DEPARTMENT OF THE INTERIOR

BULLETIN

OF THB

UNITED STATES

GEOLOGICAL SURVEY

No. 157

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1899

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIRECTOR

THE

OF

SOUTHWESTEEN MINNESOTA

BY

CHRISTOPHER WEBBER HALL

WASHINGTON

G-OVERNMBNT PRINTING OFFICE 

1899

CONTENTS.

Page.

Chapter I. Introduction.................................................... 9.

Chapter II. Summary of earlier observations............................... 12

Separate publications................................................... 12

Serial publications...................................................... 17

Chapter III. Geographical distribution and general characters of the rocks

of the Minnesota River Valley............................... 20

Courtland district ....................................................... 20

Quartzite exposures......................-...----.----..--- - .- - - 20

Exposures near Bedstone........................................ 21

Exposures near New Ulm........................................ 23

Probable continuation of the quartzite in surrounding areas......... 23

Age of the quartzite....... ........................................... 24

Gueissoid granite exposures......................................... 25

Fort Ridgely district.................................................... 25

Gneissoid granite ................................................... 25

Area northeast of Golden Gate.................................. 25

Area west of Fort Ridgely ...................................... 26

Basic rocks ................... ...................................... 26

Morton and Beaver Falls districts. ...................................... 27

Pre-Cambrian rocks................................................. 27

Lower Sioux Agency area. ...................................... 27

Morton area..................................................... 28

Beaver Falls area ............................................... 29

Redwood River area............................................. 30

Area above Redwood River...................................... 31

Origin of kaolinic gneisses ...................................... 32

Vicksburg district ...................................................... 32

Gneisses ............................................................ 32

Gabbro-schists...................................................... 33

Granite Falls district.................................................... 34

Montevideo district............... ...................................... 36

Lac qui Parle district................................................... 38

Ortonville district ...................................................... 38

Redwood River Valley district.......................................... 41

Post-Cambrian deposits................................................. 42

Chapter IV. Gneisses ...................................................... 46

General considerations .................................................. 46

. Mineral constituents of the various gneisses............................. 49

Primary quartz ....................................... .......... 49

Secondary quartz............................................... 51

Feldspars............;.................................... ...... 51

Biotite ......................................................... 53

Muscovite ...................................................... 54

Hornblende..................................................... 54

Augite.......................................................... 57

5
6 CONTENTS.

Chapter IV Continued. Page. 

Mineral constituents Continued.

Pyrite...... .................................................... 57

Magnetite ...................................................... 58

Hematite....................................................... 58

Limonite ....................................................... 58

Ferrite..................................................... .... 58

Entile.......................................................... 58

Apatite....... .'. ............................ 1................... 58

Kaolin.......................................................... 59

Epidote ........................................................ 59

ChloTite..................................i........:............ 59

Hornblende-biotite-granite-gneisses..................................... 60

Ortonville area ..................................................... 60

Vicksburg area ......................................... ............ 63

Fort Eidgely area .................................................. 64

Augite-biotite-granite-gneisses...... ......:............................. 66

La Framboise area.. ................................................. 66

Granite Falls area .................................................. 68

Area in T. 114 N., R. 37 W............................................ 69

Hornblende-biotite-gneisses............................................. 72

Mortonarea...... ................................................... 72

Beaver Falls and Redwood Falls areas............................... 75

Chapter V. Gabbro-schists ................................................. 77

General considerations .................................................. 77

Mineral constituents of the gabbro-schists.................... ....^...... 80

Hypersthene-bearing gabbro-schists..................................... 82

La Framboise area.................................................. 82

Wabasha Creek area ................................................ 84

Minnesota Falls and Granite Falls areas ............................. 85

Montevideo area .................................................... 93

Odessa area......................................................... 94

Hypersthene-free gabbro-schists ........................................ 98

Pyroxene-free gabbro-schists or gabbro-diorites.......................... 106

Chapter VI. Peridotite and serpentine...................................... 110

Peridotite .............................................................. 110

Serpentine.............................................................. 114

Chapter VII. Dikes........................................................ 115

Occurrence ........^.................................................... 115

Age.................................................. ................... 117

General petrographical characters ....................................... 118

Macroscopical................................................... 118

Microscopical ...............................\................... 119

Morton district.......................................................... 121

Vicksburg district ...................................................... 122

Granite Falls district ................................................... 123

Montevideo district..................................................... 126

Exceptional dikes....................................................... 127
ILLUSTRATIONS.

Page. 

PLATE I. Map of a portion of southwestern Minnesota, showing outcrops of

crystalline rocks............................................... 9

II. Geological map of the Courtland district......................... 20

III. Eolation of the gneissic series to the basal conglomerate near New

Ulm ........................................................... 22

IV. Geological map of the Fort Ridgely district....................... 24

V. Geological map of the Morton district ............................ 26

VI. Geological map of the Beaver Falls district....................... 28

VII. EedwoodEiver Falls ............................................. 30

VIII. Geological map of the Vickshurg district......................... 32

I.. View of rock exposures in the central valley, Vicksburg district... 32

X. Geological map of the Granite Falls district ...................... 34

XI. Geological map of the Montevideo district........................ 36

XII. Geological map of the Lac qui Parle district...................... 38

XIII. Geological map of the Ortonville district ......................... 40

XIV. Granite quarry at Ortonville ..................................... 40

XV. Geological map of the Redwood Eiver district.................... 42

XVI. A, Granite-gneiss, Ortonville quarry; B, Contorted hornblendebiotite-gneiss, Morton quarries ................................. 134

XVII. A, Chloritic gneiss; B, Unaltered hypersthene-gabbro............ 136

XVIII. A, Hornblende-biotite-gueiss muscovite formed by the alteration 

of an albite inclusion in microcline; B, Vermicular quartz within 

an albite-anorthite feldspar of low extinction....... .. .......... 138

XIX. A, Gabbro-schist hornblende in granules within a plate of diallage; B, Contact zone between microcline and oligoclase....... 140

XX. A, Porphyritic gneiss biotite in radial clusters within quartz; 

B, Granite-gneiss microcline within oligoclase; C, Hornblendebiotite-gneiss lenses of quartz in biotite....................... 142

XXI. A, Hypersthene-gabbro; B, Gabbro-diorite ...................... 144

XXII. A, Gabbro in which the monoclinic pyroxenes have been almost 

entirely altered to a granular hornblende, biotite, and quartz; 

B, Garnetiferous gabbro in typical development at the " gold 

mine " j ust south of Granite Falls............................... 146

XXIII. A, Gabbro-diorite anastomosing of veins of hornblendic material 

between feldspars; B, Diallage as a matrix containing the other 

mineral constituents as inclusions............................... 148

XXIV. A, Hand specimen of porphyritic gabbro, west part of Granite

Falls; B, Broken apatite needles in labradorite feldspars....... 150

XXV. A, Normal diabase of the Minnesota River Valley; B, Typical

ophitic diabase, from a dike at Minnesota Falls................. 152

XXVI. A, Contact of finely granular dike and hypersthene-free gabbro; 

B, Areas of quartz extinguishing simultaneously within a corroded feldspar.................................................. 154

7
8 ILLUSTRATIONS.

Page.

PLATE XXVII. A, Somewhat altered peridotite, var. saxonite; B, Serpentine derived from the alteration of saxonite .............. 156

FIG. 1. A township from the United States Government survey T. 110 N.,

R. 30 W., Minnesota............................................... 9

2. A section 1 mile square (i mile = 2,000 paces)........................ 10

3. Section from Courtland to Pipestone, Minnesota (after James Hall)... 16

4. Hypothetical section from Courtland to Minneopa .................... 24

5. Contorted hornblende-biotite-gneiss (railway cut at Morton, 150 feet

long) ............................................................. 28

6. Contact of granite-gneiss and hornblendic hypersthene-gabbro-schist

near Montevideo ........ ........................................... 37

7. Exposure of granite-gneiss a.nd hornblende-biotite-schist, showing 

alternation of the two rock species (sec. 15, 1,200 paces N., 1,600 

paces W.,T. 120 N.,R. 45 W., Minnesota)........................... 39
U. S. GEOLOGICAL SURVEY BULLETIN NO. 157 PI. I

MAP OF A PORTION OF SOUTHWESTERN MINNESOTA, SHOWING OUTCROPS 

OF CRYSTALLINE ROCKS.
H: GNEISSES, GABBRO - SCHISTS, AND ASSOCIATED 

EOCKS OF SOUTHWESTERN MINNESOTA.

By 0. W. HALL.

OHAPTEE I.

INTRODUCTION.

The territory from which the material for this paper was gathered 

embraces that part of the Minnesota Eiver Valley lying between 

Oourtland and Ortonville and that portion of the State of Minnesota 

to the west and south of the river named wherever gneisses are found. 

(See PI. I.) The area is about 5,500 square miles in extent.

FIG. 1. A township from the United States Government survey T. 110 N., E. 30 W., Minnesota.

This entire area has been platted by the land surveyors of the Government, and geologists are thus enabled to locate accurately every 

sample gathered. Wherever in the following pages a definite location 

is given, the township and range "are named, and the paces, on the 

basis of 2,000 to the mile, north and west from the southeast corner of

9
10 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

the section are stated. Occasionally, however, it was found that the 

surveyors' rno'numents and section posts had disappeared, and then 

the distances were estimated.

Although the area embraced in this study reaches well up to the 

headwaters of the Minnesota Eiver, the rock elevations do not attain 

great heights above the sea. Probably no outcrop occurs at a higher 

elevation than 1,200 feet. This portion of the State is deeply drift 

covered, and wherever the rocks appear above the present high-water 

mark of the Minnesota River they protrude in well-rounded knobs or 

bosses, sometimes of small size, and never of great extent, either in 

height or in area.. As a consequence of the heavy drift covering, we 

find few outcrops upon the prairie beyond the morainic bluffs which 

bound an ancient flood plain of the Minnesota Eiver. These few are

s.w.'4

N.W.orN£. N.E.o'N£.

\ 

 -------

S.W.OFN.E. S.E.OTN.E.

S.E.*4

3?io. 2. A section 1 mile square. One mile=2,000 paces.

the highest masses of an eroded rock surface protruding through the 

glacial drift.

The flood plain of the Minnesota Eiver is from 1 to 5 miles wide 

throughout its course from Ortonville to and beyond New Ulin, and 

was formed while the valley was a channel of outflow from Glacial Lake 

Agassiz, extending northward through the present Eed Eiver -Valley. 

Projecting through this flood plain are many exposures of gneisses, 

gabbro-schists, diorites, and diabasic effusives. In the 120 miles between Ortonville and New Ulin the rocks named " often fill the entire 

valley, 1 to 2 miles wide, rising in a profusion of knobs and hills 50 to 

100 feet above the river." l

Unless there be assumed a considerable elevation of the central portion of the continent, it must be supposed that during Glacial or preGlacial time the descent of the Eiver Warren was a gentle one. Such 

is that of its successor, the Minnesota Eiver, of post-Glacial time. 

This fact, and the presence, throughout the upper portion of its course,

1 The Minnesota Valley in the Ice age, by Warren Upham: Proc. Am. Assoc. Adv. Sci., 1883, p. 

215.
HALL.] INTRODUCTION. 11

of crystalline rocks, have determined the slight depth below the 

Warren flood plain to which the bed of the river has been eroded. 

Few rapids occur, and these are where the river crosses the strike 

of the gneisses. The Minnesota River at Oourtland stands 807 feet 

above the Gulf of Mexico, and the elevation of Bigstone Lake, a 

widening of the river near Ortonville, is 962 feet.

The few exposures occurring outside this ancient river valley lie in 

the towns of Omro, Posen, Echo, Vesta, Johnsonville, and Vaill, south 

and west of the Minnesota Eiver. The next exposures in these direC' 

tions beyond the townships named are the glaciated knobs of red 

quartzite in Oottonwood and Watonwan counties, and other quartzite 

exposures among the Coteau des Prairies called Shining Mouutai us 1 by 

the early explorers of the Northwest in Eock and Pipestone counties, 

from which, with many a superstitious ceremony, the Indian quarried 

the soft catlinite for his pipes.

North and east of the Minnesota Eiver the Leaf Hills and the uniformly 

thick beds of till cut off the Minnesota Eiver Valley series of gneisses 

and associated rocks from the granites and schists of the Mississippi 

Valley, whose most southwesterly exposures, so far as known, lie in 

Stearns County. 2

In locating the geological boundaries of the Minnesota Eiver formations, as has been done on the accompanying maps, sometimes an 

approximation only is possible. Wherever the drift covering conceals 

the underlying rocks the rock boundaries are denoted by dotted lines.

1 Travels through the Interior Parts of Forth America in the Tears 1766,1767, and 1768, by Jonathan Carver, Dublin, 1779, p. 45.

2 Preliminary report ou the geology of central and western Minnesota, by Warren Upham: Eighth 

Ann. Kept. Geol. Nat. Hist. Survey Minnesota, p. 75; and also The geology of Ottertail County, by 

Warren TJpham: Final Kept. Geol. Nat. Hist. Survey Minnesota, Vol. II, p. 546.
OHAPTEE II. 

SUMMARY OF EARLIER OBSERVATIONS.

In the mass of material published by the earlier explorers and the 

later scientific investigators who have given their attention to the 

northwestern Territories and States, little reference is made to the 

rocks which form the subject of the present study. For the purpose of 

historic and scientific reference the following list of titles and abstracts 

is made. The order is chronological.

SEPARATE PUBLICATIONS.

1. CARVER (Jonathan). Travels through the interior parts of North America in 

the years 1766, 1767, and 1768. Dublin, 1779.

Carver wrote an exceedingly interesting book, but he said very little 

on the geology of the Northwest. The St. Peter (Lower Silurian) 

sandstone is briefly mentioned, not by that name but as a snow-white 

rock crumbled outwardly by the wind and weather. The crystalline 

rocks farther up the Minnesota River Valley are not alluded to.

2. KEATING (William H.). Narrative of an expedition to the sources of the St. 

Peter's River, Lake Winnepeek, Lake of the Woods, etc., performed in the year 

1823, * * * under the command of Stephen H. Long, U. S. T. E. London, 

1825. 2 vols.

Professor Keating, of the University of Pennsylvania, geologist and 

historiographer of Major Long's expedition, indulged in many philosophical reflections on the character and constitution of the granitic 

rocks met with in the explorations of the party with which he was 

connected (Vol. I, pp. 362-382, and elsewhere). In the primitive rocks 

above Redwood Falls it seemed to Keating u asif four simple minerals, 

quartz, feldspar, mica, and amphibole, had united here to produce almost 

all the varieties of combinations which can arise from the association of 

two or more of these minerals, and these combinations were in such 

immediate contact that the same fragment might, as we viewed one or 

the other end of it, be referred to different rocks; while in some places 

granite was seen perfectly well characterized; in others a gneiss, micaslate, greisen (quartz and mica), compact feldspar (Weisstein of Werner), 

syenite, greenstone, and the syenite with addition of quartz, forming the 

amphibolic granite of D'Aubuisson, were equally well characterized. 

The only rock composed of these principles which we did not observe, 

12
HALL.] SUMMARY OF EARLIER OBSERVATIONS. 13

but which may perhaps exist there, is the graphic granite (pegmatite, 

Hauy).» (P. 365.)

Keating also advances the suggestion that the Coteau des Prairies, 

as the dividing ridge between the waters of the Mississippi and those 

of the Missouri, "is doubtless the grand dike which has obstructed the 

latter in its progress eastward and caused it to flow southwardly 

through a distance of many hundred miles, before it could again 

resume a direct course to the former." (Vol. II, p. 220.)

3. BELTRAMI (G. C.), esq., formerly judge of a royal court in the ex-kingdom of 

Italy. A pilgrimage in Europe and America, leading to the discovery of the 

sources of the Mississippi and Bloody river, with a description of the whole 

course of the former and of the Ohio. Printed for Hunt and Clark. London, 

1828. 2 vols.

Beltrami several times alludes to the rocks of the Minnesota Eiver 

Valley. Those at the Eedwood Eiver he speaks of as granite of a 

beautiful and varied quality. It is not necessary to make here any 

quotation from Beltrami's work.

4. FEATHERSTONHAUGH (G. W.), U. S. geologist. Report of a geological reconnoissance made in 1835, from the seat of government, by the way of Green Bay 

and the Wisconsin Territory to the Coteau de Prairie. Document 333. 

Printed hy order of the Senate. Washington, 1836.

Featherstonhaugh mentions gneiss at the Grand Portage (Granite 

Falls), east of Lac qui Parle, as dipping southeast, with an almost vertical inclination (introduction). He further says concerning this rock 

that it has "little or no mica" (p. 28), and.that it "resembles granite in 

every particular except its stratification " (p. 149). Granite, he says, 

was of frequent occurrence, and was the only rock seen in places beyond 

the mouth of the Waraju (Cottonwood) Eiver toward the northwest, 

the last seen described as consisting of "immense masses of granite in 

place, isolated from each other, and occupying several hundred acres. 

Some of these masses are 25 feet high; they extend 6 or 8 miles down 

the valley and give its name of Batatenka or Great Eocks to the lake 

[Bigstone Lake]." (P. 157.)

In a later publication 1 the same author mentions many localities 

between Eedstone (Courtland) and Bigstone Lake, where granites occur, 

and many details concerning them are given.

5. NICOLLET (J. N.). Report intended to illustrate a map of the hydrographical 

basin of the Upper Mississippi River. Senate Doc. 237, 2d sess., 26th Congress, February 16, 1841.

The valley of the Minnesota Eiver, together with other tributaries 

of the Mississippi, is mapped. The general trend of the Minnesota 

Eiver and the situation of its branches are portrayed with remarkable 

accuracy, considering the fact that there were no primary surveys at 

that time to give guidance to the work.

1 See T*To. 6, following page, Vol. I, Chapters XXVIII to XXXV.
14 GNEISSES, GABBRO-SCHISTS, ETC., OP SW. MINNESOTA. [BULL. 157.

6. FEATHERSTONMAUGH (G. W.). A canoe voyage up the Minnay Sotor. London, 

1847. 2 vols.

The geological observations of Featherstonhaugh in this expedition 

have already been referred to. He saw many bowlders of the primitive rocks, from escarpments of vertical granite and rugged granite 

hills "resembling parts of Dartmoor." (Vol. I, p. 3 VJ5.) Elsewhere he 

saw "immense masses of granite all quasi-stratified, in laminae about 

an inch broad." (Ibid., p. 333.)

7. ALLEN (Lieut. J.). Map and journal of, in charge of escort accompanying 

Schoolcraft's expedition to the sources of the Mississippi. American state 

papers, documents, legislative and executive, of the Congress of the United 

States from the first session of the Twenty-fourth Congress inclusive, Vol. V., 

Military Affairs, pp. 312-344.

Lieutenant Alien, in this rather detailed report, published a map of 

the region now embraced in southwestern Minnesota, which was compiled with great care from the information then at hand. The chains 

of hills and v mountains as located on the map show the explorers' 

and author's views of the situation and trend of the geological formations, which have had so marked an effect on the.topography of the 

region.

8. OWEN (David Dale), U. S. geologist. Report of a geological survey of Wisconsin, .Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory. Philadelphia, 1852.

With the advent of Owen into the Northwest we enter the time of 

more active and systematic research. This geologist, with his chief 

assistant, Dr. Norwood, and such aids as Shumard and Whittlesey, 

came for the distinct purpose of throwing light on the geological 

character of the territory comprised within the field of operations, and 

of presenting such practical results as might be drawn therefrom by 

Government officers and prospective settlers.

Shumard began an examination of the St. Peter (Minnesota) River, 

but an attack of pleurisy at the Eedwood Eiver put a stop to his 

further progress. He located the boundary between the quartzite conglomerate and the granitic rocks of the valley nearly opposite New 

Ulm. "About 1 mile in a straight line above the mouth of the Waraju, 

and 3 miles by the course of the river, we found some outliers of coarse 

conglomerate and granite a short distance from the bank of the stream. 

The conglomerate is composed mainly of quartz pebbles and bowlders, 

some of the latter 10 inches in diameter; the whole cemented with a 

siliceous material. . . . The course of the exposure is nearly east 

and west, with a dip toward the south of 20°. . . . The granite is 

100 yards removed from the conglomerate. . . . Flesh-colored feldspar forms nearly two-thirds of the granite." (P. 491.)
HALL.] . SUMMARY OF EARLIER OBSERVATIONS. 15

9. WHEELOCK (J. A.), commissioner of statistics. Minnesota: its place among the 

States. First annual report of the commissioner of statistics for the year 

ending January 1, 1860. Published hy authority of law. Hartford, 1860.

Touching southeastern Minnesota, Wheelock says: " From the head 

of Lake Superior a narrow granitic spur of this primary rock, divested 

of the volcanic debris in which the mineral wealth of Lake Superior is 

embedded, stretches southwesterly under the drift, beyond the boundary of the State, revealed only when it crosses the Mississippi at Sauk 

Rapids, the Minnesota between the Cottonwood and the Eedwood, and 

the Big Sioux at Sioux Falls. This granitic seam intervening in the 

sandstone has a breadth upon the streams which it crosses of from 40 

to 50 miles" (p. 53).

10. ANDERSON (Charles S.) and CLARK (Thomas). Report on geology and plan for a 

geological survey of the State of Minnesota. Made in accordance with a concurrent resolution passed by the second legislature of Minnesota, March 10, 

1860.

This report of Anderson and Clark merely alludes to "an extensive 

range of the primitive system of rocks" . . . which "appear on 

the Minnesota Eiver about the mouth of the Redwood River. Although 

sometimes termed a mountain by geologists, the highest elevation of 

these ranges does not exceed 2,000 feet above tide water." The breadth 

of this granitic region is out of proportion to its elevation considering 

it a mountain being about 60 or 70 miles. . . . Here are also found 

nearly all varieties of the granitic and metainorphic rocks (p. 5 et 

seq.).

This report is of historic interest in that it marks the beginning of 

geological surveys by the State of Minnesota.

11. HALL (James). Notes upon the geology of some portions of Minnesota, etc. 

Transactions of Philosophical Society, Vol. XIII, N. S., Philadelphia, 1869.

Professor Hall, in speaking of an area about Courtland, says: "Then 

rises to the surface a reddish-brown crystalline rock having a granitoid 

aspect, but when critically examined is found to consist mainly of 

quartz and feldspar without the perceptible admixture of other minerals. . . . We have, therefore, an almost continuous exposure of 

the strata for more than 100 feet in thickness. There is a distinct dip 

to the northeast." (P. 330.) Further on he adds: "I conceive it to be 

sufficiently demonstrated that the quartzites at Redstone Ferry [Courtland] on the Minnesota River those near the sources of the Little 

Cottonwood, . . . together with those of the escarpments of Pipestone, . . . are of the same age. These quartzites I regard as of 

the age of the Huronian formation of Canada and the quartzites of 

Wisconsin." (P. 338.) Turning now to the older rocks, Professor Hall 

says that around Redwood Falls they are gneissoid, and "in many 

places these rocks are intercalated by steatitic or glauconitic beds, and 

the entire mass is in a state of decomposition to the depth of 80 or 100
16 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

feet. It would be of much, interest," he continues, " to ascertain the 

relation which the dip and direction of these older gneissoid rocks bear 

to the succeeding quartzites, which evidently belong to the Huronian 

system" (p.335). (See fig. 3.)

12. CHAMBERLIN (T. C.) and assistants. Geology of Wisconsin, survey of 1873-1879. 

By the commissioners of public printing, Madison. 4 vols. Vol. II, 1877; 

Vol. Ill, 1880; Vol. IV, 1882; Vol. 1,1883.

In the discussion of the "quartzites of Barron and Chippewa counties, compiled from the notes of Messrs. Strong, Sweet, Brotherton, and 

Chamberlin," reference is made to the quartzites near New Ulm, Minnesota, and to the Sioux quartzites of Iowa. The equivalence of the 

quartzites of Barron and Chippewa counties to the Sioux and New Ulin 

quartzites is to be inferred by the reader. These southwesterly exposures, like those in. Wisconsin, are regarded as outcropping marginal 

portions of a quartzite belt extending, with some possible interruptions, through three or four States. The lithological characters of the 

quartzites of the several areas enumerated and their relationship to the 

Laurentian and the Paleozoic formations sanction their reference to 

the Huronian age. (Vol. IY, pp. 575-581.)

FIG. 3. Section from Courtland to Pipestone, Minnesota. (After James Hall.)

a. Quartzites east of the Minnesota, at Eedstone Ferry, dipping eastward, b. The outcrop of

-, quartzites near the source of the Little Cottonwood dipping to the westward, c. The outcrop of

quartzites at the Pipestone locality, dipping to the eastward, d. The interval between a and b, made

by an eroded anticline which is now occupied by Cretaceous and more recent formations along this

line. (An east-west section 40 miles north of this point shows the continuity of the Cretaceous fbrrnaation interrupted by numerous prominences of Laurentian gneiss.) e. The lower country west of the

Pipestone locality occupied by modern formations similar to d. E. Minnesota River. R'. Sioux

River. The space between the two upper lines is intended to represent prairie formation. /. The

place of the Pipestone stratum.

13. WINCHELL (N. H.). History of the Minnesota Valley. Minneapolis, 1882.

Chapter XXIX is devoted to the geology of the Minnesota Valley. 

This chapter is practically an epitome of Professor Winchell's observations in the Minnesota Valley in 1873, published in detail in the annual 

report of the Geological and Natural History Survey of Minnesota for 

that year. This report will be referred to in another place.

14. WINCHELL (N. H.) and UPHAM (Warren). The geology of Minnesota. Minneapolis, Vol. I, 1884, and Vol. II, 1888. Vols. I and II of the final report.

Mr. Upham's descriptions and field notes of the geology of the Minnesota Eiver Valley are numerous and valuable. Special reference is 

here made only to the following topics and pages: Decomposed gneiss 

and granite (of Brown and Eedwood counties) (p. 570), Potsdam 

quartzite (p. 572), the Eozoic rocks of Yellow Medicine County (p. 596), 

granite and gneiss of Lac qui Parle and Bigstone counties (p. 617), all 

in Vol. I; Archean rocks of Eenville County (p. 194), Archean rocks of 

Chippewa County (p. 210), in Vol. II.
HALL.] SUMMARY OF EARLIER OBSERVATIONS. 17

Further citations from Mr. Upham's writings will be made where the 

particular topics he writes upon are discussed.

SERIAL PUBLICATIONS.

15. Proceedings of the American Association for the Advancement of Science, Salem, 

Massachusetts, since 1848.

Iii an extremely interesting description of the Minnesota Valley 

during the Ice age, Warren Upham maintains (Vol. XXXII, 1883, p. 215) 

the ancient crystalline character of the underlying rocks of the Minnesota Eiver Valley itf these words: "In the 100 miles from Bigstone 

Lake to Fort Ridgely the strata are metamorphic gneisses and granites, 

which often fill the entire valley, 1 to 2 miles wide, rising in a profusion 

of knobs and hills 50 to 100 feet above the river."

In 1884, at the Philadelphia meeting, N. H. Winchell delivered the 

vice-presidential address in Section E on The Crystalline Rocks of the 

Northwest. In the grouping and correlation of these rocks the autbor 

says that "the outcrop of red granite near New Ulm, lying under the 

conglomerate and red quartzite, is probably the southwestward extension of ' the gabbro and red syenite at Duluth.'" These he calls the 

first group, i. e., the uppermost series of the crystalline rocks of the 

Northwest. (Vol. XXXIII, p. 364.) He further adds; "This group is 

represented by No. XX southwest of Lake Michigamme [in the upper 

peninsula of Michigan], by No. XX at Menominee [in Michigan and 

Wisconsin], and by Nos. 1 and la at Black River [in Wisconsin]."

