

.

Si02 .........................

Ti02 ...-....-...... ........

A12 03 .... .............. ......

1?o <">

FeO .........................

MnO..--. ....................

MgO...... ...................

CaO .........................

ITa-jO ........................

K2 0 .........................

H2 0(ign)...... ..............

Total..................

Sp.gr........................

Per cent.

72.48

.32

13.14

1.66

1.02

.15

1.04

4.22

4.88

.42

99 33

2.62

1 Iddings, Obsidian Cliff: Seventh Ann. Kept. U. S. Geol. Survey, 1888, p. 279. Iddings and Penfield: Am. Jour. Sci., Vol. XLII, 1891, p. 39.
WEED ANtTI /fu A-VT-rm-n QK piussoN. J GRANITE. «J)

This is in all respects the analysis of a normal granite, and the 

amount of loss on ignition shows that the rock is comparatively fresh. 

The relative amounts of soda and potash are to be noted; it is probable 

that the unstriated feldspar contains the albite molecule to some 

extent.

It is, however, in its structure, the normal type of which is shown 

'in fig. 1, PI. XV, that the rock presents the points of greatest interest. 

As noted rnegascopically, it possesses a medium degree of granularity 

for a granite, but the structure is not always entirely the hypidiomorphic one of the majority of granites. The quartz has a certain tendency to assume idiomorphic outlines5 against orthoclase an occasional 

angular contact between the two is seen, the quartz possessing the 

salient angle; at other times it penetrates the feldspar in broad tongues 

or lies against it in a rounded grain. It is also observable in this 

coarse-grained form that one finds places where several of these quartz 

areas are oriented similarly and extinguish simultaneously between 

crossed nicols. Sometimes they are connected by an intervening strip 

of quartz, and these are often surrounded by a singly polarizing plate 

of feldspar. Thus there is a rude approximation to graphic granite in 

the arrangement of these parts.

These phenomena, together with the presence of the glass inclusions 

in the quartz, all point to the younger nature of the granite, and show 

that it has crystallized under somewhat different physical conditions 

from most of the older coarse-grained rocks. The age of the feldspar 

phenocrysts can not be assigned to one particular period. They contain the older minerals, while included quartz grains prove that they 

were growing amongst the latest components of the rock. The fact of 

their existence in it does not make the rock any less a granite, since 

this is determined by the comparative coarseness and structure of the 

groundmass. It may confidently be expected that as research is more 

extended the occurrence of such porphyritic granites will be found to 

be more common. An extremely coarse-grained granite with huge 

phenocrysts of feldspar has been described by one of the writers, 1 

while a number of occurrences are mentioned by Zirkel 2 in his recent 

comprehensive work.

GRANITE-PORPHYRY.

The minerals that have been described as forming the granite remain 

the same in species and relative abundance throughout the whole 

mass, but their relative dimensions change, giving rise to variations in 

structure, as previously stated. The feldspar phenocrysts show the 

least variation, the change being most marked in the groundmass 

surrounding them. From a coarseness of grain which renders it a 

true granite, the grain becomes less and less coarse, until over large

1 Am. Jonr. Sci., Vol. XLVI, 1893, p. 363.

2 Lehrbucli der Petrograplrie, 2d ed., Vol. II, 1894, p. 17.
86 THE CASTLE MOUNTAIN MINING DISTEICT. [BOLL. 139.

areas it becomes so fine that the individual components can no longer 

be differentiated by the eye alone, and the rock thus passes into a 

granite-porphyry. Under the microscope, this change is observed to 

progress in two directions. In the first the grains simply become 

smaller, retaining the irregular hypidiomorphic structure characteristic of granites, though occasionally the quartz and feldspar show a 

tendency to assume that structure which Williams' aptly characterized 

by the term micropoikilitic. Quartz also assumes the position of a 

common phenocryst, a phase not seen in the granite. In.the other 

direction the groundinass assumes that form of regularly oriented 

intergrowths of quartz aud feldspar seen in graphic granite and 

commonly called micropegmatite.

GRANITE-PORPHYRY WITH MICROPEGMATITIC GROUNDMASS.

This form is most common in the northern and western portions of 

the mass. All gradations between it and the granite may be found. 

The tendency noticed in the granite for several grains of quartz to be 

similarly oriented increases at times till a granite of a rather fine grain, 

but of micropegniatitic structure', as shown in fig. 6, PI. XII, is formed, 

and this passes by transitional forms into a true granite-porphyry with 

this structure of groundmass. The micropegmatite occurs not only 

in fringes bordering the feldspars, but in distinct, independent areas 

freely^scattered through the groundmass.

The dark-colored minerals in these occurrences of granite-porphyry 

are extremely apt to be altered, the biotite to chlorite, white mica, 

etc.; the hornblende to chlorite, calcite, limonite, serpentine, etc.

Secondary origin of micropegniatitic structure. Some petrographers 

have been inclined to regard a micropegmatitic structure as being at 

times of secondary origin. Thus Irving 2 was disposed so to regard it 

in certain granite-porphyries of the Lake Superior district; quite 

recently Hobbs 3 has attributed its formation in a clastic rock studied 

by him to metamorphic origin; while Koinberg 4 considers the structure 

to be due to corrosive silica-bearing solutions acting on the feldspar of 

igneous rocks.

That the structure is at least generally of primary origin that is, 

formed by crystallization from a molten igneous magma is not disputed, its occurrence in recent unaltered rhyolites, as in the beautifully 

fresh material from Obsidian Cliff described and figured by Iddings, 5 

being sufficient evidence upon this point.

Examined critically, the reasons given by the authors mentioned for 

regarding the structure as of secondary origin in the rocks studied by

1 Jour, of Geol., Chicago, Vol. 1,1893, p. 176.

2 Copper-bearing rocks of Lake Superior: Mon. U. S. Geol. Survey, Vol. V, 1883, p. 114.

8 Bull. Geol. Soc. America, Vol. IV, 1893, p. 171.

«N. Jahrb. Min., Bei.-Bd. VIII, J892, pp. 314-374.

8 Seventh Ann. Kept. U. S. Geol. Survey, 1885-86, p. 274.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 139 PL. XII

a. CASTLE MOUNTAIN GRANITE FROM ELK PEAK.

Q = quartz, Or orthoclase, B biotite. Actual field 4 millimeters diameter. From a micro-drawing in 

polarized light; crossed nicols.

fc._CASTLE MOUNTAIN GRANITE-PORPHYRY.

The white areas quartz, the dark feldspar. Very coarse-grained micro-pegmatite passing into granitoid structure. 

Actual field 4 millimeters diameter. From a micro-drawing; crossed nicols.
WPIBSSOND] GRANITE-PORPHYRY. . 87

them, resolve into these: that it was so believed by them because of 

its appearance, and also because, outside of the micropegiuatite groups, 

there were areas of quartz held to be secondary which had the same 

orientation as that composing them. The whole is therefore presumably believed to be quartz deposited from silica-bearing solutions which 

had corroded the feldspar. We must imagine, therefore, a feldspar 

crystal acted upon by solutions which, eating into it, create long, narrow, bent, broken, or sinuous canals in all directions, and these canals 

filled with deposited quartz which preserves the same, orientation 

throughout. When we reflect upon the manner in which cavities are 

filled by undoubted quartz of deposition, such a process does not seem 

very probable.

As to the secondary quartz outside of the group which has a similar 

orientation, it would be quite natural that silica-charged solutions, 

coming in contact with those portions of the original micrographic pegs 

which reach the surface, would be influenced by them and deposit 

quartz with a -similar orientation. This is in perfect analogy with 

secondary quartz deposited arouud the detrital grains of quartzites and 

sandstones.

The fact that it occurs at times associated with altered and weathered feldspar can not be adduced as a proof of the secondary nature 

of this structure. If it were original and exposed to alteration the 

feldspar must decay, be changed to kaolin or sericite, but evidently 

the quartz can not alter. On the other hand, by the pressure exerted 

in orogenic forces, feldspar crystals might crack through and through 

in various directions, and especially along cleavage planes. By the 

action of capillary forces silica-charged solutions might be drawn into 

the cracks and quartz deposited, and in this way a secondary structure 

be formed which might resemble a micropegmatitic intergrowth.

While, therefore, by no means denying the possibility that this structure may at times be a secondary one, the authors desire to point out 

that those facts which have been adduced as proofs of its secondary 

origin are not in themselves conclusive, that until more positive proofs 

are added it must still be generally held to be of igneous origin, and 

that great caution should be used in pronouncing it to be secondary.

INCLUDED MASSES IN THE GRANITE.

These have been mentioned in the preceding chapter, and their mode 

of occurrence has been described. On the whole, they appear to be 

somewhat darker and richer in ferro-magnesian minerals than the main 

granite. At times they are of finer grain, at others coarser, than the 

inclosing rock. They are also less porphyritic, and sometimes entirely 

free from pheuocrysts, presenting the appearance of a fine-grained 

granite.
88 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

An. analysis of a specimen of this latter variety from, the brokendown rock masses on the ridge at the extreme head of Cottonwood 

Creek gave the following results:

Analysis of mass included in granite. 

[By L. V. Pirsson.]

Si02 .........................

Ti02 . ........................

A12 03 ---- --- ................

FeaOj...... ..................

FeO...... ....................

MnO. ........................

MgO.... .....................

CaO...... ....................

Na,,0.... .....................

TL.,0.... .............. ....i...

T> A

HjO(ign) ....................

Total ..................

Per cent!

.87

 17.20

2.35

2.43

.03

1.82

3.23

5.18

3.83

1.07

99 94

2.67

This analysis is essentially that.of a syenite somewhat rich in lime, 

iron, and magnesia, and thereby a transition toward diorite. Under the 

microscope the rock is found to consist of a hypidiornorphic mixture of 

biotite, hornblende, apatite, iron ore, orthoclase, plagioclase, and quartz. 

The ferromagnesian components are moderately abundant; orthoclase 

is by far the most common mineral, while of quarlz there is little and 

that mostly interstitial. The rock is,'in fact, a fine-grained hornblende-, 

mica-syenite:

Sections cut from the flner-textured and somewhat porphyritic forms 

show the same minerals and structure, though at times small areas of 

quartz and feldspar in micropegmatitic intergrowths are seen. The 

essential variation seems to be in the amounfrof quartz, and thereby in 

the silica in the magma, though there is .also a relatively small gain 

in lime, iron, and magnesia in the variety analyzed.

At other times the included masses seem practically the same in 

nearly all respects as the inclosing granite or granite-porphyry, and 

differentiated from it only by a somewhat denser texture and a spheroidal parting suggesting the arrangement of the scales of an onion, a 

peculiarity which renders this variety of rock quite noticeable.

The authors are incliued to attribute the formation of such bodies to 

a liquation of the liquid magma, a process which produced masses 

somewhat richer in basic oxides, and, as the temperature gradually 

fell and the cooling magma stiffened and became more viscous, were 

no longer miscible with the main portion, producing, in fact, a sort 

of emulsion of the two on a vast scale. Each portion then crystallized
WEED AND"! mORTTTi' . &Q 

P1KSSON. J DlOKlIUi. 03

in place. This is similar to the theory which Backstrom : has lately 

advanced for the origin of orbicular granites, and it seems to us that 

the large included masses which differ somewhat from the inclosing 

rock, the spheroids of orbicular granites, and those agglomerated 

masses of the dark ferromagnesian minerals so commonly occurring in 

acid rocks ("schlieren" of the Germans) are all to be attributed primarily to the same cause, while their variation in composition, structure, and texture in different occurrences is to be explained partly by a 

greater or less continuation of the process and partly by the different 

physical conditions under which they have cooled and crystallized.

It should be noted, also, that these "schliereu" or agglomerations, 

rich in the dark ferromagnesian minerals, have nowhere been observed 

in the Castle Mountain mass.

DIOEITE.

This is the predominant rock of the smaller mass occurring on the 

eastern slope of Castle Mountain, between Blackhawk and Bobinson. 

The material constituting the central portion of the stock, as exposed 

in various prospect shafts, is a rather coarse granular rock of a very 

dark-gray color and quite even grain. -The main component is seen to 

be feldspar, occurring with grains of augite and plates of a black glittering mica. The component particles range from, one to several 

millimeters in diameter.

Under the microscope, in thin sections, the rock is seen to possess a 

granitoid structure and to be composed of the following minerals, named 

in the order of their crystallization: Iron ore, apatite,.and zircon, followed by biotite, hypersthene, diallage, oligoclase and orthoclase, and. 

quartz. The iron ore is moderately common in small grains. Apatite 

is generally diffused but not abundant. Zircon has been found in 

several good-sized crystals; the smaller microlites of this mineral seem 

nearly wanting. The biotite nearly always surrounds iron ore, is very 

strongly pleochroic, and of a rich umber-brown color.

The hypersthene is probably iron poor, as it has but a very pale, 

scarcely perceptible, pleochroism and has always a very low, double 

refraction, scarcely equal to quartz. This, with its parallel'extinction, 

contrasts it sharply with the monoclinic augite, which has all the characteristics of diallage with parting parallel to (100). It is full of 

inclusions of the older constituents. Parallel intergrowths of the two 

pyroxenes have been observed in several cases, the same cleavage 

running through both; they appear to be joined on the front pinacoid 

(100). In these intergrowths the two varieties are differentiated in 

plain light by the pleochroism. of the hypersthene, which seems, perhaps by contrast, to be more pleochroic here than elsewhere, and in 

polarized light by the difference in optical relations. The monoclinic 

augite is at times surrounded by small quantities of shreds of green

1 Gcol. Foreu. i. Stockholm, Forbaiidl., Bel. 16,1894, p. 128.
90 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL'. 139.

hornblende. All of these ferro-magnesian minerals tend to group themselves around the ore grains. Jt is evident that the period of growth 

of the augite overlapped that of the biotite, since the latter at times 

incloses it in grains. In the great majority of cases the striated feldspar present is an acid oligoclase, as is indicated by the almost uniformly small angle of extinction shown by lamellse twinned according 

to the albite law in sections perpendicular to 010, and the nearly coincident extinction in individuals twinned according to the Carlsbad law. 

The same method shows that much more rarely au andesine with maximum extinction angles of 22° in this zone is also present. These 

determinations are according to the beautiful optical methods recently 

perfected by Michel-Le'vy1 . The twinning striae are generally very 

fine. The crystals are in broad tables and fairly idiomorphic, recalling 

the plagioclase of basic granites, while the spaces between them are 

filled with orthoclase. Plagioclase is the most abundant mineral, 

closely followed by orthoclase. Quartz is rarely seen filling minute 

interspaces, but is evidently an original mineral. 

An analysis, made of very fresh material, gave:

Analysis of augite-mica-diorite from Robinson. 

[By L. V. Pirsson.]

Si02 .........................

Ti02 .. .....'.. .................

A12 03 . .......................

FeiA...... ..................

FeO ..........................

MnO........... ..............

MgO. ........................

CaO .........................

BaO .........................

Na,0.. ......................

K20...... ....................

Li20. ...... ..................

H20 .........................

T> n

Total..................

Sp.gr........................

Per cent.

.46

18.30

5.58

3.63

5.31

.05

4.35

3.28

Trace

.53

99.93

2.83

The high percentage of alkali is noticeable, and it must be inferred 

that the plagioclase can not be a lime-rich one. The analysis approximates that of a basic syenite several analyses of rocks quoted as 

syenites could indeed be given which are extremely similar but plagioclase is the dominant mineral, and the dark-colored components are 

those which recall certain gabbros; they are, indeed, in habit and

'Determination dos feldspatha, Paris, 1894, Baudry etCie.
DIORITE. 91

appearance, extremely similar to gabbro rocks collected in southern 

Montana near the National Park.

The rock under discussion may then be classed as an augite-micadiorite which has a tendency toward the syenites in type.

From this type, as one leaves the center of the mass and proceeds 

toward the periphery, the rock becomes gradually lighter in color. 

Studied under the microscope, this change is found to consist in an 

increase in quartz; the hypersthene disappears, and the pyroxene 

occurs in smaller amount and is often iutergrown with a green hornblende with a brownish tone of color, whose presence may be due to a 

paraniorphic change in the augite. A good example of this variety is 

found in the rock cutting made above Robinson in building the highroad to Blackhawk. Mica becomes more prominent and the rock 

passes into a hornbleude-inica-quartz-diorite. This is the type found 

in the hill back (southwest) of Blackhawk. On the western rim of the

mass the rock has this composition, but is porphyritic; the feldspar

phenocrysts are idiomorphic, and megascopically the rock strongly 

resembles the granite already described. This resemblance is heightened by the fact that it weathers into crags, as noted in the preceding 

chapter, which, seen from a distance, lead at first to an impression of 

the eastward extension of the granite to this point.

Under the microscope, however, the pheuocrysts are seen to be a 

striated plagioclase similar to that found elsewhere. They are very 

abundant, and their interstices are filled with a granular mixture of 

mica, hornblende, orthoclase, and quartz, giving a transition form into 

quartz-diorite-porphyrite. No varieties with formation of a rnicropegmatitic intergrowth of quartz and feldspar have been observed in connection with the diorite.

The southern portion of the mass which abuts against the Cretaceous strata forms a high ridge with castellated crags, similar to the high 

hills back of Blackhawk, just described. The rock constituting these 

outcrops is, however, of a different character from the main mass. Jt 

is a most typical, granite-porphyry, with phenocrysts of orthoclase, biotite, and quartz lying in a granular allotriomorphic mixture of quartz 

and feldspar. In the hand specimen it is of a pale-gray color with a 

fine granular groundmass in which feldspar and occasionally quartz 

phenocrysts are seen. In places among the outcrops inclusions of a 

darker, more basic character appear which bear the same relation to 

the inclosing rock as those already described as occurring in the 

granite. It is possible that this granite-porphyry is not a peripheral 

fades of the diorite mass, but a dike, an offshoot from the granite cutting along its boundary. The nature of the exposures is not such as 

to definitely settle this point, but, so far as can be judged, we believe 

it to be an integral portion of the diorite mass, and its granitic 

character to be due to a local differentiation in place in the cooling 

fluid magma.
92 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

CONTACT METAMORPHISM.

Contact metamorphism naturally divides itself into two classes: 

endomorphic, or the variation in crystalline structure and texture from 

the normal type which an igneous fluid mass assumes on cooling, due 

to local variations of the physical conditions produced by the adjacent rock masses; and exomorphic, those changes induced in the 

adjacent rock masses, such as baking, recrystallization, and the formation of new mineral compounds by the heat, aqueous vapor, and other 

gases at high tension given off from the cooling injected fluid.

Endomprpliic iy, the granite. Approaching the contact with the adjacent sedimentaries, the granite becomes finer-grained and porphyritic, 

passing rapidly into a granite-porphyry. Usually the area of contact 

is eroded, so that good exposures are not numerous for purposes of 

study. So far as observed there are no phenomena which are of especial 

interest or which have not already been frequently described by various 

petrographers. One of the most interesting and instructive examples 

of endomorphic contact metamorphisin afforded by the granite is that 

which may be seen in the excellent exposure on Fourmile Creek, 

already described, where the granite abuts against the ends of the 

sedimentary beds and passes out between them in intruded sheets. 

Within a distance of 20 feet it passes from granite through transitional 

stages into granite-porphyry and into typical quartz porphyry in the 

intruded sheet. As seen in thin section, the granite at" this point is 

rather fine-grained and porphyritic; the quartz of the groundmass h'as 

the tendency already noted to assume idiomorphic form; indeed, the 

rock might be well described as an extremely coarse-grained graniteporphyry in which iron ore and biotite are the only dark minerals and 

the feldspar is somewhat kaolinized. In the next stage, passing outward at a distance of about 6 feet, or nearly 2 meters, from the margin,

the grain is perceptibly finer; the quartz grains are now rounded and 

the structure is that often seen in aplitic granites, where each component tends to assume its own form, a structure called by Rosenbusch 

panidiomorphic. In the next stage this process continues, and at times 

the quartz is observed in micropegmatitic structures with the feldspar. 

The change into the quartz-porphyry of the intruded sheet occurs 

suddenly, the groundmass sinking into a fine-grained allotriomorphic 

mixture of quartz and feldspar, while the pheiiocrysts remain of nearly 

the same size but are much fewer in number. This form is megascopically dense, with conchoidal fracture. The fine-grained quartz and 

feldspar of the groundmass are often observed in areas with micropoikilitic structure.

Quite similar transitions from granite to quartz-porphyry on the 

periphery of an intruded mass have been described by Hague, 1 with

'Geology of the Eureka district, Nevada: Mon. TJ. S. GeoL. Survey, Vol. XX, p. 225, Washington, 1892.
WpmssoND ] CONTACT METAMORiPHISM. 93

petrographical details by Iddiugs, 1 as occurring in the Eureka district, 

Nevada.

JEndomorphic in the diorite. ThQ diorite also suiters a very similar 

change at the contact. It grows finer in grain and more porpliyritic, 

and at the actual contact is generally dense and aphanitic. Seen in 

thin sections this latter form is, however, holocrystalline, and consists 

of quartz and feldspar in characteristic mlcropoikilitic structure, 

embedded in which are plagioclase phenocrysts. The rock seems to be 

peculiarly liable to decomposition at the contact, and this holds true 

for the granite as well as for the diorite. fto in the form just described 

the plagioclase phenocrysts are full of'fine leaves of white mica similarly oriented along the cleavage lines, while the biotite which was 

formerly present is entirely gone, only chlorite remaining in its place.

At other times the diorite is observed retaining its full size of grain 

up to the very edge of the contact. This was especially noticed in 

material thrown out from a prospect shaft sunk on the contact with the 

Cambrian limestone a few rods .east of the wagon road from Blackhawk 

to Castle, about one-half mile below Blackhawk, and on the knoll dividing two head branches of Bonanza Creek. The shaft is locally known 

as the "Frenchman's Claim." The material developed here is very 

fresh. The dark-gray diorite is seen sharply differentiated from the 

limestone, which is here white calcite, due to exomorphic processes.

JSxomorplii.0 changes. In the example last mentioned there is a distinct baud, about 1 inch in width, into which the substance of the diorite seems to have entered. It is a mixture of calcite with the dark 

minerals of the diorite hornblende, biotite, and pyroxene mixed with 

calcite. As previously stated, it is sharply defined, however, from the 

diorite itself. Beyond this band occurs the metamorphic rock proper, 

"which is a limestone changed chiefly into crystalline calcite, but filled 

with nests, pockets, and agglomerations of contact minerals. A pale 

oil-green grossularite garnet occurs in grains and in ill-defined crystals 

often lumped into masses, and rarely in well-defined dodecahedrons an 

inch in diameter. Associated with the garnet are masses of a brown 

vesnvianite, which was not found iu good crystals. A green pyroxene 

is also common at times in well-defined crystals, and this occurs so 

abundantly in places that a mixture of it and calcite forms rock masses 

a few feet across. A phlogopite of a pale greenish-white color is also 

abundant, scattered through the rock. A more detailed description of 

the occurrence of these and other minerals is given in the last chapter 

of this bulletin. This calcite rock, with the garnets and vesuviauite 

scattered through it, strongly recalls the similar contact rock from 

Cuautla, State of Morelos, Mexico.

In the village of Blackhawk, where the limestone has been metamorphosed not only by the diorite but also by intrusive dikes and sheets, 

which have greatly intensified the effect, it has been changed into

'Ibid., p. 342.
94 THE CASTLE MOUNTAIN MINING DISTRICT. [BOLL. 139.

marble over large areas. At times 'the calcite composing the marble has 

cleavage surfaces nearly an inch across in the vicinity of the intrusive masses. The same is true also around the town of Robinson, but 

here the garnet, vesuvianite, and the other minerals mentioned above 

have not been seen. It is in these areas that the sulphides and other 

ores described in the chapter on the mining resources of the district 

have been chiefly deposited.

The effect of contact metaniorphisin on the shales has been to change 

them, in those localities where the action is the most marked, into 

tough, hornstone-like rocks. Under the microscope these shales appear 

as a cryptocrystalliue mass. The section has a nearly uniform gray 

"pepper-and-salt" appearance; it is extremely difficult to identify 

with certainty individual minerals, but the mass evidently consists 

largely of quartz, feldspar, white mica, etc. This appearance is not 

greatly changed by the metaniorphism, but the texture is much denser, 

so that the rock has a true hornstone-like appearance with a splintery 

fracture. In many cases, however, there are seams running through 

the altered rock, tilled with green epidote in small grains' and crystals. 

In some cases layers richer in lime have been altered, and immediately 

garnet and pyroxene appear with the epidote.

Near the contact locality on upper Fourmile Creek, described on 

page 60, a rock was collected, from the talus of the lower mountain side, 

which in the hand specimen appears to be a breccia of light-colored 

angular fragments cemented by a dark-colored material. Under the 

microscope the light-colored portions appear like altered shales, and 

show a cryptocrystalline to microcrystalliue mixture of fine particles 

with larger angular fragments of quartz scattered through them, 

mingled with areas of coarser quartz particles. A banding in the 

finer-grained portions reveals the original planes of sedimentation. 

The pieces composing the breccia lie with the sedimentation planes at 

all angles. The dark-colored material cementing them consists of tourmaline mixed with quartz, which runs in bauds, veins, and stringers 

through the rock. It is in irregular shreds and patches of a grayishbrown color, highly pleochroic. The occurrence of this rock near the 

contact is of interest, as it shows the formation of a contact breccia, 

and that the upward movement of the magma to the position where it 

now rests as a crystalline mass was attended with great force, shattering and rupturing the rocks along the plane of contact a fact which 

will be subsequently noticed in the description of the effusive masses. 