16. Reports of the Chief of Engineers, U.. S. Army, Washington, since 1866.

In the records of the work of the Corps of Engineers, United States 

Army, between 1867 and 1879, many facts of geological interest are stated 

and discussed. !No engineer of the Army stationed of late years in the 

Northwest has taken a deeper interest in geologic and physiographic 

studies than Gen. G. K. Warren, after whom, "in honor and in memoriarn," the River Warren has been named. 1 No one can fail to find 

profit in studying, from a geological standpoint, what General Warren 

has set forth regarding the physical features of the Mississippi and 

Minnesota river valleys and the history of their formation.

In his report for 1868 General Warren discussed the extensive erosion which the Upper Mississippi Valley has undergone, the evidences 

which this valley presents of a former elevation of the central part of 

the North American continent, and the almost entire lack of abrasion 

which the granite bed of the Minnesota River reveals in the upper 

part of its course (pp. 305-314). In 1873 a report was made of an " examination and survey of the Minnesota River above the mouth of the 

Yellow Medicine." The average fall of the stream, the character of 

the bluffs bordering it, and the location and extent of the lakes found 

along its course are given. The granite which occurs at the foot of

1 Warren Upham, Proc. Am. Assoc. Adv. Sci., Vol. XXXII, 1883, p. 231.

Bull, 157 2
18 GNEISSES, GABBRO-SCHISTH, ETC., OF SW. MINNESOTA. [BULL. 157.

Bigstone Lake is designated feldspathic (pp. 438, 439). In 1875-76 

a very detailed report on the Mississippi Eiver was published (Vol. II, 

Part L, pp. 381-451), containing "An essay concerning important physical features exhibited in the valley . . . and upon their signification." 

This valuable paper is illustrated by several maps and charts.

In 1878 the mature results of General Warren's labors appeared in 

the report on bridging the Mississippi River, containing several maps 

and diagrams. These results bear on the glacial origin of the Minnesota 

Valley, the existence of a glacial river, subsequently named Elver 

Warren by Warren Upham, and the occurrence of silted lakes between 

Ortonville and Oourtland. They will be frequently referred to in subsequent pages.

17. The Minnesota Teacher, St. Paul, 1867-1875.

W. D. Hurlburt, a gentleman who has observed the geology of the 

State as his business carried him to various places, stated in this publication (Vol. IV, 1871), that "a band of granitic rocks forms the bed 

of the river [at Redwood Falls] similar to the exposures at Sauk 

Eapids and above [on the Mississippi River], and these indicate at 

once the shore line of the Azoic continent and the northern border of 

nearly all the sedimentary rocks in place in Minnesota" (p. 5).

18. The American Naturalist, Salem, Philadelphia, and New York, since 1867.

In Vol. II, 1868, an article by Gen. G. K. Warren, entitled, Certain 

Physical Features of the Upper Mississippi River, is discussed. The 

great excavation of the Minnesota River above Fort Snelling is cited, 

and the fact is stated that the bed of this river "for 110 miles below 

Bigstone Lake is partially granitic" (p. 497).

Emphasis is laid on the proposition that "the waters issuing from a 

lake have little abrading power, for they have comparatively little 

rubbing material to operate with. 1 Thus it was that the waters issuing 

at the old southern outlet of Lake Winnipeg (Lake Agassiz of Warren 

Upham) could make no impression on the granitic bed of the Minnesota. 

Had the granite been soft, like the Silurian rocks lower down in its 

course or like the Tertiary and Cretaceous rocks through which the 

Missouri has cut its way, then this part of the valley would have been 

worn away in the same manner, and we should have the drainage of all 

the Winnipeg Basin still to the southward. A cut of 400 feet at Bigstone Lake would have drained it, and the banks would then not have 

been as high as those of the Missouri at the Bijou Hills, which are 800 

feet above the water of the river" (p. 500).

19. The Geological and Natural History Survey of Minnesota, Annual Reports, Minneapolis, since 1872.

Under this survey (still in progress) N. H. Winchell, State geologist, 

inspected the valley in 1873. The granites were held to be of a pre1 Compare Geology of the Henry Mountains, by G. K. Gilbert, Washington, 1880, paragraph on 

Gorrasion, p. 101,
HALL.] SERIAL PUBLICATIONS. 19

vailing reddish color. That they are "schistose granites" is emphasized. The granite at La Framboise's place, described as a "type," is 

made up of the "ternary granite compound." The tendency to decompose into an impure kaolin, seen at Birch Cooley and Redwood Falls, 

is also mentioned. Since this material, provisionally called kaolin, 

prevails in the Cretaceous areas and is always present, so far as known, 

wherever the Cretaceous deposits have preserved it from disruption by 

the ice erosion of the Glacial period, its origin maybe attributed to the 

action of the Cretaceous ocean. (Eeport for 1373, p. 163.)

20. The Minnesota Academy of Natural Sciences, Bulletins, Minneapolis, since 1873.

In Vol. Ill, Bulletin No. 1, Warren Upham has described (p. 151) a 

set of "maps showing the climate, geography and geology of Minnesota." In outlining the distribution of rocks throughout the State he 

says that the rocks of southwestern Minnesota are "mainly red quartzites exposed near New Ulm and thence westward to Pipestone and 

and Eock counties in the southwest corner of the State." The Archeau system reaches from "the international boundary from the Lake 

of the Woods east to Lake Superior and extending thence southwest 

to the Minnesota Eiver between Bigstone Lake and New Ulm, but 

terminating 20 or 30 miles southwest of this river" (p. 154).

21. Bulletins United States Geological Survey since'1883.

in Bulletin No. 8, on Secondary Enlargements of Mineral Fragments 

in Certain Eocks, E. D. Irving and C. E. Yan Hise have discussed secondary enlargements of mineral fragments in certain vitreous quartzites (and sandstones) in the quartzite formation of southern Minnesota.

The similarity in character of these quartzites to the Huroniau 

quartzites of the Baraboo district of Wisconsin, and to much of the 

quartzite of the Huronian of Lake Huron is pointed out. "At Eedstone [Courtland] the transitions from argillaceous, reddish sandstone 

to completely vitrified, brick-red to purple quartzite, and from these 

again to completely loose sandstones, are frequent and abrupt." The 

fact is also emphasized that "over considerable areas the appearance 

is as if the rock at the higher levels had been vitrified by exposure." 

(P. 34.) Yet the appearance of the quartzite in the railway cuttings 

and elsewhere suggests the possibility that this vitrification may be 

due to the descent and spreading of a silica-bearing solution (p. 34).

22. The Geological and Natural History Survey of Minnesota, Bulletins, Minneapolis, since 1887.

23. The American Geologist, Minneapolis, since 1888.
GHAPTEE III.

GEOGRAPHICAL DISTRIBUTION AND GENERAL CHARACTERS OF THE ROCKS OF THE MINNESOTA RIVER 

VALLEY.

Before proceeding to a detailed description of the crystalline rocks 

under discussion, a resume of their geographical position will be given, 

and the general characters of these rocks, more especially with reference to their structural relations and peculiarities of texture, will be 

outlined.

In passing up the Minnesota River from its mouth at Fort Snelling 

to South Bend a southwesterly course is followed, the river flowing in 

a northeasterly direction. The river runs through a troughlike valley, 

whose walls, partly of Cambrian dolomite and sandstone and partly of 

glacial debris, reach a height of 200 to 250 feet. These walls are from 

1 to 4 miles apart. 1 At Sotith Bend a right angle is turned, to which 

the river cornes down from the northwest. Near this point the volume 

of the stream is enlarged by the addition of streams of some size flowing in from the south. Along the banks of one of these streams, the 

Lesueur, the explorer of that name found deposits of copper ore, from 

which quantities were sent to France for assay in the year 1700. Continuing up the river, it is not until T. 109 N., E. 30 W. is entered that 

any crystalline rocks are found. This area of crystalline rock is called 

the Courtland district, from the township of that name.2

COURTLAND DISTRICT. 

QUARTZITE EXPOSURES.

The pre-Cambrian rocks first found in passing up the Minnesota 

Eiver Valley are red quartzites (see PI. II) with associated quartzite 

conglomerates and sandstones. These exposures have been visited by 

nearly all the geologists and explorers who have traversed the Minnesota Eiver Valley. The rocks are not continuously bare from the 

river to the summits, but lie exposed in a series of terraces. In a 

rough way these terraces show different phases of rock structure and 

texture. Probably 50 feet represents the greatest thickness of strata 

shown at any one place. So far as has been seen, the outcrops lie 

wholly on the north side of the Minnesota Eiver. The Cottonwood

1 The Minnesota Valley in the Ice age, by Warren Upham: Proc. Am. Assoc. Adv. Sci., 1883, p. 216.

2 Report intended to illustrate a map of the hydrographical basin of the Upper Mississippi River, by 

J. N. Nicollet; dated February 16,1841: Twenty-Sixth Congress, second session, Senate Doc. 237, p. 18. 

20
U. S GEOLOGICAL SURVEY BULLETIN No. 157 PL

R XXIX W

GEOLOGICAL MAP OF THE COURTLAND DISTRICT

Scale

I CRETACEOUS QUARTZITK |Jg!_ J (rRANJTE AND GNKTSS
HALL.] COURTLAND DISTRICT. 21

Eiver enters the Minnesota directly south of the locality. With its 

power for erosion this stream would have laid bare any high masses 

of quartzite lying between tbe elevated bluffs that bound it on either 

side.

Exposures near Redstone. The exposure southeast of Bedstone, in 

sees. 1 and 2, T. 109 K, E. 30 W., and sees. 35 and 36, T. 110 1ST., E. 30 

W., is an extensive one. Here many steps can be seen in the transition from a loose, argillaceous and shaly sandstone to a vitreous quartzite; and again, from a vitreous quartzite of fine and uniform texture to 

a coarse quartzite conglomerate. The sandstones occupy the lower 

beds of the exposures, and the vitreous quartzite and quartzite conglomerates the higher beds. Yet there are no sharp divisions, many 

alternations of sandstone and quartzite layers occurring. The railroad 

cuts which penetrate these rocks 30 feet or more show that the quartzite is interbedded with the sandstone, and that the induration is not 

wholly a surface phenomenon. 1 There is a gradual ascent of the surface in passing away from the river, varying from 1 to 25 degrees, and 

many depressions bear evidence of unequal erosion, due to the unequal 

hardness of the rock, resulting in the terraces mentioned.

The highest portions of these exposures must be at least 175 feet 

above the Minnesota Eiver. The erosion of the upper 50 feet is unquestionably due partly to glacial action, as grooves and striae, still sharp 

and clear, show the direction of the ice currents, which, for this locality, 

was S. 33° E. But at places up to 125 feet above the point at which 

the lowermost shaly sandstone layers pass under the river, potholes 

and narrow tortuous grooves are numerous. This peculiar surface is 

clearly the result of the wear of running water rather than of moving 

ice; so the conclusion must be that the river's bed was once high up in 

this exposure, and that a large, muddy stream, with a rapid current, 

cut the gorges, gouged out the softer layers, wore the potholes, and 

chiseled into great irregularity the whole surface of the harder 

quartzite.

It is implied, in the statement of these erosive phenomena, that the 

wear of the rocks from the terrace 125 feet above the river down to 

the present water level has occurred since the Glacial period closed 

for this locality. That it has been accomplished since Cretaceous 

time is evident from the erosion of Cretaceous rocks in the neighborhood and within this same river gorge. The height mentioned, 

viz, 125 feet, certainly suggests the upper limit of erosion by water 

alone, and thus of post-Glacial work. The rocks are too hard to admit 

of rapid erosion.2 The more complete preservation of the marks of

1 On secondary enlargements of mineral fragments in certain rocks, by E. D. Irving and C. B. Van 

Hise: Bull. TJ. S. Geol. Survey No. 8, 1884, p. 34.

2 Compare The recession of the Falls of St. Anthony, by N. H. Wiuchell: Geol. Nat. Hist. Survey 

Minnesota, Vol. II, 1888, pp. 313-341; The place of Niagara Falls in geologic history, by G. K. Gilbert: 

Proc. Am. Assoc. Adv. Sci., 1886, p.222; Man and the Glacial Period, by G.F. Wright, 1892, pp.361, 

362; and many others.
22 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

water wear on the lower terras es may be due to the protection against 

glaeiation afforded by the shelving rock surfaces which confronted the 

river gorge while the ice masses were pushed over them; yet the 

stronger probability is that this condition marks the stage of erosion 

when the final ice retreat passed this point. A recent study of the 

Dalles of the St. Croix by Berkey shows that a post-Glacial work has 

been accomplished by the St. Croix Eiver l even greater than that at 

Courtland, both in its extent and in its significance as a measuring 

rod to apply to post-Glacial time.

In general the strike of the quartzites is N. 60° to 70° W., and their 

dip varies from 5° to 27° N. These figures are the average of a large 

number of observations in different portions of the area. There is 

considerable variation both in strike and jn dip, even within restricted 

areas. The dip is more marked in the lower beds those near the edge 

of the river than in the uppermost beds now visible.

All the rocks have very high silica content. Often, indeed, they 

seem to be wholly silica, except for a small quantity of ferric oxide 

and calcium phosphate in the cement, solidifying the grains of clear 

quartz sand into a nongranular rock. Noting that Prof. James A. 

Dodge found distinct traces of calcium phosphate in the supposed new 

fossil, Lingula calumet, discovered 2 by Winchell in 1885 in the catlinite interbedded with the red quartzite of Pipestone, Minnesota, a 

rock universally conceded to belong to the same horizon as this 

quartzite at Courtland, Mr. E. J. Babcock, now of the University of 

North Dakota, made tests for calcium phosphate in the Courtland 

quartzites. Five samples, showing no traces whatever of fossils, were 

taken from as many different parts of these rocks, representing the 

shale, the sandstone, the indurated quartzite, and the conglomerate of 

these beds, and qualitative analysis in all cases showed distinct traces 

of calcium phosphate. Some of them, and notably a sample of the 

conglomerate from near the top of the hill that is, from the uppermost layers of quartzite exposed yielded a very appreciable amount.

Microscopically, also, the quartzites show considerable diversity. 

The fine vitreous quartzite, as stated by Irving and Van Hise,3 shows 

the original fragmeutal grains coated with a film of oxide of iron and 

then enlarged, the cementing material becoming a constituent part of 

the reconstructed angular grains. In other places the indurating 

material is deposited independently as cryptocrystallme, partially 

chalcedonic quartz, with no regular orientation. There are still other 

places in which there seems to have been a cutting down of the original grains and the formation of a cryptocrystalliue groundmass 

inclosing scattered and corroded grains of quartz. These latter 

places appear to be confined to the surface portions of the exposure

'Geology of the St. Croix Dalles, by Charles P. Berkey: "Am. Geologist, Vol. XX, Dec. 1897, p. 368.

 Notice of the discovery of Lingula and Paradoxides in the red quartzites of Minnesota, by N. H. 

Winchell: Bull'. Minn. Acad. Nat. Sci., Vol. Ill, Pt. I, 1889, p. 103.

8 On secondary enlargements of mineral fragments in certain rocks, by R. D. Irving and C. R. Van 

Hiae: Bull. U. S Geol. Survey No. 8, 1884, pp. 34, 35.
EISSIC SERIES TO THE BASAL CONGLOMERATE NEAR NEW ULM.
HALL.] PROBABLE CONTINUATION OF COURTLAND QUARTZ1TE. 23

along the bedding planes and on the sides of the vertical joints, which 

have served as channels of percolation for subterranean waters. The 

silica thus dissolved and removed has doubtless acted as the indurating material for the underlying layers.

Exposures near New Ulm. In sec. 27, T. 110 N.,B. 30 W., near Ne\v Ulm, 

H miles northwest of the exposures near .Bedstone just described, lie 

large outcrops of coarse quartzite conglomerate. The strike and dip 

are nearly at right angles with the strike and dip of the quartzite southeast of Bedstone; the strike near New Ulm, as roughly represented by 

the direction of the exposure, being N. 15° E., with a dip varying from 

10° to 15° SE.

This rock is figured by Professor Irving, in his classic paper on Classification of the Early Cambrian and pre-Cainbrian Formations, as an 

exposure of a typical basal conglomerate. 1

There is much variation in the coarseness of the pebbles, all sizes, 

from average sand grains to bowlders 10 to 12 inches in diameter, 

being seen. These pebbles consist almost entirely of quartz, in part 

dull-red or brown jasper and in part clear white quartz. No quartzite 

masses were noticed possessing'the characters of the quartzite southwest of Bedstone, nor were any granitic pebbles seen.2 This is somewhat surprising, in view of the close proximity of the granitic and 

gneissic rocks of the valley.

PROBABLE CONTINUATION OF THE QUARTZITE IN SURROUNDING

AREAS.

There is evidence that the Courtland quartzites underlie a considerable belt of territory. Around Swan Lake (see PI. I), from 4 to 6 miles 

northeast of the exposures near Bedstone and New Ulm,.bowlders of 

red quartzite strew the ground in places. In Cottonwood and Watonwan counties, about 25 miles to the southwest, many square miles are 

underlain by vitreous quartzites, and many broad surfaces appear, 

representing a thickness of strata of at least 1,500 feet. The formation 

is, therefore, several times as thick as at Courtland, there being exposed 

at the latter place from 250 to 300 feet only/' There is no doubt that 

the extensive exposures of red quartzites in Bock and Pipestone counties, Minnesota, and in many counties of southeastern South Dakota, 

belong to this same formation.4

To the eastward little is known of the extension of these beds. At 

Minneopa Falls, near Mankato, is a well 1,000 feet deep. In boring,' 

at 575 feet from the surface a conglomerate was reached with pebbles 

of clear, fine, nongranular quartzite up to 2 or more inches in diameter.

1 Seventh Ann. Kept. U. S. Geol. Survey, PI. XLIV, facing p. 430. '

'Compare paragraphs in Geology of Sibley and Nicollet counties, by Warren Upham: G-eol. Nat. 

Hist. Survey Minnesota, Vol. II, 1888, pp. 157-160.

Preliminary paper on an investigation of tbo Archean formations of the Northwestern States, by 

K. D. Irving: Fifth Ann. Kept. TJ. S. Geol. Survey, 1884, p. 201.

 "Notes upon the geology of some portions of Minnesota, etc., by James Hall: Traus. Am. I'hilos. 

Soc., Phila., Vol. XIII, N. S., 1865, p. 338.
24 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 15?!

From tliis depth to more than 800 feet below the surface the borings 

consisted chiefly of quartzite pebbles indistinguishable in macroscopic 

and microscopic characters from the vitrified quartzites at Courtland.1 

These pebbles are essentially different from those composing the " basal 

conglomerate" beds opposite New Ulm.

In 1897, at Gleucoe, about 35 miles north of Redstone, a well, bored 

for water, yielded drillings of a decidedly red color between depths 

of 936 and 1,640 feet. The rocks there penetrated were probably 

the northward extension, beneath the Paleozoic, of these Courtland 

quartzites.

AGE OF THE QUABTZML'E.

The occurrence of the quartzite conglomerate at Minneopa Falls 

establishes the fact that the great Sioux quartzite terrane was thoroughly indurated and a subject of extensive erosion at the time the 

bottom beds of the Middle Cambrian were being laid down in the area 

now embraced in the Upper Mississippi Valley. (See fig. 4.)

FIG. 4. Hypothetical section from Courtland to Minneopa. 1, Granite-gneiss of the Minnesota 

River Valley; 2, Conglomeratic base of the quartzite formation; 3, Quartzite conglomerate much finer 

than at the base; 4, Concretionary Cretaceous limestone; 5, The Courtland exposures of red quartzite, 

with, overlying conglomeratic phases; 6,Cambrian conglomerate with pebbles of red quartzite; 

7, Dresbach (?) sandstone; 8, St. Lawrence dolomites and shales; 9, Jordan sandstone, seen at Minneopa Falls; 10, Glacial drift of varying thickness.

The lapse of an enormous interval of time between the deposition of 

the quartzites and the Middle Cambrian (Potsdam) is most strikingly 

shown by Irving for the Baraboo district of central Wisconsin, a district presumed by that writer to show features of age and stratigraphic relations identical with those of the Courtland district of 

southwestern Minnesota.2 Whether these quartzites belong to the 

Upper Huronian subdivision of the Algonkian can not be determined 

from the mere fact of their lying unconformably beneath the Middle 

Cambrian. It is sufficient, for our present purpose, to know that a time 

gap of great length intervened between the deposition of these 

great quartzite beds and the close of the erosion which partly removed 

them and accumulated their debris as masses of conglomerate pebbles 

in the bottom layers of the Paleozoic of the Upper Mississippi Valley.

1 The deep well at Minneopa, Minnesota, by C. W. Hall: Bull. Minn. Acad. Nat. Sci., Vol. Ill, No. 2, 

1891, p. 248.

2 On the classification of the early Cambrian and pre-Cainbrian formations, by R. D. Irving: Seventh 

Ann. Rept. U. S. Geol. Survey, 1886, p. 403 et seq.
U.S.GEOLOGICAL SURVEY BULLETIN No. 157 PL.

JULIUS BIEN a CO. LIT

GEOLOGICAL MAP OF THE FORT RIDGELY DISTRICT

Scale

GABBROAKD GABBBO-SCHIST fjfM. .. GRANITE AND &NBISS
HALL.] FORT RIDGELY DISTRICT. 25 

GNBISSOID GRANITE EXPOSURES.

One hundred and fifty paces west of the New Ulm conglomerate, 

and undoubtedly extending beneath it, lies the first outcrop of gneiss 

that is reached in ascending the Minnesota River Valley. Three or 

four knobs stand some 5 to 15 feet above the level of the river bottom. The several outcrops are shattered into numerous loose blocks 

by the forces of weathering. These blocks are so disintegrated and 

crumble so readily that hand specimens can with difficulty be shaped. 

They show all the phenomena of concentric weathering so common in disintegrating granites of coarsely crystalline texture. The shattered condition precludes the possibility of noting the.strike or dip of the laminae. 

No contortion can be seen. The rock is granitic in texture, of medium 

coarseness, somewhat porphyritic, and of a reddish color, due to the 

abundance of red feldspars. It is hornblendic, but contains only a 

small portion of basic constituents and a much less proportion of 

quartz than the exposures farther up the river. The feldspars are 

orthoclase, microcline, and oligoclase, and are all considerably altered 

through kaolin, the orthoclase and the oligoclase to a much greater 

extent than the microcliue. The altered areas are heavily charged 

with hematite and brown ferrite. The deepest specimens obtainable 

along the planes of fracture show a lodgment of ferric oxide, which in 

places is apparently hydrous (limonite). This lodgment is especially 

interesting around nests of basic constituents. In such places the 

intermingling of the brown and green gives a particularly weathered 

appearance, which is not, however, fully borne out by the microscopic 

examination.

FORT RIDGELY DISTRICT.

Passing up the river a distance of 15 miles from New Ulm, La Framboise's place is reached. This spot marks the beginning of some very 

extensive exposures of rocks. Here they come down to the water's 

edge, forming a good canoe lauding and marking the spot where one 

of the earJiest trading posts of the valley was located by the father of 

the present owner of the place.

GNEISSOID GRANITE.

Area northeast of Golden Gate. In the area northeast of Golden 

Gate (PI. IV), scattered over several sections in the vicinity, particularly over sees. 14,15,16,21,22, and 23, T. Ill N., K. 33 W., are many 

exposures of a gneiss which to a considerable extent is augitic and 

granitoid. It varies in texture, and over much of the area is so greatly shattered as to be unfit for systematic quarrying. Occasional 

areas are rather coarse, orthoclase crystals 5 or 6 inches in length 

being found, and in other areas a fineness unusual in gueissic rocks 

can be seen. As a rule, these exposures exhibit a rock of red color, 

but this color shades through pink to a dirty gray, and in some of the
26 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

exposures the last-named color is the predominant one. The more finely 

granular modifications are those of the gray color, and these carry 

augite. In all the rocks of this locality there is an unusual lack of biotite. Often hand specimens may be found on which not a folium of 

this mineral can be seen. The quartz is rather opalescent. The 

gneisses, as a whole, are characterized bj a high per cent of silica.

Area west of Fort Ridgely. West and south west of old Fort Eidgely 

the gneiss assumes a darker color and a more pronounced gneissic 

habit through the increase of the biotite and the hornblende. It seems 

much fresher, and in places some quarrying has been done.

Along the south side of the river, within the area of the map (PI. IV), 

several exposures of gneissic rocks are said to occur. The one locality 

visited was the old quarry out of which material was secured by the 

Government for erecting the barracks and other buildings of Fort 

Ridgely, whose ruins now stand upon a conspicuous morainic bluff on 

the north side of the river, 1 £ miles distant from the quarry. The location of this outcrop is at a sharp bend in the river, 150 paces N. and 

1/.IOO paces W. of the southeast corner of'sec. 1, T. Ill N., E. 33 W. It is 

200 paces long by 150 paces wide. The rock is medium grained gneissoid 

granite, porphyritic in texture, and is somewhat stained at the surface 

by weathering and the. infiltration of ferric oxide along secondary structures. It is somewhat shattered, and in places is decidedly granitoid. 

The more laminated portions show some contortion of the bands, which 

have a general N.-S. strike, with a westerly dip of from 40° to 50°. 

In these respects the rock agrees with the masses on the north side of 

the river in sees. 35 and" 36, T. 112 N., E. 33 W.

BASIC BOCKS.

In sec. 34, T. 112 N., E. 33 W., is a fine exposure of augite-schist. It 

can not be certainly determined whether it is a dike breaking through 

the gneiss or an interbedded layer of quite different mineral and chemical composition from the associated gneissic rocks, although it is probably the former. Only a few hundred paces northeast of the schist is 

a highly altered rock, now bearing epidote and calcite, which in both 

external and internal characters gives strong evidence of having once 

been an olivine-gabbro.

Near La Framboise's a hornblendic hypersthene-gabbro occurs in the 

augitic granitoid gneiss, and at the old Fort Eidgely quarries, NW. \ 

of KB. £ sec. 2, T. Ill K, E. 33 W., a hornblencle-biotite-schist can be 

seen which doubtless was once similar in mineral composition to the 

hypersthene-gabbro at La Framboise's. It is probable that the gabbro 

just mentioned occurs as an intrusion in the gneiss. The contact 

between the two is sharp and clear where a wall is uncovered for a 

few paces.

Everywhere in the rocks of the Fort Eidgely district there are veins 

of quartz, and in places they are very numerous. For example, in sec.
. S GEOLOGICAL SURVEY BULLETIN No. 157 PL.V

R XXXIV W R XXXIII W

R XXXIV W R XXXIII W

JULIUS BIEN 8 CO. LITH. N.Y.

GEOLOGICAL MAP OF THE MORTON DISTRICT

Scale

I CRETACEOUS DIABASE DIKES GABBRO AND GABBRO- SCHIST GHANITK AKD GNEISS
HALL.] LOWER SIOUX AGENCY AREA. 27

.2, T. Ill N., E. 33 W., running in a prevailing NW.-SE. direction, veins 

of milky quartz, varying in thickness from that of paper to 5 inches 

across and lenticular in shape, are seen to variegate the surface of the 

rock. Schistose inclusions are not infrequent. These possess a medium 

texture and vary somewhat in mineral composition.

MORTON AND BEAVER FALLS DISTRICTS. 

PRE-CAMBRIAN ROCKS.

Lower Sioux Agency area. In sees. 10, 11, 13, 14, 15, 23, and 24, T. 