The occurrence of the tourmaline is also of interest in eovinciug the 

presence of boric acid among the fumarole products of the cooling 

magma, and also in throwing light on the formation of a tourmalinequartz-porphyry, to be described later.

The Cretaceous shales, which have been altered by the intrusion of 

the granite, are well exposed at the head of Alibaugh Creek. They are 

dense, black, tough hornstones, and by the eye alone are not to be
JD] APLITIC GRANITE. 95

distinguished lithologically from altered varieties of the Belt slates. 

In thin section, however, they disclose some marked differences, being 

characterized by the occurrence of andalusite, which has nowhere been 

observed in the altered Belt rocks.

The section shows the usual fine-grained mixture of clastic minerals, 

chiefly quartz, which is so characteristic of this class of rocks. Scattered through this are knots or ovoid spots, about one-half millimeter 

in diameter, which catch the eye, as they are much lighter in color than 

the mass in which they lie. This is* due to the absence, or the local 

concentration into a few larger grains, of the dusty, sooty, organic 

matter which is elsewhere present, and also to the somewhat coarser 

crystallization of the component mineral particles. The rocks also 

contain irregular shreds of a brown mica similar to that usually found 

in contact hornstones. Scattered around among these knots are crystals of andalusite which attain an extreme diameter of 1 millimeter. 

They are bounded by the prism faces and the basal piuacoids, and with 

one exception show clear crystal outlines; the prism angles are wanting and their place is supplied by a narrowing wedge of included 

matter, which gradually tapers to a mere point as the center of the 

crystal is reached. At this point the four wedges come together, and 

the whole result is such that a basal section of the crystal shows for the 

clear andalusite substance the exact outline of a Maltese cross. This 

is, of course, only another variation of the chiastolite variety of andalusite. The mineral was proved by its optical characters.

The presence of this andalusite schist is of interest, as so few rocks 

of this nature have been described in this country, and recalls the wellknown German occurrences which have become classic from the work 

of Eosenbusch.

ROCKS OF THE INTRUDED SHEETS AND DIKES. 

APLITIC GRANITE.

This occurs in the form of very narrow dikes, rare in the granite 

mass, but common in. the diorite; and where good exposures were seen 

the latter rock was generally penetrated by dikes of this rock. An 

excellent example was obtained from the material thrown out of the 

prospect shaft on the open hillside between Blackhawk and Eobinson, 

the same locality from which the freshest and most typical diorite was 

obtained, whose description and analysis are given on page 89. Hand 

specimens show a grayish rock of a very fine grain, in which are seen 

occasional black plates of biotite. Under the microscope it is found 

to be a fine-grained, allotriomorphic mixture of quartz and unstriated 

feldspar; some plagioclase is present in idiomorphic crystals. From, 

this phase it varies, becoming still finer grained, while piienocrysts of 

quartz and feldspar appear, and a transition into granite-porphyry 

occurs.
96 THE-CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

An analysis of a typical specimen gave the following result:

Analysis of afiUtic granite. 

[By L. V. Pirsson.]

Si02 . ........................

Tin

Al_.0a . .... ............

Po2 03 .... ....................

JTeO .........................

MgO. ......... ...............

MnO...... ...................

CaO.... ......................

Na.,0 ........................

K20 ."........................

H,0............. .............

Sp.gr........................

Per cent.

79 QQ

4^

12.90

.74

1.05

.75

.05

.81

3.72

5.03

1.22

99.60

2.64

The very close analogy between this analysis and that of the granite 

is quite striking when we consider the origin of these dikes and the 

source of their supply.

It is interesting to note that along the wall and close to the contact 

they often contain large, irregular fragments of feldspar and of biotite, 

which have evidently been torn off from the inclosing rock and floated 

out in the magma in much the same way as the large crystals that are 

found- in the well-known minette dike near Aschaffenburg, in Germany, 

.though on a' much more minute scale.- Sections cut from the contact 

line and showing both the aplite and diorite indicate that the latter 

rock has suffered alteration along the contact plane. The large feldspars are altered, with formation of a considerable amount of sericite; 

the augites are changed into masses of carbonates, chlorites, etc., while 

biotite has changed to chlorite, earthy iron ores, rutile needles, etc. The 

most interesting change is that exhibited by the ilmenite or titanic iron 

ore. This is surrounded in places by leucoxene-like material, which 

occurs also in isolated masses free from the ore. This leucoxeue is not, 

however, titanite, as is generally the case. It is composed of a yellowish-brown mineral, of extremely high single and high double refraction, 

which possesses cleavages intersecting at a right angle, and is uniaxial 

and negative. Such a combination of characters could belong only to 

anatase. That leucoxene is sometimes composed of anatase was maintained J^yKosenbusch 1 and by Diller.2 The present occurrence confirms this, but in the diorite here described the mineral does not occur in 

sharply denned crystals, but in crystalline masses and aggregates. It 

is not intended, of course, to assert that the changes in the diorite 

described above are due to contact action alone.

>Mikr.Phys.,1885,p.332. . Jahrb. Min., 1883,1, p. 192.
KOCKS OF THE INTRUDED SHEETS AND DIKES. 97

QUARTZ-PORPHYRY.

Introductory. The quartz-porphyries are found chiefly as intruded 

sheets and masses around the eastern flanks of the mountain, where, as 

has been previously shown, they stand in direct genetic relationship to 

the granitic stock. In the study of a great number of intruded sheets 

and dikes it has been observed that transitions exist from the most 

typical of quartz-porphyries, with abundant quartz phenocrysts and 

quartzose groundmasses, into those where the quartz as a phetiocryst 

is lacking but is abundantly found in the groundmass, on into varieties 

where quartz practically or nearly disappears, the feldspars assuming 

a lath shape and the structure a trachytic habit. Thus the range 

extends from quartz-porphyry into quartzless or syenite porphyry. 

These forms of transition are also expressed in the change in the chemical composition of the magmas forming these bodies, but it is to^be 

noted that this change is not due alone to a simple sinking of the silica 

percentage and a cor responding rise of alumina and alkalies, but as the 

silica decreases there is also a corresponding rise in lime, magnesia, 

and iron, and this expresses itself by an increase in plagioclase and the 

ferromagnesian minerals, so that the eventual porphyries have strong 

tendencies to porphyrites. One or two occurrences, indeed, have all 

the habit and appearance of porphyrites, but unfortunately their 

greatly altered condition prevents satisfactory conclusions from either 

microscopic study or chemical analysis.

If, on the other hand, we disregard the geologic occurrence of the 

rock masses of the district and compare types of the acid rocks, both 

effusive and" intrusive, from a structural point of view, all gradations 

will be found to exist. While the intrusive acid rocks are always 

holocrystalline, careful study with high powers of the microscope not 

revealing any interstitial glass base, yet they descend to types of 

exceeding fineness of grain. The effusive forms, on the other hand, 

pass from pure glassy pitchstone into semiglassy, microspherulitic 

types, and from these into microcrystalliue groundmasses, and so on 

into granular crystalline forms in no wise distinguishable from the 

intrusive rocks and often possessing a higher degree of granularity 

than many of them. And these gradations are found not only in the 

extrusive bodies as a whole, but, as will be shown more in detail in a 

subsequent place, in a single, individual, continuous rock mass. . The 

question of their degree of crystallization or granularity is not, therefore, one of geologic occurrence, but of physical condition, and while it 

is true that, in. general, similar geologic positions cause similar physical 

conditions, and therefore similar types of structure, it is evident that 

this is not always the case and that the number of exceptions will multiply in ratio with the increase of geologic research. Castle Mountain 

furnishes a number of excellent and instructive examples of these 

exceptions. Since, however, it has been found convenient for purposes 

Bull. 139 7
98 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

of description to divide the igneous rocks of the Castle Mountain district into three classes according to their mode of geologic v occurrence, the quartz-porphyries described in the following section are 

those of the intruded sheets and dikes, while the effusive types will be 

treated later as transitional forms of the rhyolite. According to their 

distribution they may be divided into two main and separate occurrences those of Castle Mountain proper, where- as sheets and dikes 

they form peripheral fringes of the granitic stock, and the separate 

area of the North Fork of the Musselshell Eiver.

Quartz-porphyry of Castle Mountain. These rocks are generally dense, 

felsitic-lookiiig, and of various shades of light colors, pale-gray, white, 

lavender, and brown being common. They break with a conchoidal or 

splintery fracture, and on fresh surfaces the lustrous cleavage of feldspar 

phenocrysts is seen. These feldspars are small and rather numerous, 

but generally not thickly crowded. The quartz phenocrysts often break 

out of the felsitic matrix of the groundmass and then show the common 

dihexahedral form, the angles and planes roughened and rounded, never 

sharp and lustrous like the quartzes of veins and geodes. The rocks 

are much jointed, and their sharp angular blocks form great talus slides 

which cover the mountain slopes of the upper valleys of Fourmile and 

the other creeks draining the eastern half of the mountain.

Microscopically these rocks are seen to possess a monotonous regularity and simplicity of structure and mineral composition. They are 

of a type which is extremely common wherever such rocks are found, 

and which, has often been described by various investigators. Only 

such details are therefore given as will enable one to recognize the 

types. The phenocrysts are quartz, feldspar, and biotite. The quartz 

shows the common phenomena of cracking- and of deep embayments 

filled with grouudmass. It contains fluid and glass inclusions, often 

as negative crystals. The feldspar phenocrysts are of plagioclase and 

an untwinned feldspar, the latter at times singly twinned according to 

the Carlsbad law. The bulk analysis of a typical example shows a large 

amount of soda present, as indeed do all of the acid rocks of the district, so that it must be concluded that the albite molecule is largely 

present. On the other hand, the very small amount of lime proves 

that the plagioclase must be a^very acid one. Hence it is quite possible that the untwinned feldspar is a soda orthoclase. It has been 

recently shown l that in high alkali rocks, w(ith relatively low lime, a 

soda orthoclase may be present which optically is scarcely distinguishable from orthoclase. Unfortunately, in the case of these quartzporphyries the feldspar phenocrysts are almost invariably somewhat 

kaolinized, and accurate optical determinations are thus prevented.

The groundmass is a mixture of quartz and feldspar, the latter 

untwinned or twinned according to the Carlsbad law. Striated feldspars have not been seen in the groundmass mixture of these rocks.

»L. V. Pirsson: Am. Jour. Sci., Vol. XLVII, 1894, p. 342.
WEBD AND-I 

PIRSSON. J QUARTZ-PORPHYRY. 99

The feldspars are generally present in the short,' rectangular form, while 

quartz forms the cement between them. This type of structure, which 

may be called the microgranitic, is generally present, but at times 

passes into a variety where quartz and feldspar are mingled in allotriomorphic grains confusedly interlocked. The monotony of these uniform 

groundmasses is interrupted only by the phenocrysts and by an occasional grain of iron ore or a crystal of apatite or zircon, the latter 

sharply bounded by crystal planes. An analysis of an average specimen from one of the intruded sheets from the divide between the 

headwaters of Fourmile and Checkerboard creeks (Sp. No. 94) gave the 

following result:

Analysis of quartz-porphyry' 

[By L. V. PirsBon.]

Si02 . ........................

Ti02 ~.. ........ ...............

Al-jOg...... ..................

re-jOg...... ..................

FoO .........................

MnO...... ...................

MgO...... ...................

CaO .........................

Nn-jO ........................

K20. ...... ...................

Li2 0...- ..........;..........

H2 0 .........................

Total..................

Sp.'gr...... ..................

Per cent.

79 QH

.10

14.71

1 09

.82

.70

.67

4.28

4.15

.92

QQ 89

2.61

The composition of this rock is strikingly like that of the granite 

mass, and the similar relations of the alkalies to one another is very 

noticeable. It differs in being somewhat lower in lime and iron. Its 

composition allies it with some of the quartz-keratophyres.

In respect to their state of preservation, these rocks are nowhere 

found absolutely fresh. The feldspars never have the sanidiue habit, 

while the biotite is nearly always altered to a mixture of chlorite 

and iron ore, more rarely to muscovite. The groundmass, and often 

the feldspar phenocrysts, are dotted with leaves of sericite. On fresh 

fractures also the weathering of the surface is seen extending into the 

rock as a brown mantle, often to some depth.

QUARTZ-TOURMALINE-PORPHYRY.

An occurrence of a quartz-porphyry with tourmaline, sufficiently 

common to rank as an accessory component, deserves especial mention.

The rock occurs as an intruded sheet, whose outcrops and talus are 

found on the lower slopes of the mountain side forming the right-hand
100 THE CASTLE MOUNTAIN MINING DISTKICT. [BULL. 139.

wall of the deep and narrow valley of the main upper head of Fourmile Creek, and about 1£ miles south of the present road from White 

Sulphur Spring to Castle. The mountain, side is covered by such a 

heavy mass of- talus that the exact position and thickness of the many 

intruded sheets at this point can not be accurately determined; their 

outcrops project above the talus in confused masses. Among them 

are the exposures of the tourmaline-bearing quartz-porphyry. It is at 

once noticeable from its very pale, almost pure-white color, tinged 

pinkish on weathered surfaces. It is so jointed that it breaks out 

and falls in large blocks along the talus. Under the-hammer it is 

extremely tough and breaks with a splintery fracture. Megascopically the rock consists of a dense, very pale-gray felsitic groundmass. 

in which lie numerous quartz phenocrysts, from 1 to 2 millimeters in 

diameter. It is further flecked by small, whitish spots of feldspar, 

which do not reflect light from cleavage surfaces, and by many dots 

and dashes of black, averaging from one to" several millimeters in 

diameter. Examined closely, these latter are seen to consist of radial 

and stellate groups of a black, fibrous mineral with a silky luster. 

Much more rarely occasional minute masses of bright-purple fluorite 

are seen, sometimes associated with the tourmaline. There also occur 

occasionally very small patches of a pale oil-green substance of a waxy 

luster. Under the microscope the rock is found to possess a normal, 

fine-grained groundmass, composed of an allotriomorphic mixture of 

shapeless, interlocking quartz and feldspar grains. In plain light this 

has a mottled appearance, due to the fact that the feldspar is somewhat kaolinized and cloudy, and thus contrasts with the limpid quartz. 

In this groundmass are numerous quartz phenocrysts of the usual 

quartz-porphyry type. The feldspar phenocrysts are also numerous, 

and consist of orthdclase and plagioclase, the latter a very acid one, 

since it invariably extinguishes at very slight angles to the twinning 

lamellae. In plain light the orthoclase appears clear and colorless, 

while the plagioclase is always brownish from incipient kaolinization. 

The plagioclase is sharply idiomorphic and often in groups of several 

attached crystals. The orthoclase is less idiomorphic, and appears to 

have been growing in the second period of consolidation; it often 

incloses small idiomorphic plagioclases.

The occurrence of the tourmaline and fluorite is of greater interest. 

The former is composed of radial bunches of slender acicular crystals, 

strongly pleochroic, the ordinary ray a deep sea-green, often a deep 

indigo-blue or bluish-green, the extraordinary ray a pale reddish-brown. 

It evidently replaces feldspar, and often the shape of the cavity occupied by the stellate groups is that of this mineral; in other places the 

needles packed together appear as broad tongues penetrating the feldspar substance; at other times it is seen as shreds and patches diffused 

through the feldspar. The mineral is easily identified by its pronounced 

optical characters, and these were confirmed by tests before the blow-
WEED AND"! 

P1KSSON. J QUARTZ-TOURMALINE-PORPHYRY. 101

pipe, which showed the presence of boron. The fluorite appears in the 

section in precisely the same mode of occurrence, replacing feldspar. It 

is identified by its isotropic character, its cleavage, and its low single 

refraction. Small leaves of white mica are also scattered through the 

feldspars. A few crystals of apatite and zircon were observed, but no 

ferromagnesian mineral has been seen either under the microscope or 

in the hand specimen. The iron and magnesia present appear to be all 

in the tourmaline. An analysis of the rock resulted as follows:

 Analysis of quartz-tourmaline-porphyri/ from Fourmile Creek.

[By L.V.Pirsson.]

SiO. .........................

TiO.,.. .......................

MO....... ............ ......

Fe203 ........................

FeO .........................

MnO. ........................

MgO....... ..................

CaO .........................

KoO .........................

 NasO...--. ..................

H.O (igii). ............. . : ....

Total....................

Sp.gr........................

Per cent.

74.82

OR

13. 80

07

.30

10

.17

4.81

4.33

.83

99.78

2.59

The similarity between this analysis and that of the normal variety 

of quartz-porphyry is striking, the difference being in a smaller amount 

of lime, iron, and magnesia, and slightly higher silica. The alkalies 

are about the same and almost exactly like those in the granite; the 

same ratio of potash to soda obtains, and it is evident that here also 

the albite molecule plays a conspicuous part in the formation of the 

feldspars.

The chief interest in the rock lies, of course, in the presence of the 

tourmaline and fluorite. The former mineral seems to be of rare occurrence in quartz-porphyry and rhyolite, though common in granites. 

Phillips 1 mentions it as occurring in a number of quartz porphyry dikes 

(elvans) in Cornwall, where it occurs, as in the present case^, in detached 

crystals or shreds, in radial groups, and also as filling spaces left by 

decomposed feldspars. Kosenbusch 2 gives an occurrence on the Auersberg in the Hartz Mountains, while Zirkel 3 adds Wagenberg in the 

Odenwald, at Elba, near Lugano, and on Monte Mesma on the lake of 

Orta, and D'Achiardi 4 in quartz-porphyry dikes in Donoratico, in blue

1 Quart. Jour. Geol. Soc., 1875, p. 335.

2Mikro. Phys. Gesteiue, 1887, p. 305.

3 Lehrb. Petrographie, Vol. II, 1894, p. 154.

* Atti della Soc. lose. <li Sci. Nat. Pisa, Vol. VII, 1885, p. 21.
102 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

and bluish-green crystals, their identity confirmed by tests for boron. 

Of its occurrence in true effusive rhyolites we have found only one 

mention, that by Iddings l in the spherulites from Obsidian Cliff.

Fluorite seems to be of still rarer occurrence. Bosenbusch 2 mentions 

two localities: Halle on the Saale, and Altenhavn in Saxony. It does 

not appear to have been observed in rhyolite.

The origin of these two minerals is to be referred to the pneumatolitic 

action of fluorine and boron vapors included in the igneous magma and 

given off in the process of cooling and crystallization. These vapors 

have evidently attacked the compounds first formed, resulting in the 

production of these minerals. It is of interest to note here that the 

contact breccia filled with seams and veins of tourmaline is at no great 

distance, and this is a fact which also bears witness to the active 

agency of mineralizers in the^cooling magma.

MUSSELSHELL QUARTZ-PORPHYRY.

Occurrences of large masses of intrusive porphyry on the North Fork 

of the Musselshell, near the mouths of Checkerboard and Flagstaff 

creeks, present collectively a certain habit of the rock which is somewhat 

different from that of the quartz-porphyries of Castle Mountain. The 

groundmass is more granular, less deu^e and felsitic in appearance; 

quartz phenocrysts are rarely seen, and they are generally quite thickly 

dotted with small, glittering idiomorphic tablets of biotite. The rocks 

range from white to a pale pink in color, the latter being very common. 

They are abundantly filled with pale yellowish-white feldspar phenocrysts from 2 to 4 millemeters in length. There are also small rectangular spaces filled with yellowish-brown limonite, the pseudomorphs of 

some former phenocryst.

A very dense white variety occurs as a facies of the most northern 

mass on the banks of the little creek cutting through it on the east 

side of the Musselshell. With this exception the rocks are nearly 

everywhere of the type described. Under the microscope the groundmass is seen to vary somewhat in granularity in the different occurrences; in structure it ranges from allotriomorphic to microgranitoid, 

the small feldspars in the latter rather idiomorphic and short-rectangular in section, the quartz tilling the interspaces. The phenocrysts 

are quartz, which is quite rare, a plagioclase near oligoclase or albiteorthoclase, biotite similar in character to that found in the rocks on 

Castle Mountain, a little apatite, iron ore, and zircon. The dense white 

variety is almost devoid of phenocrysts.

1 Bull. Philos. Soc. Washington, Vol. XT, 1891, p. 455. 

2 Mikro. Phys. Geateine, 1887, p. 365.
WEED AND"] 

PIHSSON. J MUSSELSHELL QUARTZ-PORPHYRY. 103

An analysis of a typical specimen (No. 197), whose groundmass shows 

a tendency toward the micropoikilitic structure, gave 

Analysis of quartz-porphyry from the Mussclshell liiver. 

[By L. V. Pirsson.]

SiO., .........................

Ti02 ...... ...................

A12 0S .-.... ...............

Fo2 03 .--. ....................

FeO. ........................

MnO. ........................

MgO.. .......................

CaO .........................

K,0 .........................

LijO...... ...................

TT2 0 .........................

Total..................

Sp.gr........................

Per cent.

71.67

.10

15.82

1.18

.35

.25

4.46

4.45

1.21

99.62

2.60

This analysis shows a very striking resemblance to the analyses 

already given of the acidic rocks of Castle Mountain, especially in the 

relation of soda to potash. Although no direct geologic relationship 

can be traced in the Held between the two occurrences, this close agreement in chemical and mineral composition seems to clearly indicate that 

they have had a common origin and that the magma which formed this 

porphyry ascended through a different conduit from the same reservoir 

which formed the Castle Mountain mass.

Near the juncture of "Warm Spring Creek with Woodman Creek, or 

in other words at the head of the South Fork of the Musselshell, occurs 

a small butte formed by an intrusion of quartz-porphyry into Cretaceous strata. Megascopically and microscopically the rock is in every 

way similar to the quartz-porphyries of Castle Mountain. The grain 

is fine and has a tendency to micropoikilitic structure, but is in general 

allotriomorphic. The phenocrysts are quartz, plagioclase, and orthoclaseof the usual type. The presence of some-former ferromagnesian 

mineral biotite or hornblende, or possibly both is indicated by collections of limonite grains mixed with serpentine and at times calcite. 

These give the rock a general brownish tone. This is the most southerly occurrence of a rock of undoubted Castle Mountain type, and is 

therefore of interest.

FELSITE-PORPHYRY. 

This name is used in the sense of Tschermak, 1 and without reference 

to geologic age (of which indeed Tschermak himself makes no mention), to designate those rocks with dense groundinasses of quartz

' Sitzungsber. K. Akad. Wiss., Wion, LVI., 1807, note 11, p. 305.
104 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

and alkali feldspar in which lie ortlioclase pheuocrysts, but none of 

quartz, and which have the chemical composition of a quartz-porphyry. 

Although large portions of the Musselshell porphyry might properly 

come under this designation, at Castle Mountain itself the rock has 

only a few limited occurrences and is of such a nature that it forms 

really a transition stage into the quartz-free porphyries, to be described, 

later.

The great dike which rises like a wall above the talus-covered slopes 

of the right canyon wall of upper Fourmile Creek, and which has been 

mentioned on page 65, is an example of this type. The rock has been 

somewhat altered, and the former ferromagnesian components, which 

are thought to have been biotite and hornblende, are replaced by 

masses of hydrated iron oxide, calcite, and other decomposition products. The rock has a rusty tint from the iron oxide. The phenocrysts 

are orthoclase and plagioclase, both greatly altered, which lie in a 

groundmass of quartz and feldspar, the latter predominating. The 

relation of the rock to quartz-porphyry is shown by the fact that 

although quartz as a phenocryst is generally wanting, in a few places 

in the rock mass it is abundantly developed, and it is then precisely 

similar to that already described under the normal type of quartz-porphyry. A section cut from the contact with the Belt shales, which 

have been highly indurated into a greenish hornstone, shows in the 

porphyry a pronounced parallel arrangement of the numerous phenocrysts into a flow structure parallel with the sides of the dike. This is 

especially shown by the feldspars, which have a lath-like development. 

It proves that they had already crystallized out when the viscous mass 

was intruded. The groundmass in this portion of the dike has all the 

appearance of a^devitrifled glass, and is peppered through and through 

with dusty globulitic material.

The large dike which is found cutting the Cambrian limestones on 

the right side of the Fourmile Creek near its sharp turn to the northwest above Eeed's ranch is somewhat similar to the above, but has less 

quartz in the groundmass and is a transition form into syenite porphyry } it might, indeed, be called a rather acid type of that rock. 

Megascopically it is seen to consist of abundant large pinkish or yellowish orthoclase crystals, often half an inch long, and with a deep 

stain around their edges, with small white striated feldspars and small 

black hornblendes lying as phenocrysts in a dense gray groundmass. 

Under the microscope the rock is seen to be highly altered. It contains large amounts of calcite, some of which may indeed be material 

derived from the calcareous rocks into which it has been injected, while 

some is plainly secondary. The hornblendes are often entirely decomposed arid their places filled with calcite, which is not a confused crystalline mass, as is so often the case, but a single individual crystal with 

uuiform polarization, but with all the idiomorphic form and characteristic angles of the original hornblende. Such a crystal often includes
I FELSITE-PORPHYEY. 105

masses of chlorite material, from the alteration of the original hornblende, which has not been removed from the cavity and around which 

the calcite has grown. Its orientation does not seem to bear any relation to the original hornblende. The explanation of this rather curious 

pseudomorphism is undoubtedly to be sought in the fact that the percolating water which has been the active agent in the decomposition 

of the rock must have been highly charged with the bicarbonate of 

lime in solution, derived from the limestone strata through or over 

which it had to pass to reach the dike below, and this accounts, at 

least in part, for the large amount of carbonates in so acid a rock. 

Where the hornblende still remains it is of the common green variety 

and incloses much brown biotite, the latter also occurring idiomorphically by itself. The groundmass consists of allotriomorphic singly 

twinned feldspar, interspersed with considerable quartz, which is also 

generally shapeless, but in several cases was noted to possess clearly 

cut hexagonal outlines.

The dike which outcrops on the hill to the northeast of the above, 

and of which, indeed, it may be the continuation, though direct proof 

is wanting, is megascopically almost precisely similar. Under the 

microscope it differs chiefly in the fact that the ferromagnesian components do not appear to have formed as phenocrysts, but are scattered 

in small shreds and rods through the groundmass, the biotite predominating over hornblende. This produces, so far as the groundmass is 

concerned, a resemblance to some ininettes, but the rock is far too rich 

in silica, alkalies, and alumina, being essentially a feldspar rock, to be 

classified under that type. Considerable interstitial quartz is present, 

but less than in the former rock. It is also considerably altered. In 

reality it is a syenite-porphyry, but from its resemblance and possible 

geological connection with the dike previously described, is mentioned 

under this heading.

Another type intermediate between felsite-porphyry and a true 

syenite-porphyry is furnished by a series of rocks that occur, as shown 

on the map, below the town of Castle, intruded into Cretaceous strata 

in a series of dikes and sheets. Though they differ somewhat among 

themselves in megascopic appearance, as regards color, size of phenocrysts, and state of preservation, yet the microscope shows that they 

all belong to one type, and the best and freshest occurrence is here 

selected for description. Megascopically the rock consists of a dense 

gray grouudmass which is thickly dotted with very small black hornblende and mica phenocrysts and much larger ones of a white feldspar, 

which is not sharply idiomorphic but rather rounded in outline, and 

which averages about 2 millimeters in diameter. Under the microscope 

the groundmass resolves itself into a very fine mass of unstriated feldspar, mixed with a little quartz, generally allotriomorphic, but at times 

showing a tendency to a trachytoid structure. The quartz is sometimes 

rather idiomorphic, and occasionally, though very rarely, rises to the
106 THE CASTLE MOUNTAIN MINING DISTKICT. [BULL. 139.

position of a very small phenocryst. It plays in to to but an unimportant role as one of the rock-forming constituents. The feldspar 

phenocrysts are chiefly orthoclase with a smaller amount of an acid 

plagioclase. The hornblende is present in small, well-formed crystals 

bounded by the common forms m (110), & (010), r (Oil), and c (001). 

It is always the ordinary green variety, with a small extinction angle. 

Also in a few instances brown basaltic allanite, with high extinction 

angle, has been noticed. The biotite is of a yellow-brown color and 

possesses, for the mineral, an extraordinarily small absorption; it is 

probably somewhat bleached, and is, moreover, nearly always, even in 

the freshest examples, partially altered to chlorite. Both the hornblende and mica occur in extremely small crystals, at times nearly 

reaching the dimensions. of the groundmass. Occasional crystals of 

apatite, zircon, and iron ore complete the list of minerals. An analysis 

gave the following result:

Analysis of porphyry from Castle.

[By L. V. Pirrson.]

Si02 . ........................

Tin

A1203 ...... ..................

-Fo.£)3 . .......................

FeO. ...... ...................

MnO. ........................

MgO.. ........ ...............

CoO

Na2O ........................

K20.. ........................

HjO........ .................

Total..................

Sp.'gr. .......................

Per cent.

65.87

07

16.82

1.58

1.23

1.54

2.65

4.72

3.15

1.43

99.36

2.62

This analysis agrees with the mineral composition, but indicates that 

the albite molecule must be more largely present among the feldspars 

than is indicated by the study of'the sections.

The other occurrences at this locality are quite similar to the above; 

at times the g'rouudmass is somewhat coarser and a greater tendency 

to a trachytoid structure becomes evident, but the mineral combination 

remains the same. These other rocks are generally more altered, containing quantities of calcite, chlorite, limonite, and kaolin.

QUARTZ-FREE PORPHYRY (SYENITE-PORPHYRY).

Feldspathic porphyritic rocks free from quartz, in which orthoclase 

rules, are found in a number of occurrences in the Castle Mountain district. For purposes of description they may be divided, according to
I SYENITE-PORPHYRY, 107

their origin, into three groups: those of the Castle Mountain mass 

belonging to the Castle Mountain center of eruptive activity proper; 

those of the intruded masses and sheets of Black Butte, shown on the 

map in the extreme southwest corner; and the numerous intrusive 

sheets in the Cretaceous strata, which form the broad, open valley of the 

south branch of the Musselshell, and which, from their mineral combination and peculiar habit, are known to be of Crazy Mountain type 

and origin.

The quartz free porphyry of Castle Mountain is confined to a limited 

number of dikes. The most important occurrence is in a series of three 

parallel dikes crossing the limestone hill northeast of Blabkhawk, as 

mentioned in the previous chapter. These dikes are all composed of 

the same type of rock; megascopically it is dense but perceptibly 

granular and of a dark-gray color, spotted with white, waxy-looking 

feldspars of rounded, ill-defined form. A little<mica is seen, but the 

ferromagnesian component is generally chloritized, forming dull-greenish 

spots.

Microscopically the rock is seen to be very simple, consisting of 

phenocrysts of orthoclase and some plagioclase in a groundmass chiefly 

of unstriated feldspar having a trachytoid structure. No primary quartz 

is seen. Hornblende was probably originally present, but is now a mass 

of calcite, chlorite, limonite, serpentine, etc. The micas have also 

nearly disappeared, and in general the rock is much altered and kaolinized, though the feldspars in the most southerly of the three dikes 

are still fresh enough for distinct determination. On account of their 

altered condition no analysis of these rocks has been made. If they 

were named according to the present German classification, but without 

reference to their geologic age or mode of occurrence, they would 

undoubtedly be classed as orthophyres. From the position of these 

dikes it seems quite possible that they may be offshoots from the diorite 

mass, though their composition allies them with the granite.

Another rock which comes in this class is the dike a few rods below 

Reed's ranch on Fourmile Creek. This dike is cut through by the stream 

and the rock is intruded in the green shales that form the upper part 

of the Carboniferous series, as shown on the map. It is a brownish 

rock, with very fine-grained groundmass holding white feldspar and 

black biotite phenocrysts. Under the microscope its composition is 

seen to be very simple. It is composed of biotite, feldspar, iron ore, 

and a little quartz. The biotite has the deep-brown color and strong 

pleochroisrn found in the Castle Mountain rocks. In the feldspar phenocrysts orthoclase rules, but some plagioclase is present. The groundmass is composed of small raicrolitic feldspars which are unstriated;° 

packed in a trachytic-like felt, dusted through with ferritic material. 

.The quartz is scattered in small areas and grains and appears to be 

largely secondary. The rock is considerably altered, and if any glass 

was originally present it has been devitrified. The rock might well be 

classed as a trachyte.
_108 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

Similar to the above, but greatly altered, is the dike which is intruded 

in the Cambrian strata just north of the great bend in Fourmile Creek 

and near the large dike previously described. So also is the dike 

cutting the breccias on Fivemile Creek. In both the mica has nearly 

disappeared and great quantities of calcite are present.

BLACK BUTTB PORPHYRY.

The several sheets and intrusions at Black Butte are all closely related 

rocks, augite and porphyries inclining strongly to porphyrites. The 

rock which composes the highest point of Black Butte itself is of a 

pale-brownish color, dense in texture, and thickly crowded with feldspar 

phenocrysts, which, as they are somewhat rounded in outline and tinged 

in most cases like the rock itself, are not strongly prominent. Ferromagnesiau components are rare.

The microscope reve'als that the groundmass is made up of ragged, 

patches of unstriated feldspar with a little quartz in very fine grain. 

The feldspar phenocrysts are extremely numerous and vary greatly in 

size, from 5 millimeters in diameter to transition forms into the groundmass. The unstriated feldspar is predominant and is zonally built; the 

striated one is a rather acid plagioclase, to infer from the great number 

of uniformly small extinction angles to the twinning lines; it shows 

both albite and pericline twinning. The ferromagnesian component is a 

green diopside, like augite, very fresh and clear, but in small phenocrysts 

and never abundant. Titanite is common in characteristic outlines. 

The rock is an augite porphyry.

Numerous inclusions were observed, of what appears in the hand 

specimen to be hornblende-schist in angular fragments up to an inch 

or so long. Sections of these show that they are composed chiefly of 

a green pleochrpic hornblende, wi'th a large amount of orthoclase and 

much less plagioclase, and without quartz. The structure is schistose and allotriomorphic, the feldspars in rounded grains embedded 

in and against the hornblende so that no crystal boundaries are seen in 

either mineral. Occasional portions of the mass are of finer grain, and 

in them occurs much black iron ore, a constituent which is nearly lacking in other parts of the rock. Such areas recall the transformation 

and recrystallization of some large crystal of an inclusion, examples of 

a process which has been so beautifully figured and described by 

Lacroix 1 in his recent admirable memoirs on the inclusions of volcanic 

rocks. It would seem as if these inclusions were of some deeply seated 

rock torn off by the ascending magma and recrystallized, the minerals 

being extremely fresh and well preserved.

Below this mass, which forms the summit, several intrusive sheets 

outcrop on the north and east sides of the butte, the rocks being in 

general so greatly altered that the study of thin sections reveals nothing beyond what has been stated for the upper mass. On the east

1 Lcs enclaves des roches volcaniques: Ann. Acad. Macon, Vol. X, 1893.
CRAZY MOUNTAIN ROCKS. 109

side, at an altitude of 5,400 feet and just above the lake-bed deposits, 

there occurs in the Belt shales an intrusive mass or sheet where the 

rock, there well preserved, shows a somewhat different character. In 

the hand specimen it resembles in its habit some of the porphyrites 

which occur in the Absaroka Range. It is of a dark-gray color, thickly 

spotted with feldspar phenocrysts which rise to one centimeter in diameter. These lie in a grounclmass that to the naked eye appears finely 

granular and composed of feldspar and hornblende. Under the microscope, however, the groundmass is found to consist of these small 

hornblendes and feldspars lying in a microcrystalliue, evenly granular 

mixture of orthoclase with some quartz. If we were to judge by the 

relative sizes of the crystal components, the rock would therefore represent three stages of crystallization ; as a matter of fact, between the 

large and small feldspar phenocrysts, every gradation in size may be 

found, but it is evident that what is generally called the " mtratelluric 

stage" had progressed very far before the rock was intruded. .The 

feldspar is chiefly an unstriated one. zonally built and often showing 

some kaolinization. Plagioclase is present, but is not common. The 

hornblende is the same green variety that has already been described 

as occurring in the Castle Mountain porphyries. It is generally idiomorphic, often in very slender prisms, sometimes in shreds and patches. 

There is considerable titanite present in good crystals. It is remarkable how constant the association of this mineral is with hornblendeorthoclase rocks." Considerable iron ore is present, and some apatite; 

also a little white mica as an alteration product.

From what has been said it is therefore evident that the igneous 

rocks of Black Butte are closely related to those of Castle Mountain. 

They display, to use the apt term of Iddings, 1 a large amount of " consanguinity" with them, and the only observed variation in mineral 

character is in the zonal structure of the unstriatecl feldspar, a phenomenon rarely observed in the Castle Mountain rocks.

CRAZY MOUNTAIN KOCKS.

Under the heading of quartz-free, intrusive porphyritic rocks may be 

here briefly mentioned the occurrence of the great number of sheets 

which fill the Cretaceous strata in the southeast portion of the district. 

They are composed chiefly of acmite trachytes with very large feldspar 

phenocrysts, and of those holocrystalline rocks of tephritic character 

to which Eosenbusch 2 has given the name of theralite. Both of these 

types have been preliminarily described by Ji E. Wolff,3 to whom their 

discovery is due. A detailed description of them will be published in 

a memoir by Professor Wolff and W. H. Weed on the Crazy Mountains.

1 Origin of igneous rocks: Bull. Philos. Soo. Washington, Vol. XII, 1892, p. 128. 

 s Phys. Mass. Ges., 1887, p. 248.

3Notes on Petrog. Crazy *Mts., etc.: Northern Transcontinental Survey, 1885. Of. also Geol. Crazy 

Mts.: Bull. Geol. Soc. America, Vol. Ill, pp. 445-452. Acmite-trachyte: Bull. Mus. Comp. Zool. 

Harvard Coll. (Geol. Ser., Vol. II), Vol. XVI, No. 12,1893.
110 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

It will be unnecessary, therefore, to do more than refer to them here. 

A few porphyrites have been found in the southern part of the Castle 

Mountain area, which Professor Wolff, who has kindly examined the 

sections, pronounces to be of distinct Crazy Mountain type.

In addition to these there occur also several intrusive sheets whose 

direct source is doubtful, partly because of their decomposed state and 

partly because of their mineral nature. A certain type of these rocks 

is an augite-porphyrite, which exhibits some peculiar features. The 

rock is a dark stone-gray or blue, thickly dotted with small white feldspar phenocrysts, which are very slender and appear like white scratches 

on the surface. In thin section these feldspars are seen to be plagioclase, 

and with such a high extinction angle at times that they must belong 

to the labradorite group. Both albite and (more rarely) pericliue 

twinning are seen. The feldspars contain large cavities filled with a 

dusty brown deyitrified glass. The base also consists of this dusty 

brown devitrified glass filled with globulitic material, often arranged 

in plumose patterns. In this lie slender, lath-like microlites of an acid 

plagioclase, also twinned in stria3, scattered about like straws, and 

which in plain light are seen as colorless cuts and slashes in the brown 

base. Originally some augite Avas present, but it is completely changed 

to serpentine and calcite. Biotite is rare, being generally altered to-a 

mass of chloritic, ferruginous material.

This type occurs in several intrusive sheets, one of which is in the 

Cretaceous strata immediately below the small mass" of Carboniferous 

and the diorite on'Bonanza Creek,.and it is also found, with slight 

modifications, in a curious, winding serpentine dike cutting Cretaceous 

strata and intruded theralite sheets in the lower basin of Bonanza 

Creek. A rock of the same general character, but nearly holocrystalline, occurs in the synclinal Cretaceous basin north and east of the 

great bend in Bonanza Creek, but it is much weathered. It is believed 

to be a porphyrite of Crazy Mountain origin.

LAMPROPHYRE ROCKS. 

INTRODTJCTOKY.

It is not the intention of the authors to present here a discusion as 

to whether or not certain classes of rocks should be termed distinctively "dike rocks," as has, been done by Eosenbusch. Whether a 

given magma will take the form of dikes, sheets, or intruded masses 

depends upon the physical conditions relating to the rock masses into 

which it is forced. And if the same physical conditions as regards 

pressure and cooling are present, as they well may be, then we may 

find the same rock as a sheet, a dike, or a small intrusive mass indifferently.

Iddings 1 has pointed out that the effusive equivalents of the rocks

 ' Origin of igneous rocks: Bull. Philos. Soc. Washington, Vol. XII, 1892.
] AUGITE-VOGESITE. Ill

which have been termed "dike rocks" occur, but they were crystallized 

under different physical conditions and therefore are structurally, and 

generally inineralogically, different from the deep-seated forms. In a 

most admirable paper on the basic eruptive rocks of Gran, in Norway, 

Brogger 1 shows that not only do camptouites and bostonites occur in 

sheet as well as in dike form, but he also regards these rocks as differentiation products of an olivine gabbro, and claims, therefore, that their 

occurrence is not necessarily indicative of the presence of a definite 

type of granular plutonic rock, as had been formerly supposed. Nevertheless, since it is true that in regions where erosion has been sufficient to expose the central stocks or cores of former eruptive activity, 

there nearly always occur certain highly differentiated forms of the 

general magma in sheets or dikes, it would be well to have general 

terms by which to express the contradistinction of the basic and the 

acid types. One is distinguished by the high ferromagnesian content, 

the other by the alumina, alkali, and silica. It is here proposed to 

call the basic types simply lamprophyres, or lamprophyric rocks, without reference to their mode of geologic occurrence.2 In or around the 

Castle Mountain center these rocks are rare. They are limited to a 

few dikes, one or two intrusive sheets, and one small intrusive mass. 

So far as they correspond with well-known and already-described typ'es 

they will be named accordingly.

AUGrlTE-VOGESITE.

The basic dike which cuts the Belt shales and intruded quartzporphyry on Fourmile Creek near the contact with the central granitic 

stock has already been mentioned and a view of it given in PI. IX. 

Megascopically the rock is a very dark stone-gray with a greenish 

tone. On a fractured surface an occasional white spot is seen which 

reflects light from the cleavage surface of a small calcite inclusion. 

The grain in general is rather dense, so that the individual components 

can scarcely be recognized by the eye. The rock, following a cross 

section of the dike, has in fact three grades of crystallization, which 

pass insensibly into each other. In the central portion, which forms 

the greater part of the cross section, it is very fine granular, but feldspar cleavages can be recognized; this passes into a narrower zone on 

each side where they can with difficulty, or not at all, be discriminated, 

and finally at the contact there is a very narrow selvage band which is 

nearly black and is very dense. Microscopically the rock is seen to be 

composed of augite, hornblende, iron ore, a little plagioclase, orthoclase, 

calcite, and some decomposition products. The augite is a colorless 

diopside in phenocrysts which rise to 1 millimeter in length. They are 

all more or less altered with formation of serpentine and other decomposition products. They are quite numerous and are often gathered in.

'Quart. Jour. Geol. Soc. London, Vol. L, February, 1894, p. 15.

8 Of. Complementary rocks and radial dikes: Am. Jour. Sci., Vol. L, August, 1895.
112 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

groups. They lie in what is, compared to them in size, a groundmass, 

composed of orthoclase pegged through and through with slender crystals of a brown hornblende and dotted with the iron ore in small 

grains. The ore is usually found around the edges of the augite crystals, clothing their outlines in the section like a string of beads. It 

does not, however, appear to be secondary. The feldspar is clear and 

colorless, filled with great quantities of very long, slender, apatite 

prisms j it is allotriomorphic, and in rather broad, irregular masses 

showing Carlsbad twinning. The slender hornblende prisms are a pure 

brown in color, slightly pleochroic, and with a small extinction angleOalcite, while undoubtedly present, largely as a decomposition product, 

is also seen at times to occur in suclra manner as to suggest a primary 

origin. It is then present in the angular interspaces between the feldspars and not connected with any lime-bearing mineral. Here frequently the fresh hornblendes pjn it and the feldspars together. No 

decomposition products are present, the minerals are fresh, and it Is 

difficult to believe that the calcite has been, secondarily introduced. 

Similar facts have been observed by other investigators, and Zirkel 

gives an excellent re"suni6 of the occurrence of calcite in this class 

o'f rocks under the heading of kersantite. 1 

An analysis of the rock (Sp. No. 103) yielded the following result:

Analysis of augite-vogesite, Fourmile Creek. 

[By L. V. Pirsson.]

Ti02 . ............ ............

A1203 .-...- ..................

KO^OS-... ....................

FeO. ........................

MnO. ........................

MgO. ........................

CaO .........................

Na2 0 ........................

K2O. ........................

Li.,0....... ..................

H2 0 .........................

P2 05 . ........................

C02 .. ........................

Total..................

Sp.gr........................

Per cent.

45.15

2.80

15.39

2.76-

5.64

.14

6.38

8.83

2.C7

2.77

2.85

.56

4.27

100. 21

2.70

The carbonic acid shows the considerable amount of carbonates 

present. The large quantity of titanic oxide is also noticeable, and 

the iron ore is evidently ilmenite. It is also interesting to note that 

the ratio of potash to soda remains the same as in the acid rocks 

which the dike accompanies.

iLehrb. Petrog., Vol. II, 1894, p. 516.
MINETTE. 113

The second phase of crystallization is quite similar, save that the 

grain is finer and the augite phenocrysts are less numerous.

The narrow selvage band at the contact shows more of interest, 

however. Immediately at the contact it is dense and black and does 

not transmit light, being opaque even in thin section. It is, without 

doubt, a basic glass charged with ferritic globulites. It is crowded Avith 

augite phenocrysts, almost entirely altered to masses of serpentine and 

calcite. From this stage, as one passes inward, the degree of crystallization-becomes rapidly greater, light passes through the section, and 

the rock is here a mass of minute inicroli'tes which are so small and 

densely packed that only the ore grains can be told with certainty. 

Then occasional lath-like feldspars appear among the microlites; they 

become larger and more numerous, the grains of the ot*her constituents 

increase in size and their distances from each other become greater, 

and the rock assumes a structure and appearance not unlike many 

dense basalts, save for the greater- abundance of the ferromagnesian 

elements. Up to this point nothing which can safely be referred to 

hornblende appears in the rock; the grains seem, to be entirely 

augite and iron ore. The variations in crystallization which have been 

described occupy not more than half an inch in distance. The rock 

then passes more gradually into the types previously described. The 

quartz-porphyry does not appear to have been changed by the intrusion of the dike; its structure is micropoikilitic.

The existence of this dense saalband is of value in demonstrating 

that the magma forming the dike was injected into rocks that had 

already become comparatively cold, and when we consider that it is 

upon the immediate edge of the great central stock which must have 

been radiating heat in enormous quantities through a long period of 

time, it is evident that the intrusion of this lamprophyre dike was long 

subsequent to the acid intrusions it cuts.

A basic intrusive sheet which appears on the spur above, between 

the headwaters of Fourmile and Checkerboard creeks, has in many 

ways a close analogy to this dike, and resembles that stage where it 

is mentioned as appearing like a basalt. It is, however, so greatly 

altered, with the dark lime, iron, and magnesia components changed 

to epidote, serpentine, and other products, that no safe conclusions can 

be drawn regarding it.

" MINETTE.

The small mass which has been intruded into the Cambrian strata 

where Fourmile Creek empties into the North Fork of Smith Eiver is 

composed of a rock which, in the present generally accepted system 

of classification, should be termed a uiinette, in that it is composed 

essentially of biotite and orthoclase with phenocrysts of biotite and 

none of feldspar. It contains accessory hornblende, and thus shows a 

tendency to vogesite.

To the eye the rock is dense, of a somewhat greasy appearance, and 

Bull. 139 8
114 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

a ratlier dark gray in color. Through this groundmass are scattered 

numerous black mica phenocrysts of rather small size. The microscope 

shows the rock to be panidiomorphic in structure, and composed of 

orthoclase, biotite, iron ore, and apatite, with phenocrysts of hornblende 

and biotite. The biotite is, as usual, strongly pleochroic, and, like all 

the biotites of Castle Mountain, is practically uniaxial, at least no 

opening of the cross can be observed. The hornblende is peculiar. It 

is present in rather slender pale-green prisms, and is also zonally built, 

the outer shell being of a much paler tint than the interior. Its angle 

of extinction is small, its double refraction weak. It is, indeed, the 

same hornblende as that found in the typical vogesites from the Odenwald and Vosges, which have become classic through the researches of 

that great master of petrography, Eosenbusch. In the order of crystallization the biotite commenced, since it is inclosed at times in the 

hornblende. As the rock as a whole is extremely similar to well-known 

types of minette, it needs no further co.niinent on its mineral association. It grows richer in feldspar toward the center, and shows a tend- ~ 

ency toward the quartz-free porphyry or trachytes heretofore mentioned. The feldspar does not, however, occur in distinct phenocrysts.

At the contact the rock is a greenish-brown glass tilled with hornblende, and more rarely mica phenocrysts. The glass is dusted through 

with globulitic material. A large broken hornblende in the section 

shows clearly that this mineral was present in the viscous mass when it 

was forced into its present place.

The limestone at the contact has been changed into saccharoidal 

marble. ^

MONCHIQUITE.

This name has been given by Rosenbusch 1 to a variety of lamprophyric rocks which consist of olivine, pyroxene, and biotite or hornblende in a colorless glass base. In this country they have been chiefly 

investigated by Kemp, 2 in whose excellent papers will be found a summary of their history and a complete bibliography.

The dike which occurs in the Belt shales on the left bank of Willow 

Creek is composed of a rock which is most nearly related to the monchiquites. The outcrop of the dike has been prospected for ore, and some 

quite fresh material has been thrown out of the trench. The rock is 

black and basaltic in appearance and contains abundant grains of clear 

yellow olivine as phenocrysts. In thin section it appears at first glance 

to" be composed almost wholly of augite, so abundant and so thickly 

crowded are the crystals. Olivine, biotite, iron ore, and apatite are, 

however, also present in essential quantities, and. are lying in a colorless 

base dusted through with brownish globulitic material. The augite is 

zonally built; the larger individuals are often greenish in the center 

and surrounded by a purplish-brown mantle, which has a lesser angle

' Min. inicl Petr. Mitt., XI, 1890, p. 445.

2 Arkansas Geol. Surv., II Ann. Eept., 1890, Vol. II., Ign. Rocks, p. 394; Bull. TJ. B. Geol. Survey 

Ko. 107| Trap Dikes, etc., 1893, p, 32; also other papers mentioned above.
WEED AND~1 

PIHSSON. J MONCHIQUITE. 115

of extinction and a great dispersion of the optic axes. The smallest 

augites are similar to this mantle. One might by analogy infer that 

these and the outer shell of the large ones contained titanic acid.

The olivine is in general quite fresh, sometimes more or less altered 

to serpentine. In a few cases the augites have also been-altered to 

chlorite and calcite. The biotite is of the usual red-brown A^ariety. An 

inclined extinction was not observed in .it. The base in which these 

minerals lie is small in amount and filled with microlites of the larger 

crystals. In places it is wholly isotropic; in other spots it shows a 

feeble aggregate polarization. No outlines that could be referred to 

nephelite appear in such spots, and it is probably due to zeolitic alteration. Nothing that would suggest leucite, either in outline or arrangement of inclusions, could be found. It looks like a glass partially 

altered.