112 K., E. 34 W., on the south side of the Minnesota Eiver, occur mound 

after mound of gneissic rocks, cut by diabase dikes and associated with 

altered gabbros and peridotites. In texture the gneissic exposures 

vary somewhat. On the average, the rocks are medium grained, but 

in places orthoclase individuals 4 inches across are found. Considerable contortion was noted in the lamination, but the general direction 

is NE.-SW. The number of veins and segregations is less than around 

Fort Eidgely. The gneiss adjacent to the large diabase dikes is of 

unusual freshness. This freshness seems to be an accidental character, 

for northwest of these exposures, in sec. 15 of the same township, the 

gneiss is badly weathered. This weathering and the already existing 

jointing of the rocks produce concentric blocks of various sizes, the 

uppermost and oldest being the most spheroidal. The gneisses are of 

the normal hornblendic biotite type. Accessories rather widely found 

in granitic rocks occur, particularly apatite, pyrite, and magnetite.

On the Tracy farm, in sec. 10 of this same township, a beautiful 

chloritic gneissoid granite is found, not in large quantity, but with 

beautiful areas of radiated chlorite and chlorite plates in vermicular 

chains. (PI. XVII, A.) This rock carries many inclusions of a darkgreen color, which show a predominating quantity of chlorite.

In sec. 14 of the same township are large masses of a coarse-grained, 

dark-colored, hornbleudic rock. There is a foliation in a general 

NE.-SW. direction and a southeasterly dip of 80°, although at the surface there is much contortion. This is evidently an altered gabbroid 

rock.

In sec. 19, T. 112 K, E. 33 W., and sees. 23 and 24, T. 112 K, E. 34 

W., lies the largest clearly defined dike, save the one at Pigeon Falls, 

which the writer has seen in Minnesota. 1 Its width is 60 paces, and it 

can be traced 2 or 3 miles across the valley, standing in the midst 

of a large mass of fine, fresh, gray gneiss. In composition it is a 

badly altered quartzose diabase..

In sec. 15, associated with the gneiss, is a hornblende-augite-schist 

which contains some hypersthene.

1 This dike, crossing the Pigeon Eiver aijd forming the falls named, has been described by Norwood in Owen's Keport of a Geological Survey of Wisconsin, Iowa, and Minnesota, etc., Philadelpllia, 

1852, p. 405; and has been referred to by Irving in The copper-bearing rocks of Lake Superior: Mon. 

U. S. Geol. Survey, Vol. V, 1883, p. 371.
28 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

In sec. 9 of the same township occur the only peridotite arid serpentine seen in place in the Minnesota River Valley. Two low knobs, 

each 100 paces long, 50 paces wide, and not exceeding 10 feet in height, 

represent all of these rocks. The variety of peridotite seems to be 

saxonite, although somewhat altered. The numerous cavities in the 

rock are filled with geodized incrustations of amethystine quartz and, 

more rarely, resinous-looking calcite.

In the more exposed portions the alteration mentioned has proceeded 

until the original constituents have wholly disappeared. Indeed, a 

set of rock sections may be prepared showing every degree of alteration from a nearly normal peridotite, with only slight alteration of its 

enstatite and olivine, to a serpentinous rock in which not a trace of the 

original constituents remains.

While these knobs of peridotite are the only occurrence of the rock 

undoubtedly in place, it may be added that in sees. 32 and 33, T. 112 

N., E. 33 W., between the foot of the morainic bluff and the present 

bed of the river, stands a terrace in which are many .bowlders of peridotite and serpentine. The present condition of these bowlders points 

to the probability that they have been transported but a short distance, 

thus leading to the inference that beneath the glacial drift and the

-iso feetFIG. 5. Contorted hornblende-biotite-gneiss, from railway cut at Morion, 150 feet long.

deposits of river d6bris there lies a large mass of these heavily basic 

rocks. Every petrological character is identical with the corresponding one of the rocks in situ in sec. 9, already mentioned.

Morion area. At Morton appear huge mounds of gneiss with a contorted lamination. (Fig. 5.) This gneiss is hornblendic, and has a prevailingly dark color. In places nests of pyrite can be seen on freshly 

fractured surfaces. In coming up the river from La Framboise's to 

this place we see a gradual change in the color of the gneissic rocks 

from a reddish to a prevailingly dark gray, a change from the granitoid structure to the gneissic, a disappearance of the augite-bearing 

rocks lean in basic minerals, and the appearance of nonaugitic gneisses 

in which both hornblende and biotite are abundant. The foliation of 

these rocks in all its contortions can be seen on the surface wherever 

weathering has not so corroded it that part of the constituents are no 

longer visible. The railroad cut at Morton shows a fresh surface. The 

accompanying figure (fig. 5) was taken from the west side of that cut. 

Attention may be called to the coarsely crystalline masses of pegmatite which are rather thickly distributed throughout this fresh Morton 

gneiss. These masses are not so clearly defined as are pegmatitic
US.GEOLOGICAL SURVEY BULLETIN No 157 PL VI

RXXXVI W

JULIUS BIEN S CO

GEOLOGICAL MAP OF THE BEAVER FALLS DISTRICT

Scale

I CRETACKOUS I .DIABASE DIKES ijGABBROAN'D GABBHO -SCHIST j5lfc'«Sa GRANITE ASH GKEISS
HALL.] MORTON AND BEAVER FALLS AREAS. 29

veins, but their general characters indicate them to be such. In 

almost all cases noted they merge insensibly into the normal gneiss. 

In places they carry nests of biotite and hornblende, many of which 

are 1 or 2 inches across.

Veinlike segregations of feldspar and quartz and of quartz alone 

are everywhere numerous. They are not marked by unusual size, nor 

by the occurrence of metallic contents. There is no little diversity, 

however, in mineral constituents, as well as in texture and structure. 

On Birch Cooley some finely textured reddish veins-of true granitic 

type show a distinctly laminated structure. They are irregular in shape 

and direction. Elsewhere a pegmatitic structure prevails. In the 

altered portions of the gneissic rocks the veins, which are also altered, 

carry numerous scales and pockets of limonite. These altered veins 

show much variation in color and texture. In many large areas, where 

the color is pink or reddish gray the feldspars are abundant, and where 

reddish black the biotite and hornblende occupy a larger place than 

usual. These characters are particularly noticeable at the exposures 

in sees. 29 and 30, T. 113 B"., R. 35 W., north of the city of Eedwood 

Falls.

The tops of the large mounds of gneiss at Morton, 75 feet above the 

high-water mark of the present Minnesota Biver, display indubitable 

evidences of water erosion. For the most part these surfaces are 

smooth and rounded, but on the very top of the highest hill a pothole 

of considerable size still exists. Grooves cut by running water can 

also be seen.

At Morton (PI. Y) the gneiss is, in places, remarkably fresh. Ordinary rnetamorphic changes, as Geikie defines the term, 1 are everywhere met with among these rocks. In a few places changes are noted 

which can be traced directly to surface influences; also changes 

which have removed so much of the rocks that what is now left has 

every character of an entirely different species. The kaolin beds of 

the Birch Cooley Gorge are a case in point, because there is no doubt 

that these beds are altered gneissic rocks which at one time presented 

characters similar to, if not identical with, those of the gneisses at 

Morton which are now so fresh and firm. There is little doubt that the 

Birch Cooley exposures are portions of the same beds that appear at 

Morton.

Beaver Falls area. (PI. VI.) Along the bottom lands of the Minnesota Eiver, northwest of Morton, the gneissic rocks appear in large, 

high knobs. The higher and more exposed portions of these outcrops 

are comparatively fresh and firm, while in the more sheltered places 

the kaolinized portions are not yet removed. A distinctly gneissic 

habit prevails, with less contortion than appears around Morton. The 

strike of the lamina? is NW.-SE., and the dip is NE., with a varying 

angle. While in general there is little diiference in character between

1 Text-book of Geology, by Archibald Geikie, 3d ed., 1893, p. 595.
30 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

the gneiss of the knobs between Morton and Beaver Falls and the 

quarried rocks of Morton, slight differences may be noted. The 

porphyritic character of the gneiss at Beaver Falls is rather prominent. Feldspar crystals, orthoclase and microcline, are well defined. 

In size they reach a length of 2 or more inches. They show Carlsbad 

twinning, are much fresher than the finer-grained matrix, and resist 

weathering to such an extent that they stand out from the surface of 

the rock a half inch or more. Their whitened aspect indicates the 

kaolinized condition which even the freshest individuals exhibit. 

The gueissic, porphyritic condition which has just been noted is not 

uniform. In places a pegmatitic structure covers extensive areas. 

These coarser phases are at times sharply defined, and again pass 

gradually into the normal biotitic gneiss of the area.. In some areas 

finer granitic veins are frequent, similar in texture, color, and mineral 

composition to those seen on Birch Cooley Creek, in the Morton district, penetrating the fresher phases of the gneisses.

The surface material of the gneisses shows no appreciable amount of 

hornblende, almost all of the basic portion being biotite.

Joints are everywhere very conspicuous upon these mounds. In 

places they have produced nearly perpendicular walls. This is not 

noticeably the case on the southerly face of the large mound which 

crops out in sees. 26, 35, and 36, T. 113 N., E. 35 W. The usual direction of the joints is N. 35° W. A waterworn condition of the surface 

is rather marked. Even the highest portions of the mound referred to, 

from 30 to 50 feet above the river's present level, show potholes and 

grooves which bear conclusive evidence of water wear rather than of 

the even cutting of glacial ice. These phenomena of water action are 

not so conspicuous at this locality as on the summits of the quartzite 

areas at Courtland, described on page 21, nor so numerous as at several 

localities farther up the river (pp. 36, 42).

Along the river road from sec. 26, T. 113 N., E. 35 W., westward to 

Beaver Creek, there are a number of unimportant weathered exposures 

of the same general character as those above described.

Redwood River area. Near the mouth of the Eedwood Eiver (PI.VII) 

the gneisses, showing a typical, distinctly laminated, and uncontorted 

structure, appear in large areas and cause many tortuous windings in the 

Minnesota Eiver. The strike is NE.-SW., in places N. 75° E., with a 

dip sometimes as high as 70° SE. The lamination at this point is 

conditioned by the parallelism of the constituent minerals. At the 

northern border of this area of gueissic rocks there is an intercalation 

of hornblende-schist. It is characterized'by a rather coarse texture 

(sp. 5260), and consists chiefly of quartz granules, triclinic feldspars, 

and green hornblende, the last occasionally carrying cores of augite. 

It bears many evidences of descent from the bedded gabbros of the 

valley. Numerous feldspathic veins form an anastomosing network 

throughout the whole exposed area of this rock.
U. S. GEOLOGICAL SURVEY

REDWOOD RIVER FALLS.
HALL.] REDWOOD RIVER AREA. 31

Near the Redwood Eiver, from its mouth to the falls in the city of 

Eedwood Falls, there are many places where the altered gneiss is promneutly exposed to view. One spot shows such a complete kaolinization, 

and at the same time such a staining by ferric oxide, that considerable 

material has been extracted for the manufacture of brown mineral 

paint. In sec. 29, T. 113 N., E. 35 W., some quarrying has been done 

in a gueissic rock which in state of preservation stands about midway 

between the rock just mentioned as serviceable for a paint and the fine, 

fresh gneiss at Morton and above Beaver Falls.

The Eedwood Eiver has cut a picturesque gorge for itself for a 

distance of from 1J to 2 miles from the south wall of the old Eiver Warren to the present Eedwood Falls.1 The walls of this gorge are massive 

gneissic rocks, reaching 150 feet in height. In all the places cited 

above almost every stage of decomposition may be seen, from the freshest and finest gneiss down to the impure green kaolin of Birch Cooley, 

or the "red paint" below Eedwood Falls. Usually the quartz constituent of the gneisses is quite fresh and holds its place in lenticular 

laminae through the soft material which now makes up the mass of the 

rock exposures.

Area above Redwood River. Along the river bottoms above Beaver 

Falls and Eedwood Falls, on both sides of the Minnesota Eiver, there 

are numerous exposures of biotite-gneisses. In sees. 19 and 20, T. 113 N., 

E. 35 W., are some large and high rock masses which have to no little 

degree modified the course of the river, as a glance at PI. VI will 

show. In the southeastern corner of sec. 20 are some notable mounds, 

caused by concentric weathering. Spherical masses of tons' weight lie 

around. The rocks are, as a rule, of a decidedly fresh aspect; a few 

are thoroughly disintegrated at the surface, yet have considerable consistency and toughness at the center. They are well laminated. 

The gneisses exhibit very little variation from the types in the southeastern portion of the Beaver Falls district (PI. VI). But interlaminated with the hornblende-biotite-gneisses are some interesting hornblende-schists. The schists weather rather more rapidly than the 

gneissic rocks, and as a consequence they are found in the lower portions of the exposures. They show some variation in texture, in 

some cases a rather coarse condition being prominent. The structure 

and mineral composition of the schists are distinctly gabbroid. Hornblende and a plagioclastic feldspar close to labradorite in its optical 

characters are the chief constituents. The form and structure of these 

minerals are the same as in ordinary hornblende gabbros, the only 

difference being in the complete alteration of the pyroxenic into amphibolic constituents. The strike of the rocks, gneisses and schists alike, 

where it shows any regularity, is NE.-SW,, and the dip is SE. But in 

places-there is great contortion.

 Compare Geology of Brown and Eedwood counties, by Warren Upharu: Geol. Nat. Hist. Survey 

Minnesota, "Vol. I, p. 564.
32 GNEISSES, GABBRO-SCHISTS, ETC., OP SW. MINNESOTA. [BULL. 157.

Origin of Jcaolinie gneisses. It may be well to note here the altitude 

of the kaoliiiic gneisses, as compared with that of the fresh gneisses, at 

Morton, Beaver Falls, and several other places along the valley where 

the fresh rocks and the kaolinic varieties are found. Nowhere does 

the kaolinic stand at a greater altitude than the undecomposed rock 

at some place in the immediate neighborhood. At Morton the 

knobs of fresh gneiss in sees. 30 and 31, T. 113 1ST., E. 34 W., must 

reach an elevation at least 100 feet higher than the kaolinic gneiss 

in the Birch Cooley Gorge. At Beaver Falls dam the fresh, bright 

horublende-biotite-gueiss is certainly 75 feet higher than the decayed 

material beside the road running along Beaver Creek, between the 

village of Beaver Falls and the Minnesota Eiver bottoms. The 

same relative altitudes are shown in the gorge of the Eedwood 

Eiver between Eedwood Falls and the Minnesota Eiver, and, although 

to a less degree, near the village of Minnesota Falls and in the city of 

Granite Falls.

These facts of position tend to show that in the decomposition, which 

must have been pre-Glacial, the breaking up of the old crystalline rocks 

could not have been a leveling process, as we understand that term, but 

that, either through a difference in the hardness of the rock or by reason 

of other qualifying circumstances, the corrosion reached to great depths 

in certain places, while in others near by but little effect was produced.

VICKSBURG DISTRICT.

Continuing in a northwesterly direction up the Minnesota Valley 

(Pis. VIII and IX), there is found, in T. 114 K, E. 30 W., and especially 

in that portion lying on the north side of the Minnesota River south of 

the post-office of Vicksburg, an extremely rocky area. The valley 

here broadens to 4 or 5 miles, with large areas so rock covered that 

these sections were marked by the land surveyors as "rock barrens." 

The rocky mounds stand high and bare. Their tops, from 50 to 100 

feet above the present river, show many potholes and tortuous grooves 

from river erosion. As one enters this township from the southwest 

he finds an extremely interesting series of gneisses and schists. The 

former are rather acidic in composition, carrying hornblende as^the 

chief basic constituent, and the latter are gabbro-schists of somewhat 

varying types.

GNEISSES.

Taking the district as a whole, the strike of the gneisses is NB.-S W. 

Their dip, according to several measurements below .Redwood Falls, is 

SE. at an angle of 70°, while elsewhere the angle was as low as 25°. 

The general direction of these rocks, both in strike and in dip, is shown 

on the maps by dotted lines. The gneiss is, as a rule, highly siliceous 

and strongly banded, 27 laminae having been counted in a single 

inch. It is not so contorted as the gneiss around Beaver Falls and
U. S.GEOLOGICAL SURVEY BULLETI N No 157 PL.VIM.

UUS BIEN & CO. L

GEOLOGICAL MAP OF THE VICKSBIIRG DTSTRIGT

Scale

I DIABASE DIKES pGABBRO AND GABBRO SCHIST W'l'WlS GBANITE ANl^ GNEISS
o m

2 H
HALL.] VICKSBURG DISTRICT. 33

Morton. In places it is quite epidotic, and almost everywhere it is 

porphyritic.

In the almost continuous series of exposures in this vicinity the 

rocks present great diversity of type. In places the gneiss is so massive that no lamination is seen for several feet, while not far away, 

it may be, a decidedly schistose habit prevails., Large areas are seen 

with few traces of veins, whether granular or quartzose, while in other 

and contiguous areas veins are so numerous that scarcely a square 

inch of surface can be seen which does not carry these segregations. 

Some rock masses are very fine in texture, while others are coarse and 

crumbling. Many of the gneisses show an abundance of quartz and 

potash feldspars, with only a small proportion of the basic minerals, 

hornblende and biotite; elsewhere hornblende and plagioclases constitute the bulk of the rock, with almost no quartz present. Some 

stretches of rock surfaces present a prevailingly red color, while others 

are a decided gray.

In several exposures of the gneisses pyrite and chalcopyrite were 

seen as accessories. Karely the latter occurs in sufficient quantity to 

give a gray or blue color to its associated minerals through the alteration to carbonate of copper.

In one locality, in sec. 8, T. 114 N., E. 37 W., on these same rock barrens, occurs a rock of much interest (sp.5372-, 5373,5374), which is fully 

described later (pp. 69-72). It is an augite-biotite-granite-gneiss of not 

very great extent. The grain is coarse, and the color is a mottled dark 

green and light green. Streaks of a veinlike mixture of feldspar (micro- 

. cline) and hornblende appear, with individuals 3 and 4 inches across.

GABBRO-SCHISTS.

In the gabbro-schists associated with the gneisses we have rocks quite 

different in many of their aspects from either ordinary gneisses or 

ordinary schists. In texture the schists are medium to fine, and in 

structure they are distinctly laminated. The strike of the lamination is 

KE.-SW., and the dip is 25° SE. The rocks are laminated gabbros, 

and so far as general characters are concerned they appear much like 

the flaser-gabbros of the German lithologists, particularly those 

described by Lehmann.1 These gabbroid rocks must be very ancient, 

since they are broken through by the diabase dikes so common in the 

valley, and are apparently interbedded with the gneisses with which 

they are associated. They will be described further in Chapter Y.

In this vicinity, in sec. 33, a white variety of hornblende treinolite 

was seen in one rock, an occurrence somewhat rare in the valley. Near 

it occurs a garnetiferous hornblende rock which carries diallage, the 

mineral from which, by its alteration, a portion of the white hornblende 

springs.

1 Enstehung d. altkryst. Gesteiue, 1884, p. 194.

Bull. 157 3
34 GNEISSES, GABBRO-SG'HISTS, ETC., OF SW. MINNESOTA. [BULL. 157. 

GRANITE FALLS DISTRICT.

Directly on the line between T. 115 N., E. 38 W., and T. 115 N., E. 39 

W., in sec. 19 of the former and in sec. 24 of the latter (PI. X), gabbroschist occurs. The rock stands in nearly vertical position. It is so 

shattered that measurements of strike and dip are somewhat uncertain. 

The strike was determined as N. 65° E., the dip being SE. at an angle 

ranging from 75° to vertical. The texture is very fine, and geodized 

masses of quartz occur. Only one other place is known in the valley 

where a schist as finely textured as this occurs in place. That is in sec. 

17, township of Omro, 115 N., E. 43 W. This peculiar rock is not infrequent in the form of drift bowlders in various parts of Minnesota. 

Large masses have been found at Litchfield, T. 119 ]$"., E. 31 W., at Minneapolis, and at many other places. These occurrences point to a somewhat wide distribution of this hornblende-schist beneath the drift.

A mile and a half northwest of these gabbro-schists of sees. 19 and 24 

begins an interesting series of gneisses, diorites, and pyroxeuic schists 

of a markedly gabbroid character. Through Minnesota Falls and 

Granite Falls a succession of folds can be traced in which this series of 

interbeclded gneisses and schists dips alternately S. and N. at angles 

usually less than 35°. To the south of Minnesota Falls the dip is 

southerly, changing to northerly in Minnesota Falls itself. In Granite 

Falls the dip again changes to a southerly direction, and this in turn 

gives place to a northerly dip, as the long series of billowy masses of 

crystalline rocks finally disappears under the masses of glacial drift 

and river silt which conceal all the rock exposures between the northern 

part of Granite Falls and Montevideo.

The gneissic rocks take the leading place in point of area underlain. 

In texture and composition they exhibit some modifications which are 

of interest. They are nowhere granitoid. The lamination is clear and 

distinct, with a strike varying little for many miles, being ENE.-WSW. 

The color is gray or reddish gray, and the texture medium grained. 

The proportion of quartz among the mineral constituents is very 

high. The feldspathic constituent is largely plagioclastic, of the sodalime variety, and in places is somewhat kaolinized, thus giving a 

bleached appearance to the mineral. The kaolinization seems to pervade orthoclase and plagioclase alike. The basic minerals are both 

hornblende and biotite, the former predominating.

On the north side of the river, in sec. 28, T. 116 N., E. 40 W., a great 

many small garnets appear within the gneissic rocks. These show all 

stages of alteration into biotite; and biotite remote from the garnet 

patches is comparatively rare.

Inclusions and bands of hornblende-augite-schist are rather frequent 

in the gneisses.

As shown by microscopical examination (see pp. 85-93) the darkcolored rocks of this locality are schistose gabbros. So far as can be 

seen these schistose gabbros conform in position with the gneisses and
U S.GEOLOGICAL SURVEY BULLETIN No. 157 PL.X

JULIUSBIEN6 CO. LITH N.Y.

GEOLOGICAL MAP OF GRANITE FALLS DISTRICT

Scale

I CREOACKOUB t DIABASE DIKES 5 GABBRO AN1> GABBRO-SCHIST 

s INCIA'UI.NG CAUBHO DiOKITE
HALL.] GRANITE FALLS DISTRICT. 35

diorite-schists. They carry numerous garnets. At the surface the 

augite is, to a considerable extent, altered to biotite. It is probable that 

the garnets also, by their decomposition, have added materially to the 

amount of biotite present in the rocks. In the deeper beds the schists 

have a fresher look. They carry little biotite, but are rich in hornblende. A heavy stain of ferric oxide is everywhere present on the 

weathered surfaces, both of the surficial rock and along internal fractures exposed in quarries and railway and highway cuts.

Evidences of water erosion, potholes, tortuous grooves, etc., are found 

at a considerable height above the present river.

This district can not be passed without mentioning the bed of gabbro-schist, described in detail on pages 87 and 88, which occurs in 

sec. 32, T. 115 K, E. 40 W., 1,750 paces K, 250 paces W. It covers 

an area of small extent, the exposed surface being only 25 paces long 

and 20 paces wide. Over its whole surface this rock is profusely veined. 

While its contact with the surrounding rocks can not be seen, it is no 

doubt conformable with the beautifully contorted gneiss which stands 

out to view only 40 paces to the north. The gabbro-schists throughout 

the district present as pronounced a lamination as do the gneisses 

themselves. In color they vary from dark green to black, and they 

fade out very perceptibly on weathering. These rocks are further described on pages 85-93.

All the rocks, gneisses and gabbro-schists, are cut through by the 

dikes. The number of dikes crossing the valley through this district 

is large. There is great diversity in their width as well as in their 

massiveness and texture. They have one general direction, from the 

northeast to the southwest, but locally the direction varies to some 

extent. All these dikes are diabasic, or were originally, although 

many of them are now much altered. The change is more marked in 

the southern portion of the area. This difference may be due to a long 

time interval between the intrusion of the dikes of the southern and 

northern areas.

Veins are frequently seen in all the different rocks of this district. 

The coarse granitic material of several of the veins is noticeable, 

while the milky quartz veins, which are not infrequent, have been found 

to carry traces of gold. Much quartz, in lenticular masses or segregations, is to'be seen in these rocks, as well as in the deposited masses in 

the areas bordering the fissures.

While there is no evidence at hand to show that the rocks of this 

district are eruptive, all indications, if any ever existed, of a sedimentary origin for any of them, whether basic gabbro schists, semi-basic 

diorites, or acidic gneisses, have been obliterated in the subsequent 

transformations through which they have passed. The characters of 

every class of rock in the area are such that most geologists would 

regard them as evidence of eruptive origin. All evidences of any different condition at any previous geologic period point to the alteration
36 GNEISSES, GABBRO-SCHISTS, ETC., OP SW. MINNESOTA. [BULL 157.

of some other crystalline rock rather than to the upbuilding from an 

originally clastic mass into the present perfectly crystalline condition. 

Still, no undoubted evidence of eruptive origin exists here. Lawson 

has pointed this out 1 in the case of similar rocks in his upper division 

of the Archean rocks of Ontario.

The area about Minnesota Falls has been, at least twice within its 

geologic history, the scene of profound volcanic disturbances. At the 

first, great quantities of rock were intruded. At least a portion of 

these rocks was basic. During the profound foldings which the 

Archean crust has undergone throughout the entire Northwest, these 

rocks were made schistose and were infolded with the associated acidic 

and gneissic rocks. It was after such folding that another period of 

volcanic activity ensued, and dike after dike of basic material appeared. 

Whether an overflow of extensive beds of this dike material occurred 

can not be determined, but the probabilities are negative 5 nor will it 

be known whether all these dikes were formed at the same epoch. 

Observations have not shown any two dikes crossing each other, and 

this bears strongly against successive epochs of intrusion. Many 

things point to the probable Keweenawan age of these outbreaks. 

Whether the Minnesota River Yalley dikes be or be not of Keweenawan 

age, this much at least can be said, the Cambrian rocks of the Northwestern States have not, up to this time, disclosed a single dike of 

volcanic material. On the other hand, in many places rocks unquestionably of igneous origin have been discovered beneath the horizontal 

sandstones, shales, and dolomites affirmed by all paleontologists to be 

of Cambrian age.

MONTEVIDEO DISTRICT.

In the western part of Chippewa County there is an area of biotite 

gneissic rocks of sufficient interest to receive attention. (See PI. XI.)

At Montevideo the gneiss, which is strongly banded, has a red color. 

The strike in the vicinity is very nearly NE.-SW., with a uniform dip 

varying from 30° to 50° SE. .The surfaces of the rocks are much 

waterwprn, more even than at Granite Falls. Potholes and irregular 

grooves are numerous.2 One of the potholes is at least 8 feet deep and 

3 feet in diameter. Deposits of modified drift surround these worn 

surfaces. In many places, as at Montevideo, these evidences of water 

erosion are at a considerable height above the present river, being at 

least 50 feet at a railway cut near Montevideo, and not less than threefourths of a mile distant from its channel.

The peculiar kaolinized gneiss, which occurs in many places farther 

down the river, seems here to have entirely disappeared, the last 

exposures of such kaolinized gneiss occurring at Granite Falls, at the

'The internal relations and taxonomy of the Archean of central Canada, by A. C. Lawson: Bull. 

Geol. Soc. America, Vol. I, 1890, p. 178.

2 Geology of Swift and Chippewa counties, by Warren Upham: Geol. Nat. Hist. Survey Minnesota, 

Vol. II, 1888, p. 212.
U.S.GEOLOGICAL SURVEY BULLETIN No 157 PL.XI.