An. analysis of the rock gave the following result:

Analysis of monchiquite from Willoiv Creek. 

[By L. V. Pirsson.]

Si02 .........................

TiOg.... .....................

A12 03 ...... ..................

Fe203 ... .....................

FeO .........................

MnO. ........................

Mg6...... ...................

CaO .........................

Na 20 ........................

K2 0 ......................;..

LijO...... ...................

C02 -.:. ......................

Po06 ...... ...................

:EL,0 .........................

Total..!...............

Sp.gr........................

Per cent.

42.46

2

/L7

32.04

9 1Q

5.34

.16

12.40

19 14

1 91

2.68

.55

.84

4.03

2.94

From the large amount of titanic acid it is evident that the iron ore 

must be ilmenite. The water must be derived, in part at least, from 

the partially zeolitic base, for although in some cases the olivines are 

changed to serpentine and the augites are chloritized, on the whole 

there are certainly not enough decomposition products present to account for so much water. As a matter of fact, the rock is very fresh 

for one of this class.

The relation of the magnesia to the lime is exactly what should be 

expected from the great preponderance-of augite.

A consideration of the analysis, especially if the ratios of the oxides 

be taken as shown on page 136, indicates that if the residual base had 

crystallized it would probably have formed some feldspar. The amount 

of alumina is almost double that of the alkalies, and some may there-
116 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

fore be reasonably expected to be present in the augite. The biotite 

also would demand some. In all these cases, however, the alumina 

demands an equivalent of alkalies, and it seems quite possible, therefore, that, had the crystallization been complete, melilite \vould' have 

been formed.

On treatment with acids the rock powder gelatinizes. On account of 

the olivine and the small amount of the isotropic base, the authors are 

not able to say positively that the latter is soluble in acids, but from the 

amount of gelatinization they believe that it is.

A comparison of this rock, and especially of its analysis, with the rocks 

described by Eosenbusch and Kemp shows that in mineral composition 

it approaches most nearly the rocks described by the latter. Professor 

Kemp has kindly examined a section of it, and states that it resembles 

most remarkably many of those described by him. In chemical composition, however, it approaches most nearly the rock from Eio do Ouro, 

Serra de Tingua, of which an analysis is given by Eosenbusch. It differs 

from it in being somewhat higher in magnesia and lower in soda.

On comparing it with the analyses given by Kemp it became apparent that there were serious discrepancies between the petrographical 

description of the rocks given by him and the accompanying analyses. 

On communicating with Professor Kemp regarding this point, the authors received in reply the'folio wing note, which he desired them to 

insert at this point:

In Bulletin 107 of the United States Geological Survey on the Trap Dikes of the 

Lake Chaniplain Region the analysis of the monchiquite dike (No. 14) contains elements of improbability, and it was felt necessary to. have the analysis repeated. This 

has been kindly done with great care by Dr. H. T. Vult6, instructor in chemistry in 

Columbia College, with the results given below. These differ in important essentials from the results of the earlier analysis, and show the rock to be provided with 

the normal amount of magnesia, thus conforming to the general type. This latter 

analysis should therefore replace the earlier one, and the one given of Dike No. 2 

(likewise on page 34) by the same chemist is to be considered of very questionable

accuracy.

Analysis of monchiquite from Shellurne Point, Vermont.

Si02 ...... ...................

Ti02 ...... ...................

ALj03 ...... ..................

~fi\\ n

FeO

MnO...... ...................

CaO .........................

MgO.... .....................

K,0 .........................

Total..................

Per cent.

45.58

Not (let.

15.87

4.65

6.37

9.91

8.32

3.42

1.61

3.14

98.87

As observed by Professor Kemp, this new analysis agrees fairly well 

in its important details with those given by Eosenbusch. From this
WEED AND~1 

PIHSSON. J DIABASE. 117

point of view it is difficult to understand why Zirkel 1 objects to rocks 

being placed in this class when they have the same mineral composition 

and structure, if they do not possess the same exact chemical composition, as the types described by Eosenbusch. A very great variation in 

composition in important particulars would of course throw any rock 

out of a given class, but a certain latitude is necessary and must be 

allowed in any system of classification. Under the miuettes, ZirkeP 

presents as typical examples rocks which vary in percentage composition as follows, taken in round numbers:

Si02 .......................

A12 03 - .».. ................

Fo2 03 .---. .................

FeO .......................

MgO......................

CaO....... .................

N.^0 ......................

K,0........................

Per cent.

45 to 57

11 to 16

0 to 8J

5& to 10£

5J to 12

£to 3

3 to 10

If such large variations as these are not sufficient to cause rocks showing them to be placed in separate classes, it does not seem to the writers 

logical to object to rocks being placed together whose variation is much 

less and which have the same structure and mineral composition.

The petrological significance of this 'rock in its present occurrence 

will be discussed in the following chapter.

' DIABASE.

The two occurrences of this rock have been mentioned in the preceding chapter. In the hand specimen it appears quite fresh on fractured 

surfaces, but in general the sheets have been much jointed and on such 

surfaces have weathered to a rusty brown. The rock is so coarse 

grained that the ophitic arrangement of the feldspars may be seen with 

the naked eye.

Under the microscope the rock appears as a quite fresh typical diabase in all respects. No oliviue is present. In the occurrence at the 

head of Warm Spring Creek, where the rock is rather Coarse grained, 

micropegmatitic inlergrowths of quartz and orthoclase are common in 

the angular interspaces between the feldspars. The rock is thus precisely similar to the occurrence on Smith "River (Deep Creek) described 

by Lindgren.3 The descriptions, indeed, of the diabases observed by 

Lindgren in this region apply so well to the occurrence at Castle 

Mountain that no further comment is necessary. Diabases containing 

micropegmatitic intergrowths of quartz and feldspar have since been 

described'by G. H. Williams.4

1 Lehrb. dor Petrog., Vol. Ill, zweite Aufl., 1894, p. 6.

"Idem, Vol. II, p. 349.

3 Tenth Census, Vol. X^7", p. 7BG.

4Bull. U. S. Geol. Survey No. 02, Greenstone Schista, etc., 1890, p. 141.
CHAP TEE VII.

MICROSCOPICAL PETROGRAPHY OF THE EXTRUSIVE 

ROCKS, OR LAVA FLOWS AND BRECCIAS.

The lava flows are confined to rhyolites and basalts, and the field evidence indicates that the former are the older. The mode of occurrence 

of the rhyolite, and facts connected with its micro-structure, indicate 

that it was ejected in an extremely thick, viscous condition, while the 

basalt, on the contrary, was quite fluid and mobile facts in accord 

with the observations of other geologists. The details in regard to 

the geologic occurrence of these rock masses have been given in a 

preceding chapter.

RHYOLITE.

In outward appearance this rock shows considerable variation. Not 

only do the various occurrences differ from one another, but even in the 

same one there will be found at times a variation from place to place. 

Thus, in ascending the great hill of rhyolite which lies on the righthand side of Fourinile Creek, and around which it curves,, one finds 

on the western slope a talus gf dark-yellowish to reddish-brown angular fragments. These show most characteristic lines of flow motion 

wavy, twisted, and sinuous brought out best on a weathered surface by a difference in color depending on the iron oxide in the rock. 

Toward the summit the character gradually changes, the rock becomes 

lighter, more dense, and breaks with a broad conchoidal fracture. The 

outcrop at the top has a nearly white color. Down the slope, in the 

direction of Fiveinile Creek, the character changes again, the phenocrysts of feldspar become more numerous, and the rock has a perceptibly granular appearance, which indicates that it is crystalline. At 

times the rock forming the debris heaps which represent the brokendown outcrops are more crystalline, appear richer in feldspar phenocrysts, and more basic than the rhyolite. These .rocks may occur as 

dikes, but are really believed to be drawn out, included masses, such 

as occur in the granite.

All of these changes are accompanied by corresponding variations 

in structure and crystallization, as brought out by study with the 

microscope. Again, in the glacial drift on the lower slopes of the hills, 

varieties of the rhyolite occur which, although the proof is positive 

that they came from the Castle Mountain effusions, are not exactly 

similar to any of the masses now in place. It is often surprising to see 

118
LOWER FOURMILE RHYOLITE. 119

how fresh the material in these glacial pebbles is, while the outcrops 

from.which they came (which can in a few cases be identified) have 

been deeply altered by the elements.

In structure there are all gradations from such rocks as those mentioned above to a pitchstone glass, and in crystallization from holocrystalline through microspherulitic forms to amorphous glass. The 

more coarsely spherulitic or puiniceous forms which constitute so 

prominent a feature of the great rhyolite masses of the Yellowstone 

National Park to the southward have not, however, been found in this 

district. If they formerly existed, as they might well have, they have 

been swept away by erosion and glaciation.

As previously stated, there are three principal occurrences of rhyolite in the district besides the one just mentioned: the masses near the 

mouth of Fourinile Creek, the glassy flows occurring at the forks .of 

Checkerboard Creek, and a great flow on upper Bonanza Creek resting

on the Cretaceous strata.

LOWER FOURMILE RHYOLITE.

The mass which occurs on the right side of Fourmile Creek, forming 

a hill near the crossing of the road to Martinsdale and about two miles 

southeast of where Fourmile empties into Smith Eiver, presents a rock 

of a pale-gray color on a fresh surface. It is extremely dense in texture, breaking with a conchoidal fracture. Small, rather sparsely scattered quartz and feldspar phenocrysts are to be seen, and occasional 

minute black dots.

Under the microscope it is found to consist of quartz, feldspar, tourmaline, and a little iron ore and zircon. The feldspar phenocrysts are 

fresh and unstriated. They appear to have exerted an influence on the 

crystallization of the groundmass, in that they are surrounded by a 

narrow but distinct aureole of quartz and feldspar granules, in which 

the latter are oriented like the phenocryst. The quartz is similar and 

of the usual type found in such rocks. The tourmaline occurs in small, 

formless shreds and patches, rather regularly scattered through the 

groundmass, but very insignificant in total bulk. It is of an olivebrown color and strongly pleochroic. That it is not hornblende or 

mica is shown by its lack of cleavage and its optical properties. The 

groundmass is an allotriomorphic mixture of quartz and unstriated 

feldspar in small, formless, interlocking grains between the larger 

ones, of which the more minute granules are collected in small micropoikilitic structures. This is quite similar to the Musselshell porphyry 

described above. The iron ..ore and zircon are rare. The tourmaline 

and occasional patches of ferritic material are believed to represent 

the former presence of biotite, which was destroyed during the second 

period of consolidation by mineralizing vapors given out on cooling. 

It is believed to have been biotite rather than hornblende from the 

fact that the former is the common ferromaguesian component in the
120 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

majority of the acid rocks of the district. An analysis of the rock (No. 

237) gave the following result:

Analysis of rhyolite from Fourmile Creek. 

[By L. V. Pirsson.l

Si02 .........................

Tifi

A\^.... ....................

r^o3 ....... .................

FeO. ........... ..............

MnO. ........................

MgO...... ...................

CaO...... ....................

Na-iO. ...... ..................

KaO. ...... ...................

 Li2 0 .........................

H2 0...... ....................

Sp.gr..................

Per cent.

74.90

.15

13.04

.66

.50

.61

4.22

4. 04

Trace.

.33

99.05

2.61

This analysis is almost identical with that of the quartz-tourmaliueporphyry previously described; it would, indeed, make an excellent 

duplicate of it. It is also closely similar to that of the normal variety 

of quartz-porphyry given on page 99. It shows the same even relations of soda to potash exhibited by the 'majority of the acid rocks of 

Castle Mountain, and is, therefore, a contemporaneous phase of the 

eruptive magma. The prevailing feldspar must be a soda-orthoclase, 

since no striated feldspars have been observed. In structure, minerals, 

texture, degree of crystallization, and chemical composition this rock 

is precisely identical with the common type of quartz-porphyries of the

district, and in a natural system of classification would be called such, 

though it has been found convenient to describe it here under the head 

of the effusive rhyolites.

The rock composing the small masses which rest as caps on the limestone forming the north side of the canyon of Smith River below the 

mouth of Fourmile Creek, is in general quite similar to the above, 

though lacking in phenocrysts. It also contains tourmaline. The outcrops are noticeable on account of their white color.

About a mile north of the main mass mentioned above, and near the 

mouth of Fivemile Creek, there is a small cap of rhyolite on the point 

of the hill. It is in connection with basaltic breccias at this point, as 

mentioned on page 73. The rock is much brecciated, and under the 

microscope is seen to consist of a glass, colored reddish-brown by a 

large amount of ferritic material distributed through it. It is much 

cracked and the cracks are filled with secondary calcite deposits. The 

glass shows a considerable amount of feldspar microlites dispersed
FOURMILE RHYOLITE. 121

through it, which polarize very feebly. Other areas show the feeblest 

possible aggregate polarization, and may be portions where devitrification is going on. Collections of opacite material may represent some 

former ferroinagnesian mineral. JSTo crystals which could be called 

phenocrysts were observed.

FOURMILE RHYOLITE.

The rhyolite on the right side of the canyon of Fourmile Creek, 

which forms the high hill previously mentioned, varies in the different 

occurrences. The talus slope and outcrops furnish a lithoidal rhyolite, 

with lines of flow structure which appear under the microscope very 

similar to many typical and well-known occurrences of parallel rocks, 

such as the felsophyre from Elfdalen in Sweden, or that from Tharandt 

in Saxony. With low powers, in ordinary light, the section has a 

structure recalling-many woods with knotted, wavy, sinuous grain, 

used for ornamental purposes. This appearance is caused by alternating bands of material with different colors and structures. One set 

appears reddish, being highly charged with ferritic material, while the 

other is grayish, from collection of the ferritic material through crystallizing processes into excessively minute ore grains. 'In their interrelations these bands display great variety; they knot and wind and 

twisfraround and pass one another; at times they lie quite parallel in 

long strings, and at other times they bend up abruptly or break off' in 

cuspidate forms. Around the phenocrysts, which are quartz and uhstriated feldspar, they always bulge out, as wood fibers pass around a knot. 

The phenocrysts are nearly always broken and cracked; often, indeed, 

only detached angular fragments are seen, and at times a quartz which 

has been completely cracked up is distributed in a heap of chips along 

the plane of flowage, appearing somewhat like a mass of secondary 

quartz filling a cavity. The whole phenomenon, in short, is that caused 

by an excessively tough, viscid mass pulling and stringing along with 

great force, caused by its own enormous weight. With higher powers, in 

polarized light, it is seen that the lighter-colored gray bands are finely 

crystalline, a mixture of quartz and feldspar in allotriomorphic structure. Darker-gray bands give only a cryptocrystalline polarization, 

while the yellowish bands are apparently the much-discussed microfelsite; that is, while they do not act on polarized light by the highest 

powers, they are granular, composed of scales, shreds, fibers, etc., of a 

brownish color. The writers are inclined to believe that, in the present 

case at least, they represent collections of globulitic ierrite material 

in the glass, and that in the grayish" bands, where the glass itself has 

actually partially crystallized into minute particles of quartz and 

feldspar, this has further agglomerated the ferrite particles into minute 

grains of iron ore.

At other times the grayish bands are composed of rows and masses 

of microspherulites with negative interference cross; or again they are
122 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

formed of fibers not arranged in spherulites, but in parallel growths, 

arranged perpendicularly to the length of the band i. e., to the plane 

of flowage or in fan-shaped form.s. In these cases there is always a 

double set in each band, and often between them are small grains of 

quartz, or they are separated by a layer of crystalline granular quartz 

and feldspar. The fibers are generally negative in their optical orientation, and in this case maybe safely referred to feldspar; but occasionally they are positive, and may be either quartz or feldspar, as 

shown by Cross' and Iddings.2

Toward, the top of the hill the dense rock encountered in the outcrops 

does not possess the banded structure just described; nevertheless, in 

thin section the lines of flowage are quite as plainly shown by the 

granules of iron ore, which represent in ordinary light a sort of 

dotted drawing of these flow structures. In polarized light the 

same microspherulitic layers are found, but the material which separates them is no longer glassy, polarizing with low powers with a 

uniform gray tone, which, with the highest objectives, is found to.be 

due to a minute cryptocrystalline aggregate. It resembles the sections of slates formed by the deposition of excessively fine silt. It has 

all the appearance of the devitrification of a glassy base.

DEVITRIFICATION.

At the present day many petrographers are inclined to attribute to 

devitrification of an originally glassy base all those fine-grained groundmasses in rhyolites which consist of an allotriomorphic mixture of 

quartz and feldspar, and especially those which show.also spherulitic 

and other structures which form in rapidly cooling igneous magmas. 

This devitrification is supposed to be a secondary process, generally 

due to weathering, and to be distinguished from original crystallization. t

In many cases it is not always clear in exactly what sense the word 

"secondary" is used. Of course, strictly speaking, there is no moment 

when a rock remains absolutely passive, unacted upon, unchanging. 

From the moment that crystallization sets in until, decayed, the rock is 

turned into soil, processes of various kinds are acting upon it; and in 

this sense there is no time boundary between processes original and 

processes secondary. Still, practically, we may say that original rockmaking processes cease when the molecules lose the motion derived 

from the heated mass and come to rest either as a stiffened glass or in 

crystal symmetry.

The question of solidity must also be considered. Of course, when 

the molecules of a body fall into symmetrical, crystal position, it then 

becomes solid and generally rigid. This is not the case with acid 

glasses, however, for wi'th them no exact line dividing the liquid from

1 Bull. Philoa. Soc. "Washington, Vol. XI, 1891, pp. 426-427. 

2 Ibid., p. 457.
--_ -!°] DEVITRIFICATION. 123

fluid state can be drawn, and rigidity or capability of movement seems 

to be, in such cooling material, largely a function of time and mass.

Again, spherulites are undoubtedly bodies which form very rapidly 

when the conditions of the cooling fluid are right for their formation, 

and this, as remarked by Cross, 1 seems to depend upon a certain 

viscosity of the medium. Anyone who has studied the phenomena of 

crystal growth in solutions will recall how rapidly crystallization takes 

place in those which are heavily saturated, when the temperature 

reaches the proper stage.

There is in the writers' possession a mass of glass, spherulite-filled, 

which resulted from the accidental breakage of a Pennsylvania glass 

furnace a few years ago. Similar specimens from Staffordshire, England, are seen in the museums. The interesting feature with .respect to 

this specimen is that the spherulites, which consist of an undetermined white mineral and vary in size from small shot to a musket ball, 

are drawn out into lines of flowage, sometimes, indeed, deformed by 

its dragging effect. The glass thus consists of nonhomogeneous layers, 

some pure glass, others glassy and filled with spherulites, and thus 

presents a perfect analogy to the glassy rhyolites. But we know that 

these spherulites were not present in the molten mass in the furnace 

and that they formed before flowage of the material ceased, and therefore their production must have been a relatively rapid one.

But if there is enough fluidity left for flowage after spherulites have 

formed in acid glasses, there is also enough left for crystallization. 

Moreover, the force of chemical affinity, acting within limited distances, 

is the most powerful one with which we are acquainted. It is easy to 

imagine that a resultant crystallization might convert the remaining 

glassy base, still hot and enormously viscid, into a macrocrystalline 

allotriomorphic mixture of quartz and feldspar.

Thus we might have the formation of spherulites and other forms 

of rapid growth that occur in rhyolites, and then, as the mass became 

too viscid for their formation, a further fine-grained crystallization, all 

as the result of continuous processes in nowise to be regarded as 

secondary.

Though they can not definitely prove the fact, the authors believe 

this to have been the case at Castle Mountain in occurrences of the 

rhyolites under discussion. It is in nowise intended by this to deny the 

fact that glasses become devitrified by secondary processes of weathering, etc.; we have, indeed, positive evidence, in tbe devitrification of 

artificial glass, that it does occur by such means. It is intended only 

to point out that the occurrence of spherulites, etc., in rhyolites with 

fine-grained holocrystalliue bases does not necessarily imply devitrification by weathering, as seems generally accepted at present.

At the summit of the rhyolite-capped hill at the lower forks of Fourmile Creek the rock becomes more coarsely crystalline, passing, in fact,

1 Bull. Philos. Soc. Washington, Vol. XI, p. 433, June, 1891.
124 THE CASTLE MOUNTAIN MINING DISTRICT. [DULL. 139.

into a quartz-porphyry with a microgranitic groundrnass. In the 

direction of Fivemile Creek the rock heaps at times consist of material 

very similar to the dikes of felsite and quartz-free porphyries already 

mentioned. Quartz is found only in the groundmass, and then ofteu 

sparingly. The orthoclase phenocrysts are larger and much more numerous than in the rhyolite just described; they contain intergrown 

albite lamelke, ,80 frequently seen in the feldspars of eleolite-syenite. 

Biotite was formerly present, but is nearly always altered to ferruginous 

products. Such variations bear a close analogy to the felsite dikes 

and to the included masses in the granite. The transition from the 

rhyolite was found in one case to be gradual and not abrupt, as might 

be expected from the presence of a dike, and they are regarded as partly 

differentiated masses occurring in the rhyolite, though the outcrops 

are not sufficient to decide the question definitely.

RHYOLITE OF CHECKERBOARD CREEK.

The geologic occurrence and general appearance of the masses of 

glassy rhyolite through which the forks of Checkerboard Creek have cut 

their way have been described on page 71. A typical specimen of this 

rock shows a dense semivitreous-lookiug groundmass, of a brownishgray color, embedded in which' are irregular-shaped fragments of a 

black, glassy pitchstone resembling anthracite coal. These fragments 

are of all sizes, from little pieces like fine grains of sand, which thickly 

crowd the gray-brown mass, to chunks several inches long. Besides 

these there are many small fragments of reddened Belt shales aud of 

gray limestone. All these materials are arranged so,that they indicate 

the lines of flowage, though this is not so well perceived on a hand 

specimen as on the rock face of the outcrops. The larger fragments 

of black pitchstone have a pronounced lamination along- the flo\?age 

planes, and this is shared to a lesser degree by the entire rock mass. 

The rock is also much cracked and very brittle, making it difficult to 

procure good specimens.

Under the microscope the black fragments of pitchstone, which make 

up a quarter to a third or even more of the rock, are found to consist 

of a clear, colorless glass, peppered tbrough with minute inicrolitic 

grains of iron ore. At times these reveal a flow structure in' the glass. 

Generally these glass fragments are sharply bounded against the mass 

in which they lie, but in a few instances the boundary is indefinite and 

a partial remelting took place. The evidence gained from a study of 

these fragments shows that though they were too viscous aud cold to 

be remelted, they still possessed plasticity enough to be deformed in 

their shapes by the dragging motion of the flowage, and also to become 

laminated.
WEED AND"! 

PIRSSON. J

RHYOLITE OF CHECKERBOARD CREEK. 125

An analysis of a good clear piece of this glass gave the result shown 

by the following figures:

Analysis of rhyalite from Checkerboard Creek. 

[By L. V. Pirssoii.]

Si02 ..........................

Ti02 ...... ...................

A12 03 .~... ..................

FojO,. .......................

FeO .........................

MnO... ......................

MgO.... .....................

CaO .........................

¥a20 ........................

K20...- ......................

Li20...... ...................

H2O .........................

Sp.gr........................

Per cent.

72.56

.20

12.33

.80

.82

5.36

3.68

4.59

99.74

2.37

That no minerals except iron ore were observed in this pitchstone is 

due in part to the almost total lack of lime and magnesia. The preponderance of soda over potash is a marked feature of the analysis; 

in this respect this rock stands alone among the acid effusives and 

iutrusives of the district. Disregarding the water and comparing the 

silica, alumina, and total alkalies with those of the other acid rocks, 

the similarity is seen to be marked and shows the common genesis of 

the types.

The grayish-brown material in which these fragments lie does not 

polarize, neither does it appear definitely glassy. With the highest 

powers of the microscope there appear myriads of tiny globulites, which 

are believed to be iron ore, and these and the decomposition products 

resulting in places from their hydration, mingled with a glassy base, 

are believed to be the composition of this material. It shows pronounced flow structures, and contains green hornblende, plagioclase, 

unstriated feldspar, and quartz as rather infrequent phenocrysts, all of 

which are more or less cracked and appear at times in angular fragments.

The inclusions of limestone are generally but little altered. There 

has been little or no absorption of their material by the magma and no 

new minerals have been produced in them. They are generally slightly 

marmorized. The fragments of shale also appear little altered. More 

interesting are a few inclusions of holocrystalline igneous rocks that 

appear to have been torn off and taken up during the upward movement of the magma. One of these shows a mica-trachyte.inclining to 

andesite, lath-shaped feldspars in trachytic structure, a considerable 

proportion of which are plagioclase. Mixed with these are idioinorphic
126 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

biotites and some iron ore. Several fragments of this rock were seen, 

but always very small, about 1 millimeter across. Other inclusions are 

those of a quartz-porphyry. All these fragments are angular and 

clearly enallogenetic. 1

No analysis was attempted of the grayish-brown base or of the rock 

as a whole, owing to the impossibility of obtaining even moderately 

homogeneous material. The analysis of the clear pitchstoue glass 

given above must, however, represent the rock as a whole, since it was 

clearly a portion of the flow that, stiffening first, was cracked up, rolled 

under, and kneaded through the mass.

From the foregoing description it will be seen that these rock masses 

on upper Checkerboard Creek present the remains of a large lava flow, 

the material composing which is a very fresh and typical rhyolitic flow 

breccia.

BHYOLITE OF UPPER BONANZA CREEK.

The large flow, through the eastern portion of wliich Bonanza Creek 

has cut a deep drainage channel into the upturned Cretaceous qnartzites on which it rests, presents a rock which megascopically differs in 

habit from the rocks previously described.

Atypical specimen shows an ocher-yellow to brown-colored, dense, 

crackly groundinass, in which lie imbedded numerous white feldspar 

phenocrysts averaging 2 to 3 millimeters in diameter, with somewhat 

less numerous ones of quartz. The base is more or less altered, and at 

times some what'earthy. Its yellowish color and numerous phenocrysts 

are its chief characteristics. Fragments collected from the glacial 

drift on lower Bonanza Creek are very fresh, however, and do not differ 

materially, except in a darker gray color, from specimens obtained from 

other parts of the district.

Under the microscope the sections appear like varieties previously 

described. The groundinass is always very flue granular crystalline, 

sometimes tending toward a micropoikilitic structure, sometimes microgranitoid. The phenocrysts are orthoclase in large crystals, and there 

is a lesser amount of an acid plagioclase, always quite idiomorphic, and 

quartz which has all the characteristics that obtain in such rocks. 

The ferromagnesian mineral is a, brown biotite, generally greatly 

altered to ferruginous decomposition products, sometimes to white 

mica, with separation of iron ore. The grouudmass is more or less 

stained with limonite, which gives the rock its yellowish color.

In the denser material from the glacial drift, which "was evidently 

swept from higher levels in the mass, the groundmass shows a cryptocrystalline polarization and appears like a devitrified glass. The micas, 

which are altered .to muscovite and iron ore, and the other phenocrysts 

are arranged in lines which show flow structure.

'Lacroix: Les enclaves ties roches volcaniques: Ann. Acad. de Macon, Vol. X, 1893, p. 8.
BEECCIAS AND TUFFS. 127

The microscopical study of the sections shows so close an agreement 

with the other rhyolites of the district that no further remarks are 

necessary.

BRECCIAS AND TUFFS.

The breccias, which occur so frequently on the foothills drained xvby 

the head waters of Checkerboard and Fiveinile creeks, are almost uniformly of one type. They consist of fragments and angular masses of 

all sizes, of various limestones, Belt shales, and, less commonly, igneous rocks held together by a light-yellowish to grayish cement, which 

appears greatly kaolinized and is somewhat cavernous from the decay 

of former inclusions. The proportion of inclusions to cement is very 

large. Sometimes the fragments are several inches long, but in the 

main they are comminuted Belt shales, which pepper the mass through 

and through.

The microscopical examination of such material affords little that is

conclusive. The inclusions seem to be little altered. In the.cement 

are found broken quartz crystals, fragments of feldspar, and occasional 

hornblendes or decayed micas. The cementing material appears greatly 

altere'd and seems to be silicified as well. Stringers and veins of quartz 

run through and along planes which may represent bedding planes of 

the ash or lamination planes of an original glassy fragmentf Much 

secondary silica is deposited in fibers, and these seem to be of chalcedony. Much of the material is extremely fine and indeterminable.

From what has been already stated concerning the occurrence of a 

flow breccia, it is evident that cases may arise in which, if the material composing the cement is as greatly altered as in the present case, 

it may be doubtful whether one has to deal with an originally explosive 

air breccia or with a flow breccia.

The material composing the breccia deposits on Fourinile Creek and 

on which the upper rhyolite hill rests is much better preserved. There 

are the same sedimentary fragments, but the cement joining them, 

which has a grayish color, is full of lapilli,-shown by their shape, etc. 

It contains numerous fragments of quartz-porphyry and other igneous 

rocks, which are, however, as a rule very small in size. Broken crystals 

of quartz, feldspar, hornblende, and mica leaves are scattered through 

the mass. The lapilli were many of them once glassy and show flow 

structures, which end abruptly against the adjoining portions.

RHYOLITIC TUFFS.

The great quantity of this material and the difficulty of properly 

delimiting it have been mentioned in Chapter V. The best exposure, 

which is illustrated in PI. XI, is composed of a brownish, somewhat 

friable material, of nearly the consistency of chalk and as easily cut 

with the knife. Close examination will enable one to detect an occasional crystalline face or cleavage of some mineral fragment in this soft 

brown background.
128 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

Under the microscope the tuff is seen to be more or less decomposed. 

It consists of a brownish material which does not act upon polarized 

light, scattered through which are fragments of pumiceous glass. 

These are clear and transparent, of the well-known lune and cuspidate 

shaped forms, and at times consisting of short sections of strings-pulled 

out by the viscous material, similar to the well-known Pele's hair. 

When examined with high powers these short pieces are often seen to 

contain a core not homogeneous with the rest, which has also been 

pulled out. They then appear at times strangely vermicular. Such 

pieces occnr much more rarely than the well:known cuspidate forms. 

Numerous minute, broken fragments of quartz, plagioclase, sanidine, 

and green hornblende occur.

Occasionally there occur also brownish fragments, which are undoubtedly those of excessively comminuted Belt shales. The result of an 

analysis of this tuff is shown in the following table, column A:

Analysis of tuff from Checkerboard Creek.

[By L. V. Pirsson.]

sio2 ..........-.:...........

TiO2 ....... ................

A1A............ ..........

T?« O

~I?AO

MiiO- ...... ................

MgO....... ................

CaO .......................

INa-jO ......................

K2O .......................

Li20- ......................

H20 .......................

A.

Per cent. 

61.21

.56

15.07

4.06

fiO

.10

1 KQ

2.18

1 £.1

2

7K

10. 20

100. 50

B.

Per cent. 

68.28

17.48

4.53

69

1.76

2.43

1.76

3.07

100.-00

The large amount of water is especially noticeable, and shows that 

considerably more than could possibly be in the glass must be present, 

and undoubtedly much kaolin is also present. If we disregard the 

water and reduce the main oxides to 100 per cent we obtain the results 

given in column B. This does not show a composition exactly like that 

of the acid rocks previously described; but, on the other hand, the figures are such as should be expected when it is recalled that a considerable portion of alkalies, especially soda, would be lost by processes of 

kaolinization, which would cause the water to rise correspondingly; 

and the high alumina may also be partly explained by this and by the 

admixture of Belt shales. So far, then, as the chemical composition is 

concerned, we are warranted in referring these tuff deposits to the 

rhyolitic outbursts, with which, indeed, their geologic position and mode 

of occurrence most clearly agree,
MICROSCOPICAL PETROGRAPHY. 129 

BASALTS.

These rocks possess the usual dense black appearance so commonly a 

feature of basalts wherever found. The tops of the flows are more or 

less vesicular, grading into compact material as one descends over the 

outcrops toward the bottom of the flows, and a progressive increase in 

crystallization and granularity accompanies the change.

They are all, however, fine-grained rocks in which phenocrysts of 

olivine are common, while those of augite are rare.. Under the microscope a typical and fresh specimen taken from the lava flow of Volcano 

Butte, near the mouth of Fivemile Creek, shows a structure consisting 

of lath-shaped plagioclase mixed with granular pyroxene. The broad 

laths of feldspar with the augite filled into the interspaces suggest at 

times an approach to ophitic structure of diabases, though it is never 

exactly attained. Olivine in good-sized, idiomorphic crystals, solely 

of the first generation, appears freely scattered through this. It is generally very fresh, though at times a little serpentinized along cleavage 

cracks.

The augite, of the usual basaltic type, appears in small grains, but 

never in large phenocrysts. The feldspar is fresh, and its extinction 

angle, rising to 35°, shows it to be labradorite. Iron ore and apatite 

are also present. Occasional small patches in the angular interspaces 

between the feldspars are of a brownish color and are filled with globulitic material. They represent partially devitrified glass. Occasionally 

other interspaces are filled with a mineral which is colorless and transparent, of extremely low single refraction, and of a double refraction so 

low that it appears at times isotropic. Its double refraction is best 

studied under the sensitive tint. It does not carry any interpositions 

or included microlites, audits form is determined by the shape of the 

space in which it is found. It is therefore not leucite, and the difference between it and those spaces which are clearly filled with glass, 

together with its lack of color and general anisotropic character, show 

that it is not a glass. It may be nephelite, or a zeolite resembling analcite, but with optical anomalies. It is present in too small amount and 

too minute masses for any satisfactory decision. The rock appears to 

be very fresh, and thec amount of water shown by the analysis would 

seem to indicate the latter as the more probable of the two. It is also 

to be noted that where the ends of the feldspar laths project into this 

mineral they are not sharp and clear in outline, but appear rounded, 

corroded, with little bays eaten into them. The low percentage of 

alumina shown by the analysis seems to be the only argument against 

the presence of a high alkali-alumina silicate.

An interesting feature is the occasional appearance of sporadic quartz 

grains. Their occurrence in basalts has been so fully discussed by 

Diller, Iddings, and Andrew, l and more recently in such detail and

iladings: Am. Jour. Sci., Vol. XXXVI, 1888, p. 208; Bull. U. S. Geol. Surrey Ko. 66,1890. Diller: 

Am. Jour. Sci., Vol. XXXIII, 1887, p. 45; Bull. U. S. Geol. Survey No. 79, 1891. Audrete, Zoitaclir. 

Deutscli., geol. Gesell. 3892, p. 824.

Bull. 139 9
130 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

with such beautiful plates by Lacroix, 1 that the present example adds 

but little to what has already been observed. The grains are corroded, 

cracked, and surrounded by the usual corona of pyroxene, which is 

precisely similar in all details to the typical cases mentioned by Lacroix. 

They have been also at times entirely melted, and their former presence is indicated by the x>yroxene grouping. No inclusions, vitreous 

or gaseous, appear in the quartz, which is pure and limpid, a fact which 

seems to be contrary to the general case. In regard to the question 

of the primary origin of these sporadic quartz grains, the writers can 

add nothing to what has already been said, from any facts which this 

occurrence affords. Since the grains have been partially absorbed, however, it is evident that whether they were foreign substances caught up 

by the magma, as thought by Lacroix, or were primary secretions in the 

molten fluid, as held by Iddings, the subsequent phenomena or crystallizations around them would be similar, and these phenomena alone are 

therefore not sufficient to decide the question.

In the present case their number is too infrequent to exercise any 

influence on the chemical composition of the rock, which was found by 

analysis to be as follows:

Analysis of basalt from Volcano Butte. 

[By L. V. Pirssou.]

QiH

Ti02 ..-- .....................

X.......... ..................

A12 03 -. ......................

Fe2 0s . .......................

FeO .........................

MnO. ........................

MgO....... ..................

CnH

Na2O .......................

K20...... ....................

Li2 0. ...... ............'......

HjO ........................

P

f\

co2 ...... ....... .............

Per cent.

46.52

.73

10.48

.11

10. 58

9 49

3.12

1.55

1.79

jw

100. 37

-2.99

The symbol X refers to those earths which, obtained with titanic acid, 

are not dissolved by cold water after a bisulphate fusion. They may 

consist of tantalic and niobic acids and are naturally more or less 

impure, with traces of zirconia, titanic acid, etc. As they exercise no 

influence on the petrographical questions for which the analysis was 

made, no attempt has been made to separate and determine them.

!Les enclaves des rochea volcaniques: Ann. Acad. Macon, Vol. X, 1893, p. 17.
WEED AND"! « A C A r TQ PIRSSON. J UAbALlb.

The large amount of titanic acid points clearly to ilmenite as being the 

chief iron ore. The trace of carbonic acid is due to a slight amount of 

infiltrated calcite, coming probably from the waters of Smith Kiver, 

which drain the limestone plateau of the Little Belt range to the northward, and from those of Fivemile Creek. The phosphoric acid is, of 

course, from apatite, which is present in considerable amount. The 

low percentage of alumina and the large amounts of lime and magnesia 

are characteristic features of the above analysis, and they serve to 

explaiu the abundance of olivine and pyroxene in the rock. The low 

percentage of alumina, together with that of the silica, also indicates 

that it can not be a very feldspathic rock. Chemically it is closely 

related to the lamprophyric dike rocks whose analyses have been already 

given and which will be found compared on page 136. and in some 

respects is intermediate.between them. The writers believe it to represent the same general stage of differentiation of the Castle Mountain 

magma, and that.had it solidified in dike form it would have been a 

rock of pronounced lamprophyric character.

The pumiceous, scoriaceons material which composes so large a part 

of Yolcano Butte is too greatly altered for petrographic examination, 

but the more solid portion of the upper end of the lava streams which 

terminate here is quite similar to the above except for its much finer 

grain in the groundmass. The size of the olivine phenocrysts remains, 

however, the same as would naturally be expected. It also contains 

some brown biotite, a mineral not observed in the coarser-grained forms. 

The constituents also show, by a more or less parallel and oriented 

arrangement, that some of them had formed previous to or during the 

flow, so that they were arranged by it.

The inclusions of Belt shales, which are in places quite thickly distributed, do not seem to be greatly altered as a rule. Smaller ones, 

however, contain a mineral in grains without form, but of high single 

and double refraction. They are too small to be determined with 

certainty, but appear like pyroxene.

The other basalt flows which occur in the northern portion of the 

district are quite similar in type to that already described. They show 

at times a conversion of the otivine iuto the red mineral to which Lawson1 has given the name of iddingsite, or rather, one should say, into a 

red mineral corresponding in the main to the description of iddingsite. 

In the present case it is, however, clearly a decomposition or alteration 

product of olivine, and precisely similar to an occurrence previously 

described.2 As the material is not of such a nature as to furnish this 

mineral in quantity or in quality proper for analytical investigation, it 

would be useless to speculate further upon its exact nature. It is quite 

possible that it is not the iddiugsite of Lawson at all.

'Geol. Cainelo Bay: Bull. Geol. Dept. Univ. Cal., Vol. T, 1893, p. 30. 

« Am. Jour. Sci., 3d series, Vol. XLV, 1893, p. 381.
CHAPTER VIII.

GENERAL PETROLOGY OF THE CASTLE MOUNTAIN

DISTRICT.

In the foregoing chapters it has been shown that the igneous rocks 

of Castle Mountain all belong to one center of eruptive activity. Taken 

as a whole, they constitute a unit, separated geologically and petrologically from other neighboring centers of igneous action in this portion 

of the Eocky Mountain region. A study of their geologic disposition 

and mode of occurrence must convince the observer that no matter how 

diversified their mineral nature, no matter how different their chemical 

composition, they have all had a common origin. On no other supposition could we reasonably explain the intimate association of such 

varied types.

The fact that from the same center of activity rocks varying both 

chemically and minerally may arise, has, at the present time, been 

shown in so many different occurrences that it is only necessary to call 

attention to it in passing. Admitting, then, as well grounded, the 

assumption that the Castle Mountain rocks are varied phases of a single 

molten mass below, it remains to inquire Avhat bearing the facts observed 

have upon the general question'of the differentiation of molten magmas 

and the origin of igneous rocks.

It is to be clearly noted at the outset that in the comparative study 

of the rocks of any district, in the endeavor to trace their relationships 

and to determine the laws of their formation, the student is met at 

the very beginning by a great and insuperable difficulty. The facts 

observed and the theories advanced must result from an examination 

of what is, comparatively speaking, the surface alone. Whether the 

locus of fusion and sources of eruptive material are deeply seated, as 

claimed by some, or are in the superficial crust of the earth, as argued 

by others, is, petrologically speaking, of little moment, since the volume 

of material exposed at the surface or concerning which safe inferences 

. may be drawn, is, or may be, minute compared with what may exist 

below, even in the relatively superficial crust. Thus our results can 

never be quantitative, but must always be qualitative, and possibly even 

then not complete. It is true that Brogger, in his striking work on the 

basic eruptive rocks of Gran, 1 has arrived at some results which are 

quantitative, but the localities where such work may be performed are 

necessarily limited, and for the present a certain reserve will naturally

1 Quart. Jour. Geol. Soc. London, Vol. L, 1894, p. 15. 

132
GENERAL PETROLOGY. 133

be held imtL. concurrent facts from a number of such districts show the 

views held by Brogger and others to be of general application.

The chemical examination of the rocks of any distinct locality does, 

however, afford data for qualitative results of magmatic variation, which 

are of extreme value in showing how far and in what directions such 

variation has taken place. It is necessary to remember only that such 

results are qualitative, not quantitative, a fact often overlooked by 

theoretical petrologists.

It is unnecessary to detail here the various theories that have been 

held by different geologists as to the origin and cause of the variation 

of igneous rocks. They have been admirably reviewed with great 

fullness and detail by Hague 1 and by Iddings 2 . As stated above, the 

view that is commonly held at the present time is that the variations 

observed at any eruptive center are due to the differentiation of a 

single^origiually homogeneous molten mass. This theory, as extended

by some writers, further supposes that the mass has in the beginning 

a composition in type intermediate between the acid and the basic 

rocks that it is similar to an andesitic lava. As the differentiation 

progresses the iron, magnesia, and in great part the lime molecules 

are concentrated in certain areas, while the silica, alumina, and alkalies 

are thereby increased in the remaining portions. Duriug*the process 

of concentration the oxides are supposed by some to be in a state of 

combination as definite mineral molecules, while by others they are 

supposed to be in a dissociated condition. Recently the volcanologist, 

. John son-La vis,3 has propounded a theory by which the variations 

observed at a given center are referred to the solvent action of a 

magma on the containing crust walls; that is, as the magma rises in 

the conduit or remains molten in a cavity its composition Avill change 

by the absorption of the country rock. Various obstacles to this view 

might be pointed out,-and on the chemical side none seems to present 

greater difficulties than a consideration of the alkalies. For, considering the theory as true, it is evident that if Ave deal with an originally 

trachytic magma, suggested by Johnson-Lavis, any addition of country 

rock must tend to lower the alkali content. Therefore, in a center 

ejecting phonolites with 15 per cent of alkalies, this must be supposed 

to be the original magma. Since these are accompanied, however, by 

very basic rocks with, low alkalies, it appears that the amount of dilution by foreign material would have to be enormously beyond what we 

may well consider possible. The extreme range of alkalies at Castle 

Mountain is, in molecular proportions, as 125:50; in percentages, 9:4.25; 

and to reduce the former to the latter would require, supposing the 

most favorable, though improbable, case, that the absorbed material 

should contain no alkalies and that the magma should absorb somewhat 

more than its own weight of country rock.

1 Geology of the Eureka District, Nevada: Moil. U. S. Gcol. Survey, Vol. XX, 1893. 

2 Origin of igneous rocks: Bull. Philos. Soc. Washington, Vol. XII, 1892, pp. 89-214. 

sNatural Science, Vol. IV, Fob., 1894.
134 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

The theory would also necessarily require that the acid magma 

should give evidence of having possessed the greatest amount of heat, 

while the state of the included foreign fragments at Castle Mountain, 

as pointed out in the preceding chapter, evinces the contrary j observations in agreement with those of Lacroix. 1 Finally, as pointed out 

by the author himself,2 we should expect that in case an igneous rock 

mass varied locally it would do so in accordance with the more acid or 

basic nature of the enveloping rock, while in the case of the intruded 

diorite area at Castle Mountain exactly the contrary is to be observed; 

the center of the stock is a diorite with 56 per cent of silica, which 

grows steadily more acid toward the periphery, till at the contact it 

has the facies of a very acid quartz-porphyrite, though intruded into 

limestones and very basic shales.

Quite recently Backstrorn3 has proposed a modification of the theory 

of differentiation by which this process shall depend on the separation, 

at a certain stage of cooling, of the originally homogeneous liquid 

magma into two or inore raiscible fluids which form a kind of emulsion. 

To support this view he adduces the behavior of certain artificially 

formed liquids and presents arguments against the theory which causes 

differentiation to depend on dissociated molecular diffusion or crystal 

separation. * This view has been previously referred to.

It is now in order to consider what bearing the facts brought out 

by the study of the Castle Mountain 9rocks may have on theoretical 

petrology. For this purpose it will be necessary to consider the analyses, 

and for convenience they are presented together in the accompanying 

table on page 135.

1 Les enclaves lies roches voJcaniques.

2 Loc. cit., p. 139.

'Jour, of Geol., Chicago, Vol. I, 1893, p. 773.
WEED AND"! 

PIRSSON. J

GENERAL PETROLOGY. 135

Table of analyses of Castle Mountain rocJcs. 

[Analyses made by L. V. Pirsson.]

Si02 ......

Ti02 ....--

X. ........

A12 03 -----

EeA-----

FoO ......

MnO......

MgO......

PiO

Na^O .....

K2 0 ......

Li.,0 ......

5,0......

co2 .......

pao6 ......

Total . .

Sp.gr.....

I.

74.90

.15

13.64

.66

.50

Tr.

Tr.

.61

4.22

4.64

Tr.

.33

99. 65

2.61

II.

74.82

.25

13.80

.37

.30

Tr.

.10

.17

4.33

4.81

Tr.

.83

99.78

2.59

in.

72.48

.32

13.14

1.66

1.02

Tr.

.15

1.04

4.22

4.88

Tr.

.42

99. 33

2.62

IV.

72.38

.10

14.71

1.09

.82

Tr.

.70

.67

4.28

4.15

Tr.

.92

99.82

2.61

V.

72.56

.20

12.33

.80

.82

Tr.

Tr.

Tr.

5.36

3.08

4.59

99. 74

2.37

VI.

72.88

.45

12.90

.74

1.05

.05

.75

.81

Si. 72

5.03

......

1.22

99.60

2.64

VII.

71.67

.10

15.82

1.18

.35

Tr.

.13

.25

4.40

4.45

1.21

99. 62

 2.60

VIII.

65.87

.37

16.82

1.58

1.23

Tr.

1.54

2.65 _. .

IX.

61.21

.56

15. 67

4.06

.62

.10

1.58

2.18

4. 72 1. 57

3.15

......

1.43

Tr.

99.30

2.62

2.75

Tr.

10.20

100. 50

......

X.

61.87

.87

17.26

2.35

2.43

..03

1.82

3.23

5.18

3.8?

Tr.

1.07

Tr.

99.94

2.67

XI.

56. 80

.46

18.30

1.64

5.58

Tr.

3.63

5.31

4.35

3.28

Tr.

.53

Tr.

99. 93

2.83

xn.

46.52

2.98

.73

10.48

4.40

7.79

.11

10.58

xni.

45.15

2.80

15.39

2.76

5.64

.14

6.38

9. 49 8. 83

3. 12

1.55

Tr.

1.79

Tr,

.83

100. 37

2.99

2.67

2.77

Tr.

2.85

4 ?,7

.56

100.21

2.70

XIV.

42.46

2.47

12.04

3.19

5.34

.16

12.40

12.14

1.21

2. 68

Tr.

4.03

r.K

.84

99. 51

2.94

I. Rhyolite between Fourmile and Myemilo 

creeks, near Smith River.

II. Qnartz-tourmaline-porphyry, upper Fourmile Creek.

III. Granite, Elk Peak.

IV. Quartz-porphyry sheet, ridge between 

ITourmilo and Checkerboard creeks.

V. Ehyolite-pitchstone, forks of Checkerboard 

Creek.

VI. Aplitic granite dike cutting diorite between 

Blaokhawk and Robinson.

VII. Quartz-porphyry, Musselshell Canyon. 

VIH. Feldspar-porphyry dike below Castle. 

IX. Rhyolitic tuff, near, the forks of Checkerboard Crock. 

X. Included sycuitic mass in granite, head of

Cottonwood Creek. 

XI. Diorito, between Blackhawk and Robinson.

XII. Basalt of Volcano Butto.

XIII. Vogesite dike, upper Fourmilo Creek.

XIV. Moncliiquito-liko dike, west side of upper 

Willow Creek.

In connection with these analyses, it may be remarked that the rarer 

elements have been sought with care, and that they are either wanting 

or exist in such minute quantities that they could exercise no influence 

on the-purpose for which these analyses have been made, and therefore 

they have not been determined. Of these, barium and strontium, may 

be cited as examples, the last four rocks containing traces of them.

These analyses present several points of interest, among which the 

relations of silica, iron, and titanic acid may be mentioned. It will be 

observed that the amount of the latter oxide is closely dependent upon 

the iron present, and that these increase in proportion as the silica 

decreases. The phosphoric acid also appears to depend on the lime 

and magnesia, being absent, or practically so, where these are very 

low, and increasing with them toward the basic end of the series.

In order to better exhibit .the relations of the chief rock-making 

oxides toward one another, -they have been separated from the rarer 

less essential elements, and are presented by themselves in the accompanying table, reduced to 100. To compare them molecularly, a table of 

their molecular proportions is added.
136 THE CASTLE MOUNTAIN MINING DISTRICT.

Percentages of rock-making oxides reduced to 100.

[BULL. 139.

Si02 ......

AM),.....

FezOg.....

FeO ......

M>0

CaO ......

JsTaaO .....

K2 0 ......

I.

76.43

12.98

.84

.86

Tr.

Tr.

5.64

3.25

II.

75.81

13. 98

.38

.31

.10

.17

4.38

4.87

III.

75.52

13.75

.67

.51

Tr.

.62

4.25

4.68

IV.

7445

13.18

.75

1.07

.77

.83

3.81

5.14

V.

73.52

13.33

1.68

1.03

.15

1.06

4.28

4.95

VI.

73.26

14.89

1.10

.83

.71

.68

4.33

4.20

VII.

72.90

16.08

1.20

.35

.13

.25

4.54

4.55

VIII.

68. 28"

17.48

4.53

.69

1.76

2.43

1.76

3.07

IX.