R XLI W

GEOLOGICAL MAP OP' THE MONTEVIDEO DISTRICT

Scale 

o i

DIABASE DIKES GARBRO AND GABBRO SCHIST

JULIUS 8IEN & CO. LITH N.Y

ffHANITE AND GNEISS
HALL.] MONTEVIDEO DISTRICT. 37

corner of Prentice and Thursey streets, and in a gorge opposite Minnesota Falls, on the north side of the Minnesota Eiver, in the west half 

of sec. 2, T. 115 N., E. 39 W.

Dikes occur having a NE.-SW. direction, as in-the Granite Falls 

district, which show some interesting phases of composition. Taken 

as a whole they are not so much altered as at Granite Falls. Some of 

them are heavily loaded with quartz. Their coarseness varies with 

their width.

At two or three localities in this district the gabbro-schist rocks 

appear. One locality is near the line between sees. 18 and 19, T. 117 

N., E. 40 W., where the rocks stand in rounded knobs 30 feet or more 

above the river. There seems to be in this exposure an abnormally 

large amount of quartz. The presence of this mineral, bearing every 

evidence of infiltration, is probably the cause of the resistance to 

erosion shown by these knobs, for the rocks around them have been 

eroded to a depth which must be considerably below the surface of the

20

GRANITE-GNEISS

HOffNBLENOIC HYPERSrHENE-GABBRO

FIG. 6. Contact of granite-gneisa and liorublendic hypersthene-gabbro-schist near Montevideo. 

present alluvial beds of the river valley. Another interesting exposure 

of this type of rock was seen in sec. 31, T. 118 N., E. 40 W., 2£ miles 

due north from Montevideo.

The area is small, only 60 paces long and 30 paces wide. The rock is 

dark gray, somewhat imperfectly laminated, and of medium texture. 

The strike of the lamina} is N. 30° E., and their dip 55° SE.. The laminae are bent sufficiently to give a wavy appearance, but not enough to 

make a contorted schist. Indeed, nowhere in the valley have these 

gabbro-schists exhibited a degree of irregularity in direction of lamination that would entitle them to be called contorted. (See fig. 6.) 

While the gneisses are contorted in many places, this condition has 

neve* been noted where the gabbroids occur in proximity. In mineral 

composition this rock is a hornbleude-hypersthene-gabbro. It is fully 

described in Chapter V.
38 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157. 

LAC QUI PARLE DISTRICT.

For several miles above Montevideo there are few rocks in sight. 

The bluffs on either side of the Minnesota Eiver slope off in smooth 

and rounded outlines from the prairie level to the bottoms, a descent of 

about 100 feet. Lac qui Parle, the " Echo Lake" of the Indians, is 

simply a broadening of the Minnesota Eiver for several miles of its 

course, as the result of a dam of silt deposited across the valley by the 

Lac qui Parle Eiver, which empties into the Minnesota some little 

distance below the foot of Lac qui Parle. (See PI. XII.)

The sloping and rounded bluffs on opposite sides of the river are 

from 1 to 3 miles apart. With the long lake and the meadows between, 

these bluffs outline a landscape of rare beauty. Along the north side 

of the lake there is a large tract which is in much of its extent rather 

swamplike and in times of flood is covered with water. A great 

portion of it, "however, makes highly productive farms. The rocks 

throughout the valley are mostly covered. The few exposures show a 

gneiss with distinct lamination, having a strike K. 70° E., with a dip 

as high as 80° SE. Most of the rock is a hornblende-biotite-gneiss. 

It is microscopically described in Chapter IV. There are both finely 

textured and coarsely textured gneisses in this district. The exposures 

at the lower end of the lake and along its south side seem to be much 

the finer and lighter colored.

Everywhere the rocks carry an unusual number of veins. These 

veins are sometimes several feet across, and again almost of paper 

thinness. In places they are quartz, and elsewhere they are granitic 

in composition. The quartz veins are seldom of great width, and as a 

rule are lenticular within the laminae of the inclosing gneiss.

ORTONVILLE DISTRICT.

Passing from Lac qui Parle toward the Odessa district, on the north 

side of the valley, one finds no exposures between those in sec. 32, T. 

119 N., E. 42 W., and those in sec. 20, T. 120 N., E. 44 W. In the Marsh 

Lake area large masses of gneissic rocks appear on both sides of the 

river. Their position seems to mark an ancient channel of the glacial 

Eiver Warren. The large rounded knobs show beautifully glaciated 

surfaces, with striae nearly parallel with the course of the valley.

In the center of this district (PL XIII), along a line extending from 

sec. 31, T. 121 N., E. 44 W., in a SW. direction through sees. 9 and 15, 

T. 120 N., E. 45 W., there seems to be an axis of a subordinate anticline; at least to the east of this line the dip of the schistosity is 

southeasterly, while to the west for se7eral miles there is a northwesterly dip. Within the townships just named there is considerable 

diversity in the character of the rocks. The larger portion is a hornblende biotite-gneiss with interlaminated bands of biotitic schist.

In sec. 9, T. 120 N., E. 45 W., there stand two or three knobs of a bio-
U S.GEOLOGICAL SURVEY 

GEOLOGICAL MAP OF LAC QUI PARIJE DISTRICT, SHO

Scale

X 

_ 

' O I______________

iiMwfB GBANITE AND GNEISS
HALL.] LAC QUI PAELE AND ORTONVILLE DISTRICTS. 39

titic quartz-pyroxene-schist. So far as mineral constituents are concerned, this rock is very similar to the gabbroid schists from the several 

localities in the valley already mentioned.

We have here, as well as in the districts already described, rocks 

which for convenience have been termed "squeezed eruptives" by the 

lithologists who have studied them for other regions. Large areas of 

rocks, possibly spread out as flows of igneous matter, together with 

rocks of different chemical and mineral composition, were squeezed in 

the continental movements subsequent to their extrusion to such an 

extent that a schistose structure was induced. Subsequent conditions 

have no doubt played their part in bringing these rocks to their present 

miueralogical and structural states. 1 . The tine crumpling so often seen 

in this structure is absent in the schistose rocks of the Minnesota Eiver 

Valley.

In the old gorge of the Eiver Warren, in sees. 9,15, and 16, T. 120 

$"., E. 45 W., around the mouth of the Yellowbank, a small creek

FIG. 7. Exposure of granite-gneiss and hprnblende-biotite-schist, showing alternation of the two 

rock species, sec. 15,1,200 paces K"., 1,600 paces W., T. 120 N., E. 45 W., Minnesota.

which enters the Minnesota Eiver near this place, are some interesting 

alternations of gneiss and biotite-schist, though so small a part of the 

rocks of this locality is in sight that only occasional glimpses of them 

could be gained. The two types of rock are interlaminated, sometimes in belts several feet in width arid sometimes in bands less 

than an inch wide; while lenticular inclusions of the schists in the 

gneiss are frequently to be seen. Fig. 7 shows this relation. Here 

and there a pegmatitic condition of the more feldspathic gneissic portions stands out in sharp contrast with the bordering schistose belts.

The gueissic condition is evidenced, not in the ordinary lamination 

presented by alternating biotitic and feldspathic bands, which as a 

rule characterize the gneisses of this valley, but in the prevailing 

direction of the large and porphyritic feldspar crystals.

Around Odessa are several gneissic exposures of interest. To the 

east, where the Chicago, Milwaukee and St. Paul Eailway comes down 

from the prairie around Appleton and Oorrell to the river bottoms, 

which it follows from Odessa to Ortonville and Bigstoue, the gneisses 

outcrop in fresh, firm masses.

1 Geology, Chemical, Physical.and Stratigruphical, by Joseph Prestwich, Vol. I, 1886, p. 270.
40 GNEISSES, GABBRO-SCHISTS, ETC., OF SW..MINNESOTA. [BULL. 15?:

In sec. 31, T. 121 N"., E. 44 W., 700 paces N., 1,350 paces W., within a 

few paces of milepost 168 of the railroad, a porphyritic granite and 

gneiss appear, with inclusions of schist. The.main mass of the exposure is gneiss; in places it's granitic. The feldspar crystals are frequently 1£ inches in length. Their color is a light pink, and they 

abound in inclusions of quartz and even of other feldspars. Sulphide 

of iron is so abundant that almost every fresh fracture discloses it. 

The species is presumed to be marcasite, since it readily decomposes 

on exposure to the air and covers the rock surfaces with* an iron rust. 

The schist inclusions exhibit a fine-grained dark-colored hornblendebiotite rock with secondary fine-grained quartz and several feldspars.

Only 300 paces from milepost 169, in sec. 36, T. 121 N., E. 45 W., is 

another set of exposures showing similar types of rock. A distinct 

gueissic structure prevails, the lamination running N. 20° E., with a 

SE. dip. Schistose inclusions abound. In this respect these exposures 

are essentially the same as the large bowlder mentioned by Upham, 

and referred to on the next page.

In the low, swampy area along the river, south of the knobs just 

located, are many outcrops of gneissic rock almost identical with those 

near the railroad, just noted. In places a sharp lamination is seen, 

and again the rock becomes so granitic that it is only in the parallel position of the feldspar individuals that any gneissic structure is 

disclosed.

A short distance west of the station at Odessa several small knobs 

occur, different in no essential particular from the gneiss and granite 

near mileposts 168 and 169, above mentioned. The word u granitoid" 

well describes the texture. The strike of the masses is nearly N.-S., 

and the dip is 40°-45° E. The rock as a whole is not in so good a state 

of preservation as that nearer the river a fact very commonly noted 

in rocks lying along river courses.

Within the 3 miles of the valley immediately below Ortonville there 

are many large areas of a granitoid gneiss. The rocks generally dip 

toward the northwest at a high angle. This gneiss has a prevailing red 

color (see PI. XVI, A), varying at times to gray through the increase 

in the quantity of biotite and hornblende. The feldspar crystals are 

frequently large. Below the railway station at Odessa are two or 

three glacial moraines which carry an enormous amount of granitic 

and schistose material. Bowlders of these rock types constitute a 

large portion of the material in many places. The proportion of the 

crystalline rocks in this glacial debris is evidence of enormous glacial 

erosion of the Archean rocks lying in the region stretching northward and northeastward from the Minnesota Eiver. Warren TJpham 

mentions this accumulation of bowlders, 1 and explains it as amorainic 

plain whose material, accumulated in an early epoch of the great Ice 

age, was partly buried beneath the debris of subsequent southeast1 Geology of Bigstoue and 1 

Minnesota, Vol. 1, 1884, p. 626

ac qui Parlo counties, by Warren Upham: Geol. Nat. Hist. Survey
BULLETIN No 157 PL.XIII.

GEOLOGICAL- MAP OF THE ORTONVIIJ.E DISTRICT

Scalp

:| GABBHO AND GABBRO SCHIST GRANITE AND &NKTSS
U. S. GEOLOGICAL SURVEY

GRANITE QUARRY AT ORTONVILL
HALL.] REDWOOD RIVER VALLEY DISTRICT. 41

ward extensions of the ice. The surface of this plain stands 50 to 75 

feet above the river and 50 feet below the top of the bluffs, or the general prairie level. The bowlders composing this de"bris are quite large 

in places. Uphain mentions one 30 feet or more in diameter, and 

another, of gray granite, measuring 20 feet, "in which are seen at one 

side several fragments of hornblende-schist up to 3 and 4 feet in diameter." 1 The writer and his assistant, Mr. J. E. Manchester, took a midday lunch in the shade of a bowlder of granitoid gneiss lying near 

the bluff on the south side of sec. 11, T. 120 N., E. 45 W., which measured 

15 feet in height, 34 feet in width, and 43 feet in length. Its Aveight 

was estimated to be over 100 tons.

In this mass of glacial drift there is much fine material, apparently 

to some extent calcareous, intermingled with occasional calcareous 

bowlders. Most of this calcareous material has, no doubt, been torn 

from the Minnesota shales and limestones of Cretaceous age.

The surfaces of the granitoid gneisses immediately below Ortonville 

are well worn and rounded by glacial erosion. Indeed, no better exhibition of the roches moutonne"es of glaciated districts can be seen in 

the whole valley. Any unevenness in the hardness of the rock is 

clearly indicated by the grooves and protuberances on the surface. 

The direction which the ice took down the Minnesota Eiver Valley 

was almost the course of the present stream. There is no doubt that 

the Eiver Warren occupied the same course. The grooves and striae 

which indicate this direction can be distinctly seen on the surface of 

almost every rock within the first 3 miles below Ortonville, and also in 

the old river channel now in part occupied by the unruly and tortuous 

stream, Yellowbank Creek, which empties into the Minnesota Eiver on 

the south side of sec. 34. T. 121 N., E. 45 W.

To the south west, of Ortonville, in sec. 16, T.*121 N., E. 46 W., stands 

a considerable mass of granitic gneiss, which is the most westerly 

exposure of this rock now known in this valley. Indeed, this is the 

last granitic outcrop in the whole Northwest until the great uplift of 

the Black Hills of South Dakota is reached. Deep wells bored in several South Dakota towns to the west and southwest show these rocks 

at from 50 to 100 feet below the surface. Millbank, 10 miles west of the 

Minnesota State line, has bored several wells with that result.

REDWOOD RIVER VALLEY DISTRICT.

Having passed in brief review the most notable rock exposures of 

the valley as they lie bounded on either side by the sharp ridges 

of iDorainic material, we will now note the few outcrops of crystalline rocks known to stand upon the prairies to the south and west 

of the Minnesota Eiver and north of the quartzites which are probably 

continuous from Coartland to Pipestone. The gorge of the Eedwood 

Eiver from the south border of the Minnesota Eiver bottoms into the

 Geology of Bigstone and Lac qui Parlo counties, by Warren TJphani: Geol. N»t. Hist. Survey 

innesota, Vol. 1,1884, p. 620.
42 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

middle of the city of Bed wood Falls is one of great natural beauty and 

interest. The rock is a gneiss, so altered in places that it has become 

an impure kaolin, sometimes green and sometimes red. From that 

completely changed condition to one of a fresh and tolerably firm 

building stone there is a well-graded series. Cretaceous beds overlie 

these gneisses in places. The length of the gorge must be at least 2 

miles, although the air-line distance is considerably less. At the head 

of this gorge is a cascade 25 or more feet in height. (See PI. VII.) 

The rock here is much fresher than at points farther down the stream. 

Above this principal cascade are several rapids where the water 

tumbles over floors of gneiss. Beyond, a few hundred paces, no rocks 

appear, save occasional bowlders, until miles of the slowly ascending 

prairie have been traversed.

Beyond the falls of the Eedwood Eiver in the city of Eedwood Falls 

oue loses sight of all rocks, the bed of the Eedwood Eiver resting upon 

the glacial till of the prairies. Artesian and deep wells sunk in Redwood Falls and several places on the prairie to the west, whenever 

deep enough, pass entirely through the glacial drift and all other fragmental deposits and disclose the giieissic rocks, which lie at depths varying from a few feet to 300 or 400 feet. So it may be positively maintained 

that gneissic rocks underlie the wbole district. In three or four townships south of the river small areas of gneiss are seen at the surface 

(PI. XV). These are air areas of a distinctly laminated, very quartz- 

.ose, hornblendic, and biotitic gneiss, in places considerably contorted. 

Their surfaces are unusually well preserved. They must stand from 

1.050 to 1,200 feet above the sea, and thus between 200 and 350 feet 

above Bigstoue Lake, which is practically the head of the valley. 

Where the covering of glacial debris has been but recently removed, 

beautifully polished surfaces are disclosed. In sec. 32, T. 113 TsT., E. 

38 W., the glacial striae show a direction S. 65° E.

POST-CAMBRIAN DEPOSITS.

In the Morton and Beaver Falls districts (Pis. V and VI) there 

are several exposures of supposed Cretaceous sandstones and shales. 

Along the Minneapolis and St. Louis Eailroad, from the prairie town 

of Frankliu down to the river bottoms at Morton, several cuts have 

been made in a medium-grained white sandstone. Beside the wagon 

road between the Franklin station and the bottoms in sec. 10, T. 113 N., 

E. 35 W., in the bluffs, where the glacial debris has been removed in 

the construction of the highway, there are exposures of a white sandstone. Again, in the gorge of Birch Cooley Creek, on the line between 

sees. 32 and 33, and also just above the old mill in sec. 28, all in T. 113 

N., E. 34 W., are exposures of horizontal, well-bedded white sandstone 

of medium grain and slight induration. These exposures are undoubtedly a continuation of the same beds that are seen along the railroad 

at the same and somewhat lower altitudes, The exact thickness of
U.S.GEOLOGICAL SURVEY BULLETIN No. 157 PL.XV.

R XXXVIII W

R XXXVIH W

GEOLOGICAL MAP OF THE REDWOOD RIVER DISTRICT, SHOWING GRANITE AND GNEISS

Scale

GRANITE AND &NEISS

JULIUS BIEN 8 CO. LITH N V.
HALL.] POST-CAMBKIAN DEPOSITS. 43

this sandstone can not be stated, but nowhere is an exposure more 

than 12 or 15 feet thick.

In sec. 5, or vicinity, T. 112 N., E. 34 W. 7 there is reported "an 

exposure of sandy marl, horizontally stratified, seen in the road that 

descends from the Lower Sioux Agency to the ferry [across the Minnesota Kiverj." 1 Along Crow Creek shales appear, and along the Eedwood Eiver both shales and seams of lignite occur.2 These Cretaceous 

rocks are not well compacted. The exposures stand at some little 

height above the river, and appear to rest in protected spots where the 

higher gneissic bluff protected them from glacial erosion. This rock 

was first observed by Keating,3 who mentions a fine white sandstone 

as forming the bluffs at a place corresponding to this portion of the 

valley. "In the sandstone exposures I saw no fossils, nor were other 

facts observed which could bear on geological age." Geographical 

situation and lithological characters naturally assign them a place 

with the beds along the Cottonwood Eiver, already mentioned in the 

discussion of the New Ulin area in the Cretaceous.4

The lignites have at various times excited considerable local interest, 

but all efforts at profitable working have been thus far in vain.5 No 

systematic exploration has yet been done to determine the economic 

value of the deposits or to work out the fauna or flora.

Between the quartzite exposure near Bedstone and the "basal conglomerate" of sec. 27, T. 110 N,, E. 30 W., and near the line between 

sees. 27 and 32, there is a bed, apparently thin, of poorly compacted 

limestone intermingled with shales and sand. On the south side of the 

Minnesota Eiver, in a terrace stretching to the southeast nearly parallel with the river, is another deposit of the same lithological character. 

No doubt the two deposits are portions of the same bed, which has been 

cut in two by the Minnesota Eiver. The present existence of this limestone is doubtless due to the protection afforded by the quartzite bluffs 

- to the north and east, which stand considerably higher than any limestone exposures observed. The rock is used for making lime at several 

Mlns. As taken from the quarries its appearance is peculiar. It 

occurs in corroded and vesicular blocks and in crystalline masses. 

The color of these masses on freshly fractured surfaces is seldom 

uniform, but varies from white to brown. It does not appear to be an 

extensive formation, either in thickness or in area.

Along the Big Cottonwood Eiver, to the west of the exposures named, 

and probably separated from them by river and glacial erosion, are 

beds of sandstone. They are horizontal, fairly well bedded, unevenly

1 Geol. Nat. Hist. Survey Minnesota, Vol. 1,1884, p. 577.

2 Compare The geology of the Minnesota Valley, by N. H. Winchell; Geol. Nat. Hist. Survey Minnesota, 1873, pp. 187,188; also Warren Upham, Final Kept., Vol. 1,1883, p. 572, et seq.

3 Narrative of an Expedition to the Sources of the St. Peter's Eiver, Lake Winnepeek, Lake of the 

Woods, etc., by William H. Keating, London, Vol. 1,1825, p. 362.

4 Ante p. 21.

6 Geology of Brown and Redwood counties, by Warren Upham; Geol.Nat. Hist. Survey Minnesota, 

Vol. 1,1884, p. 577.
44 . GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

textured, and generally white in color, although some of the layers are 

brown, particularly in the center of the exposed beds of the river gorge 

in sec. 36, T. 110 IS"., E. 31 W. At this location, in the lenticular layers 

of sandstone on the north side of the river, where a shady condition is 

favorable for their preservation, many fossil leaves have been found. 

These are Populites elegans Lesq., P. litigiosus Lesq., Ficus austiniana 

Lesq., Magnolia alternans Heer, Protophyllum credneroides Lesq., P. intergerrimum Lesq., and Laurus plutonia Heer, indicating that the rocks 

are of the Dakota formation of the Cretaceous. !

In general the data regarding the stratigraphy of western Minnesota, 

accumulated through the records of deep wells, go to show that a large 

portion of this district has a series of Cretaceous beds sandwiched 

between the drift deposits and the Paleozoic and pre-Paleozoic formations, which extend under ground westward from the central and eastern portions of the State. These Cretaceous beds are not at present 

of great thickness, nor are they of uniform composition and succession, 

as is shown by a comparison of borings from different localities. This 

area, being doubtless on the eastern border of the great Cretaceous 

gulf, could never .have received the enormous sedimentation which 

accumulated through the western portion of the Dakotas and southward. The present thickness of the Cretaceous and of the drift accumulations together represents at most only a few hundred feet of sediments. Owing to their soft character, these partially compacted 

deposits easily yielded to the abrasive action of Glacial time. It can 

easily be noted by the geologist crossing Minnesota from east to west 

that the character of the drift debris gradually undergoes a change. 

In the east it is predominantly siliceous. The pebbles are chiefly from 

the great terranes of the pre-Cambrian series, and where calcareous 

pebbles occur they seem to be wrought out of the Cambrian and overlying formations of the Paleozoic, their fossils proving their age.2 

Water in the streams is somewhat hard, yet not objectionably so. In 

the west a fine calcareous shale, seldom appearing in large bowlders, 

gives a different texture to both the till and the modified drift. The 

color of the morainic material is different. Water from its springs is 

hard and far from pleasant for domestic usei Near Ortonville are 

springs which yield sufficient quantities of the carbonates of calcium 

and magnesium to deposit considerable masses of travertine. Travertine deposits are not infrequent in the upper portion of the valley. 

Magnesium salts and other compounds are usually seen in dry weather 

as efflorescences covering low grounds and desiccated sloughs throughout western Minnesota.

North of the Ortonville district Paleozoic limestones and fossils constitute a considerable portion of the drift material. At Morris and

1 Geology of Brown and Redwood counties, by Warren TJpham; Geol. Nat. Hist. Survey Minnesota, 

Vol. I, 1884, p. 573. See, also, Cretaceous fossil plants from Minnesota, by Leo Lesquereaux: Geol. 

Nat. Hist. Survey Minnesota, Vol. Ill, Pt. 1, 1895, pp. 1-22.

z Paleozoic fossils iu the drift, by F. W. Sardesou: Bull. M.inu. Acad. Nat. Sci., Vol. Ill, 1891, 

p. 317.
HALL.] POST-CAMBRIAN DEPOSITS. 45

at Fergus Falls large numbers of fossils have been found and identified 

as Silurian and Devonian species. These formations underlie and are 

more resistant than the Cretaceous, and hence they must be referred 

to a subordinate place in our search for the source of the material of 

the. glacial deposits. Since the Cretaceous is presumed still to remain 

over much of the area, the Paleozoic may also be presumed still to 

remain beneath it. Upham is confident that it does, 1 and that the 

bowlders we find represent the disruption of its edges only. Suffice it 

to say that throughout the entire valley above Courtland no exposures 

of Paleozoic rocks were seen, and no Cretaceous rocks were noted in 

place above° Redwood Falls.

While the glacial strias seen on the rocks have been noted from time 

to time in this geographical sketch, the following table may be found 

convenient as containing all the observations made in the explorations 

of the valley by the writer and others, so far as their observations are 

at hand. The writer is the authority unless another name is given. 

Starting at Bigstone Lake, the order is down the valley :

Glacial strice observed in the Minnesota River Valley.

Locality.

Sec. 15, T. 121 N., R. 46 W ........................

Sec.21,T.121 N..R.46 W ........................ 

Sec. 22, T. 121 N., R. 46 W ........................

NE. J S W. i sec. 26, T. 121 N., R. 46 W ............

NE. J sec. 12, T. 120 N., R. 45 W ...................

NW. J SW. 1 sec 16, T. 120 N., R. 45 W.. ..........

SE. J SE. J sec. 22, T. 120 N., R. 45 W ..............

NW. i NE. i sec. 22, T. 120 N., R. 45 W. ...........

SW.JNW.Jsec.l4,T.118N.,R.42W............

East Granite Falls ...............................

NE.jNE.Jsec.4, T. 115 N., R. 39 W .............

N. J sec. 11, T. 115 N., R. 39 W. ...................

NW. i SW. J sec. 29, T. 113 N.,R. 38 W .. .........

Posen, T. 113 N., R. 39 W ........................

Echo, T. 113 N., R. 38 W ..........................

NE. J NE. J sec. 32, T. 113 N., R. 38 W ............

N W. J sec. 10, T. 113 N., R. 35 W. .................

NW. J NE. J sec. 30, T. 112 N., R. 38 W. ...........

NE. i SE. J sec. 6, T. Ill N., R. 38 W ..............

SE. J NE. J sec. 12, T. Ill N., R. 38 W. ............ 

E J sec. 12, T. Ill N., R. 38 W. ....................

SW. £ SW. J sec. 1, T. Ill N., R. 32 W. ............

NE. J SW. J sec. 35, T. 110 N., R. 30 W ............

Direction.

S. 403 E.

S.40°E. 

S.40°E.

S. 350 E.

SE.

S. 45° E.

S. 30° E.

S. 40° E.

S. 40° E.

S. 45o E.

S.45°-50°E. (TTpham.) 

S. 50° E.

S. 50° E.

S. 52° E.

S.50°E. (Upham.)

S.50°-55°E. (Upham.) 

S. 53° E.

S.60°E. (Upham.)

S. 61° E.

S. 55° E.

S. G3° E. 

S.50°-60°E. (Uphara.)

S. 48° E. 

S.60°E. (Upham.)

S. 33° E.

 Geology of Ottertail County, by Warren Upham : Geol. Nat. Hist. Survey Minnesota, Vol. II, 1888, 

p. 543.
CHAPTER IV.

GNEISSES. . 

GENERAL CONSIDERATIONS.

The laminated granitic rocks designated gneisses constitute the great 

bulk of the crystalline rocks stretching along the bottoms of the Minnesota Eiver Valley between New Ulm and Ortonville and standing 

in a few isolated knobs in the higher prairie district south and west of 

the river.

As a rock type, gneiss probably presents more varieties than any 

other known species. In addition to all the chemical and mineral 

phases which the granites are known to possess, the gneisses exhibit 

all the modifications which the lamination of a rock may develop. 

Structurally a series is recognized from the granitoid masses where 

only the imagination of the geologist or his observations covering 

extensive rock areas can distinguish between a gneiss and a granite, 

to that other extreme where the discrimination between a gneiss and a 

schist is based on equally uncertain data. As Hager truly says in discussing the granites and gneisses of Vermont: "Cases occasionally 

occur when it is very difficult to decide whether the rock be stratified 

or not. Even those rocks which all geologists concede to be granites 

. evince such a disposition to split in certain directions that the 

 workmen generally regard them as stratified rocks." :

There is scarcely a step in the structural series of gneissic rocks that 

is not represented by the gneisses of this valley. The granite-gneisses 

of Ortonville, Yellowbank, and Fort Eidgely, the porphyroidal exposures of Beaver Falls and Odessa, the normal and strongly banded 

masses at Morton, the schistose bands of many other localities, as along 

the lake shore below Vicksburg (sec. 29, T. 114 IsT., R. 36 W.), all express 

certain well-known phases of this rock type.

Some exposures are intricately contorted and others evenly banded. 

The rocks of some localities are shattered throughout, and in other 

localities there are immense areas where for long distances no fault or 

joint is seen.

In mineral composition there is even greater diversity than in structure. In the valley are hornblende-gneiss, hornblende-biotite-gneiss, 

augite-biotite-gneiss, biotite-gneiss, and biotite-muscovite-gneiss, with 

every possible transition stage between them. The chemical composition varies from a highly siliceous to a basic gneiss.

'Economical geology of Vermont, by A. D. Hager: Geoloiry of Vermont, 1861, Vol. II, Pt. IX, p. 736. 

46 ^
BALL.] GNEISSES. . 47

In using the term '' gneiss" to cover rocks with so great a range of 

structural and mineral characters, it will at once be seen that its application must be much wider than that given it in the early days of 

geology. Excluding those dioritic and gabbroid rocks which are sometimes regarded as gneisses,1 there remains the vasfc series of laminated 

rocks consisting of feldspar of several species without exception more 

than one is present quartz in one and two generations, the micas or 

hornblende, or both, and occasionally some other representative of the 

bisilicate constituents.

The investigations made by the writer show the gneisses of the 

Minnesota Elver Valley to be peculiar. They differ from the gneisses 

of the Muhl circuit, since in that district muscovite predominates in 

one division and biotite in the other; 2 from the Korean gneisses, which, 

according to Both, are to a great extent biotitic and associated with 

hornblende and other schists; 3 from the pyroxene gneisses of Varberg, 

which Tornebohm recently described; 4 from those of Scotland, described 

particularly in their relation to the attending granite by J. J. H. Teall; 5 

from the varieties of Vermont gneisses enumerated by Hitchcock; 6 

from the gneisses of New Hampshire described by Hawes,7 and from 

the hornblendic gneisses of the Farmington Canyon mentioned by 

King.8

The geologic age of the rocks under discussion can only be assumed. 

The fact that they are geographically isolated from every other district 

where similar crystalline rocks occur has already been emphasized. 

The glacial drift everywhere spread over them conceals their relations 

to the horublende-biotite-granites of the Sauk Eiver Valley to the 

northeast, and to the epidote-granites of Ashley and Ward to the north. 

Their stratigraphic relation to the quartzites of Oourtlaud and southwestern Minnesota has already been pointed out. Cretaceous limestones around New Ulm, and sandstones and shales below Kedwood 

Falls, in the Birch Cooley Gorge, and toward Franklin, lie close to 

them, if not immediately upon them. They are believed to represent a 

portion of the vast stretches of Archean rock formations underlying 

Minnesota and the neighboring States of Wisconsin, Iowa, and the two 

Dakotas.

By means of artesian-well borings made in many portions of the 

States named, the crystalline rocks have been discovered and their

1 British Petrography, by J. J. H. Teall, London, 1888, p. 295.

2 Materialien znr Orographie und Geognosie des Muhlviertels, lleferat, by Hans Commender: 

Neues Jahrbuch fur Mineral., Bronhard, Vol. 1,1885, p. 417.

3 Beitriige zur Petrographie von Korea, by J. Roth: Kept, in Neues Jahrbuch fiir' Mineral., Bronhard, Vol. II, 1887, p. 104.

4 Naagra exempelpaapyrorenforande graniter och gneiaser, by Tornebohm: lleferat, Neues Jahrbuch fiir Mineral., Bronhard, Vol. I, 1881, p. 70.

'British Petrography, London, 1888, p. 326.

6 Azoic rocks, by C. H. Hitchcock: Geol. of Vermont, Vol. I, Pt. Ill, 1861, p. 454.

'Mineralogy and lithology, by Geo.W. Hawes: Geol. of Now Hampshire, Vol. Ill, Pt. IV, 1878, p. 209.

Systematic geology, by Clarence King: U. S. Geol. Expl. Fortieth Par., Vol. I, Chap. II, Archean, 

p. 51.
48 GNEISSES, GAEBRO-SCHISTS, ETC!, OF SW. MINNESOTA. [BULL. 157.

depth from the surface has been measured. In this way it has been 

found that the Paleozoic rocks of Wisconsin and Iowa form only a thin 

sheet, from a few feet to a few hundred feet thick, over the pre-Paleozoic 

rocks beneath. In the Dakotas on the west the gneisses and quartzites 

slope gradually westward from near the west line of Minnesota beneath 

the water-bearing Cretaceous sandstones, with their overlying shales, 

until at the James Kiver Valley at Aberdeen, South Dakota, they are 

1,220 feet belovT the surface. Toward the north also, at Moorhead and 

at Fargo, granitic rocks are reached at 375 feet. The slope toward the 

south, beneath the Paleozoics of Wisconsin and Iowa, is shown in a 

similar manner to be a gradual one. Granitic rocks are reached at 524 

feet beneath the city of La Crosse, and quartzite is said to be reached 

at 768 feet beneath the surface at Spencer, Iowa, while to the east and 

northeast surface exposures are soon reached in Wisconsin. With such 

a comparatively extended plain on which the Paleozoic sandstones and 

limestones were laid down, the formation and deposition of this sediment must have been a process of vast time extent, and it is only natural 

to find the floor of fresh rock distinguished by very marked irregularities after the scorings of the successive ice invasions.

There is no reason to believe that in any geologic period since Cambrian time there has been any marked or unequal uplifting of this 

region, but as a vast, central continental plain it has been subject to 

the ordinary oscillations of level illustrated by those of the Glacial 

epoch. It is believed that nowhere in the States named save, perhaps, for a part of the area of the Dakotas and a portion of the Mississippi Valley does the upper limit of the Archean rocks sink below 

the present level of the sea.

The metamorphosed condition of the rocks points to an enormous 

time gap between the close of their building processes and the first 

deposition of Cambrian, sediments on this part of the North American 

continent. Walcott has placed these first sediments in the Upper 

Cambrian, 1 while Hall and Sardeson refer them to the Middle Cambrian.2

 Nowhere have any indications been found that the crystalline rocks 

can be regarded as altered sedimentary strata, as was recently 

announced by Director-General Geikie for some of the gneisses of the 

northeast Highlands of Scotland.3

The considerations above given, together with the mineral composition and other lithological characters, lead the writer to the opinion 

that the ancient gneisses under discussion must be regarded as of 

Archeau alge. Indeed, no geologist has thus far placed them in any 

other division of geological time. Owen, who calls these "the crystal1 Fauna of the Lower Cambrian or Olenellus zone, by C. D. Walcott: Tenth Ann. Kept. TJ. S. Geol. 

Survey, 1890, p. 548.

2Magnesian series of the Northwestern States, by C. W. Hall and F. W. Sardesou: Bull. Geol. Soc. 

America, Vol. VI, 1895, p. 170.

a Nature, Vol. XLIV, 1891, p. 480.
HALL.] MINERAL CONSTITUENTS OF THE GNEISSES. 49

line and inetamorphic rocks," Auderson and Clark, who designate them 

"the primitive system of rocks," and others, down to Van Hise, 1 place 

the Minnesota River Valley rocks in the bottom division of the geological series. The term "crystalline and metainorphic, rocks" of Owen 

and the "primitive system" of Auderson and Clark meant the same 

thing as does the word Archean, first proposed to geologists by Dana 

in 1874 in his subdivisions of geological time.2

The subdivision of these rocks into series of greater or less independence and importance can be made without much difficulty. In 

traversing them across their strike, as one must in ascending or 

descending the valley, many modifications of the gneissic type are 

encountered. These modifications lie partly in the structural phases 

which have already been pointed out and partly in the lithological 

characters which are now to be discussed.

Our studies will lead us to the conclusion that we have had under 

investigation two series of rocks, the one exposed at the two ends of 

the valley, typified in the exposures at and around old Fort Eidgely and 

at Ortouville, and the other in the central part, at Granite Falls and 

at Minnesota Falls. The Granite Falls series is represented by the 

gneissic exposures in Delhi and neighboring townships (PI. VIII) and 

along the bottoms to a point several miles above Granite Falls, in sec. 

14, T. 116 N., E. 40 W. (PI. X). The Ortonville series is represented by 

the heavy and generally granitoid exposures seen stretching northwesterly from New Ulm to the great rock barrens, reaching into sees. 

12 and 13, T. 114 N., E. 38 W. (Pis. IV, V, VI, VII, and VIII). Again, 

in the river bottoms below Montevideo the same series reappears and 

continues uninterruptedly to Ortonville (Pis. XI, XII, and XIII). Exposures of the granite-gneisses cover several square miles below the 

latter place. Their extent and favorable condition for study suggested 

the name Ortonville that has been applied to the series so extensively 

developed in the valley. Nowhere do these rocks rest in contact with 

those appearing in massive exposures around Fort Eidgely, Morton, 

Beaver Falls, and Montevideo.

In the study of the rock succession represented in the valley, the rock 

phases will be described according to the petrographical characters. 

Such a classification will be found exceedingly simple and at the same 

time progressive. Starting with the massive or granitoid types, the 

discussion will follow to the schistose and semibasic varieties.

MINERAL CONSTITUENTS OF THE VARIOUS GNEISSES.

Primary quartz. In many typical hand specimens of the hornblendebiotite-gneisses quartz is the most conspicuous mineral among the constituents. This is noticeably the case in the granitoid gneiss localities.

'An attempt to harmonize some apparently conflicting views of Lake Superior stratigraphy, by 

C. R. Van Hise: Am. Jour. Sci., 3d series, Vol. XLI, 1891, pp. 117-137. 

'Manual of Geology, by J. D. Dana, 2d ed., 1874, p. 139.

Bull. 157 4
50 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BOLL. 1W..

In places among these rocks quartz seems to constitute at least 

two-fifths of the whole bulk (sp. 5268, 5342). Again, there are isolated 

areas where the quartz occurs in lenticular masses almost wholly 

unmingled with other minerals. Typical localities are at Hindarmann's farm (sp. 5183), a mile or so above old Fort Bidgely (sp. 

5184 A), near the mouth of Birch Cooley (sp. 5208), and at other places. 

These occurrences will be discussed in another place under vein material. The usual mode of occurrence of quartz, however, obtains here 

that is, as interstitial material generally distributed among the other 

constituents.

Frequently there lie along the sides of fractures zones of quartz containing cavities, usually filled with liquid but sometimes empty. The 

origin of the quartz by pressure is thus indicated. As a rule the cavities are in clusters or bands. The quartz is broken up into numerous 

interlocking individuals, some of which are of large size. The liquidfilled cavities often occur in streams crossing both the quartzes and the 

adjoining feldspars until they lie scattered across the entire slide (sp. 

5237, 5301, 5308, 5344, and others).

Very generally, too, in these primary quartz areas there are numerous inclusions of minute, needle-shaped crystals, which lie in every 

possible direction, and are too small to be studied with any positive 

results. They are believed to be rutile needles, both from kindred 

observations in other localities and from the fact that Prof. James A. 

Dodge, of the University of Minnesota, made a very careful qualitative 

examination of some of this quartz from Hindarmann's place (sp. 

5183,1,750 paces K., 1,750 paces W., sec. 22, T. Ill N., E. 32 W.) and 

discovered in it distinct traces of TiO2 .

In many places throughout the valley, particularly around Fort 

Kidgely and Ortonville, the quartz has a remarkably opalescent, bluish 

aspect, giving a peculiar skimmed-milk appearance to the mineral. The 

quartz is white, and at the same time has a watery, limped look not 

usual in milky quartz. Occasionally an amethystine tint is also present. The opalescence is seen in the gneissic quartz and in the veinlike segregations alike. Where there is the highest degree of opalescence the needle-shaped crystals mentioned are most abundant. The 

conclusion was therefore drawn that the opalescent character depends 

on the abundance of the mineral rutile within the quartz.

Such inclusions in the quartz as have been pointed out occur only in 

those large yet allotriomorphic individuals with which all the gneisses 

of the valley abound. The individuals, usually regarded as primary, 

may or may not be one of the original mineral constituents of the rock. 

Their existence and present condition afford no evidence touching the 

origin or development of quartz as an essential constituent mineral. 

The quartz might have been present at the original crystallization of 

the rock or it might not have been. Time and the alteration processes 

to which the rocks have been subjected might have developed large 

and numerous areas of this mineral as it has of others.
HALL.] MINERAL CONSTITUENTS OF THE GNEISSES. 51

Secondary quartz. In addition to the large areas of quartz just noted 

there are almost always present segregated areas of a finely crystalline 

phase of this mineral. These areas are at times around the borders of 

the feldspars, again, in the midst of the feldspars themselves, especially 

within the orthoclase and microcline individuals, and at other times in 

the areas of altered basic constituents intermingled with the hornblende, biotite, magnetite, etc., which have resulted from the breaking 

up of the original mineral constituents of the rock. In these areas the 

quartz exists in very minute individuals, highly transparent, and, under 

crossed nicols, brightly polarizing. They are sutured together, and are 

exceedingly free from impurities of every kind.

Three forms of secondary quartz are noticeable:

1. Fine, granular segregations, perfectly transparent, compact, and 

almost wholly free from impurities.

2. Fine, brightly polarizing, isolated granules and pegmatitic areas 

in the midst of and around the feldspars. They occur more frequently 

associated with the potash feldspars, i. e., orthoclase and microcline, 

than elsewhere, although they are frequent in individuals of the albite 

type.

3. Vermicular quartz in and around the feldspathic individuals, more 

particularly those which show strong evidences of alteration.

Of peculiar interest in these granitoid rocks is this vermicular quartz, 

which is very widely distributed. As a rule it is microscopic, very 

rarely appearing in areas sufficiently large to be detected with the 

unaided eye. Almost without exception its secondary origin is evidenced by its distribution, for it is found within the boundaries of the 

older rock constituents, particularly the feldspars. 1 Within this group 

of minerals no particular preference seems to be shown; it as frequently 

invades orthoclase or microcline as it does the plagioclastic species. 

The contorted, vermicular passages through which the silica has made 

its way into the corroded constituents can be seen on PI. XVIII, J3, 

taken from the horn blende-biotite-gneiss (sp. 5396) of sec. 30, T. 112 K, 

K. 38 W., the town of Vesta, in the prairie region of western Redwood 

County.

JFeldspars. This mineral group is represented by several members. 

There is no apparent law of occurrence; scarcely a sample was taken 

that did not show several species. In few places did the rock show any 

single species as especially predominant.

In size of individuals there is the greatest diversity, the apparently 

primary ones reaching in places the size of several inches, while others 

are very small. The former are usually filled with the products of 

their own decomposition, mingled with and modified by those of the 

other mineral constituents of the rock. The outlines of the individuals 

"of primary consolidation," as Fouque* and L6vy call them, 2 are seldom

1 Min6ralogie Micrographique, by Fouqu6 et L6vy, Paris, 1879, p. 193.

2 Op. cit., p. 151.
52 GNEISSES, GABBRO-SCHISTS, ETC., OP SW. MINNESOTA. [BULL. 157.

clearly defined. They usually merge into a mingled mass of quartz and 

feldspar individuals or into a zone of quartz alone.

At a few places in the valley for instance, around Ortonville orthoclase seems to have suffered alteration more than any other feldspathic 

constituent. Yet amid all the changes it has undergone this mineral 

has not completely lost its identity.

Microcline plays a very important part in the constitution of these 

gneisses. It is frequently in porphyritic crystals an inch or more long. 

The surface usually has a pale reddish tint, and the fine fractures are 

coated with limonite. In point of quantity microcline seems greatly to 

exceed orthoclase in this group of gneisses.

While orthoclase and the plagioclases are, as a rule, more or less 

altered to kaolin, muscovite, and other products, with few exceptions 

the microcline areas are fresh and transparent. In this respect these 

rocks are no exception to the condition observed in northern Michigan. 

Where the microcline has altered to a preceptible extent it contains 

many small areas of clear quartz. These areas are either closely compacted or in the vermicular forms. 1 Wherever the feldspars are secondary and such places are numerous microcline takes the foremost 

place in quantity as well as in the development of its characteristic 

features.

In its distribution microcline accompanies both orthoclase and the 

acid plagioclases. Usually the boundaries of the contiguous individuals are sharply defined, yet not infrequently the sharp definition disappears and the line of demarkation can not be seen. Such a merging 

of an oligoclastic individual with microcline is seen in PL XIX, B, 

from a specimen taken below Ortonville. Again the merging of the 

microcline into orthoclase is seen. Several slides from fresh specimens 

of gneiss show orthoclase grains of large size surrounded on two or 

more sides with microcline. The two feldspars grade into each other 

so imperceptibly that the boundary between them can not be followed. 

The former thus constitutes the core, and the relative situation strongly 

suggests some genetic relationship between the two.

It still more frequently happens that in the midst of large individuals of orthoclase or plagioclase bright, fresh grains and granules of 

microcline appear. (PL XX, B.) Less frequently similar small patches 

of other feldspars, and even other mineral species, are seen. The outlines of these individuals are well defined. All the areas of microcline 

found within a single individual of orthoclase or oligoclase extinguish 

together, showing their possible connection in the third dimension.' 

While the relationship in position holds good under the circumstances 

named, the clusters of microcline, orthoclase, plagioclase, and quartz, so 

frequent in more altered material, show no such parallelism. Yet it is 

by no means clear that the smaller included individuals are secondary 

to the larger ones. They may represent minute quantities of micro1 The greenstone-schist areas of the Menominee and Marquette regions of Michigan, by G. H. Williams: Bull. TJ. S. Geol. Survey No. 62,1890, p. 111.
HALL.I MINERAL CONSTITUENTS OF THE GNEISSES. 53

clinic material caught up in the original individualization of the rock 

constituents, and maintained to the present time in their mineral 

integrity. Excellent illustrations of this phase of mineral association 

are shown in specimens 5315, 5320, and many others (see PI. XX, B).

Several varieties of plagioclastic feldspars are present in varying 

quantity. At one extreme, in the large number of individuals measured, the extinction angle along the twin lamellae is between 1 and 4 

degrees, pointing to a soda feldspar.

The macrocrystalline feldspars, while occasionally appearing in the 

larger individuals of feldspar, are as a rule distributed among the 

grains of quartz, also microcrystalline. They are always much more 

transparent and more strongly polarizing than the primary individuals 

just described; in optical characters they are almost identical with 

them.

Some features shown by the microcrystalline feldspars are marked. 

For instance, the outlines of all the grains are sharply defined by contact with neighboring grains. Occasionally an interlocking of material 

occurs, yet this habit is not so common as among the quartzes. Very 

few impurities or intergrowths are seen in these areas, and they are 

poor in bisilicate individuals. Accessory minerals are also rare, apatite 

being almost unknown in these portions of the slides.

Biotite. This mineral is everywhere present and often bears unmistakable evidence of being secondary. As a rule it is associated with 

hornblende. The latter mineral occupies the major place among the 

bisilicates in the sections cut from the freshest rocks; more altered 

rocks show the biotite and hornblende in about equal proportions; and 

samples taken from those portions nearest the surface and most thoroughly altered show little hornblende, but biotite is plentiful. In these 

weathered portions the biotite is well individualized, yet even here the 

areas are either small in size or complex in structure. Where the areas 

are not complex they are noticeably scattered through the rock, much 

more than are the individuals of quartz and the several feldspars.

The general characters of biotite so often described obtain here; 

therefore little need be said on that subject. Several specimens show 

a marked radial arrangement of the biotite. The individuals appear 

decidedly acicular in form, and shoot out from, a common center in all 

directions, forming rosettes of long, slender crystals. A well-marked 

porphyritic gneiss (sp. 5262) from the middle of the valley, shows in 

.one of the quartz individuals a decidedly resetted arrangement of the 

biotite (PI. XX, A).

< Inclusions of rutile, or some mineral similar in habit, are rather 

frequent in the biotite of the most altered gneisses. As a rule this 

mineral assumes the form or position of a sagenite web, but many 

needles were also noted, identical in their external characters with 

those seen in the quartzes and feldspars associated with the biotite.
54 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

Another mineral forming an interesting inclusion in the biotite of 

several localities in the southeastern portion of the valley is quartz. 

Beautiful lenticular aggregates of clear grains of this mineral are seen 

in certain exposures (sp. 5186 and 5199, from the Fort Eidgely district; 

PI. XX, C) toward the lower end of the valley. Epidote as well as 

quartz occurs in this situation, and in many cases it is difficult to tell, 

without a close examination, whether the lenticular aggregates are 

composed of the one or the other of the two minerals named.

Muscovite. Nowhere in the whole valley, so far as the rocks were 

sampled, is muscovite as prominent as biotite and hornblende. Wherever it is present it is associated with biotite or is its representative. So it can not be said that this is an essential rock constituent in 

the valley. It may be added, in passing, that muscovite is surprisingly 

rare in the gneisses and granites of the whole State of Minnesota. 

Within a restricted area southeast of Ortonville, in the old channel of 

the Minnesota Eiver, or its predecessor, the Eiver Warren, it occurs in 

nearly equal amount with biotite (sp. 5336 and 5337). In these specimens it bears every evidence of being a secondary mineral derived in 

large part, if not wholly, from orthoclase and microcline. . As a rule it 

may be said that the muscovite appears as small secondary folia 

within the large idiomorphic feldspar individuals or around their borders (sp. 5338; PI. XVIII, A}. It is rare that large, independently 

formed individuals of muscovite are seen. The folia, as they lie within 

the feldspar matrix, are clustered either in the centers of their hosts 

or in somewhat irregular bands about halfway from the center to the 

circumference, the bands taking the shape of the contour of the feldspar individuals. This clustered condition of the muscovite individuals has been noted by G. H. Williams in the granite-porphyry of the 

Horse Eace, 1 although there the muscovite clusters occupied the centers of the orthoclase crystals.

The optical characters of this mineral in these rocks are not different 

from those described by investigators who have worked in other 

regions. Usually the color is white, and the areas are free from inclusions and are very transparent, so that with ordinary light it is only 

by the detection of the cleavage striae that the mineral would be 

recognized.

Hornblende. This mineral occurs in a few localities in large isolated 

idiomorphic individuals. While these individuals suggest a porphyritic rock, the evidence that these rocks are porphyries is still 

lacking. The individuals are sharply denned. Their internal organization shows a homogeneity of structure, indicating a continuity of 

building noted in the old crystallines only, in which that building has 

been practically a continuous process. From the material at hand it 

is not shown that the formation of this mineral took place in two generations a process necessary in the development of a porphyry.2 The

' Bull. U. S. Geol. Survey No. 62,1890, p. 117.

2 Synopsis of Rosenbusch's new scheme for the classification of massive rocks, by W. S. Bayley: 

Am. Naturalist, Vol. XXII, 1888, p. 209.
HALL.] MINERAL CONSTITUENTS OF THE GNEISSES. 55

usual characters of hornblende, as described by Rosenbusch, Fouque" and 

L6vy, and Eutley, are seen.1 The mineral appears in clusters of small 

individuals (sp. 5255, 5310). The color is, without exception, green. 

The dichroism is strong, giving a brownish dark-green when parallel to 

c and a bright green in the direction of a. In the arrangement of these 

hornblende individuals there is little regularity. When the gneissic character of the rocks is brought out through the parallel position 

of the porphyritic feldspars, hornblende takes a subordinate place, 

both in quantity and in function, and displays a less tendency to 

parallelism than do the feldspars.

Another interesting phase of the hornblende of these gneissic rocks 

is the fibrous variety. This gives the uralitic character first described 

by G. Eose many years ago.2 The chemical composition of this variety 

of hornblende has not been determined. A typical locality, if the 

writer understands the type, is found in the quarry recently opened 

at Granite Falls, jnst west of the tracks of the Great Northern Railway. The rock is rather dark colored, possesses a distinct gneissic 

structure, and is rather coarsely granular. The bisilicate constituent 

was once a pyroxene, and even now some traces of that mineral are 

seen. For the most part it has changed to the fibrous modification of 

hornblende. The color of the alteration product is light green in some 

fibers, while others show an almost entire lack of color. The fibers lie 

in approximately parallel positions, although often a tendency to a 

radial arrangement is seen, especially where some portion of an old 

pyroxene (augite) area serves as a nucleus of crystallization.

In other localities for instance, at La Framboise's a similar occurrence of the fibrous hornblende is to be seen. The relations of the 

hornblende to the small areas of augite which are seen at this locality 

must remove all doubts as to its origin from this mineral, even though 

doubts remain concerning the secondary nature of the few isolated 

biotite folia which most thin sections of these interesting augite-biotitegranite-gneisses disclose.

The fibrous hornblende is not so frequent.in the gneissic rocks of the 

valley as it is in the gabbroids described in Chapter V. It may be said, 

however, that in the gneisses represented by the specimens collected 

there appears to be a rock which stands on the borderland between the 

great gabbro family, so widely distributed in northwestern Minnesota, 

and the great mass of Archean gneiss typified in the exposures of 

central Wisconsin.

The relation of this mineral to the feldspars indicates that the hornblende took a secondary place in the process of crystallization. Along 

the lines of contact the feldspar individuals are bounded by surfaces 

which in many instances closely approximate to crystal faces, while the

1 Mikroskopiacne Physiographie d. pet. wichtig. Mineralien, by H. Roaenbusch, 3d ed., 1892, p. 556 ct 

seq. Ibid.. Iddinga's translation of 2d ed., 1888, p. 244. Mineralogie Micrographiquo, by Fouqu6 

and Michel-L6vy, 1879, p. 365 et seq. llock-forming Minerals, by Frank Eutley, 1888, p. 184 et seq.

"Beisen iiacli dem Ural, 1837 and 1842. .Also Neues Jahrbuch fur Mia., 1833, p. 424; 1834, p. 652,
56 GNEISSES, GABBRO-SCHISTS, ETC., OF SW, MINNESOTA. [BULL. 157.

hornblende lying in contact with them assumes the shape thus determined. Occasionally smaller individuals of hornblende are well formed 

and show very complete sections of crystals, but this is a rare phenomenon. In color there is much variation, the green usually prevailing. In the gneisses, as a rule, the hornblende is unusually free from 

inclusions, although in the gabbro series it abounds in them, as will be 

stated in Chapter Y. Where mineral impurities occur, epidote, rutile, 

and biotite occupy the leading place.

Very frequently a close relationship is seen between the biotite and 

the hornblende. As has already been mentioned, the last-named 

mineral shows every indication of secondary origin. It lies in clustered aggregates, and seldom is an area composed of a single individual. The hornblende individuals bear no evidence, in their association and structure, of being primary. They can not be found in these 

rocks associated with some pyrogenic constituent and the steps of 

change from the latter be followed step by step, as can be done in the 

gabbroid granites of Ehinelander, Wisconsin, and in the horublendebiotite-granites of central Minnesota; but in studying the series as a 

whole, from one end of the valley to the other, every stage in the alteration process can be seen, from the fresh augite individuals to those 

firmly knitted, brown hornblende areas which represent the mature 

stage of this mineral. From the general composition of the hornblende 

clusters, from the association of other minerals, and from the presence 

among them of secondary quartz and feldspar granules the secondary 

origin of the hornblende is most confidently assumed.