67.53

17.24

1.62

1.25

1.57

2.71

4.84

3.24

X.

63.15

17.62

2.40

2.48

3.30

5.29

3.91

XI.

57.44

18.51

1.66

5.64

3.67

5.35

4.41

3.32

XII.

50.40

17.18

3.08

6.29

719

9.86

2.98

3.09

XIII.

4S.54

11.10

4.68

8.29

11.26

10.10

3.32

1.65

XIV.

46.42

13.17

3.49

5.84

13.56

13.27

1.32

2.93

Molecular ratios of the oxides in the above analyses.

Si02 ......

A12 03 ----- 

FeO ......

MjrO......

CaO ......

Na20 ..... 

K2 0 ......

NajO^... 

K20 .'..-.-

I.

1,273

126 

21

91 

34

] 125

II.

1,263

135 

9

2

3

71 

52

123

III.

1,258

133 

15

H

68

50

118

IV.

1.241

128 

24

19

15

61 

55

116

V.

1,225

130 

21

4

19

69 

53

122

VI.

1,221

144 

25

18

12

70 

45

115

VII.

1,215

156 

20

3

5

73

48

121

VIII.

1,138

170 

66

44

43

28 

33

61

IX.

1,125

167 

38

39

48

78 

34

112

X.

1,052

171 

64

46

59

85 

41

126

XI.

957

179 

85

92

95

71 

35

106

XII.

840

167 

126

178

176

48 

33

81

XIII.

825

108

187

281

198

52 

18

70

XIV.

773

128 

125

339

237

21 

31

52

I. Pitckstone.

II. Quartz-tourmaline-porphyry. 

HI. Rhyolito. 

IV. Aplitic granite.

V. Granite. ^ 

VI. Quartz-porphyry. 

VII. Musselshell quartz-porphyry.

VIII. Khyolitic tuff. 

IX. Feldspar-porphyry.

X. Syenitic mass in granite. 

XI. Diorito. 

XII. Vogesito dike.

XIII. Basalt.

XIV. Monchiquite-like dike.

For the sake of simplicity the iron in the above table of molecular 

proportions has been calculated entirely as ferrous oxide. It wilt be 

noticed that ferric oxide does not appear to show any definite relation 

to alumina, while, as may be seen in the analyses, the proportion of ferrous to ferric oxides remains quite constant in th'e acid rocks ; and 

then, as in the basic ones the silica falls, the proportion of ferrous over 

ferric rapidly increases.

In regard to the alumina, it will be noticed that it rises gradually 

with decreasing silica until very near the basic end, when it suddenly 

falls. The table of molecular proportions shows that the alkalies in 

sum total remain pretty constant, with slight variations until the basic 

end is reached, when they also fall like the alumina. In the case of 

No. VIII, it should be recalled from the preceding chapter that this tuff 

is largely mingled with shale and limestone, which tend to seriously lower 

the alkalies and silica and increase the alumina and bivalent oxides. 

Therefore, although this analysis is here included for comparison, it can 

not be depended on in drawing any general conclusions. It is interesting to note, in the analyses of the pitchstone, that as the alkalies are to
WEED AND*! 

PIRSSON. J GENERAL PETROLOGY. 137

the alumina as 1:1 and to silica as 1:10, and as magnesia and lime are 

wanting, the rock, if crystallized, would consist essentially of one molecule of feldspar to four of quartz, and that a little iron ore would have 

been the only other compound. The feldspar would.be an anorthoclase 

or a mixture of Ab. and Or. as 2.7:1.

From the above statements it will be seen that no absolute relation 

exists between alumina, silica, and the alkalies, since as the silica 

decreases the alumina rises, and the alkalies remain about the same 

until the basic end is almost reached, when all fall together.

The relation of potash to soda shows some facts which are interesting to study. Iddingsl has called attention to the fact that this relationship is characteristic for regional rock groups, and our study of the 

analyses of the rocks of Electric Peak, as given in his paper, shows 

that K2O:Na2O: : 1:3.94 to 1:1.79; or, in round numbers, the ratio 

varies from 1:4 to 1: 2; and a consideration of the twenty-one analyses

given shows that the variation is most rapid near the two extremes, 

while between K2O = 1 and Na2O = 2.8 to 2.4 it is very gradual and 

embraces a majority of the analyses, thus showing for this volcanic 

center a tendency to the ratio expressed by K2O: Na2O:: 1: 2.6. The 

same author gives a series of analyses for another eruptive center at 

Cranclall Basin, where the K2O: Na2O varies from 1:2.9 to 1:1.13. Here 

again the variation is most rapid at the extremes, while between 2 and 

1.65 it is very gradual, showing a tendency for K2O: Na2O:: 1:1.8.

The analyses given by Iddings are valuable for a comparative study 

of this nature because they show at each of the given centers the composition of a variety, of rocks differing in geologic position and degree 

of crystallization, which latter are seen to apparently have no influence 

upon the relation of soda to potash. At Castle Mountain the relations 

are as follows:

Number of analysis.

XIII ......................

I...........................

IX..........................

X.. ........................

XI.........................

VI...........:.............

vn.. ................... ...u

XII .......................

ii .........................

in........................

v.. ........................

IV.........................

XIV. ................ .:....

KjO : Nn^O.

1 : 2. 88

1 : 2. 69

1:2.41

1 : 2. 07

1:2.03

1 : 1. 55

1:1.,52 

. 1:1.45

1:1.36

1:1.36

1:1.30

1:1.11

1: .68

Rock.

Musselsholl porphyry sheet.

As before, the variation is rapid around the two extremes, while in 

a very considerable proportion of the analyses it varies gradually

'Origin of igneous rocks: Bull. Philos. Soc. Washington, Vol. XII, 1892, p. 138.
138 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

between 1.55 and 1.30. Here again we see that the relation does not 

depend upon geologic position or coarseness of crystallization.

A consideration of the above in connection with the table of molecular 

proportions seems also to indicate that the relation does not depend 

upon the other elements present, and this theory is borne out by analyses from other localities. Thus the very basic vogesite dike has nearly 

the same proportions of soda and potash as the very acid rocks with 

which it so intimately occurs.

It will be noticed, also, that the two effusive rocks, basalt and pitchstone, standing at the acid and basic extremes, show about the same 

relation of potash to soda, and both differ considerably from the granite, while the rhyolite is very close to it. Again, the syenite-like mass 

included in the granite has a relation like that of the diorite, and both 

differ considerably from the granite..

From a consideration of the foregoing facts it does not appear, therefore, that the proportion of potash to soda obeys any law which can be 

brought into direct connection with the variation of the other elements 5 

but, on the other hand, it would seem as if there was a tendency for a 

more or less definite relation to exist between them and for the most 

common rocks to approach this mean. This mean appears also to be 

characteristic for this center of eruption and to differ from neighboring 

ones, so far as these are known. Such analyses of Crazy Mountain 

rocks as are at present available indicate a higher proportion of soda 

to potash, 1 while that for centers farther to the southward, as mentioned 

by Eddings,2 have already been given. Before this point can be considered settled, however, further analytical and petrographical work will 

be necessary, and the future may show that this portion of the Eocky 

Mountain region, embracing various centers of eruptive activity, is distinguished by a general relation of the alkalies that is characteristic, 

as Brogger 3 has already shown for the district of southern Norway and

Backstrom 4 for Iceland. In such cases we may expect local variations 

in neighboring center's without 'disturbing the general law.5

In regard to the bivalent metallic oxides, iron, lime, and magnesia, it 

will be observed from a study of the table of molecular .proportions 

that, with very slight irregularities, they all increase steadily and 

together from the most acid to the most basic end of the series.

When, from a consideration of the, chemical and geologic data previously given, we endeavor to present to ourselves the composition of 

the original maguia from which the Castle Mountain rocks have been 

derived by differentiation, it at once becomes evident that it must have

1 J. E. Wolff, Petrography of the Crazy Mts.: North Trans. Survey, 1885. Ibid, Acmite Traehytc of 

the Crazy Mts.: Bull. Mus. Comp. Zool. Harvard Coll., Vol. XVI, 1893, No. 12.

2Loc. cit.

3Min. der Syenitpeg.: Zeitsch. f. Kryst., Vol. XVI, p. 84,1889.

4Beit. z. Kennt. Island. Gest.: Geol. Foreningens Stockholm, Forhandl. Vol. XIII, 1891, p. 669.

8 Since the above was written the authors have shown that at Yogo Peak and Square Butte, where 

highly differentiated magmas have been intruded, soda and potash have been differentiated with respect 

to each other, the potash increasing in the more basic types.
GENERAL PETROLOGY. 139

been extremely acid, if the volume of material exposed at the surface 

be taken as a criterion. If the whole country covered by the map 

should be planed down to the level of the lowest point in the district, 

and all of the igneous rocks fused together to a homogeneous mass, it 

would differ but little in composition from the granite. It would be 

somewhat lower in silica and a little higher in lime, iron, and magnesia, 

but on crystallizing it would still form, a typical granite.

The huge intrusive bodies forming the cores of the Crazy Mountains 

to the southward are\)f diorite. The field work of the authors in 1893 

shows that some 15 miles to the east of Castle Mountain, in the Big 

Belt Range, occurs an. immense granitic intrusion of very similar character, and around Neihart, some 30 miles to the north, new eruptive 

centers with great intrusions of acid and moderately acid rocks are 

again encountered. Iu: all of this area basic rocks play but an insignificant role, and are confined to relatively small intrusive sheets, dikes,

or stocks and to thin extrusive sheets of limited area. The report of 

Messrs. Dana and Grinuell, 1 together with the work of Mr. Lindgren2 

and reconnaissance work by W. H. Weed, indicates a prevailing abundance of acid trachytic types of rocks in the isolated mountain groups 

of the country to the eastward fronting on the Great Plains region, while 

the medium character of the Highwood group to the northward may be 

seen from a paper on that region lately published by the writers.3 If 

these are all assembled into a petrograpllical province 4 whose igneous 

rocks have been furnished by some huge reservoir of molten material 

far below, the varied types being formed by differentiation of the fluid 

magma contained in this reservoir, which was originally homogeneous, 

then the original composition of this-initial mass must have been of a 

quite acid type, and with rather high alkali content. At Castle Mountain the material was still more acid than the probable mean of the 

whole district; but by a concentration of the lime, iron, and magnesia 

molecules a process in. some portions complete enough to entirely 

deprive the acid magmas of these elements relatively small bodies of 

basic rocks were formed. In drawing such conclusions, however, as 

remarked before, it must always be borne in mind that our knowledge 

relates to surface exposures only.

In regard to the order of succession of the varied partial magmas at 

Castle Mountain, the rock exposures are not always connected in such 

a manner as to yield complete, definite data upon this point. It is cer~ 

tain, however, that the diabase is the oldest rock which appears in the 

district, but whether it should be directly connected with the Castle 

Mountain rocks in rnagmatic relations is at least questionable.

'Kept, of a Keconnoissance from Carroll. Mont., to Tellowstone Park, Ludlow, War Dept., Washington, 1876, p. 91.

"Tenth Census, Vol. XV, p. 719; Proc. Gal. Acad. Sci., ser. 2, Vol. Ill, JJ90, p. 39; Am. Jour. Sci., 

Vol. XLV, 1893, p.- 280.

'Bull. Geol. Soc. Amor., Vol. VI, 1895, p. 389.

4 Judd: Quart. Jour. Qool. Soc. London, Feb., 1886, p. 54.
140 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

The evidence seems to show that after this and the earliest of the 

Castle Mountain intrusions is the diorite stock, which was succeeded 

by the granitic intrusion, which in turn developed the outflows of rhyolite and pitchstone. - Succeeding these came the outbreaks of basalt 

and the intrusion of basic dikes, which from evidence already given 

occurred at a considerably later date, when the acid rocks had become 

cold. These two, from their later appearance and close chemical relationships, we are inclined to consider simultaneous. So far, then, as 

these facts go, they conform to the general rule of the Eocky Mountain 

region, that the outflows became successively more differentiated into 

acid and basic material as they followed each other.

It can not be said that the Castle Mountain rocks, taken as a group, 

offer any of those marked peculiarities of minerals or structure to which 

Iddings 1 has given the name of "consanguinity;" the majority of them 

are acid rocks of \very simple mineral composition, and it could scarcely 

be expected that they should.

Some phases of the rhyolites are extremely like those occurring in the 

Yellowstone Park, while others, together with the quartz-porphyries, 

could not be distinguished either in hand specimen or section from similar pre-Tertiary rocks occurring in Germany. The basalt appears to be 

of a rather common, type, while the correspondence of the diabase with 

similar rocks has already been pointed out. The remaining types are 

so few in number that no general conclusion can be based upon them.

This is not, however, in any sense an argument either for or against 

evidences of "consanguinity" in general, since every district or individual group of rocks can not be expected to exhibit such peculiarities 

as will serve to distinguish it from all other districts or groups of 

igneous rocks. On the other hand, the preliminary study that has been 

made of the rocks occurring in the eruptive centers of the Little Belt 

Eange near Neihart seems to show that they exhibit certain peculiarities 

of minerals and mineral association which will strongly differentiate 

them from the types described in this memoir,'and in general from previously described rock groups; and this is also very markedly the case 

in the High wood Mountains immediately to the northward.

When we consider the relation of the facts observed at Castle Mountain to the differentiation of molten magmas and the origin of igneous 

rocks, it at once becomes evident that this process has been a deeply 

seated one. There has been, it is true, some local and comparatively 

superficial differentiation. For example, the diorite mass grows more> 

acid toward its contact. This is not according to the more general case 

where intruded masses grow more basic toward the outer edge; but 

cases where they reverse this order are known and have been mentioned 

in the literature. The less acid masses in the granite-granite-porphyry main2 mass are also local in their formation, but in general the

"Bull. Philos Soc. Washington, Vol. XII, 1892, p. 128.

s Seo Min. Mitt., XII, 1891, p. 390; also, Zeitschr. f. Kryat, Vol. XVJ, 1889, p. 45.
GENERAL PETROLOGY. 141

occurrences are such that the process of differentiation took place at 

greater depths.

No facts have been observed which would lead to the belief that differentiation has taken place by crystal separation, or by processes of 

crystallization, locally concentrating certain materials. On the other 

hand, all the facts are opposed to it. It is by no means intended by 

this to express a disbelief in the actuality of such processes and their 

capability of performing a differentiation which would give rise to rocks 

minerally and chemically different from a single homogeneous magma.

The fact is patent that the more the occurrences of igneous rocks are 

studied with reference to their origin the more evident it becomes that 

under the term "differentiation," as now employed, not one but several 

processes must be included, by any one of which, or a combination of 

all, varied rock types may arise from an originally homogeneous molten 

mass. Though "originally homogeneous" is a term very commonly 

employed by petrologists, as a matter of fact we have no means of

knowing whether there is or has been any such thing in the sense in 

which this term is used. Its existence is a pure -assumption, based, it 

is true, on good reasoning, but nevertheless an assumption.

One fact has been made patently clear by the chemical researches 

on rocks during the past half century, and that is that in proportion 

as silica and alkalies increase in a magma, the lime, magnesia, andiron 

sink; and that high silica, high alumina, and high alkalies go hand in 

hand, while high magnesia, iron, and lime and low silica go together. 

There are, of course, exceptions to this, but as a general law it is certainly true. Thus we should not expect to find an igneous rock with 

75 per cent of silica and 25 of lime, iron, and magnesia, with alkalies 

and alumina practically wanting.

In general these facts seem to be due to the tendency of magmas, of 

whatever composition, to separate themselves into these two general 

products the acid end or negative pole and the basic or positive one. 

The process is certainly not a stoichiometric one, and evidently a great 

variety of modifying causes may be also present. The original composition of the magma is of course the chief factor in determining the 

end products. At Castle Mountain, as already shown, it was measurably acid, and hence we find the basic products in but relatively small 

amount.

The exact cause that produces such a process is evidently not at 

present determinable. It may be, as suggested by Backstrom, 1 by 

processes of liquation, a method which commends itself by its simplicity, or by this in connection with other circumstances.

THE DIKE ROCKS.

The occurrence of the basic dike rocks presents several facts of general petrologic interest. One of these is the comparative rarity of such

1 Jour, of Geol., Vol. 1,1893, p. 773.
142 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

rocks in this district as compared with that of the neighboring Crazy 

Mountain area, a fact previously mentioned. This rarity is evidently 

due in great part to the fact that the intrusions took place in strata 

already flexed and broken, and the tendency has been for the peripheral, fringing material to assume the position of sheets rather than dikes.

But the comparative absence of attendant basic rocks has been due 

much more to the extremely acid, simple character of the intruded 

magma, which was already, so to speak, greatly differentiated at its 

appearance toward the surface.

The association of the vogesite dike and intruded mass of rninette 

is of interest in this connection, as it repeats a fact elsewhere often 

observed of the relation of such rocks to intruded stocks of granite. 

The occurrence of the iniuette, however, shows that such rocks are 

not necessarily confined to dike form, and we have, indeed, elsewhere 

observed in the Little Belt Mountains that if the structural relations are 

suitable they may appear almost entirely in the position of intruded 

sheets.

The occurrence of the monchiquite-like dike is interesting in this 

connection because such rocks have been referred to parent stocks of 

eleolite-syenite; yet here we find it in close geological connection with 

granite. The work of Brogger on the rocks of Gran, already quoted, 

tends to show, however, that such genetic relationships are not always 

invariable, although the mass of evidence tends to prove that they are 

at least very common.

Recently an attempt has been made 1 to formulate a general theory 

for the origin of lamprophyre dikes, in which the process depends on the 

local concentration of lime, iron, and magnesia on the borders of the 

magma basin below, giving rise to interior and exterior zones of more 

highly differentiated material than that forming the initial intrusions.

Later outbreaks of these more highly differentiated partial magmas 

through fissures, which may form in Vt^rious ways, would give rise to

rocks on the one hand more acid, more feldspathic, and on the other 

more basic, richer in lime, iron, and magnesia, than the earlier central 

masses with which they are found.

The vogesite dike on the border of the central granitic mass, with its 

glassy saalband, seems to the writers to furnish proof pointing to this 

process, while the basaltic outflows which stand intermediate in chemi-, 

cal composition between the vogesite and monchiquite dikes may be the 

results of fissures furnishing conduits to the surface for the outer basic 

zone of magma.

'L. V. Pirsson: Complementary rocks and radial dikes: Am. Jour. Sci., 3d series, Vol. L, 1895, pp. 

116-121.
CHAPTEE IX. 

GLACIATION OF CASTLE MOUNTAIN.

Evidences of former glaciation are conspicuous on many parts of 

Castle Mountain. Like many other portions of the mountain region 

of Montana, it nourished a number of local glaciers that flowed down 

its flanks, filling the valleys and covering the adjacent bench lands. 

The glaciation of the mountain was local in character. The surrounding region is uuglaciated, and the southern limit of the two great 

ice sheets of the continent is over 50 miles farther north. The extent 

of these local ice streams, as shown by their moraines, is surprising. 

The mountain is relatively low, its highest point, 8,606-feet above sea 

level, being 2,500 feet less than the culminating peaks of the Crazy 

Mountains and surpassed by several summits in the neighboring Belt 

ranges. Yet its former glacial covering was relatively larger than that 

of the Crazy Mountains, while the Belt ranges were without any 

glaciers worthy of comment.

Bowlder moraines constitute the most important and-conspicuous 

evidence of the former glaciatiou of the region. Striated rock surfaces 

-are rarely seen, for with the exception of the granites and diorite of the 

mountain summit, the soft and readily eroded character of the rocks 

rendered their preservation impossible. The granite is the rock of 

which the bulk of the bowlders are formed. The craggy, castellated 

masses into which its peripheral portions are carved by weathering 

furnished abundant material, while the resistance to the ordinary processes of disintegration leaves the bowlders still intact.

A study of the mountain shows that the ice sheets had their birth in 

the amphitheaters and recesses that indent the sides of the mountain 

summit. On the mountain top itself no signs of glacial erosion were 

noticed. The amphitheaters still hold extensive snow banks in early 

summer, and the granite shows planed and rounded surfaces that are 

clearly the work of moving ice. The detritus that was no doubt 

abundant in pre-Glacial time over the summit and projecting ridges of 

the mountain has been, entirely swept away, and the former crust 

of partly disintegrated and decomposed rock has been planed off.

Below these rock-carved cirques the high mountain valleys are 

largely occupied by the tumultuous heapings of glacial rubbish (largely 

overgrown by pine and spruces) that mark the declining phases of the 

ice. These morainal deposits do not, however, extend down the valleys and are not a continuous part of the moraines found on the lower

143
144 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

slopes. They occur only in the mountain basins whose united rills 

form streams which have cut out gorges below.

In every case noted the ice gathered in and flowed down preexisting 

depressions in the mountain flanks. The higher parts of the ridges 

show no signs of glaciation, but lower down, at 7,000 feet and below, 

the mountain spurs expand into broad, flat areas cut out of the upturned 

and folded sedimentary rocks. These benches show but little drift. 

An occasional bowlder is seen, but the surface has been swept clean''by 

the ice, which, filling the mountain gorges, overflowed these highland 

benches and ground off their irregularities of surface. Neither the 

Belt shale nor the Paleozoic limestones retain, however, any of the 

detailed evidence of such action; it is in the broader features that this 

work is recognizable.

Below 7,000 feet the bowlder moraines become prominent. The 

change from the smooth limestone areas to drift-covered, bowlderlittered bench land is quite abrupt on the southeastern side of the mountain. The northerly slopes, being cut in a variety of rocks, are less 

regular, and there the occurrence of morainal deposits is less striking. 

The lower limit of glacial drift is 5,400 feet on Alibaugh Creek, and 

somewhat less on the northern slopes. The areas covered by morainal 

deposits are shown on the geologic map.

Fine examples of bowlder moraines are crossed by the roads leading 

to Castle, and one is seen at the forks of JFourmile Creek. The Townsend stage road also crosses a morainal bench between Warm Spring 

and Alibaugh creeks. Near the town of Castle the moraines attain 

their best development. Ice streams, coalescing at the time of their 

greatest expansion, have left a mantle of drift and bowlders extending 

from Warm Spring Creek to Bonapza Creek Ibhat completely obscures 

the underlying rocks on the bench laud between the creeks, and very 

nearly those of the valley walls. The° moraine between Alibaugh and 

Warm Spring creeks is particularly striking. Large erratics of 

granite dot the surface and remind one of the glacial areas of New 

England. The soil consists largely of sand, formed by the disintegration of the granite bowlders, and is abundantly grassed, bringing them 

into strong relief. (PI. XIII shows a photograph of this morainecovered bench.) This moraine terminates abruptly in a steep bowlderformed slope. The summit, where it is crossed by the road, has an 

elevation of 200 feet above the Cretaceous bench, and maintains its 

abruptness, with slightly diminished height, eastward to the mouth of 

Alibaugh Creek. There could be no stronger contrast than that shown 

along the front of this moraine between the bowlder-formed slope and 

the gentle monoclinal ridges eroded out of soft Livingston shales that 

trend at right angles to the moraine margin and disappear beneath it. 

The moraine summit is slightly undulatory and has a gentle rise toward 

the mountain.

In the vicinity of Castle, Alibaugh Valley is wide and open, and"
U. 8. GEOLOGICAL SURVEY 

MORAINE-COVERED BENCH SOUTH OF TOWN 
GLACIATION. . 145

glacial drift is uncommon in the valley bottom. The inclosing slopes 

are, however, dotted with bowlders up to 5 or 6 feet in diameter, and 

show successive parallel terraces with few bowlders and grassy surfaces between steep bowlder-strewn slopes. ' These mark successive 

pauses in the melting and recession of the ice. The summit of the ridge 

to the north is drift-covered and bowlder-strewn, quite like that of the 

bench lands south. While the harder rocks stand out in bold relief in 

the valley bottom and lower slopes, no exposures whatever are seen on 

the higher valley wall or summit of the bench. Alibaugh Creek flows 

through a narrow valley in a bowlder-choked channel in its lower course.

A short distance above the outskirts of Castle, the termination of a 

moraine is seen at the forks of the creek. This moraine front is represented in PI. XIV, which shows the general character of all the morainal 

deposits of the mountain. It is a true terminal moraine, formed of 

large and small bowlders of granite, and the moraine closes in the upper 

mountain valley of this the chief branch of the stream. The moraine 

illustrated is, however, one formed during the recession of the glacier.

The abrupt upper limit of the drift is nowhere better shown than on 

the ridge northeast of the town of Castle. Near the Cumberland mine 

the summit of the flat-topped bench shows excellent exposures of the 

readily eroded clays and shale of the Carboniferous and overlying 

Mesozoic beds. A short distance eastward the drift mantle obscures 

all exposures.

The slopes below the rhyolite flow of Bonanza Creek show an abundance of drift, probably glacial, derived from the flow itself, but no 

granite was seen. The moraine forms a flat-topped ridge near the 

ranch on Bonanza Creek, and extends up to 6,950 feet, where it abruptly 

ceases. Drift of possible glacial origin was also seen at the head of 

Coyote Creek, but no other evidence of such an eastward extension of 

an ice stream was seen.

On upper Fourinile Creek the glacier formed at the head of the 

stream brought down granitic debris that now lies scattered over the 

adjacent slopes and fills the lower valley bottom. .South of the road 

the valley of the main or easterly branch of the stream shows on the 

west a morainal slope, largely covered with pines, in which the white 

granite and porphyry bowlders show conspicuously. The east wall 

shows a nearly continuous straight slope of debris, encroaching upon 

the moraine bench of the valley bottom.