Again, among these gneissic rocks phases, with many intermediate 

steps, appear, in the one of which the genesis of the hornblende can 

not be read, in the other of which it can be read with high assurance of 

correctness. The relations between these two types are so close, and 

their mineral conditions are so nearly identical, that one can not well 

avoid carrying the conclusions reached in the study of the latter phase 

over into interpretations of the former, and asserting the same conclusions in respect to it. Hence the writer has reached the conclusion that there is no such thing in the Minnesota River Valley as 

original or primary hornblende. For other portions of the Northwest, 

where the so-called Archean rocks appear, investigators have announced 

similar conclusions. For instance, Irving stated, while studying the 

Wisconsin Valley:

"Mr. Van Hise's study shows now that augite, so far from being a 

stranger to such rocks as gneiss and granite, is nearly as common a 

constituent, so far at least as this region is concerned, as hornblende 

or mica, and that all of the hornblende of the rocks of this region is 

but altered augite." l

Wadsworth, who examined the gabbros of the Northwest, believed 

"all the hornblende seen . . . to be secondary ... In color

1 Geology of Wisconsin, Vol. IV, Part VII, 1882, p. 714.
HAH.] MINERAL CONSTITUENTS OF THE GNEISSES. 57

it varies from a light or yellowish, green to a dark brown or yellowish 

brown . . . the darker stages being simply the more perfected 

condition of the greener or uralitic stage." 1 Lawson, for the Eainy 

Lake region, recently studied by him, said that "it seems not improbable that all the hornblende of the hornblende-syenite and granite 

[granite gneiss] of this region is of secondary origin." 2 Further, Geo. 

H. Williams, Judd, Schuster, and many others have noted the alteration of augite into hornblende in other parts of the world. Laboratory experiments 3 have been made, extending through a long series of 

years, in endeavors to show that hornblende represents the stable molecular arrangement of this already much-studied metasilicate compound.

The hornblende and biotite are, in all the granitoid gneisses sampled, 

extremely sparse. The only exception is seen in the rocks of the 

bottoms just above old Fort Eidgely, where a liberal sprinkling of 

hornblende and biotite can be seen over two or three areas. Basic 

inclusions were found in the rocks in their ordinary phases, while none 

were found in the acidic gneisses. In sec. 2, T. 11 N., E. 33 W., there 

are a number of inclusions of various sizes, well denned, schistose in 

structure, rather fine in texture, and a hornblende-biotite-schist or 

gneiss in mineral composition.

Augite. In their present altered condition the normal gneisses of the 

Minnesota Eiver Valley carry augite in only two or three localities, so 

far as the gathered hand specimens show.

Of these localities, the first to be mentioned is that around La Framboise's and Hindarmann's farms, below old Fort Eidgely, in sees. 15,16, 

22, and 23, T. 109 N., E. 32 W. (sp. 5191,5192, 5721, 5722). The second 

locality is at Granite Falls, in sec. 31, T. 116 ST., E. 40 W.' (sp. 5768 

and 5769).

Only a small quantity of augite is present in the freshest of these 

exposures, and even this shows partial alteration, save in one or two 

slides from the La Framboise locality. Two or three hand specimens 

from this place present the alteration of augite into biotite without 

disclosing the intermediate stage of hornblende-building. At Granite 

Falls the alteration into fibrous hornblende of some mineral which is 

presumed to have been augite has already been discussed. It may be 

remarked here that in all the slides cut from gneissic rocks where 

augite is present the proportion of basic minerals is extremely small.

Pyrite is to be seen nearly everywhere, but, as a rule, it is sparse. 

Under the railway tracks near Odessa, sec. 31, T. 121 N., E. 44 W., 

there is a comparative abundance of iron sulphide, probably marcasite. 

It occurs in segregations of small size. Some blocks have been blasted 

out and their surfaces are stained with iron rust from the alteration of

1 Preliminary description of the peridotytes, gabbros, diabases, and andesytes of Minnesota, by 

M. E. Wadsworth: Geol. Nat. Hist. Survey Minnesota, Bull. No. 2, 1887, p. 66.

8 Eeport on the geology of the Eainy Lake region, by Andrew C. Lawson: Ann. Eept. Geol. Nat. 

Hist. Survey Canada, 1887-88, p. 125F.

3 Geo. H. Williams has enumerated the chief of these. On the paramorphosis of pyroxene to hornblende in rocks, by Geo. H. Williams: Am. Jour. Sci., 3d series, Vol. XXVIII, 1884, p. 259.
58 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

these segregations. This alteration is so rapid that only on comparatively fresh surfaces can any of the iron sulphide be seen. Numerous 

cavities on the older surfaces, and the deep stains of iron rust, point 

to the former existence of the compound. In other localities clusters 

of pyrite crystals are to be seen, but nowhere in sufficient quantity to 

deserve particular notice, and probably in no place save the one mentioned in such quantity as to have a deleterious effect on the stone for 

economic uses.

Magnetite appears in many slides, but nowhere in quantity visible to 

the unaided eye. In its occurrence it bears evidence of secondary 

origin. It usually lies clustered about the spots where bright-green 

hornblende and minute, clear grains of secondary quartz are segregated. In several sections beautifully segregated masses occur, notably 

in the larger dike west of Granite Falls (sp. 5286).

Hematite is present in about the same quantity as magnetite. It 

appears in the surficial, most-altered portions of the rock. Where 

the rock crumbles the heaviest accumulations of this mineral appear. 

In such situations the cleavage planes of the feldspars are stained, as 

well as the opened contact zones between these and the other constituents. More rarely the hematite stains hornblende and chlorite 

clusters so that the appearance is that of a fibrous or radiated hematite.

Limonite is distributed as is hematite. Indeed, these minerals 

are usually distinguished by their colors. So far as their associations 

go, hematite and limonite merge into each other in many cases in such 

a way as to suggest that the only difference between them lies in the 

hydrated condition of the latter through the influence of percolating 

waters.

Ferrite. Occasionally around the areas of hematite one sees a whitish or light-brown, finely granular, closely packed iron substance, 

sometimes arranged in well-defined areas, as if the material were a 

pseudomorph after some preexisting mineral, now totally removed. 

Not infrequently this preexisting mineral would seem to be magnetite. 

From the difficulty found in separating it from a stained kaolinic 

decomposition product, almost everywhere abundant in the muchaltered gneisses, no satisfactory determination of the mineral can be 

made, and so it is given the general name/ern'te.

Rutile is present in minute, needle-shaped crystals which penetrate 

all the leading constituents of the granitoid gneisses. It was only in 

the biotite that any regularity whatever was noticed in the arrangement 

of these crystals. Here occasionally a strongly marked network of 

needlelike crystals can be seen in such sections as are cut at a low 

angle to the cleavage. In the quartz rutile is a most frequent impurity, especially around Fort Eidgely and Ortonville, as has been elsewhere stated.

Apatite, in the usual needles, is a frequent accessory. Of ben a broken 

condition indicates great pressure exerted at some early or middle 

transition stage in the alteration of the rocks.
HALL.] MINERAL CONSTITUENTS OF THE GNEISSES. 59

Kaolin. This is an extremely interesting alteration product. It is 

very general in its distribution. The freshest specimens disclose only 

traces, but the gneisses of Birch Cooley, Eedwood Falls, and thence 

locally as far as Granite Falls, are so completely broken down as to be 

soft and unctious to the touch. Here kaolin is one of the two or three 

abundant secondary minerals present. Between those two extremes 

the fresh, firm rocks on the one hand and the completely decomposed 

varieties on the other lie numberless phases of alteration. Ordinarily 

the kaolin occurs in minute elongated leaflets, which resemble very 

closely in all outward characters minute folia of rnuscovite. They are 

very clear, white, and strongly polarizing. Indeed, in many cases it is 

more than probable that muscovite and kaolin are intermingled.

Epidote is a mineral of no small interest in the study of these rocks. 

In all the sections examined not one crystal of this mineral was seen. 

There are two modes of occurrence: the granular, where the mineral 

lies in isolated or segregated individuals of varying size, from the 

.extremely minute to those which compare in area with those of quartz 

and feldspar; and the radial, where from a central point radiating 

grains shoot out into the substance of the host, giving a bright, rosettelike field in polarized light.

Epidote is singularly free from inclusions. Rarely fluid inclusions 

can be seen. Here and there an intergrowth with other minerals was 

noted. Nowhere does epidote present the features of a primary constituent of the rocks carrying it. It is most abundant in the extremely 

weathered rocks. Thus beneath rock surfaces long exposed to the 

weather and in proximity to thin bands or veins in the gneissic masses 

epidote should be expected; but in the deeper alterations, like those 

which have resulted in the kaolin of Birch Cooley, only occasional 

traces of it are to be found. It is sometimes present in the midst 

of feldspar individuals, much altered from their original chemical and 

physical conditions.

Chlorite also is an accessory. Although rather frequently met with, 

there are only two or three localities where it is sufficiently abundant 

to receive special consideration; On Tracy's farm, sec. 10, T. 112 1ST., E. 

34 W., is the most conspicuous locality found. Here one knob of gneiss 

with coarse andlndistinct foliation, somewhat jointed, with large feldspar individuals, and abounding in dark-colored inclusions, exhibits 

the most beautiful individuals of chlorite thus far seen by the writer in 

Minnesota. Eadial areas and chains of hexagonal plates are beautifully developed, and show the characteristic optical characters of 

the mineral. (PI. XVII, A.) It seems to spring from the decomposition of both basic and feldspathic constituents, although it is doubtless true that the basic minerals play the leading rdle. 1 It is associated 

with quartz, and lies usually in clusters and bands of secondary 

formation.

1 Compare Rosenhusch, Physiographic d. pet. wichtigeu Mineralien, 3d ed., 1892, p. 508; also ibid., 

Iddings's translation of 2d edition, 1888, p. 188.
60 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

The other localities in the valley where chlorite appears are the 

neighborhood of old Fort Bidgely, Bedwood Falls, Beaver Falls, and 

at several localities around Granite Falls and Minnesota. Falls. At all 

these places both the hexagonal plate and the radial cluster are 

absent. Occasionally a large area having the ordinary characters of 

hornblende has the optical reactions of chlorite; again, small isolated 

individuals are discovered.

HORNBLENDE-BIOTITE-GRANITE-GNEISSES.

In ascending the Minnesota Valley the first gneissic rocks one meets 

are granite-gneisses. There are four areas where the rocks are more 

distinctly granitoid than elsewhere in the valley: Around old Fort 

Bidgely (PI. IV), below Ortonville (PI. XIII), on the "rock barrens" in T. 

114 N., Bs. 37 and 38 W. (PI. VIII), and around Vicksburg, 5 or 6 miles 

southeast of the "barrens" and practically midway in the valley between 

New Ulm and Ortonville (PI. I). In all the areas named the granitegneisses carry inclusions, sometimes of large size, of schistose rocks. 

They are, save at Ortonville, in contact with the ubiquitous gabbroid 

schists. Everywhere within them are seen numerous quartzose and 

granitic veins. The granite-gneisses are everywhere hornblendic, and 

usually carry so much of that mineral associated with the biotite that 

they are hornblende-biotite-gneisses.

ORTONVILLE AREA.

Beginning a little below the city of Ortonville, and extending for 10 

or 12 miles down the Minnesota Biver Valley, lies a series of knobs of 

granite-gneiss. There are places where for a mile and more no bed 

rocks can be seen, the d6bris of the ancient Biver Warren and of the 

Minnesota having covered them. The largest exposures are several 

hundred paces in extent, and occasionally reach heights of 50 to 75 

feet above the bed of the river. The map, PI. XIII, locates the ledges 

with sufficient detail save when some special character is to be noted. 

The peculiar billowy aspect produced by the glaciated surfaces of these 

gneissic rocks immediately below Ortonville has already been mentioned; so, too, has the direction of the glaciated striae, together with 

the morainic movements which produced them. In structural characters there is some variation. On the ledges nearest Odessa there is 

seen a massive structure which only occasionally discloses a genuine 

lamination. Yet as one looks at the polished surfaces of the rock a 

distinct parallelism in the position of the idiomorphic feldspars is seen.

Passing eastward to the series of exposures between the village of 

Odessa on the north and the Yellowbank Creek on the south side, one 

finds a distinct gneissic structure prevailing. The strike of the laminse 

is, on the most typical exposure, nearly N.-S., with a dip of from 40° to 

45° E. (sp. 5320,1,400 paces N., 600 paces W., sec. 30, T. 121 'N., B.45 W.). 

The strike and dip cited for that exposure do not, however, everywhere
HALL.] HORNBLENDE-BIOTITE-GRANITE-GNEISSES. 61

prevail. A variation to a strike NE.-SW. can be seen within two-thirds 

of a mile, and doubtless exists much nearer. Still farther to the east 

and south are exposures showing conspicuously the parallelism of the 

idiomorphic feldspars mentioned as so plainly seen near Ortonville. 

Under the railway track, near milepost 168 from Minneapolis, on the 

Chicago, Milwaukee and St. Paul Railway, lies a considerable mass of 

granite-gneiss (sp. 5312, 5313). It contains inclusions of a schist, and 

is interlaminated with a more distinctly gneissic type of rock. It has 

been quarried for trestle work, and a good view of the fresh surface can 

be had. A mile or so to the west, and just south of the railway, lies 

another low exposure of very similar habit. The mineral constituents 

are here very fresh. One mile south of this spot, in a bend in the river, 

are several exposures of a similar granitoid type, carrying considerable 

biotite. The rock here has a reddish color and is rich in feldspars. 

The direction of these feldspars is NE.-SW., with scarcely a trace of 

mineral segregation into gneissic laminae. Yet only a few hundred 

paces to the southwest a knob of sharply gneissic rock stands up in 

the bottoms. The individuals of feldspars there are several species 

in these rock masses are quite fresh, and their outlines are welldefined against the matrix. All species alike show the usual inclusions, 

even with the unaided eye.

In the old river valley, a mile and more to the south of the Tellowbank 

Creek, is a series of exposures. The most southerly knobs are almost 

entirely devoid of foliation; for instance, those on the Parmenter farm, 

in sec. 22, T. 120 N., K. 45 W. (sp. 5337), and another a half mile west 

(sp. 5336). Some show a parallelism, of the feldspars (sp. 5338), while 

others exhibit iuterlaminated schists and a varying gneissic condition 

of the acidic rock.

The easternmost exposures represented on the map (PI. XIII) consist of a pair of knobs of considerably finer grain than the gneisses to 

the west. A sharply defined parallelism marks a NE.-SW. direction, 

with a dip to the SE. of 40° to 45°. Smoothly glaciated surfaces of 

some of the highest knobs show striae pointing S. 30° E. to S. 40° E., 

which is the general trend of the valley in this upper portion.

The jointing of these rocks was everywhere noted. Broadly speaking, there are two systems of joints, one having a NE.-SW. direction, and the other a NW.-SE. direction; but there are minor joints. 

Around Ortonville joints are comparatively rare. The view of Baxter 

& Son's quarry, shown in PI. XIV (p. 40), gives an accurate representation of the massive character of the granite-gneiss at this place.

In color the granite-gneisses of the upper valley are a light red. PI. 

XVI, A, shows the color and texture in natural size. Exposure modifies its tone somewhat, but where river silt and glacial debris have kept 

the surface covered it is strikingly fresh. Examination shows that the 

color is due to films and flecks of ferric oxide infiltrated along the contact surfaces and cleavage planes of the mineral grains. The origin
62 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

of the ferric oxide is not so clear. The first source to suggest itself is 

the covering of glacial drift, which everywhere contains a perceptible 

percentage of this compound. But in the deeper parts of the extensive 

quarry at Ortonville the rocks are as brightly colored as near the 

surface.

The microscopic characters of these rocks have already been briefly 

described in this chapter.

Ordinarily a medium texture prevails. This becomes somewhat 

coarser in certain lenticular areas which are doubtless due to a segregation process. At other places rather finely crystalline, almost microcrystalline, patches are seen.

Feldspar is the leading mineral. The larger individuals are mostly 

idiomorphic, frequently affording bright cleavage surfaces and minor 

fractures. Their transparency is so great that they remind one of 

shattered quartz. Some mineral inclusions are present, the most frequent being quartz, in spherical and elliptical grains, and other species 

of feldspar. For instance, within the oligoclase areas are small, fresh 

microcline individuals. Somewhat rarely the poly synthetic twinning 

of the triclinic individuals is clearly seen. The alteration products of 

the feldspars, save the quartz just mentioned, can not be seen with the 

unaided eye. Where the quartz is of the first generation it has the 

skimmed-milk blue color so common iu the granitoid rocks of the entire 

valley. A peculiar opalescence results from this, which is noticeable 

on fresh fractures. In the granite-gneiss of ordinary texture quartz 

of the peculiar opalescent phase is abundant. In the finest-grained 

specimens the quartz is subordinate, the feldspars, with a few areas of 

biotite, making up the mass of the rock. Where the rock becomes 

coarsely granular or pegmatitic, quartz assumes an increased importance. In these places it is more transparent than is usual. It extends 

iu irregular bands and masses between the feldspars, and thus plays 

the role of a matrix.

The bisilicate constituent, biotite, is not a conspicuous one. In many 

instances hand specimens fail to disclose its presence. Again it 

appears in such quantities as to make the darker-colored bands giving 

the foliated condition seen on many exposures.

Hornblende is not abundant in the Ortonville exposures. It is seldom seen in the surface specimens, and plays a very unimportant part 

in those taken from the depth of the quarry. Enough is present, however, to give a hornbleudic character to the rock. It is usually scattered about in isolated grains. An apparent exception is seen in the 

clustered condition of the granules in the rare bisilicate nodules. Here 

the hornblende and biotite are mingled in irregular and thickly strewn 

folia, all assuming one general direction.

The leading accessory mineral in the granite-gneisses of the upper 

portion of the valley is an iron sulphide. Yet even this mineral occurs 

only rarely around Ortonville or south of the Yellowbank. At the
HORNBLENDE-BIOTlTE-GRANI'fE-GNEiSSES.

railway cut in sec. 31, T. 121 N., E. 44 W., it is more plentiful. When 

blocks have lain exposed to the air for some time heavy stains of iron 

rust appear. From the readiness with which the sulphide decomposes 

it is believed to be marcasite rather than pyrite.

A single chemical analysis of the Ortonville granite-gneiss was made 

by Mr. A. D. Meeds, of the University of Minnesota. The results confirm those of the microscopic examination as to the nearly equal proportions of potash, soda, and lime feldspars, the sparse distribution of 

the bisilicate constituents, and the comparative richness of the rock in 

quartz.

Analysis of Ortonville granite-gneiss.

Constituent.

SiO., .............................................

ALA ............................... ..........:..

Fe.03 ............................................

FeO

CaO. .............................................

McO

KyO...... ........................................

-Sa^O....... ......................................

R20. .............................................

Total ......................................

Per cent.

14.06

3.84

1 23

-1 73

0.29

2.17

0.2G

qq fit

VICKSBURCr AREA.

Around Vicksburg for several miles there lie huge masses of granitegneiss. In places a very massive appearance is seen, and again a distinctly foliated condition prevails, where great contortion characterizes 

the surface. There is almost no interruption in the occurrence of 

exposures as one approaches the Vicksburg ferry from Kedwood Falls. 

Some of the exposures stand high above the river bottoms, reaching 

well toward the height of the bluffs of glacial debris, and others are 

low in altitude and insignificant in area. Strike and dip here, as everywhere else, vary considerably. In sec. 13, T. 113 N., R. 36 W., a reddish, clearly foliated gneiss (sp. 5384) of medium texture strikes N. 80° 

E. and dips 30° S. The outcrop stands on a bench in the bluff, and 

reaches an altitude of 45 to 50 feet above the bottoms through which 

the present river has cut its channel. The chief mineral constituents 

of this rock are quartz and microcline. Other feldspars are extremely 

sparse. Quartz is present in at least two generations. The few small 

areas of hornblende are partially chloritic, and in nearly every instance 

carry minute grains and needles of what was regarded as titanite. 

The rock shows evidence of alteration of its feldspars and bisilicate 

constituents. Some of the more-altered feldspars of the ancient generations contain small folia of secondary muscovite.
64 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157. 

FORT RIDGELY AREA.

Opposite New Ulm, in sec. 27, T. 110 N., R. 30 W., is a granite -gneiss 

which is weathered to an extent much beyond that of the ordinary 

gneisses of the valley. This is due largely, no doubt, to its protected 

position at a lower level than the beds of basal conglomerate lying in 

its immediate neighborhood. An apparent bedding prevails, dipping 

15° to 20° toward N. 50° B. The jointing is extremely marked and 

the rock, as a whole, is greatly shattered. The habit of the rock is 

porphyritic, orthoclase crystals being often an inch or two in length. 

They crumble easily. The red color characteristic of this group of 

exposures lies in the feldspars. Considerable white quartz is seen in 

the form of veins. These veins are, as a rule, lenticular in shape, yet 

sometimes they are in narrow, tortuous bands. The basic constituents 

consist largely of hornblende, with occasional grains of epidote. The 

quantity of biotite present is small, and only a few small folia of muscovite were seen.

In the neighborhood of old Fort Kidgely the granitoid gneisses are 

much fresher, firmer, and more massive than in the exposures near New 

Ulin. At the old ferry and ford, where some years ago quarrying was 

done for the barracks at the fort, and a little farther up the river, on the 

line between sees. 1 and 2, T. Ill N., E. 33 W., He extensive exposures. 

The rock shows a lamination in the somewhat rare bands of more basic 

material and in the arrangement of the large porphyritic feldspar individuals (sp. 5415). While the rock shows in places considerable contortion, the general direction of the laminae is N.-S. The dip is 

westerly, averaging 50°. The red color which here largely prevails is 

due to a staining by ferric oxide which has penetrated the cleavage 

crevices of the feldspars.

Both original and secondary feldspars are present. In many original 

feldspar individuals the inclusions are so large that they can readily be 

seen with the unaided eye. They are chiefly granules of quartz, hornblende, and biotite, although some interesting intergrowths of the 

feldspars were seen (sp. 5187, 5188). Frequently the sides and ends 

of the idiomorphic feldspars are corroded to such an extent as to show 

marked irregularities in outline. There is little orthoclase, the primary and in large part idiomorphic individuals which the rock contains being mostly microcline and one or more plagioclastic varieties. 

In size they are frequently an inch or more in length. The characteristic crosshatching of the microcline is sharply drawn, few individuals 

being so decomposed as to have the marking obliterated. Frequently 

crystals show beautifully both the Carlsbad and the polysynthetic twinning. In places there is a narrow reaction rim around these microclines as they come in contact with the albites and quartzes. As the 

result of a number of measurements, the triclinic feldspar is found to 

vary between 1° and 12° in extinction angle, and is therefore regarded 

as oligoclase albite. It shows more alteration than the microcline.
HALL.] HORNBLENDE-BIOTITE-GRANITE-GNEISSES. 65

The genetic relations of the two varieties were not studied at this particular locality. The twinning striae are very fine and are often considerably altered along their boundary edges. Instances are noted 

where half of an individual is strongly twinned and the other half 

is entirely free of twinning striatious. Other cases occur where on 

either side of a feldspar area the strong crosshatching of microcline 

can be seen, while the middle part of the grain bears every evidence 

of orthoclase.

The secondary feldspars consist of small, transparent, brightly polarizing areas, and are of very frequent occurrence in all varieties of the 

mineral. These secondary individuals are usually free from inclusions. 

They are arranged in strings and bands in the midst of the interstitial 

matter which surrounds the minerals of primary consolidation.

Quartz occurs in two generations. Biotite and hornblende are present 

in nearly equal proportions, the former in much better-defined areas 

than the latter. In many of the hornblende areas there lie many 

granular inclusions of an opaque material which bears every evidence 

of secondary origin. Its nature could not be clearly determined; it 

may be pyrite. Cubes of pyrite are occasionally seen. Sphene in small 

quantities was noted. In the areas once occupied by the primary basic 

constituents are clustered hornblende granules mingled with secondary 

quartz, biotite, and some opaque material, probably either magnetite 

or pyrite. The position and associations of these granules suggest 

their derivation through the alteration processes which have obliterated 

the original minerals and made possible the nests of clustered secondary 

minerals. *

Veins or lenses of quartz were noticeable in one or two exposures, 

particularly at 1,500 paces 1ST., 750 paces E., sec. 2, T. Ill ST., E. 33 W. 

They vary in width from a fraction of an inch to 4 and more inches. 

Horizontally they rarely extend more than 6 or 8 feet. Their general 

direction is JST. 65° W. The quartz of these veins is somewhat opalescent, although less so than in the veins crossing the augite-gneiss at 

La Fraraboise's and Hindarmann's farms 5 miles down the river. This 

character is due, doubtless, to the minute crystals regarded as rutile 

which abound in this mineral. Jointing is very conspicuous in the 

neighborhood of Fort Eidgely. Upon the mass just located conspicuous 

joints run E". 80° W. across the water-eroded surfaces. They dip southward at an inclination of 65°. Another system of joints, at right angles 

to the first, dips westward at the same angle, viz, 65°. Joints with 

similar directions appear at the old Government quarry in the same 

section and only 300 paces away to the northeast.

Along what seems to be a boundary between the dark-colored and 

reddish phases of these gneissic exposures lie several interlaminated 

masses of a hornblende-biotite schist. These masses vary greatly in 

size, some of them being no larger than hen's eggs, while the largest 

extended so far downward and under the turf that its boundaries could 

Bull. 157 5
66 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

not be determined. The texture of these inclusions is medium; their 

structure is distinctly schistose. As they weather at the surface they 

seem to bleach, through the alteration of the dark-colored constituents. 

These inclusions occur in many other places over the surfaces of these 

exposures than at the contact zone named ; yet along this zone they 

seem to be segregated. The contact between the inclusions and the 

granite-gneiss containing them is sharply drawn. A porphyritic condition of the rocks is elsewhere shown through the development of 

the feldspars. So far as the basic constituents appear to the naked 

eye, they are finely granular and lie in the angles and along the contact 

zones of the feldspars. The quartz areas, vherever they can be seen, 

assume a like position, but this position is noted in the clusters of 

grains as they lie in the rock rather than in the direction of the 

crystallographic axes of the individuals.

\UGITE-BIOTITE-GRANITE-GNEISSES. 

LA FE.AMBOISE AREA.

In several sections around La Framboise's farm, 15 miles in an air 

line northwestward from JNew Ulm, occur many large masses of a fresh, 

firm, granitoid gneiss. Near the stage road, and just under the projecting morainic northeast wall of the valley, stands a very siliceous 

red rock (sp. 5193). The whole exposure is shattered into rhomboidal 

blocks by joints running N. 75° E. and N. 10° E., the former set dipping 60° SB. and the latter 70° SW. The lamination of the rock, 

which is very obscure, strikes ST. 20° to 30° W. and dips at an angle of 

20° S. Toward the river the rocks assume a grayer color and are much 

less shattered. Their surfaces, which are waterworn and not glaciated, 

are grooved and undulating, and disappear at a gentle angle beneath 

the turf. Yet several abrupt faces 30 to 50 feet high show considerable erosion. The texture is medium, but it varies in places to a finely 

granular, granitic type. Microcline individuals several inches across 

are met with. Inclusions of the gabbroid rocks also appeal, with 

sharp contacts wherever contacts can be seen, but their area could not 

be made out, owing to the covered condition of the rocks. A banded 

structure appears at times, but the usual evidence of gneissic character is the parallel arrangement of the constituent minerals, particularly 

the feldspars.

At Hindarmann's farm, a half mile west of La Framboise's, a distinct 

lamination is developed through the segregation of biotite. More 

rarely, too, a lamination is produced by the development of lenses of 

quartz in the shape of thickenings of siliceous veins. In color the 

more laminated portions of the exposures are the darker, the color 

appearing to lie as much in the feldspar as in the quantity of biotite 

and its representative. Where the color is red, it is due to infiltrations 

in the interstices of the feldspars and in the crevices between the feld-
HALL.] AUGITE-BIOTITE-GRANITE-GNEISSES. 67

spars and the quartzes. The gray color varies with the condition of 

the feldspars.

A microscopical examination shows these granitic gneisses to be 

pyroxenic, and /that the pyroxene is to a considerable extent hyper - 

sthene. This constituent occupies but a small place in the mineral 

composition of the rock. Only now and then will a section show its 

presence. The grains are small and irregular. On the cleavages and 

other fractures of the grains alteration has changed the pyroxenic 

material to a fibrous biotite. Rarely the product of the alteration is a 

hornblende, and in two or three places chlorite was seen. This pyroxenic constituent is of two kinds, apparently about equal in quantity, 

one of which shows more or less dichroism (depending on the thickness 

of the section), a parallel extinction, and the sharp prismatic cleavage 

of hypersthene; the other appears in more or less rounded grains, 

irregular cleavage, little or no pleochroism, and with the high extinction 

angle of augite. The alteration products of the two minerals seem to 

be identical, their color and degree of transparency being about equal. 

Inclusions, except as they bear some relation to the alteration processes, are very rare and insignificant. Surrounding most of the pyroxene grains of both kinds are the alteration products, biotite and small, 

brightly polarizing quartz grains, with occasionally some minute individuals of hornblende.

Several varieties of feldspar are found in the rock. The plagioclases, 

when measured in the zone, all exhibit the low extinction angle of the 

albite end of the series, from 2° to 10° being the usual measurements. 

The plagioclase seems to show more alteration than the orthoclase and 

microcline. Cloudiness generally obscures the twinning planes, and 

areas of clear quartz of minute size frequently stud their surfaces, 

derived, apparently, through the decomposition of the feldspar areas in 

which they lie embedded. Microcline is present in many large, welldefined individuals. More rarely areas show the optical characters of 

orthoclase. These two varieties are free from alteration phenomena, 

yet they exhibit undulatory extinction to an unusual extent, and in 

places (sp. 5179) induced cleavage phenomena are disclosed.

Quartz plays an important rdle. The larger and more conspicuous 

individuals exhibit large numbers of liquid inclusions and many long, 

needlelike crystals of rutile. This last-named mineral is found in large 

quantities in the feldspars also. In many areas in proximity with the 

feldspars a microcrystalline condition of the quartz is rather prominent, showing all the indications of a secondary origin. Isolated 

minute grains occur frequently within the feldspars, particularly the 

plagioclases. Vermicular quartz is conspicuous in. many slides (sp. 

5179, 5193), and seems to grade into the granular quartz already 

noted. One character which is noticeable in all the slides is the sharp 

outline of all the quartz areas against the discolored edges of the feldspars. Apatite in needlelike crystals is sparsely present.
68 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

As might be expected from the small proportion of the basic constituents to the whole mass, the rock is acidic. Specimen 5179 was analyzed 

by A. O. Dinsmore and W. P. Milliken in the laboratory of the University of Minnesota. It was selected as a representative sample of 

the granitoid type. It contains quartz and a mineral that acts optically 

like orthoclase feldspar, both in large proportions. A plagioclase of low 

extinction angle occurs in a few grains so altered that only in places 

could the twinning strife be clearly made out. The alteration products 

are a kaolinic substance and vermicular quartz. The basic constituents 

appear in exceedingly small quantity. They are biotite and hornblende.

Mr. Otto K. Folin analyzed specimen 5723, which represents the more 

schistose portion of the rock at La Framboise's. Taken near the surface, it shows some discoloration, due to weathering, and doubtless a 

larger proportion of biotite is present. In field characters this specimen can not be separated from the granitoid phases with which it is in 

several places associated.

Analyses of yranitoid gneisses.

Specimen 5179.

Constituent.

Si02 ...... ..................

ALA ...................

TTo O

MgO.......... .............

PoH

Na20 ...................... 

K20.................. ......

H20...... ..................

Per cent.

72.30

15.40

3 in

O.C5

% 71

3.40

2.00

100. 60

Specimen 5723.

Constituent.

Si02 ........................

AIA...... .......... '....:..

Fe2O3 . ......................

EeO ........................

PnO

MgO........................ 

Na/) .......................

3v2 0...... .................. 

H20. ......... ...............

Per cent.

69. 012

13. 730

2.080

3.701

1.080 

4.196

2.330 

0.130

The schist (sp. 5721) at Hindarmann's, less than a mile away, shows, 

as an average of two determinations, 70.56 per cent of silica. Mineralogically the rock is practically identical with specimen 5723. Specimen 5721 is the more altered. Both are much shattered.

GRANITE FALLS AKEA.

Near Granite Falls are localities where rock similar to that at 

La Framboise's appears. Along the Great Northern Railway, recently 

built, some quarrying had been done. Extensive work was abandoned 

when it was found that the rock was so shattered that dimension stone 

could not be quarried. This is a typical gneiss so far as pertains to 

structure, although considerable variation can be seen. Inclusions of 

schist, some of them many feet in extent, are present.

The gneiss is, in general, a rock of medium color. It is strongly foliated by the alteration of quartz-feldspar bands with those consisting
HALL.I AUG1TE-BIOTITE-GRANITE-GNEISSES. 69

largely of the bisilicate minerals. The quartz and feldspar present the 

usual characters of' these minerals. The latter is represented by orthoclase, niicrocline, and a plagioclase of albitic extinction. The bisilicate 

constituents are muscovite, biotite, hornblende, and one or two pyroxenes. The muscovite is present in small .folia scattered through a few 

altered feldspars, and none was seen that did not have that position. 

The biotite exhibited some folia of large size as compared with the 

coarseness of grain of the rock. In places a finely granular condition 

is conspicuous in thin sections. These areas are in such relation to 

the hornblende and pyroxene that their derivation from these minerals seems clear. The hornblende, while not abundant, is present in 

numerous places and always in association with the pyroxene. Nowhere, 

save in the schistose inclusions, was it seen in good-sized independent 

individuals, but rather as a fibrous uralitic variety, with indistinct 

optical reactions.

The pyroxene reacts like hypersthene; it shows the dichroism and 

usually the parallel extinction of this mineral. It does not occur in 

large individuals, as is the case in the gabbroid rocks of the valley, 

but the areas are segregations of numerous granules of varying size. 

Their edges are frequently fringed with the beginning of decomposition, 

as shown in the uralitic alteration product. The central portions of 

the segregations, as well as the individual grains, are rather fresh. 

Accessory minerals are few and of the same species as are met in nearly 

every gniessic exposure of the valley.

In the same areas in which the above-described augitic granite-gneiss 

occurs there is also present the normal hornblende-biotite-granite-gneiss. 

The color, texture, etc., as well as the petrological characters, are different, but there is no place where the field relationships can be seen, 

and thus a basis be formed for discussing their genetic relations.

AREA IN T. 114 N., R. 37 W.

A rock of special interest occurs in the "rock barrens" in sees. 7 and 

8, T. 114 JST., K. 37 W. It is a mass of rock quite different in outward aspects and internal structure and constitution from any of the 

gneisses which lie around it. It varies from them in color and texture, 

as well as in general habit, to a very marked degree. The color of the 

rock is reddish, except where augite or biotite predominates, when it 

becomes greenish or black, or where the feldspar becomes inicrocline, 

when a dirty white prevails. The texture is in places very coarse and 

pegmatitic, and again it is fine and sharply crystalline. Where the 

texture is coarsest some work has been done in a search for gold, but, 

so far as could be learned, entirely without success. In this work an 

excavation of several feet had been made (Ellingson's mine), in which 

a series of specimens was secured. Here were noted cleavage planes 

on some of the feldspars which were several inches across. Augite 

crystals were also broken out which, while in an imperfect condition, 

were more than an inch in diameter and of still greater length.
70 GNEISSES, GABBROSCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

Beneath the road, just north of the south line of sec. 7, in the township cited, some blasting has been done. Here the rock has a medium 

texture, a bright, fresh color, and the feldspars are greatly in predominance. In general features this rock has every appearance of a gabbro. 

The color and habit of the feldspars, the presence of augite, the tendency in this mineral to alter into hornblende and biotite, the segregated 

condition of the mineral constituents, all tend to give a decidedly gabbroid and nongneissic aspect to the rock.

The boundaries of this group of exposures are hard to follow, as the 

surface is to a great extent covered with grass. Everywhere that rocks 

were sampled at short distances from the above exposures they proved 

to be the ordinary red hornblende-biotite-gneiss or simply biotitegneiss. It is clear, therefore, that this series of specimens (5372-5375, 

inclusive, and 5748-5754, inclusive) does not represent a rock variety of 

great extent or wide distribution. The only, other place in the valley 

where similar features were seen was at Morton, where certain inclusions in the ordinary gneiss gave an appearance similar to this. Yet 

at Mortou a gueissic arrangement of the mineral constituents is very 

pronounced, while here, on the rock barrens at the Ellingson location, 

the gneissic character is due partly to the direction of the feldspars in 

the ordinary texture of the rock.

There is nothing of note in the weathering of this rock variety. All 

its phenomena are identical, so far as observed, with those of the ordinary gneisses of southwestern Minnesota. Where the texture is coarse 

the corrosion or weathering has been greatly accelerated.

There are observed various gneissic phases from the granitoid to the 

well-defined gneissic alteration of bisilicate and feldspathic bands of 

mineral constituents. A strike of the banding N. 70° E. is seen, and 

the dip at Ellingson's mine is N., while in the road in sec. 7, T. 114 N., 

E. 37 W., the strike is NE.-SW., with a vertical dip. Some variation 

from the above strikes and dips was seen at other places.

In microscopical characters this rock is interesting. There is little 

quartz present, and what was seen gave every evidence of secondary 

origin. It occurs in the areas of pyroxeuic alteration to amphibole 

and where biotite has been formed. There are also bands of quartz 

grains around the borders of the frayed-out feldspars in the altered portions of the rock. The feldspars themselves are largely microcline and 

partly a plagioclase of low extinction, regarded as anorthoclase or 

albite, with an occasional area of labradorite habit. In quantity the 

feldspar varies greatly, some hand specimens showing but a small proportion, while others are made up almost wholly of the feldspars and a 

few scattering grains of quartz. The pegmatitic samples show the 

feldspathic constituent to be chiefly microcline, which carries numerous 

intergrowths of albite and occasional inclusions of some variety of 

higher extinction well toward the anorthite end of the series.

The augite is green «iu color, weak in dichroism, and rather strongly 

cleaved. It possesses the physical characters of the ordinary variety
HALL.] AUGITE-BIOTITE-GRANITE-GNEISSES. 71

of the mineral. One crystal, more than an inch through, measured very 

nearly 87° and 93° on its prismatic angles. The usual extinction angle 

on prismatic cleavages was 39° to 43°. Alteration has begun in many 

places, and green hornblende is the first resultant product. Its first 

appearance was in small fresh areas in the midst of the augite individuals or in similar condition around their borders (sp. 5372, 5374, 5748, 

5751), followed in later changes by well-defined individuals (sp. 5749, 

5750) in which all the characters of common green hornblende are well 

displayed.

Biotite plays a somewhat important role in this series of specimens. 

It is found in large, well-marked folia, even in those sections where 

hornblende is just appearing along the edges of the augite individuals 

(sp. 5372, 5751). The relation of the biotite to the augite in such sections as were made of specimen 5748, where small areas of biotite lie in 

the midst of large augites, and its entire absence in other specimens 

where hornblende is strongly developed, give strong suggestion of its 

being a secondary mineral, and that its host is augite, and possibly 

in places hornblende also.

Sphene and epidote are both present in small quantities. The former 

is clear cut and wedge-shaped where the sections are in proper position. 

The favorite situation of the spheue is the circumference of the hornblende nests where associated with granules of quartz. It forms the 

boundary zone between these nests and the feldspars which make up 

the mass of the rock. Apatite is seen in the usual well-defined crystals, some of them attaining considerable size.

It is difficult to determine the relationship of this rock to the ordinary 

hornblende-biotite-gneisses around. Nowhere could the contact of 

the two be seen; yet the abrupt change in texture and mineral composition shows them to be widely different rocks. This augitic rock does 

not show evidence of being ordinary inclusions in the gneisses, like 

the masses which Mr. A. H. Elftman has found in the granites of 

northeastern Minnesota, 1 nor like those which the writer has seen 

around Bhinelander, Wisconsin. 2 Begarding the resemblances of the 

latter rock, Professor Van Hise noted these points:

The microcline is very plentiful and the feldspars in extinction angles and species 

are the same as in the granites [of central Minnesota]. The areas of biotite, augite, 

and hornblende are large and numerous, the three minerals together composing more 

than one-half of the rock (sp. 5374). -The biotite occurs in large brown flakes, 

exactly as in the hornblende-biotite-granites. As in these, the hornblende is secondary to the augite, but this specimen differs from any of them in the great relative 

abundance of angite, not more than one-fourth or one-fifth of this mineral having 

altered to hornblende. If this alteration from augite to hornblende had proceeded 

further and a portion of the feldspar had been replaced by quartz, this rock would 

have become a hornblende-biotite-granite. 3

1 Preliminary report of field work during 1893 in northeastern Minnesota, by Arthur Hugo Elftman: 

22d Ann. Kept. Geol. Nat. Hist. Survey Minnesota, 1894, p. 157.

2 Geological excursion into central AVisconsin, by C. W. Hall: Bull. Miim. Acad. Nat. Sci., Vol. 

Ill, No. 2, 1891, p. 259.

8 Van Hise, unpublished manuscript.
72 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

The foregoing very accurately expresses the conditions, and the writer's 

opinion is that here is a group of rocks which bears strong evidence of 

being a mass erupted under the same conditions, and possibly at the 

same time, as the great granite masses of central Minnesota, which have 

their typical development around St. Cloud, Sauk Kapids, and Watab.

HORNBLENDE-BIOTITE-GNEISSES.

With no perceptible change in the conditions and occurrence, the 

granite-gneisses of sec. 13,JT. 112 K, E. 34 W., and the chlorite-gneisses 

of sec. 10, T. 112 N., E. 34 W., give place to the sharply contorted 

hornblendic biotite-gneisses of Morton and Beaver Falls. That the 

rocks around these two villages reach a considerable height above the 

present level of the river has already been pointed out in Chapter III. 

They stand nearly to the height of the prairies on both sides of the 

river valley. Indeed, this prominent position of the knobs of contorted 

gneiss can be seen in many places; a few miles above Eedwood Falls, 

on the south side of the Minnesota; in the gorge of the Eedwood Eiver 

at and below Eedwood Falls; on both sides of the Minnesota Eiver 

around Vicksburg; at Minnesota Falls; at Granite Falls; and in the 

bottoms below Montevideo. The prairie exposures located on PI. XV 

are all of this mineral type. The sharp contortion prevailing in certain 

localities locally obscures the strike and dip phenomena. In spite of 

this obscurity the general NE.-SW. trend of the pre-Cambrian formations across this portion of the State can not be doubted, nor can 

the series of crustal waves whose crests appear several times between 

New Ulm and Ortonville. In many places the hornblende-biotitegneisses are very fresh. Elsewhere there is complete obliteration of 

all the normal characters of a gneissic rock, save perhaps in the lamination, through the complicated alteration process which the rocks 

have undergone. The two extremes of the series, the fresh and the 

completely altered, as well as every intermediate stage, can be seen 

where the Pacific division of the Minneapolis and St. Louis Eailroad 

crosses the Minnesota Valley from the prairies of the north side to 

those of the south side of the river. This is along the south side of T. 

113 N., Es. 34 and 35 W. In the quarries and railway cuts at Morton 

occur some of the freshest exposures of the valley, or, for that matter, 

of the whole Northwest, and in the Birch Cooley Gorge, only 1£ miles 

away, can be seen equally fine and typical examples of the very extreme 

of alteration. Within a radius of 4 miles of Morton every intermediate 

phase of alteration can be seen. These phases have been considered in 

another place, and here only the mineral and physical characters of the 

normal gneiss will be discussed.

MOBTON AREA.

In the quarries and railroad cut at Morton were found the exposures 

of freshest and apparently least-altered gneiss seen in this portion of 

the valley. Fig. 5 (p. 28) shows the contorted condition, with schistose
HALM HORNBLENDE-BIOTITE-GNEISSES. 73

intrusions, and lenticular inclusions of coarser and scarcely laminated 

material. PI. XVI, B, shows the mineral lamination of the rock as seen 

in the natural coarseness at the Morton quarries. The general color 

is dark gray, due to the light-pink hue of the feldspar and the large 

proportion of biotite. The arrangement and the thickness of the mineral bands vary considerably. Lenticular masses are frequent, composed largely of biotite, hornblende, and quartz. These are, as a rule, 

schistose and of medium grain. Less frequently the inclusions are 

decidedly hornblendic, actinolite being a prominent constituent. Individual areas within these inclusions shade off from green to white. 

They extinguish at angles of from 9° to 17° from the position parallel 

to the principal cleavage. Such actinolitic inclusions are not large, 

nor were they observed near the surface. It was in the deepest portions of the quarry that the specimens were taken (sp. 5737). Scarcely 

any mineral save hornblende is present in these lenses.

The coarser pegmatitic inclusions, consisting chiefly of orthoclase and 

microcline, are more frequent in some places than in others; for 

instance, at the railroad cut at the bank of the river they abound, while 

in the quarry some rods away they are rare. The bands of the normal 

foliated gneiss in places abut directly against these pegmatitic masses; 

again they wind around them and conform to their shape. Still elsewhere the bands fault and continue from a point some little distance, 

even 2 or 3 inches, away. The concretions of biotite are as uncertain 

and irregular in their distribution as are the pegmatitic masses. They 

are seldom over three-fourths of an inch across and carry rather large 

folia of biotite and a comparatively small proportion of other mineral 

constituents. The general direction of the laminae immediately around 

Morton village is B.-W., bending to the south and north of this direction sufficiently to give a NE.-SW. trend to the formation of the area 

as a whole. The dip is, on the whole, northerly and varies from the 

horizontal to 50° and more.

The specimens gathered from the bottom of the Morton quarry and 

at as great a distance from the joints as it was possible to secure them 

show a less degree of alteration than do those taken from near the surface. The difference is not so much in the freshness of the constituent 

minerals as in the predominance among the latter of microcline and 

finely crystalline secondary quartz. As representatives of the freshest 

material, specimens 5734 and 5735 will serve. In these specimens the 

feldspars are orthoclase, microcline, and a plagioclase, in nearly equal 

proportions. The plagioclase is near the albite-oligoclase type, with its 

inconspicuous twinning and low extinction angles. The orthoclase and 

plagioclase seem to be more corroded about their borders than are the 

i^icroclines. The result of this corrosion seems to be finely crystalline 

quartz, with a few minute, fresh microcline and orthoclase areas, 

which are brightly polarizing and free from inclusions. The larger, and 

perhaps primary, microcline areas contain less of the microcrystalline
74 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

accessories of decay and alteration largely kaolinic material than do 

the other feldspars named.

Biotite is the chief bisilicate mineral present. It occurs in isolated 

folia rather than in the segregated form seen in the gabbroid series of 

the valley. As a rule, the individuals are comparatively free from impurities, yet now and then a light-brown mineral of either clustered or 

reticulated distribution, thought to be rutile, is seen. This mineral is 

oftener, however, an impurity in the hornblendic constituent than in the 

biotite. Aside from these two, the rutile and the hornblendic mineral, 

accessories are unusually rare.

Specimens 5210, 5211, and 5212 show the condition of the rock at or 

near the surface. The proportion of secondary minerals is much 

greater and their individuals are much larger than the corresponding 

minerals in specimens 5734 and 5735. The orthoclase and plagioclase 

feldspars are considerably corroded. The microcline individuals are 

fresh and free from inclusions. Their size is greater than that of the 

other varieties of feldspar. They are rough and uneven on their borders, like the other feldspars. They contain inclusions of kaolinic 

material and small quantities of muscovite, as do the orthoclases, and 

in other ways they show evidences of alteration. Undulatory extinction is often seen. Biotite is the leading bisilicate constituent; indeed, 

it is practically the only one. The folia are small and sharply defined. 

As they lie beside each other they are bounded by even planes of 

contact. Inclusions are comparatively rare. The individuals are prone 

to segregate in clusters and bands. Not infrequently biotite inclusions 

occur in the feldspars, particularly the albite-oligoclase variety; indeed, 

they assume a fibrous condition, and appear between the twinning 

lamellae.

Here and there a chloritic phase represents the bisilicate constituent, 

and rarely hornblende clusters are seen. The ordinary green variety 

of hornblende is the one present. It can not be determined that the 

deeper gneisses are of the usual hornblende-biotite type, although the 

analogy afforded by the valley as a whole gives strong reasons for such 

an assumption.

Pyrite is present in small quantities, and apatite is even more rarely 

seen. In addition to these two accessories, another is present, which 

at once attracts attention as thin sections are examined. It occurs in 

numerous circular areas, thickly studded with minute, dark, opaque spots. 

These circular areas are small, ranging between 0.075 and 0.250 mm. 

in diameter. Within them lie thickly clustered crystal granules of 

various shapes, together with an enormous number of black, opaque, 

spherical granules. The individuals are all so small that it is extremely 

difficult to determine the species present, but feldspar, hornblende, and 

biotite can be named with much assurance. A quantity finely pulverized and tested with the magnet gave a generous supply of black magnetic grains. When these circular areas were first noted, it was thought
HORNBLENDE-BIOTITE-GNEISSES. 75

that they must have been formed through the action of substances 

derived from the decomposition of iron-sulphide crystals and crystalline 

granules distributed plentifully throughout the rock. The granitic 

character of the rock would facilitate the spherical habit which these 

bodies invariably possess. The several processes involved in such an 

alteration might be followed with, some positiveness were it possible to 

piove the mineral identity of the several species distinguished.

In chemical composition the rock as a whole shows a more basic 

character than the less foliated gneisses and granite-gneisses in other 

portions of the valley. This character will be seen by comparing the 

analysis below with the analysis of the Ortonville granite-gneiss, on 

page 63. A. D. Meeds, of the University of Minnesota, analyzed an 

average sample taken from the Morton quarry and reported as follows:

Analysis of the hornblende-liotite-yneiss of Morton, Minnesota.

Constituent.

Si02 ..............................................

A1203 ............................................

Fo203 ............................................

FeO... ...........................................

CaO........ ......................................

MgO.............................................

KjO....... .......................................

NajO.......... ...................................

H20..............................................

T> n

Total ......................................

Per cent.

C3. 61

10. 71

5.C9

2.78

4.03

1.63

2.49

1.68

0.01

0.25

99.48

BEAVER FALLS AND REDWOOD PALLS AREAS.

At Beaver Falls the conditions are almost identical with those at 

Morton and along the Birch Cooley. At Kedwood Falls also the same 

is true so far as the rock is in a fresh condition. The occurrence of 

altered gneisses shows essentially the same conditions and phenomena 

in all three of the localities; thus they are all conspicuous for the succession of changes easily traced from the fresh to the kaolinic condition.

The small exposure of gneiss in the woods between the road and the 

Minnesota Eiver,'along the bottoms between Beaver Creek and the 

Eedwood Falls ferry, is worthy of notice. Specimen 5252 is from this 

locality. The knobs of rock extend in a direction N. 60° E. The 

largest mass of the group is only 175 feet long and 15 feet high. The 

prevailing color of the rock is red. While in general the appearance 

is that of a very fresh, firm gneiss, almost entirely unaffected by the 

kaolinization process which so conspicuously marks the gneisstc 

exposures in the near vicinity, particularly along the banks of Beaver 

Creek, yet the entire mass is so shattered that not even a 3 by 4 inch 

hand specimen could anywhere be taken. The texture is medium, with
76 GNEISSES, GABBRO-SCHISTS, ETC., OP SW. MINNESOTA. [BULL. 157.

a slight tendency to porphyritic condition. It has a sharp angular 

fracture. The porphyritic condition disappears under the microscope. 

It is due to the cleavage of the scattered remnants of the feldspar 

individuals. Excepting these remnants, the entire mass is composed of 

secondary minerals. The quartz is entirely secondary and lies in 

minute, brightly polarizing, limpid areas. The few feldspar cores 

remaining are of the orthoclase and the anorthoclase-albite types. 

They are spherical or elliptical in shape, ragged, corroded on their 

margins, and somewhat altered within. The clustered and isolated 

secondary individuals are brightly polarizing, and represent microcline 

in large proportion, with scattering grains of andesine-labradorite.

The quantity of basic constituents is extremely small. No hornblende was seen. The biotite occurs in minute, sparsely scattered 

clusters of fibrous material, which generally disclose the radial extinction so frequently characteristic of chlorite. Magnetite is plentiful in 

minute, clustered, rounded granules, showing every evidence of secondary origin. Frequently around these clusters is red hematite. 

Some other accessories were noticed, particularly apatite, but they 

seem to cut no figure in the mineral composition of the rock, and are in 

all cases apparently the results of alteration processes.

The fractured condition of these exposures affords many narrow 

fissures disposed in every direction through the rock. Along the walls 

of these fissures can be seen numberless crystals of quartz and the 

several feldspars. In size they are small, rarely reaching a millimeter 

across. They proved too minute for microscopical determination, and 

too few were gathered to admit of a chemical analysis.

In all essential particulars these occurrences are identical with those 

noted down the river at La Frainboise's farm, where the microcline 

crystals were 3 to 4 inches in size.

The altered gneisses mentioned as occurring around Beaver Falls 

and Morton (see pp. 29, 32, 59) show, in chemical composition, considerable variation from the fresh phases of the rock. The following 

analysis was made by A. D. Meeds:

Analysis of impure kaolin (sp. 5220} from Birch Cooley, Minnesota.

Constituent.

Si02 .............................................

AiA- ...........................................

Fe.203 . ...........................................

FeO.. ............................................

CaO.... ..........................................

MgO. ............ ........ ........................

Xa,20......... ....................................

K,0.. ............................................

H20..............................................

Total ......................................

Per cent.

41.71

34.61

4

KQ

6

QQ

1.16

0.22

0.11

12.69
CHAPTER Y. 

GABBRO-SCHISTS.

GENERAL CONSIDERATIONS.

Throughout the valley, intermingled with the normal acidic gneisses, 

are numerous occurrences of gabbro-schists, varying considerably both 

in mineral constituents and in chemical composition.

The salient microscopic characters of the gabbro-schists as a whole 

are the following: A dark-green to black color, due to the abundance 

of hornblende; a schistose structure, dependent in large degree on the 

parallel arrangement of the hornblende individuals; a freshness of 

appearance, due in part to the unweatherecl condition of the rock, and 

in part, no doubt, to the fact that the hornblendic constituent and 

some others are of a secondary origin.

Wherever these schists occur they exhibit a pronounced foliated 

structure, which in some places represents a gneissic rock and in 

others a schistose one. This difference seems to arise from the varying 

proportion of the feldspathic constituents on the one hand and the condition of the arnphibole-pyroxene constituents on the other. Wherever 

the former predominate the arnphibole-pyroxene minerals are segregated in bands, precisely as hornblende and biotite mark off the darkercolored lam in ce in the normal gneisses, and where the feldspars dimmish 

in quantity the rock becomes more and more homogeneous in constitution until a normal hornblende-augite-schist appears. This is the case 

in several localities, notably south of Odessa, in sec. 9, T. 120 N., 

E. 45 W., at several places around Minnesota Falls and Granite 

Falls, and in sec. 15, T. 112 N., E. 34 W. In dip and strike these rocks' 

always appear conformable with the normal gneisses. While the normal gneisses display in places great contortion and diversity of direction, the gabbro-schist series, as these amphibole-pyroxene-bearing 

rocks may be called, are to a great extent free from that irregularity.