Glacial bowlders are found on all the slopes south of the road following Fourrnile Creek, but the moraine is best seen near the first forks of 

the creek, where the drift covers the slopes of the rhyolite hill north of 

the road. The slopes are dimpled, having a knob and sink topography. 

The bowlders extend 500 feet up the slopes of the hill, showing that 

the ice sheet at this point still had a considerable thickness. The drift 

is mainly granitic, but includes some metamorphosed shale. But one 

bench was observed on this steep slope. Above the bowlder moraine 

Bull. 139 10
146 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

the rocks show glaciated surfaces.. The north slope of this hill consists 

of a succession; of sloping 'glacial benches, showing however no bowlders, but merely small drift.

Buried valleys form an interesting.feature of the northern slopes. 

Fourmile Creek is shown'to possess a number of forks draining vak 

leys cut back in the mountain side (see PI. XY). .The two westerly 

branches of the' stream take their rise, however, in large springs but 

a short distance up these lateral valleys. Above 'these; springs the 

valley still persists, but its bottom is smooth and flat, without any 

stream channel whatever. The .adjacent slopes show no bluff walls^ 

such as characterize the lower end, but rise gently to the summit of the 

ridge. A few miles .up .the'se park-like valleys the ' meadows end 

abruptly'against a heavily timbered moraine that completely fills the 

bottom. This is evidently the source of the bowlders occasionally 

seen below, and it is clear, that-the pre-Glacial gorge has been: filled by 

the moraine, and lower down the finer debris carried by the glacial 

streams has filled up the old 'valley, whose present drainage flows 

through the.glaciaLsands and rubbish: to emerge near the main valley as springs. The moraine filling the valley and the de"bris or wash 

filling its lower part belong, however, to the. retreating phase of the 

ice'sheet, as the bowlders found at the Grasshopper mine and on the 

slopes, east of the moraine prove-an earlier and much greater thickness 

and extension of the ice.

Remnants of the moraine formed by a glacier flowing down Willow; 

Creek, on the northwest side of the mountain, exist on the flat-topped 

hill west of the creek near the mouth of the canyon, and are also 

abundant on the synclinal area between: Willow and Fonrmile creeks, 

the drift consisting -of bowlders i of granite 3 to 6 feet across, with 

smaller drift of baked shale and of quartzite, the drift forming wellbenched slopes. Owing, probably, to the ready erosion of the Mesozoic shales/ this moraine,has'been nearly destroyed. This latter area 

may in part represent the drift left by a tongue of the. Fourmile 

glacier, which overflowed its immediate valley near the site of the 

Grasshopper mine and flow,ed westward:

In ^reviewing the evidences of 'glaciation at Castle Mountain, it 

appears that the glacial streams occupied1 preexisting stream valleys, 

and that glacial erosion consisted chiefly in the removal of weathered 

material from the upper slopes, mainly from the granite area. No 

larger accumulations of assorted drift occur. The morainal deposits 

consist of a heterogeneous gathering of bowlders left as a mantle over 

the flat-topped ridges between stream courses and :as benched slopes 

the lateral moraines of the diminishing glacier. Terminal moraines 

are not differentiated. The morainal mantle is in part of terminal and 

in part of lateral origin.

The maximum extension of the ice is, of course, indicated by the 

lowest level'at which'-glacial drift is found. Successive phases of the
U. S. GEOLOGICAL SURVEY 

TERMINAL MORAINE ON ALIBAUQH CREEK, NEAR TOW
U. S. GEOLOGICAL SURVEY 

GORGE OF WEST FOURMILE CREEK.
GLACIATION. 147

melting ice sheet are marked by terrace lines upon the valley slopes. 

There is, however, one feature that deserves comment. The moraine 

front (pictured in PL XIII) is the terminal heaping- of an ice stream 

coming from the large amphitheaters on the south side of the mountain 

summit. The recent aspecfr of the moraine is in marked contrast to 

the appearance of the adjacent bowlder moraines previously noted. 

It fills the drainage valley and marks the. front of an area in which 

the glacial rock-polishing and other evidences of glaciation are remarkably fresh and well-preserved:. Both the contrast in appearance and 

the relative position of this moraine, with the moraiual remains found 

on the adjacent slopes, show that they iare of very different age. The 

same evidence is presented in 'the moraine on the head of the west 

branch of Fourmile Creek. Those moraines found in the upper part of 

the valleys of Fourmile Greek, noted in the mention of buried valleys, 

indicate a renewal of glacial activity after a long period of decline. The 

accumulations are of very recent aspect, and show a period of stability 

during which the ice maintained a nearly constant front.

These facts clearly indicate two periods of glaciation, one of which 

is much older than the other, the later being of much less severity. 

Although in this particular region no interglacial deposits have been 

found, the facts just noted are in accord with observations made elsewhere in the province, and in entire accord with the observations of Dr. 

Dawson and other observers in the Eocky Mountain region of Canada,1 

\vhere the observations .show that the second period of glaciation was 

comparatively mild and the glaciers relatively small and insignificant 

during the second maximum of glaciation of the cordillera.

1 Trails. Koyal Soc. Canada, Vol. VIII, Sec. IV, 1890, p. 25 et sen-
CHAPTEE X. 

ECONOMIC GEOLOGY.

COAL. 

CHECKERBOARD CREEK.

A thin seam of coal occurring beneath the Dakota quartzite near 

the forks of Checkerboard Creek has been mined in a small way for 

Messrs. Spencer, Main & Heitmann, of White Sulphur Springs. The 

seam is hardly thick enough, or the product of sufficiently high grade, 

to warrant working under ordinary circumstances; but owing to the 

distance of White Sulphur Springs from the railroad the cost of transportation of coal from other points has been prohibitive, and this seam 

furnished the only available source of supply.

WARM SPRING CREEK.

On the southern side of the mountain, where the Cretaceous rocks 

cover a large area, it may be confidently expected that coal seams will 

be discovered and developed at a number of localities. The most favorable coal-bearing horizon for prospecting is the Laramie, whose lightcolored sandstones and gray shales lie in contact with the dark-colored 

beds of the Livingston. At the mouth of Warm Spring Creek a coal 

seam has been exposed. The seam is of sufficient thickness and purity 

to be workable, but the attitude is not favorable, the beds being vertical and the exposure but a few feet above probable water level. The 

outcropping sandstones can, however., be traced up the slopes southward, and the area will furnish an abundance of fuel when the demand 

warrants exploitation.

A seam of impure coal is also found above the charcoal kilns, on 

Warm Spring Creek, where it is close to the wall of massive, white, 

heavy-bedded limestone. It is too impure for fuel, however.

Coal seams have also been located at the Doucet ranch at Warm 

Spring Creek, above the main Castle road, but the material is bony and 

of little value as a fuel, though it has been learned that small quantities have been taken out and sold in Castle. The bed is 5 feet thick, 

dips from 40° to 90°, has a strike northwest, and occurs in a series 

of grits and impure sandstones of dark-gray or greenish color. The 

sandstone over the seam is formed of angular grains and resembles 

the volcanic grits of the Livingston formation. On the bench west of

148 - -
COAL AND MINERAL SPRINGS. 149

the stream the seain has been opened by a shallow shaft, but the quality 

is too poor to warrant working.

A short distance to the northeast the same seam occurs near an eastern branch of the creek, but is not workable. It is here associated 

with a highly ferruginous sandstone that has been quarried as a flux 

for the smelter. Coal is also found on the ridge west of Eobinsou 

Creek, southeast of Castle, but the seam is too impure to work.

POLISHING POWDER.

The volcanic dust forming the tuff beds of Castle Mountain, and of 

which a large part of t*he lake-bed strata consist, are worthy of notice 

as of economic value. This dust consists of fine, angular particles of 

volcanic glass. This material is useful as an abrasive, and could be 

used for polishing wood, stone, and metallic surfaces, and for making 

scouring soap.

MINERAL SPRINGS.

The White Sulphur Hot Springs, from which the county seat takes 

its name, are situated on the bench land south of Smith Kiver, at the 

northwest base of Castle. Mountain. The springs, formerly called 

Brewers Springs, have been known for many years and have acquired 

a considerable local reputation for their therapeutic properties, the 

water being largely used internally, as well as for bathing purposes.

The waters, which issue in considerable volume from a number of 

openings, are quite clear, but possess a strong odor of sulphuretted 

hydrogen. The temperature ranges from 103° to 125° F. (40° to 57° C.). 

Algre growing in the springs near the vents are thickly coated with 

sulphur, forming delicate yellow skeins of soft, silky filaments that 

oscillate in the rapid-flowing streams. Farther from the source orange 

and green algse are also found in some of the springs.

The springs are owned by Mr. Hirshfield, of Helena, and several 

acres of land about them are fenced in and reserved as a public park. 

A comfortable brick bath house has been built, with large pools, in 

which the water is a delicate opal blue from the suspended sulphur. 

The temperature in the baths is too high for comfort, unless the waters 

are allowed to stand and cool in the tank.

The waters are alkaline, saline, and sulphuretted. There are nine 

large springs <and several seepages, whose combined flow is estimated 

at 13,000 gallons per hour. The water used for drinking purposes and 

to supply the public baths has a temperature of 123£° F. (51° 0.).

The first analysis 1 (I) on page 150, made by Dr. E. B. Eiggs in the 

laboratory of the United States Geological Survey, shows the composition of these waters.

The second analysis (II) is that published by the White Sulphur 

Springs Hotel Company.

1 Mineral springs of the United States, by A. C. Peale: Bull. U. S. Gool. Survey No. 32,1885, p. 180.
150 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

Analysis of:water of White Sulphur. Springs,-Montana. 

I.

Grams per 

liter.

Sodium carbonate..........................r.- -. 0. 5571

Calcium carbonate.............................. 0.1280

Magnesium carbonate......................... 0/0438

Sodium sulphate............................... 0.4463

Sodium chloride............................... 0.2460

Potassium..................................... 0.0807

Silica..........................................: 0.0330

Sodium silicate.............-..............-.".... 0.0194

Hydrogen sulphide............................ Trace.

Total.................................... 1.5543

II.

	Per cent.

Fixedsalta .................................... ' 0.166

Sodium........................................ 0.05258

Magnesium........................1............ 0.00178

Lime ..............................:........... 0.00254

Potassium..................................... 0.00468

Chlorine...................................-..; '0.01782

Bromine.....................................'...? Trace.

Iodine.......................................... ' Trace.

Lithia ......................................... 0.0008

Sesquioxide of iron............................ 0.0007

Alumina..........................-........'.... Trace.

Silica.......................................... 0.00404

Carbonic acid.............................:.... 0.074

Sulphuric acid................................. 0.02538

The springs issue from Miocene lake-bed deposits which conceal the 

geologic structure of the rocks below. The calcareous shales of the 

Belt formation outcrop within 'a few rods of them, however, in exposures along the bank of Smith Eiver. The proximity of the Castle 

Mountain massif and the recency of volcanic activity at Volcano Butte 

render it probable, however, that the source of heat is connected with 

volcanic rocks rather than with the great fault of Willow Greek, which 

lies a couple of miles to the east. There are no deposits of siliceous 

sinter or of calcareous tufa. The marshy areas about the springs are 

white with an efflorescence of carbonate of soda, and the peculiar sulphurous odor of the springs and the marshy'meadow itself are characteristic features of many hot-spring localities and readily recognizable

signs of their proximity.

PRECIOUS METALS.

The Castle mining district was for some years regarded as the most 

promising: silver-lead producing area of Montana. Notwithstanding
PRECIOUS METALS. . 151

tiie distance from the.railroad 75 miles and the high cost of transportation of coke and supplies, the Cumberland and Hensley smelters 

produced gold to the value of $16,550 and silver to the value of 

$36,355, in 1889. 1 In 1890 the Cumberland, the principal mine of the 

district, produced ores that yielded 500,000 pounds of argentiferous 

lead and over 20,000 ounces of silver.2 The former was increased to 

5,000,000 in 1891, making this mine the largest single producer of the 

State. The output fell to 300,000 pounds of silver lead in 1892,3 when 

the mine was closed.

The ores of the district are largely argentiferous galenas, or their 

decomposition products, which require reduction by smelting, a process 

rendered expensive by the extraordinarily high cost of fuel. The district has, therefore, been one of the earliest to suffer by the reduced 

price of silver.

These conditions, together with the lack of cheap railroad transportation and the unsatisfactory showing of the ore bodies thus far 

developed by prospecting, have, in the past, made the district a very 

disappointing one. In spite of favorable geologic conditions, of promising surface indications, and of extensive prospecting, the only large 

ore body which has been found is that of the Cumberland.

CHARACTER OF ORE.

The ores consist of comparatively few minerals. Galena and its oxi- 

' dation products sulphate and carbonate of lead constitute the chief 

ore minerals, together with copper and iron pyrites and an oxidized 

manganese mineral. These are described under the notes on the minerals of the district. The gangue is usually siliceous jasper, which 

occasionally passes into quartz.

DISTRIBUTION.

0

The following facts are noticed with regard to the distribution of the 

ore bodies of the district. The ores occur only in the sedimentary rocks 

that have been altered by the eruptive bosses of granite and diorite, 

and which form a zone of metamorphosed rocks about these eruptive 

masses. The deposits that have thus far proved valuable occur in the 

altered limestone, at the contact with dikes of porphyry or as ore 

bodies having no perceptibly direct connection with eruptive rocks. 

A number of prospects are located on decayed basic dikes. These have 

not in any instance, so far as known, shown any considerable amount 

of ore. ;:The deposits thus far discovered in the. altered Belt shales are 

not workable, though considerable prospecting has been done in-such 

rocks.

1 Report Director of Mint for: 1889.

' Idem; 1890.

3 Report Director of Mint for 1892; Mineral Resources TJ. S. 1892, p. 124.
152 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139. 

STRUCTURAL CONDITIONS.

Exploration of the Castle Mountain ore bodies has not been sufficiently thorough to develop any regular mode of occurrence, but from 

observations made by the writers it is evident that they belong to the 

class of ore pockets of galena and associated sulphides occurring in 

the altered zone of limestones adjacent to the main granite mass and 

generally connected with intrusions and offshoots of the granitic rock. 

It should be borne in mind, however, that the closing of the various 

mines has prevented underground explorations, and we are obliged to 

rely on the information furnished by the miners for the facts concerning 

the location of the ore bodies.

In general, it is noticed that throughout the Castle Mountain district 

the contact between the sedimentary rocks and intrusive masses of the 

igneous rocks is marked by more or less alteration, showing that the 

contact has afforded a passage for ascending vapors and mineral solutions. The nature of the decomposition seems to show that the vapors 

or mineral waters have been alkaline in character and deep-seated in 

origin, and none of the products of descending acid waters are found 

throughout the district. On the other hand, the deep alteration of the 

Cumberland ore body from galena, of which its lower portion is composed, to the carbonates and sulphates of lead is clearly due to the 

oxidation of the ore body.

The ore occurs in small, irregular bodies scattered through the limestones, so that the prospectors generally say that they are on a "bowlder" of ore and have not struck the ore body yet. The large size of 

the Cumberland deposit, which is a long, pod-shaped body, described 

later, led the early prospectors to believe that these small ore bodies 

were only detached fragments of the main ore mass, which they confidently expected to find on further exploration. In common phraseology the ore occurs in pockets in the limestone or near the contact of

limestone and igneous rock. So far as the writers' observations could 

be carried, they show that the general conditions of ore deposition are 

those established by Emmons for the Leadville deposits, which have 

since proved so generally applicable. Briefly stated, these are as 

follows:

1. The ores were deposited from aqueous solution.

2. They were deposited as sulphides.

3. The process was a replacement of limestone by ore a metasomatic interchange of rock by ore.

4. The mineral solutions or ore-carrying currents followed beddingplanes or planes of fracture which were or were not filled by dike 

material.

NOTES ON THE MINES.

The Cumberland. This is the largest and most important mine of the 

region. The history of the Cumberland is that of the Castle district 

itself. Discovered im 1884, it was in 1891 the largest single producer
U. S. GEOLOGICAL SURVEY BULLETIN NO. 139 PL. XVI

CUMBERLAND MINE.
WEED AND! 

PIRSSON. J PRECIOUS METALS. 153

of lead ores in the State. Complete mining and smelting works were 

soon erected (see PI. XYI) and every preparation was made for successful working. The high cost of smelting the ores, however, led to the 

shutting down of the smelting' plant, as it was considered wise to 

await the building of a railroad and ship the ores rather than incur the 

expense of treatment at the mine. The Cumberland Mining and Smelting Company was, moreover, a stock company, and diverse interests, 

each fearful of the other, sought to control the property. In 1892 Mr. 

J. Kennedy Tod, of New York City, purchased control, and a careful 

investigation of the mine and its ore reserves was made by the wellknown expert, W. B. Parsons. Operations were, however, suspended 

early in 1893, after the expenditure of considerable amounts of money 

for further development and exploration of the property. It was 

generally understood at that time that a railroad would soon be built 

connecting with one of the transcontinental lines, but the impending 

financial panic, combined with the 

low price of silver, prevented the realization of this hope, and the district 

was in 1894 practically deserted save 

by a few sanguine miners who remained, hoping each month would see 

a change for the better and the building of the promised railroad.

The ore body is a remarkably regular, pod-shaped mass, with elliptical 

cross section, dipping at an angle of 

approximately 60°, its greatest width 

roughly conforming to the bedding of 

the limestones in which it occurs. 

Theaccornpanying figure (fig.9) shows 

the relations of the ore body to the 

limestone and to the granite mass. 

The early workings were started at the outcrop, an inclined shaft being 

sunk upon the ore body and a considerable quantity of ore extracted. 

Later a perpendicular shaft was sunk from the slopes near the stream 

bottom to the east, the shaft reaching the ore body at about 500 feet 

below its outcrop. Development has progressed until the extent of the 

ore is now well known. Upraises extend through to the surface, and 

winzes have followed down the lead, which ends not far below the 500- 

foot level, the ore passing into a narrow zone of pyritiferous vein matter 

lying between a porphyry dike and the limestone. A crosscut at the 

500-foot level runs westward to the granite mass, cutting two dikes of 

greatly decomposed porphyry and tapping the variable zone of decomposed rock carrying pyrite that marks the contact between the granite 

mass and the upturned sedimentaries. The observations seem to

FIG. 9. Diagram of ore body of Cumberland 

mine, Castlo Mountain.
154 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL.ISO.

indicate that the ore body has been formed by solutions ascending 

along the contact between the porphyrite dike and the limestones. 

The contact between the limestones and the granite mass is shown by 

the underground workings to have a dip of about 65° under the limestones. In the lower working's the sedimentary rocks have been much 

more greatly altered than they have at the surface, a condition which 

is believed to be due to the proximity of the main mass of granite and 

to the number of dikes or intrusive sheets which .are offshoots from the 

granite but which do not reach the present surface. PI. XVII shows 

the situation of the mine with reference to the surrounding slopes of 

limestone and granite.

Great Eastern. Ore body of galena on contact between porphyry 

and brecciated limestone. Improved by shaft 100 feet deep and a short 

drift. Horse-whim hoist.

Yelloivstone. This is one of the oldest and largest mines of the district. It is situated on a spur of the mountain between Hensley and 

Hamilton creeks, at an elevation of 7,200 feet. The surface rock is 

"porphyry, part of an exposed intrusive sheet. The material on;the 

dump heap shows that the ore body is on a contact between white crystalline limestone and the porphyry. A two-compartment shaft, with 

bucket hoist, has been sunk about 300 feet deep, but owing to the water 

in the mine the workings could not be visited. There is said, however, 

to be about 200 feet of drifting. The company have well-constructed 

buildings containing engine and boilers and a pumping outfit, and a 

number of buildings for offices, boarding houses, etc.

The California is a mine showing good quality of ore, but has not 

been developed sufficiently to show the extent of the ore body.

The Blackhawk group of mines includes the Legal Tender, Judge, 

Iron Chief, Blackhawk, Bondholder, Alice, and many minor claims. 

These claims are in the zone of metamorphosed rock east of the diorite 

intrusion.

Blacltliaw'k. The workings include about 200 feet of east and west 

drifting and a 100-foot south crosscut. The shaft is 130 feet deep,:the 

levels running out at a depth of .100 feet. The water stopped further 

working. The ore body is said to, be large, and to lie between a porphyry wall and' the altered limestones, and .to have a dip of 50°. Massive black manganiferous ore, running about 5 to 15 ounces of silver.

Legal Tender. This mine is said to have^been sunk on a small .ore 

body that has not yet been carefully explored. There is a 200-foot 

shaft with crosscuts at right angles to ore (i. e;, east and west). The 

ore is a mixture of galena and carbonate. The mine has a complete 

:outfit of hoists and pumping machinery, and a.3-inch pump for handling 

the water, but the shaft is not-well built or proportioned.

The Judge is situated on the flat-toppediridge east of the settlement 

of Blackhawk, in metamorphosed Cambrian limestone.. It is provided 

with a two-compartment shaft^240 feet deep, and some 40 to 50 feet of
U. S. GEOLOGICAL SURVEY 

CUMBERLAND MINE, FROM THE NORTH
D] ' PRECIOUS: METALS. 155

crosscutting has been done, the ore being hoisted in a bucket. The 

mine is still being worked (1894) by two men, who take out the ore, sort 

and sack it, and ship it by wagons to White Sulphur Springs. The ore 

body is said to have outcropped at the surface. It is 15 feet wide, but 

pay ore forms a chute from 8 inches to 4 feet wide and 20 feet long.

Bondholder. This is.the name of a mine useful only as a means of 

working the adjacent claims, as it has struck but a small body of rnanganiferous ore. A crosscut connects the shaft with the workings of 

the Alice. The outfit, consisting of hoist, pumping machinery, etc., 

probably ranks second to that of the-Cumberland.

Alice. Prospect shaft, 85 feet deep, shows a body of ore 3 by 6 feet in 

cross section.

Iron Chief. Ferruginous outcrop. Ore said to yield 50 per cent lead 

and 20 to 30 ounces of silver. Two-compartment shaft, 250 feet deep, 

with outfit of bucket hoist, 40-horsepower engine, boilers, etc.

Hidden Treasure. Galena and carbonate ore in small body. Prospect shaft, 200 feet deep, with crosscut. No machinery save horsewhim.

On the high bench between the headwaters of Warm Spring and 

Alibaugh creeks there are.several prospects, among them the Princess, 

American, and Battler claims. A little high-grade ore has been found, 

but no bodies of sufficient size to warrant development have thus far 

been discovered, nor do the conditions appear favorable at this point.

On the open grassy mountain spur between the diorite and granite 

intrusions there are a number of claims located. The rocks are altered 

Belt shales cut by intrusive sheets of porphyry and basic dikes, a number of the claims being located upon the decomposed outcroppings of 

the latter.

, Tip-Top, the highest claim on the mountain, is located on a contact 

of dense, baked Belt shale and a decayed minette dike. The shaft is 

160 feet deep and as yet shows but little ore. The contact rocks contain epidote and other minerals.

Little May, located January 3,1893, shows frothy-white and ferruginous-brown quartz, with brown jasper stained with ore. JS"o workable 

ore seen at dump. The ore occurs as chalcopyrite, a decomposed 

minette dike cutting the Belt shales and porphyry sheets.

Several other prospects were examined on the ridges, but none showing workings sufficient to enable'One to judge of the size and character 

of. the ore bodies, though scattered bits of copper-stained ore were 

noticed at several prospect pits. One or two claims have been located 

on pyritiferous seams in the diorite mass, though but little work has 

been done and the ore extracted looks unpromising. The principal 

claims are those back (i. e., west) of Blackhawk settlement and on the 

mesa between Blackhawk and Eobinson.

Grasshopper mine. Wherever the white Paleozoic limestones are 

found in the contact zone about the granite mass the locality is marked
156 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

by prospect pits. This is particularly the case along the northern 

flanks of the mountain, where mine cabins and dump heaps are prominent features of the landscape. On the low limestone ridge east of the 

most westerly branch of Fourmile Creek there are several of these prospects in the slightly altered rocks, and on the divide between this 

stream and Willow Creek is the claim known as the Grasshopper mine. 

The ore is manganiferous, with some galena, and occurs in a pocket in 

slightly altered black Silurian limestones. A shaft house has been 

erected over the mine, and several cabins are located near by. PI. XV 

shows the upper gorge of Founnile Creek, from a photograph taken at 

the mine, and shows the heavily forested slopes, steep gorges, and 

crags of the granite, which are in such strong contrast to the limestone

areas.

COPPER.

The copper veins of the region north of Castle Mountain have been 

known for many years. As early as 1867 they attracted the attention 

of the pioneer fortune-hunters of the State, and in 1874, when seen by 

Grinnell and Dana, there was a shaft 40 feet deep on one of the claims, 

two men taking out ore to be shipped by steamer down the Missouri 

for smelting at Baltimore. For many years maps of Montana have 

shown the name of Copperopolis, a supposed settlement on the site of 

these veins, which really consists now, as it did in 1875, of a mining 

shaft and deserted log cabin: Notwithstanding this long period during 

which these reefs have been known, mining has in the interval been 

confined to mere surface digging or "gophering" of the deposits. A 

few pits have been sunk for the purpose of holding the claims, but without any attempt to gain a better knowledge of the ore bodies. In consequence as little is known to-day as in 1874. There are, in fact, no 

mines whatever j a number of prospects are located, and a few claims 

have been patented, but the place is all but totally abandoned.

The copper veins are all in the shales of the Belt formation. At a 

number of places this formation has been found to be penetrated by 

small fissure veins that are sometimes on shear planes. On Sixteenmile Creek, at the south end of the Big Belt Eange, as at the present 

locality and on Upper Spring Creek, the shales are traversed by narrow veins of quartz and calcite carrying cupriferous sulphurets, with 

oxidized minerals at the surface. Nowhere do these veins pass upward 

into the overlying formations, for which reason they must be regarded 

as antedating Middle Cambrian times. Their narrowness is perhaps 

due to the fact that they are the roots or lower parts of veins whose 

upper and wider parts have been worn off during the long processes 

of degradation that have removed thousands of feet of rock and produced the present features of the land.

The ores, if only found in sufficient quantity, are rich enough to 

warrant exploitation even in this remote location, but the openings 

thus far do not expose enough to warrant any favorable predictions as 

to the value of such development.
COPPER. 157 

MINING CLAIMS.

Copperopolis. The veins here are fracture planes in Belt shales. In 

the coulee these shales are of pinkish-lavender color and dip 20° into 

the hill.

The following claims are located along the outcrop of one of the 

veins: Copper Duke,now known as the Virginia; the Ohio, a patented 

claim, covering 2,200 by 100 feet, and the vein here having a trend of 

S. 57° 30" E.; the Albany; and the Standard.

The St. John is a patented claim 2,200 by 100 feet. The Northern 

Pacific lies to the northeast, and the Darling Fraction to the southwest.

bearing Copper Creek the lead bends, and the Hecla and East Hecla 

are located upon it, the ledge running S. 69° E. and crossing the creek c 

diagonally.

Calumet claim has a trend of N. 75° E. magnetic. It is a calcite 

claim, dipping 60° to northwest, and is in gray shales reddened at surface. The lead is 4 feet wide where opened by shallow pit, and shows 

crushed shales impregnated with copper stain.

The Northern Pacific is an old mine, opened in 1867, when ore was 

shipped down the Missouri River. The ore is said to average 45 per 

cent and to run as high as 75 per cent copper. The drift and shaft on 

this claim have both caved in, and though there are several openings 

but a few feet apart along the lead, it is not well exposed. The lode 

matter is largely calcite, and the width 10 feet, the calcite being 

streaked with ore.

The Darling Fraction is opened by a shaft 75 feet deep, with a log 

shaft house. It shows at best 4 feet of sulphurets. A little ore was 

shipped from here four or five years ago, and bags of it now lie in the 

shaft house. East of this claim the vein is opened by two pits about 

30 feet deep.

The Copper Duke, now called the Virginia, is a claim on the hillside, 

a half mile from these. The ore is a sheared shale impregnated with 

quartz, forming a seam 3 feet wide of good-looking oxidized ore.

On the slopes north of the Musselshell, northeast of the stage station, 

there are a couple of openings on narrow seams. At one the shaft is 

30 feet deep, 3 by 6 feet, and timbered. The lead is 2 feet wide at the 

outcrop, has well-defined, clean walls, and shows quartz, rust-stained 

on cracks, with gray copper and pyrite; a rather lean ore. The lead 

runs east and west magnetic, and lies in soft gray shales. The slopes 

of the hill are bare, rounded, and smooth.

Below the mouth of Checkerboard Creek there are a few copperstained leads in the slopes north of the Musselshell; trend N. 65° E., 

hade 75° K, 15 to 10 inches wide. Ore pyritic, occurring in a crushed, 

soft gray shale along a shear zone. This is owned by J. W. Hinckley.

The Richmond is another lead, trends S. 65° W., is mineralized, shear 

plane in clay; sulphurets, malachite and azurite. Shaft, 30 feet deep.

In the Belt area forming the mountain basin of Spring Creek the 

shales are crossed by narrow ledges stained with copper; not of 

sufficient importance to warrant development.
CHAPTER. XI.

BRIEF NOTES ON MINERALS OF THE CASTLE MOUNTAIN

DISTRICT.

The following list includes only those minerals which have been seen 

in the progress of the survey or have been reported by trustworthy 

observers. It does not, of course, pretend to be iu any way complete. 

To make a thorough examination of the various mines, ore heaps, and 

dumps, would have required far more, time than would have been warranted by the results that could have been obtained. It is therefore to 

be expected that in time this list will be greatly enlarged.

ALBITE: NaAlSisOa. This feldspar has been observed only as a secondary rock constituent in the porphyry.

ANGLKSITE : PbSO.,. Lead sulphate has not been observed by the writers, but is 

stated to occur at several mines. Occurs associated .with cerussite and is-pro> 

dnced by the oxidation of the galena (PbS), the chief ore of the district.

ANORTHOCLASE : (NaK) AlSi308 . This feldspar is one of the chief constituents of the 

granite, as mentioned in chapter vi, pages 82-83.

APATITE : Ca(ClF)Ca4 (PO)3. So far as observed, occurs only as minute crystals scattered through the igneous rocks.

AZURITE: 2CuCO:i, Cu(OH)2 . The blue carbonate of copper is found in several places 

but not in large quantities. The prospect shafts at Copperopolis show it in 

massive form mixed with malachite and chrysocolla. At the Eichmoud prospect 

shaft, east of the Musselshell Canyon, it occurs in small crystals with malachite. 

The crystal faces are apt to be dull and rounded, and the crystals are hence unsuited for measurement. They are composed chiefly of the faces c (001), d (101), 

and a (100).

BARITE: BaS04 . Barium sulphate or heavy: spar is a rather common mineral in 

many of the ore veins and deposits, and is especially associated with the copper 

ores, of which it often forms the gangue. Good crystals not observed.

BIOTITE: This black mica exists only as a rock constituent in small crystals. The 

variety called phlpgopite, poor in iron or free from at, occurs, as mentioned on 

page 93, as a contact mineral in the limestones at Blackhawk. It strikingly 

resembles the same mineral from Edwards, N. Y. It occurs in plates an inch or 

(at times) more in diameter.

CALCITE: CaCOa. Drusy cavities in the limestones are frequently lined with crystals of the carbonate of lime. It is^ahnost invariably. crystallized in the common' scaleneohedron v I3 (2131)j occurs also in cavities in the ore veins and in 

amygdules in the vesicular basalt of Volcano Butte.

CERUSSITE : PbC03 . The carbonate of lead is a very common constituent of the ore 

bodies in the limestone areas. It occurs as an alteration product of the galena, 

and is commonly associated with it. Splendid crystallizations are found at the 

Judge Miue at Blackhawk, though, unfortunately, at the time of the writers' 

visit, as active work was not being carried on, they were -unable to see or secure 

some of the best. Nevertheless) through the courtesy of Mr. Davis, the foreman 

of the mine, some excellent material was obtained. This has been studied under 

the writers' direction by Mr.. J. H. Pratt, 1 of the Sheffield Scientific^School, and 

the results of his investigation are here given : - 

"The specimens are in the form of rough masses, at times as large as one's fist,

i See also Am. Jour. Sci.j Vol. >XL VIII, 1894, p. 212. 

158
WEED AND"] 

P1KSSON. J

NOTES ON MINERALS. 159

showing an occasional broad crystal face. These fragments are remarkably 

clear and free from foreign inclusions. Attached to these are small, bright crystals, both simple and twinned. Their faces are generally smooth, giving very 

good reflections of the signal when, measured on the reflecting goniometer.

" Specimens of the simple crystals, which are represented in fig. 10, were 

found so attached to the rest of the mineral that the faces on both ends of the 

lateral axes were developed and could be measured. The forms observed.on 

them were

6 (010) i-4 i (021) 2-t

c (001) 0 v (031) 3-t

ro(110) I p (111) 1

x (012) H? .

"The greater part of the crystals observed, however, were twinned, the 

twinning following the common method in which the twinning plane is the 

unit prism m, I, (110). These crystals are represented by fig. 10 1. As shown in

Fid. 10. Cerussite crystals, a, simple crystal; 6, crystal twinned ou 110. Measured and drawn by

J. H. Pratt.

the figure, they are extended in the direction of the twinning plane, and are 

so attached against the smooth surfaces of larger crystals that the twinned 

pinacoids at the other end are wholly wanting, the crystal being cut square off 

at this point, as in the figure. They resemble strongly the untwiuned ones. 

The following table shows the identification of the forms by calculated and 

measured angles.

"For obtaining the calculated angles the elements of Koksharov1 have been 

used, in which

a: 1: c = 0.609968 : 1: 0.723002.

Calculated.

mf\in

m/\p

P/\P

o/\x

o/\i

c/\v

m/\m

«AJ>

HOAllO

HOAlll

111 A 111

001A012

001A021

001A031

HOAllO

llOAlH

0

62

35

49

19

55

65

(twin) 54

(twin) 43

/

45

46

59

52

20

15

28

36

a

50

30

30

30

30

o

62

35

49

19

55

65

54

43

Measured.

/ 45 "

46

58

49

19

9

28

394

o

62

35

49

19

55

65

54

i

45

45

574

51 '

19

11

29

29

o

62

35

54

/

454

464

26

'As given in Dana's.Mineralogy, 6th ed., 1892, p. 286.
160 THE CASTLE MOUNTAIN MINING DISTKICT. [BULL. 139.

CHALCOPYRITE : CuFeS2 . Copper pyrites occur in a number of the leads, and in the 

Copperopolis prospects is the chief ore after the water level is reached. The 

other ores of copper are products of its alteration. Distinct crystals have not 

been observed.

CHLORITE : This greenish micaceous mineral is a common alteration product of several silicate minerals. Occurs abundantly at the Little May prospect back of 

Robinson, disseminated as a scaly product in the earthy, decomposed material 

on the dump, which is stained by the alteration of copper ores and appears to 

be a greatly decomposed basic rock of lamprophyre character. Occurs also as a 

common alteration product in many of the igneous rocks, giving them their 

greenish tone.

CHRYSOCOLLA: The silicate of copper is found, with other copper ores, at many 

points in the district. Excellent specimens occur at Copperopolis, and on Sixteenmile Creek at the Copperopolis lead are beautiful examples associated with 

barite, quartz, etc.

EPIDOTE: A common product of the alteration of the Belt shales by the intrusion 

of the Castle Mountain granite. Occurs in thin seams usually coating joint 

faces of the rock masses of the characteristic yellowish-green color. Small 

crystals have been observed, but not of sufficient size or excellence to merit 

further description.

FLUORITE : Occurs as a rock-forming mineral in the quartz-porphyry of Fourmile 

Creek, as mentioned on page 99. Is reported also as occurring in the form of a 

gangue mineral in some of the leads of the district, but was not observed by 

the writers.

GALENA : PbS. The sulphuret of lead is the chief ore of the district, as described on 

page 151. Is generally massive, but was observed at times in the usual cubic 

crystals. .

GARNET: The only occurrence of this mineral seen by the writers was in the metamorphosed limestone near Blackhawk. This was the lime-alumina garnet, 

grossularite, of a pale oil-green color, usually massive, but at times in distinct 

dodecahedrons, associated with vesuvianite and white phlogopite. (See p. 93.)

GYPSUM: CaS04 .2H20. The hydrated sulphate of lime occurs in thin seams in the 

Belt Mountain rocks about 4 miles north of White Sulphur Springs. Is fibrous 

in structure.

HEMATITE : Fe^Os. This ore of iron does not occur to the writers' knowledge in any 

quantity in the district, though a common coloring; nicaterial in many of the 

slates and shales.

KAOLINITE : Clay is found at a number of places in the district, but not pure enough 

to become a commercial product.

LABRADORITE : Occurs only as a rock-forming mineral in the dark-colored igneous 

rocks of basaltic character.

LIMONITE : 2F20«, 3H2 0. This ore of iron occurs as an alteration product in many 

of the ore veins, and is disseminated through the stratified rocks, though nowhere 

occurring in quantity.

MAGNETITE : Fe304 . This iron ore occurs as a rock-forming mineral and is the chief 

constituent of the black sandstone occurring with the impure seam of coal 

found on Warm Spring Creek (see p. 149), where it was mined as a flux for the 

Castle smelters.

MALACHITE : CuC03.Cu(OH)2 . The green carbonate of copper is a common mineral, 

being found in many of the mines and prospects, and especially at Copperopolis. 

On Sixteenmile Creek at the Copperopolis lode it occurs in distinct crystals; 

elsewhere it is usually massive^ A large amount of useless labor has been performed by-the prospectors in this region from inability to realize how large an 

amount of rock may be colored vividly green by a very small quantity of thia 

mineral. Like the azurite, it results from the alteration of copper pyrites.
WEED AND! 

1'IKSSON: J

NOTES ON MINERALS. 161

MINIUM: PbaO.|. This rare mineral, the red oxide of lead, occurs at, the Little 

Jumbo mine near Castle. It is found in pulverulent earthy masses of a vivid 

red color. It occurs at or near the surface and has been supposed to be an alteration product of cerussite from forest fires, an origin which must be regarded as 

doubtful.

MUSCOVITE : This mica occurs as an alteration product of biotite as a rock-forming 

mineral. Has not been otherwise observed.

OLIGOCLASE : Found only as a constituent of the igneous rocks.

ORTHOCLASE : See page 83.

PIILOGOPITE : See Biotite.

PYRITE : FeS2 . Iron pyrite is an extremely common mineral in most of the mines in 

the district. Was observed in small but excellent crystals in the prospect shafts 

sunk in the diorite on the hill southwest of Blackhawk. The crystals show the 

usual striated combination of the cube and pyritohedron.

PYROLUSITE : Mn02 . The black oxide of manganese occurs in the hill north of Blackhawk, where it forms an ore body of undetermined size. It has been mined below 

the Judge and Legal Tender mines, the rock in which it occurs being thought 

suitable for fluxing. Occurs in soft earthy forms, and massive distinct crystals 

wore not observed.

PYROXENE: As has been previously stated, pyroxene occurs onupper Bonanza Creek 

as a product of the rnetamorphism of the Cambrian limestone by the intrusion

m a m

FIG. 11. Pyroxene crystals of altered limestone.

of the diorite mass. A prospect shaft on the hillside south of the stream, about 

a mile below Blackhawk, shows excellent material. The limestone is completely 

converted iut6 masses of pyroxene and calcite at this point. In some of'these 

masses the crystals of pyroxene are so thickly embedded in the calcite that at 

first glance the rock appears as if completely made of them. 

 The crystals are usually small, not exceeding 1 centimeter in length, except 

where projecting into a cavity. In color they are dark-green, and are clearly of 

the diopside variety. The faces are apt to be dull in luster, as is so often the 

case in examples embedded in calcite. Except where crowded so thickly together 

as to interfere with one another's growth, they are usually symmetrically developed, both ends being present, and they have, as a rule, the habit shown in 

fig. 11 a. At times, however, they are stouter and show a greater complexity of 

forms, as illustrated in fig. 11 I). It often-happens, indeed, that in such crystals 

the zone of pyramids has its faces curved and rounds down into the prism, the 

crystal then assuming a seed-like form. A few instances of twinned crystals 

were noted, which were, however, of quite different habit from the aboA'e. One 

of them is shown in fig. 11 >c. 

Bull. 139 11
162 THE CASTLE MOUNTAIN MINING DISTRICT. [BULL. 139.

The_ following forma have been identified on .these crystals: 

a 100 « 111 

& 010 o 221 

m 110 A 331 

z 021

Of these, the form A(331) is by no means common. The faces of these crystals do not as a rule reflect light well when placed on the reflecting goniometer, 

giving blurred and confused reflections, owing to minute pitting, etc. The 

measurements are, however, sufficiently accurate to identify the forms, as may 

be seen by the following table of calculated and measured angles.

Calculated. Measured.

oAm 100A110

m/\z 110A021

oAo 22IA22I

AAA 331 A 331

mAA 110A331

m/\o 110A22I

mAs HOAllI

46 25

48 6

84 11

91 35

24 27

35 29

58 48

46 26

47 55

84 27

91 48

24 43

35 03

58 20

46 41

24 18

34 51

QUARTZ : SiOs. Is of so common occurrence that it needs no further mention. 

Large, well-formed crystals have not been observed.

EUTILE : Ti02 . Only as a rock constituent.

SERPENTINE : Only as a rock constituent.

TOURMALINE : Occurs only as a rock constituent.

VESUVIANITE : This silicate occurs with the garnet previously mentioned, in brownish grains and masses showing an occasional crystal face, but not in distinct 

crystals. (Seep. 93.)

WULFENITE : PbMoO-j. This mineral occurs at the California mine, and the writers 

are indebted to the courtesy of Prof. F. W. Traphagen, of Bozeman, Mont., for the 

opportunity of examining a specimen. It occurs in cavities in massive galena 

and is of the usual red-orange color. The luster of the crystal faces is dull, asif caused by superficial alteration. The crystals are octahedral in habit, being 

composed of the unit pyramid n(lll) alone, and the largest seen was nearly half 

an inch long.
INDEX.

Achiarcli,A.d', cited..................... 101

Algonkian rocks, description of........... 31-34

Alice mine, description of................ 155

Analyses of rocks.... 84,88,90,96,99,101,103,106,

J12,115,116,120,128,130,135,136 

Andrera, A., cited ........................ 129

Anticlines of the district................. 23

Aplitic granite, occurrence of............ ' 95-96

analyses of........................ 96,135,136

Ash deposits............................. 77-78

Augite-mica-dioriio, analyses of......... 90

Augite-vogesite, occurrence and character

of................................ 111-113

analysis of............................ 112

Biickstrom, H., cited...........'... 89,134,138,141

Basalt, occurrence and character of. 71-73,129-131 

analysis of ....................... 130,135,136

Belt formation, character, age, and thickness of.......................... 31,32-34

Benton formation, description of......... 46

fossils of...............:............. 46

Black Butte, rocks at..................... 108-109

Blackhawk mine, description of.......... 151

Bondholder mine, description of.......... 155

Bowlder moraines, examples of........... 144

Breccias and tuffs, occurrence" and charactor of.......'............. 73-76,127-128

Brogger, W. C., cited................ Ill, 132,138

Cambrian rocks, section of............... 36

description of........................ 34-37

Carboniferous rocks, description of...... 38-43

Castle, section near...................... 47-48

evidences of glacial action near...... 144-145

Castle Mountain, description of...... 15-16,19-20

' discovery of ore deposits at.......... 17,18

geological structure and history of... 22-29 

Cerussite, figures of crystals of.......... 159

Coal, beds of............................. 148-149

Checkerboard Creek, columnar section on. 36, 37

analysis of rhy olite from............. 125

Colorado group, description of rocks of.. 45-46 

Contact metamorphism, examples of. 62-64,92-95 

Cope, E.D., cited......................... 53

Copper, deposits of ...................... 156-157

Copperopolis, mines at................... 157

Crazy Mountains, rocks of......... 78-79,109-110

Cretaceous formations, table 'of......,..- 30

Cretaceous rocks, description of......... 44

Cross, W., cit&d.......................... 122,123

Cumberland mine, description of......... 152-153

Pago. 

Dakota beds, description of............... 45

Dana, E.S., cited......................... 17-18

Dana, J". D., cited......................... 82

Dana and Gfinnell, cited......... 17-18, 53,79,139

Dawson, Gr. M., cited ..................... 147

Devitrification, character of.............. 122-124

Devonian rocks, description of........... 38

Diabase, occurrence and character of..... 76-77

Dike rocks, character of.................. 141-142

Dikes, occurrence and character of....... 65-69

Diller.J.S.,cited......................... 96,129

Diorite,analysis of...................... 135,136

occurrence and character of......... 61-62,89

Drainage of the district, character of.... 20-21

Eldridge, G-. H., cited.:...:............... 18

Ellis limestones, description-of........... 43

Extrusive rocks, description of .......... 69-73

Feldspar-porphyry, analysis of........... 135,136

Felsite-porphyry, occurrence and character of ......!..............' ....... 103-106

Flagstaff Creek, fold and fault exposed on. 24 

Flathead formation, description of....... 34-37

Foldsof thedistrict..................... 24-26

Fossils identified............... 37,43,45,46,52,55

Fourmile Creek, glacial deposits on. . 145-146 

Fox Hills sandstoneSj description of...... 47

Gallatin limestones, description of........ 37

fossils of............................. 37

Glaciation, evidences of.................. 143-147

Glacial drift.-lo'wer ltmit-of..:..;......... 144

Gold, production of....................... 151

Granite, occurrence and character of..... 58-61,

81,82-85 

analysis of........................ 84,135,136

included masses in................... 87-89

(aplitic), occurrence of ............... 95-96

(aplitic), analysis of............... 96,135,136

Granite-porphyry, occurrence and character of .............................. 85-87

Grasshopper mine, description of........ 155-156

Great Eastern mine, description of....... 154

Grinnell and Dana, cited......... 17-18,53,79,139

Hague, A., cited.........................

Hidden Treasure mine, description of....

133

155

Iddings, J-. P., cited......................... 67,

86,102,109,110,122,129,133,137,138,140 

Igneous rocks, description of......... 56-79

Iron Chief mine, description of .......... 155

Irving, E. D., cited ...................... 86

163
164 INDEX.

Page. 

Jefferson formation, description of....... 37-38

Johnson-Lavis, cited..................... 133

Judge mine, description of............... 154-155

Juratrias rocks, description of........... 43-44

fossils of............................. 43

Kemp, J.F., cited ....................... 114,116

Kokscharow,cited ....................... 83

Kootanie beds, description of............ 44

Lacroix, A., cited.................... 126,130,134

Lake-bed deposits, description of........ 53-55

Lamprophyre rocks, occurrence and character of............................ 110-117

Laramie formation, description of........ 48-49

Lava flows, description of................ 69-73

Lawson, A. C.,cited.......:.............. 131

Legal Tender mine, description of........ 154

Lindgreu.W., cited...................... 117,139

Little May mine, description of.......... 155

Livingston formation, description of..... 49-53

Ludlow, Captain, cited................... 17-18

Madison formation, description of........ 39-41

Metamorphism, occurrence and character

of.............................. 62-64,92-95

Michel-L6vy, A.,cited ................... 90

Monchiquite, occurrence and character

of.................................. 114-117

analyses of............... 115,116,117.135,136

Montana group, description of rocks of.. 47-48 

Moraines, examples of ................... 144

Micropegmatitic structure, secondary origin of.............................. 86-87

Microscopical petrography............... 80-131

Minerals of the district, list of........... 158-162

Mineral springs, location and character

of.................................. 149-150

Mines of the district, notes on........... 152-156

Minette, occurrence and character of..... 113-114

analysis of........................... 117

Miocene bods, description of............. 53-55

Mnsselshell Canyon, section on.......... 41-42

Musselshell Eiver, course of...........^. 20-21

section on............................ 40

analysis of quartz-porphyry from.... 103

ITewberry, J. S., cited..............

Niobrara formation, description of.

Ores of the district, character and distribution of........................... 151

Peale, A.C., cited........................ 149

Petrology of the district.................. 132-142

Phillips, J. A., cited...................... 101

Pierre shales, description of............. 47

Pirsson, L.V.,cited...................... 98,142

analyses by.....................'.... 84,88,90,

96, 99,101,103,106,112,115,120,128,130,135 

Pitchstone, analysis of...... '. ............ 135,136

Polishing powder, deposits of............ 149

Porphyry, intruded sheets and dikes of.. 64-69

Page.

Porphyry, occurrences and characters of. 97-110 

analysis of....................... 106,135,136

Pyroxene crystals, figures of............. 161

Quadrant formation, rocks of............. 41-43

Quartz-free porphyry, occurrence and

character of........................ 106-108

Quartz-porphyry, occurrence and character of...................... 81,97-99,102-103

analysis of.................... 99-103,135,136

Quartz- tourmaline-porphyry, occurrence

and character of.................... 99-102

analysis of....................... 101,135,136

Rhyolite, occurrence and character of.... 69-71,

81-82,118-127

analysis of................... 120,125,135,136

Rhyolite-pitchstone, analysis of.......... 135,136

Rhyolitic tuffs, occurrence and character

of.................................. 127-128

analyses of............................ 135,136

Riggs, It. B., analysis by................. 149-150

Romberg, J., cited ........................ 86

Rosenbusch, H., cited...... 78, 96,101,102,109,114

Scott, W.B., cited ....................... 53,'55

 fossils found by...................... 55

Sedimentary formations, description of.. 30-55 

table of................................ 30-31

Sheffield Scientific School of Tale University, acknowledgments to....... 13

Silurian rocks, d escription of............. 37-38

Silver, production of............ ........ 151

Smith River, character of................ 21

naming of............................ 28

section at head of.................... 49-51

Stanton, T. "W., fossils identified by.. 43, 45,46, 52 

Stratified rocks, description of........... 30-55

Syenite-porphyry, occurrence and character of................................. 106-108

Syenite, analysis'of...................... 135,136

Three Forks shales, description of....... 38

Tip Top mine, description of............. 155

Tschermak, G., cited..................... 103

Tuff, analysis of..................... 128,135,136

Tuffs and breccias, occurrence and character of...................... 73-76,127-128

Vogesite, analysis of..................... 135,136

Vogt, J. H. L., cited..................... 67

Volcano Butte, analysis of basalt from... 130

Warm Spring Creek, coal beds on..'.. '.... 148-149

Waters of mineral springs, analyses of.. 150-151 

Weed, W. H., cited .................... 44,49,139

Williams, G. H., cited ................... 86,117

Wolff, J. E., cited.................. 18,78,109,138

Yale University, acknowledgments to

Sheflield Scientific School of........ 13

Tellowstone mine, description of........ 154

Zirkel, F., cited................ 80,85,101,112,117