Structurally these rocks are subject to all the conditions of jointing, 

faulting, and vein carrying which obtain among the gneissic rocks. 

They are broken through by dikes and are apparently as little altered 

by the 'contact as are the gneissic rocks.

At several localities visited the gabbro-schists show no marked evidence of schistosity. Elsewhere, and generally, the foliated character 

is the most prominent feature of the rock. Thus the gabbro-schists 

appear massive in places at the "gold mine" in Minnesota Falls. A

77
78 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

few paces in either direction from the quartz vein at this place the 

schistose structure appears with a less number of garnets than lie 

beside the quartz vein. The schistose gabbro in places becomes porphyritic, as do the gneisses, and even more conspicuously so than do the 

latter rocks. One place in particular is of interest in this connection. 

Feldspar crystals of the plagioclastic type, 3 or 4 inches across, are 

frequent. This spot is in Granite Falls, the precise location being 

1,750 paces north and 250 paces west of the SE. corner of sec. 32, T. 

116 N., E. 39 W. The area referred to is 25 paces long by 20 paces wide, 

and is only a few paces from a large diabase dike, which extends 

from the river toward the southwest across a hill that is a conspicuous 

topographic feature in the western part of the city of Granite Falls. 

No relation could be seen between the schist and the dike. At this 

same locality the schist has been shattered and welded by innumerable 

quartz and feldspar veins. These veins, which are of almost paper 

thinness, extend through the rock and cross one another in every conceivable direction.

Occasionally one sees the peculiar segregations of feldspathic 

material characteristic of the "augen-gneiss" of the German geologists. 

This, however, is a character somewhat restricted to the ordinary 

acidic gneisses of the valley.

Considerable diversity in texture can be seen. Although the ordinary condition is that of a medium-grained rock, it varies on the one 

hand to a very coarse schist or gneiss, and on the other to an extremely 

fine-textured one. A crushing of the rock and the subsequent welding together through infiltration has produced in places a peculiar 

shattering. This phenomenon appears in the gneisses also. Texture 

seems in no way to be associated with or dependent upon the present 

mineral composition of these rocks. Occasionally, when a pyroxenic 

constituent breaks up into complexes or becomes changed into hornblende, as the case may be, a semblance of fineness of texture is given. 

In such cases the feldspars in their unchanged size maintain the general 

coarseness of the rock, even when they have undergone a profound 

molecular change or a complete transformation from one feldspathic 

type to another: and the infiltrations of quartz, when this process has 

been carried on to an appreciable extent, exhibit similar large areas. 

As compared with those of other districts, the gabbros of the Minnesota 

Eiver Valley possess a fine texture. They are much finer than the same 

rock which lies in enormous sheets at the base of the Keweeuawan eruptivesin northern Minnesota; they are finer than the gabbros of Maryland which Williams has described, 1 and of which that author has 

kindly furnished hand specimens; and they are of a far more compact 

and finer texture than the Delaware gabbros described by Chester.2

>The gabbros and associated hornblende rocks occurring in the neighborhood of Baltimore, 

Md., by G. H. Williams: Bull. U. S. Geol. Survey No. 28,1886, Chapters II and III.

2 Tlic gabbros and associated rocka in Delaware, by F. D. Chester: Bull. IT. S. Geol. Survey No. 59, 

1890, pp. 10-19.
HALL.] GABBRO-SCHISTS. 79

In comparing the Minnesota Valley rocks with others of similar 

mineralogical composition the most noticeable features are the fresh, 

firm, bright condition of these rocks and their general laminated 

character. All weathered portions have been removed through 

glacial erosion, and a remarkable freshness is noticed at once on 

removing the outer film of recent decay where exposure to the air 

and storms has been .constant. But the distinctive characters of igneous rocks are not seen here as they prevail in the gabbros of Delaware, described by Chester;' in those in the neighborhood of Baltimore, 

Maryland; 2 in those of New Hampshire, described by Hawes; 3 in those 

of Wisconsin,4 described by the writer; in those of'northeastern 

Minnesota; 5 in those of western Scotland and' neighboring islands, 

described by Judd; 6 in those of Saxony, described by Lehmann; 7 and 

in those of the west coast of Africa, described by Leuz.8 Yet, so far as 

mineral composition is concerned, every attribute of a typical ancient, 

basic, igneous rock is represented.

Arranged according to their mineral composition, there are several 

varieties in this gabbro-schist series. Originally there were probably 

but two a hypersthene-bearing and a hyperstheue-free gabbro and 

these two approach each other by a scarcely perceptible gradation. 

All other modifications are subsequent. These alterations consist of 

the complete disappearance of the pyroxenic constituents and of the 

substitution in their place of hornblende and biotite. Every step in 

this substitution can be seen in the series of rock samples gathered. 

Meanwhile the plagioclases are as fresh in the most altered as in the 

comparatively unaltered members of the series.

So far as the field observations could show, there was no difference 

whatever in the environment of the two original types. The hypersthene-bearing rocks show the same general external features and have 

precisely the same relations to the gneisses and diabases associated 

with them as do the exposures which carry no hypersthene. Indeed, 

this division into types may have no ground in fact. It is possible, 

indeed probable, that this classification is based upon mere accident. 

Additional slides made from the same localities may show an entire 

absence of hypersthene where the slides examined show it to be present, 

and areas of the mineral may be disclosed where now it is considered 

absent.

'The gabbros and associated rocks in Delaware, by F.D. Chester: Bull. U. S. Gool. Survey No.59, 

1890, p. 41 et seq.

"G. H. Williams, ibid, p. 11.

8 The geology and lithology of New Hampshire, by G. W. Hawes: The Geology of New Hampshire, 

Vol. Ill, Part IV, 1878, p. 165 et seq.

4 Notes of a geological excursion into central Wisconsin, by C. W. Hall: Bull. Minn. Acad. Nat. 

Sci., Vol. Ill, No. 2, 1891, p. 250.

6 Preliminary description of the peridotytes, gabbros, diabases, and andesytes of Minnesota, by M. 

E. Wadsworth: Bull. Geol. Nat. Hist. Survey Minnesota, No. 2, 1887, pp. 47-69.

6 On the Tertiary and older peridotites of Scotland: Quart. Jour. Geol. Soc. London, Vol. XLI, 1885, 

p.358.

7 Untersuchungen iiber die Entstehung der Altkrystallinischen Schiefergesteine, by Dr. J. Lehmann, Bonn, 1884, pp. 191,192.

8 Gabbro von der Westkilste Afrikas, by 0. Lonz: Vcrhandl. K.-k. geol. Eeichsanstalt, Wien, 1878, 

p. 52.
80 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157. 

MINERAL CONSTITUENTS OF THE GABBRO-SCHISTS.

The mineral constituents of the gabbro-schists are labradorite, pyroxene, both the orthorhombic and the inonoclinic, with its alteration products, and hornblende, magnetite, and various accessory minerals 

pyrite, apatite, titanic iron, garnet, and quartz.

The feldspathic constituent is the basic type, usually near to labradorite in its extinction angle and in its poly synthetic twinning. This 

last character is always present and distinctly visible, except when the 

sections are cut parallel to & (010), or in rare cases where alteration 

has obscured the lamination. In places the feldspars present some 

inclusions of foreign particles; but as a rule they are singularly free 

from impurities, save where these are the result of alteration. Such 

alteration products are chiefly kaolin and smaller quantities of epidote. 

The feldspars do not occur in lathlike forms. Were there no other 

means of distinguishing the rock from the dike bodies, this would prove 

an unerring one. The areas of this mineral show little difference in 

breadth and length.

Normal augite generally appears in thin sections. The exceptions 

are the hypersthene-gabbros soon to be described. It usually possesses 

the light-brown color characteristic of the rock-forming condition of 

this mineral, but in some places it varies to a pink, and only in polarized light can it be distinguished with certainty from the common 

garnet of the valley. Its cleavage is, as a rule, very distinct. Its 

alteration phenomena are of the usual kinds. Occasionally, however, 

the augite is badly altered to hornblende; in fact, hornblende has in 

places completely replaced its host.

Diallage, which Zirkel,' Eosenbusch,2 von Lasaulx,3 von Giimbel,4 Naumann,5 and others have regarded as an essential constituent, is always 

present in the Minnesota Valley gabbro-schists. It has in all cases 

undergone alteration, but the several stages of this change exhibit 

marked differences, described later, from those characterizing the 

hypersthene. In the diallage the alteration is to hornblende, but the - 

process goes on in the middle of the crystal grains as well as along 

cleavages and fractures. A molecular change is seen. PL XIX, J, to 

some extent indicates this process. The fresh-looking diallage alters, 

molecule by molecule, to the dichroic green hornblende. In places the 

hornblende granules or, as they more usually are, folia have attained 

considerable size, while in other places they are so minute as to require an

'Gabbro von der Westkuste Afrikas, by 0. Lenz: Verhandl. K.-k. geol. Keichsanstalt, Wien, 1878, 

Vol. II, p. 110: also Mikroskopiache Beschaffenheit der Mineralien und Gesteine,- by Dr. Ferdinand 

Zirkel, 1873, p. 441.

2 Mikroskopische Physiographic der Mineralien und Gesteine, by H. llosenbusch: Zweite Auflage, 

1886, p. 132. Compare Microscopical Physiography of the Rock-Making Minerals, by H. Koseubusch: 

Iddiugs's translation,New York, 1888, p. 239. "In the Archean rocks diallage is seldom met with. 

It is here confined * * * to certain amphibolitos which may be considered as probably dyuamometamorphic gabbros."

3 Elemente der Petrographie, by Dr; A. vou Lasaulx, Bonn, 1875, p. 310.

4 Geologie von Bayern, by Dr. K. Wilhelm yon Giimbel; Erster Theil, Grundziige der Geologie, p. 129.

6 Lehrbuch der Geognosie, by Dr. Carl Friederich Naumann, Vol. 1,1858, p. 573.
HALL.] MINERAL CONSTITUENTS OF THE GABBRO-SCHISTS. 81

immersion lens to separate them from the diallagic matrix in which 

they are embedded.

The peculiar double cleavage of diallage is not seen on every granule of inonoclinic pyroxene. Very likely many individuals, through 

the faintness of the secondary lines in the direction oo P co, may be 

regarded as the crystallographically intermediate forms between normal augite and typical diallage.

In a few places, as has already been anticipated, orthorhombic pyroxene, hypersthene, appears, but never to the exclusion of the monoclinic 

pyroxenes augite and diallage. In extent of alteration the hypersthene 

always takes precedence over all others. Earely is an individual 

found without a strongly marked corrosion along its cleavages or the 

fractures which it carries. This alteration continues until the individuals are completely obliterated. The change is accompanied by 

the formation of areas of magnetite and fibers and broader bands of 

green hornblende. Other minerals result, but they are less in quantity 

and of minor inineralogical importance.

Both pyroxenes and feldspars are granular, neither seeming to play 

the part of matrix, but both being hypidiomorphic in their derivation 

and form. In the dike rocks, on the other hand, the augite is 

wedged in between the well-formed crystals of the feldspar constituent, 

or, in broad areas, it serves as a matrix in which the lathlike feldspar 

crystals have developed a very perfect form. PI. XXV, A and B, show 

these differences very clearly.

The distinction here made between gabbros and diabases was first 

hinted at by Zirkel in 1866, when he described the labradorite of diabases as forming tabular crystalline individuals of distinct cleavage, 

varying color, and showing the characteristic twinning striae of the 

basal face, while the labradorite of the gabbros exhibits distinct twinning strioe and occurs in coarse crystalline grains. 1 The pyroxenic 

constituent was, according to Zirkel, augite in diabase and diallage in 

gabbro.2

Magnetite, which is an almost universal constituent of the gabbroschists, in places occurs in angular crystallized areas, seldom in single 

crystals but more commonly in rounded masses of very small size. 

It rarely shows traces of alteration into hematitic granules. That 

magnetite is a primary constituent of these rocks is by no means 

certain, so far as investigations now show. There is little doubt of 

its secondary nature where the hypersthene is altered; on the other 

hand, in northeastern Minnesota primary titaniferous magnetite in 

large masses occurs in the gabbros of Keweenawan age. It is probable

1 Lehrbucb der Petrographio, Dr. Ferdinand Zirkel, Bonn, I860, Vol. II, p. 78. "Der Labradoritbildet 

krystallinische tafelformigo Partieen von deutlichcr Spaltbarkeit und weisser, graulichweisser oder 

griinlicliweiasor 3?arbe; sind die Krystalle noch friscb, so gelingt es oft die charakteristischo Zwillingsstreifung auf der basischeii Spaltungsflacheu wahrzunehmen."

2 Tbid, Vol.II,p. 110. "Der Labradorit 1st gliinzend weisslichgrau oder etwas ins bliiuliclivioletto, 

und besitzt iu groberu krygtalliuisclien Korueru deutlicbo Zwilliugsstreifung."

Bull. 157 6
82 GNEISSES, GABBKO-SCHISTS, ETC., OF SW. MINNESOTA. lBULL.157.

that a portion of the opaque material which is visible in every slide 

is ilinenite or some closely related species, since titanic acid seems to be 

a common constituent of the gabbro-schists of Minnesota.

Accessory minerals occur everywhere in the schistose gabbros. The 

leading ones are pyrite, apatite, garnets, and quartz. Others appear 

as the modifying circumstances vary. Titanic iron, or meuaccauite, 

doubtless occurs in small individuals in some specimens. Somewhat 

rarely the pyritic accessory has the golden-yellow color of chalcopyrite. 

The garnet and quartz are not contact minerals, as Teall seems to 

regard them, 1 but rather accessories and alteration products, to be 

found wherever alteration appears, whether brought about by weathering or by the deeper-seated change of metamorphism. The garnets are 

undoubtedly present in all contact alterations also, and perhaps have 

a still wider distribution than either weathering or deeper-seated alteration alone would produce. The quartz is regarded as partly an infiltration product and partly a by-product of the alteration of the augite 

to hornblende or biotite, or, as is often the case, to both.

Among the minerals which may be regarded as purely resultant 

products of decomposition, hornblende should certainly occupy the 

first place. Yet it is not safe to assume that all the hornblende is of 

this nature, for there is no sure means of determining the primary or 

the secondary origin of portions of this material. There is no doubt 

of the secondary development of by far the larger portion. It lies 

around the areas of pyroxene in such a manner as to preclude all doubt 

of its origin ; it takes the place of pyroxene where such areas have 

become completely changed to a finely granular, bright-green, hornblendic segregation; and, finally, it shows a fibrous condition and a 

semigranular state within the feldspar areas, thus pointing to the 

breaking up of many individuals of this mineral. It may be said, in 

passing, that the parallel growth of augite and its resulting hornblende 

is very rarely seen; perhaps a half dozen sections were noted in the 

examination of these rocks which show this parallel position of host and 

product.

Passing to matters of more detail in the study of these gabbro-schists 

and to points which possess a somewhat local interest, we shall first 

describe the hypersthene-bearing gabbros; secondly, the hypersthenefree gabbros, and lastly the pyroxene-free gabbros or gabbro-diorites.

HYPERSTHENE-BEARING GABBRO-SCHISTS. 

LA FRAMBOISE AREA.

A few paces from the river bank at the old La Framboise landing, 

in sec. 22, T. Ill HT., E. 32 W., a mass of gabbro (sp. 5191, 5724) lies in 

the midst of the gneisses, constituting the large series of exposures in 

sees. 15,16, 21, and 22 of this township. The relations of the gabbro

'British Petrography, by J. J. H. Toall, London, 1888, pp. 383, 385,
HALL.] HYPERSTHENE-BEARING GABBRO-SCHISTS.' 83

to the gneisses could not with, certainty be made out, though, as will be 

seen later, the gabbro is probably intrusive in the gneisses. There is a 

sharp contact line where the two are seen side by side, which, however, 

can be followed a' short distance only, 'Owing to the covered condition 

of the rocks.

The gabbro here shows little of that schistose character almost 

everywhere present in the valley. The texture is medium, with perceptible tendency to porphyritic habit; to the unaided eye it is decidedly granular. The color in the mass is dark, a dark-green ground tone 

prevailing, with numerous white flecks which disclose the presence of 

feldspar, since they display, on some of the larger individuals, the 

polysynthetic twinning of the plagioclases. The dark-green grains 

show clearly the perfect cleavage of hornblende. The remaining constituents are not easily recognized with the unaided eye nor, in some 

cases, even with the microscope.

The rock is superficially weathered. The outer coating is of a reddish 

or dirty gray color, due to the alteration of the constituent minerals. 

When this coating nowhere more than one-eighth to one-fourth of 

an inch thick is removed, the rock is very fresh and sparkling. The 

green color just noted, and a tendency to crush to a light-gray powder 

under the hammer, suggest, however, that an alteration has really taken 

place and gone farther than the appearance of the rock would indicate. 

On account of the sharp contrast shown between this rock and the 

augite-gneiss in which it lies, and the general aspect which the rock 

carries of an eruptive species, the eruption of this mass into the gneiss 

can not well be doubted. The naked eye distinguishes a hypidiomorphic habit of the constituent minerals and the granitoid character of 

the gabbro rather than the ophitic or porphyritic character of a diabase.

The leading feldspars in this gabbro are near the labradorite-anorthite 

end of the plagioclase series. In shape the individuals are broad rather 

than lathlike. Twinning strise are sharp and clear. The extinction 

angle on M is frequently 40° and more. Other areas cut on the pinacoidal plane give the extinction angle characteristic of mixtures with 

the formula abi an 8 to abi an 12 ; in other words, they extinguish around 

30°. TJndulous extinction occurs in a few individuals, particularly 

those cut in such direction that the twinning lamellae do not appear, or 

if present are very far apart. This, therefore, is not a strong character 

of the rock.

In the several thin sections prepared there is little material which 

can be regarded as an alteration product from the feldspars. Now and 

then clusters of secondary grains and small crystals can be seen, but 

usually the inclusions within the feldspars are easily recognized as a 

pyroxene or hornblende. Microcline and plagioclases of low extinction, 

pointing to a high percentage of soda, were not noticed either as primary 

or secondary constituents.

The pyroxenes of this gabbro are two, and probably three, in number hypersthene, diallage, and probably augite.
84 GNEIS'SES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

Hypersthene composes more than half of the bisilicate constituents of this gabbro. The characteristic dichroisni of this mineral is 

sharply shown, as is also the parallel extinction of the grains. It is in 

the neighborhood of the hypersthene area that the grains and aggregates of hornblende are chiefly seen. Some of the hornblende individuals are isolated, and thus their genetic relationship to the hypersthene 

is obscured. It is possible that some individuals are not thus derived; 

yet the varying quantitative proportion of the mineral and the fact 

that its derivation from hyperstheue can be clearly proved at other 

localities in the valley are the grounds for assuming such origin at this 

locality. There are no characters displayed by the hornblende so 

different from those at other exposures as to require further mention.

Diallage has suffered greatly through alteration. Grains have 

become diminished in size by the changing of their borders or interior 

portions, or both, into hornblende. The external characters of augite 

and cliallage, save in one or two respects, are so similar that a sharp 

distinction between these two pyroxenes was not made. Where the 

characteristic cleavage of diallage was seen the identification of this 

mineral was considered sufficient. Many grains with a rather high 

extinction angle are entirely lacking in this character. The presence 

of augite is therefore assumed.

Pyrite is present in a few isolated and segregated grains.

WABASHA CREEK AREA.

Passing to the next district up the river from La Framboise's, we find, 

in sec. 15, T. 112 IS"., E. 34 W., a small exposure of hornblendic gabbroschist (sp. 5399). Its lamination is not sharp and clear. Only a few 

paces to the north, and again a short distance to the south, stands a 

highly acidic gneiss in moderately high exposures, decidedly abrupt on 

the north side, and weathering in large, concentrically breaking blocks 

to a crumbling, dirty mass of feldspar chips. The direction of the 

schist exposure is NW.-SE., and the dip is apparently NE. at a medium 

angle.

In the gabbro-schist the individuals of feldspar vary greatly in 

size. Occasionally those of unusually large size are seen, but more 

frequently a finely granular condition prevails. The labradorite type 

abounds. A general freshness characterizes this mineral. Even where 

individuals are fractured and the joints have become filled with veinlike material there is no trace of decomposition along the margins. 

The twinning striae are very strong. While this is true of most of the 

feldspars, there are individuals showing decided marks of disintegration. The product is a mass of finely crystalline, brightly polarizing, 

transparent material. Often in the altered individuals a fibrous radial 

arrangement of the decomposition product can be seen under crossed 

nicols. It is apparently a single mineral rather than a cluster of 

different species. This appearance leads to the opinion that the
HALL.] HYPERSTHENE-BEARING GABBRO-SCHISTS. 85

process is a zeolitization of the feldspar rather than a saussuritization, 

since this process develops an aggregate of distinct minerals. 1

The diallage is well marked with the usual characters of that mineral. 

In some individuals it has undergone alteration to hornblende to such 

an extent as to impart a green color to the residue of the decomposing 

grains. The double cleavage and the parting bauds are not always 

distinctly seen; therefore some of the pyroxene may be assumed to 

be augite.2

Hypersthene occurs both in nests and in isolated grains. Where the 

latter appear they seem to have absolutely no crystal form; a decidedly 

granular condition is always present, and the ordinary prismatic cleavage is frequently indistinct. Where the hypersthene is segregated, 

several grains may lie side by side, having precisely the same plane of 

extinction and showing the same ragged and corroded outlines toward 

the feldspars as do the isolated individuals. Many hypersthene grains 

are completely altered into hornblende and some associated mineral, 

which may be ferrite; others are only partially altered. They carry 

within them small individuals exhibiting the cleavage and optic extinction of hornblende. Being considerably shattered, alteration first seizes 

the hypersthenes, producing along the cracks fibrous hornblende, which 

usually lies parallel with the cleavage striae of the host. Around the 

borders of the_ changing hypersthene individuals, lying as though 

expelled by the alteration process from the midst of the hypersthene 

material and the resulting hornblende, is a granular mineral somewhat 

transparent and of light-brown color. Its minute size prevented further 

identification.

More rarely yet as a rule, in the vicinity of these alterations are 

larger masses of magnetite. Clusters and single grains exhibit the 

usual characters.

This locality further shows an interesting vein formation, evidently 

resulting from the joint action of crushing and mineral alteration. The 

mineral playing the leading rdle in the vein filling is hornblende. The 

anastomosing of the hornblendic material is marked, and is seen in 

PI. XXIII, A. Crystal grains are broken, and between their fragments 

the minute leaves of hornblendic matter are seen, or grains are pressed 

apart and the interspace is packed full of the vein material. These 

veins were seen only in the much-altered gabbro-schists.

MINNESOTA FALLS AND GRANITE FALLS AREAS.

Around Minnesota Palls and Granite Falls there is a remarkable 

development of the hypersthene-bearing gabbros. One approaches a 

conspicuous belt of these rocks in ascending the river on the north 

side. The chief rock in sees. 11 and 14, T. 115 K., E. 39 W., is a gabbro. 

In the southeast corner of sec. 13 stands an area of well-banded gneiss

1 Ueber Saussurit, by A. Catbrein: Zeitschr. fur Kryst. und Min., Vol. VII, 1883, p. 248. 

"British Petrograpby, by J. J. H. Teall, London, 1888, p. 132.
86 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

150 paces in extent which exhibits patches of coarse veinlike material. 

This gneiss is conspicuous for its small content of bisilicate constitu 

ents, its fresh microcline, and the undulatory extinction of its quartz 

areas. Along, the boundary between sees. 11 and 14 and thence northward to the river are many patches of a black, distinctly laminated, 

hornblendic hypersthene-gabbro. The strike of these exposures is 

about N. 80° E. Along their southern border the dip is southerly, in 

sec. 12 reaching as high as 80°, while in the southern part of sec. 11 it 

is considerably less. Along the northern side of this section, while the 

strike is nearly the same as farther south, the dip is zero.

Throughout this whole area and it embraces all the rock south of 

the great bend in the Minnesota Eiver at Minnesota Falls, just below 

the site of the old dam and mill granitic veins or dikes are numerous. 

In places, as along the section line between sees. 11 and 14, the granitic 

material is parallel with the lamination of the gabbros, and here and 

elsewhere the coarse texture of the former, as well as its sharp color 

contrasts, mark it as a phase of rock formation quite distinct from the 

latter.

Everywhere the lamination of the dark-colored hypersthene-bearing 

gabbro-schists is sharp and clear. The alteration to hornblende on the 

part of the pyroxenes has been carried so far as to give hornblende 

the leading place among the basic constituents, both macroscopically 

and microscopically. This altered condition of the rocks remains the 

same northward to the high, ragged, half-covered masses on the Russell farm, SE. | sec. 11, T. 115 K, E. 39 W. On this farm the rocks 

have a northerly dip of 20° or more, with little contortion and with 

little special modification, even where they are broken through by 

dikes. This unaltered condition in the proximity of dikes can be well 

seen around the spot 1,300 paces N. and 1,800 paces W., sec. 12, T. 115 

N., E. 39 W., where a dike (sp. 5280) 20 feet wide cuts through this same 

bed of gabbro-schist that first crops out on Russell's farm. Here and 

there a lighter color prevails, on account of the increase in the amount 

of feldspar, and in places through the light pea-green color of the 

secondary hornblende. A good illustration of these lithologic conditions is seen at the spot just indicated, near the contact of the dike. 

The gabbro-schist (sp. 5278, 5279) is hypersthene-bearing.. Locally 

this bisilicate constituent almost wholly changes, leaving a rock largely 

composed of labradorite-anorthite feldspar and the alteration products 

biotite and hornblende (sp. 5279). Elsewhere the hypersthene and its 

alteration product, hornblende, are present in normal force. The rock 

then becomes a good example of a partially altered hypersthene-gabbro 

(sp. 5278, si. 2237), and represents an intermediate stage of rock 

alteration. (See PI. XXI, A.)

Crossing the river into Yellow Medicine County and into the ruined 

village of Minnesota Falls, one sees comparatively no change in the 

characters of the rocks. Near the old dam, swept away in the flood of
HAH..] HYPERSTHENE-BEARING GABBRO-SCHISTS. 87

1882, which carried away the village itself there are both a reddish, 

highly siliceous gneiss (sp. 5236, 5237) and a gabbro-schist (sp. 5238). 

The contact of the two rocks is here as clearly marked as at La Framboise's place, already described. The distinct interbedding, or at least 

interpenetration, of the two rocks is seen at the river bank at the south 

end of the old-dam. Both the gneiss and the gabbro-schist are here 

more schistose than at La Franiboise's, yet, barring certain modifications incidental to such schistose condition, they are the same in mineral 

composition.

The monoclinic pyroxene of the gabbro-schist has sometimes a greenish tint, and again it appears very transparent and free from color save 

the usual brown tinge seen in the thicker sections. It is now mostly 

of the diallage type, though frequently the foliation partings are indistinct or even wholly absent. The individuals are granular, never 

exhibiting crystal planes. Hyperstheue seems rather more abundant 

at Minnesota Falls than at La Franiboise's, and alteration has not crept 

into the internal structure of the grains with such marked effect. In 

both places the mineral is characterized by rounded grains instead of 

extended areas more or less interrupted by other minerals. Hornblende 

is the common product of the alteration of the pyroxenes. It is almost 

wholly in the compact condition, usually dark brown in color, and somewhat strongly pleochroic. .The grains vary in size from the most 

minute, embedded in the hypersthene areas, to those as large as the 

pyroxene individuals themselves. Cleavage is perfectly developed and 

is always seen in cross sections of the prisms. Inclusions are rather 

plentiful.

A few hundred paces southwest of the ruined village of Minnesota 

Falls' the gabbro-schists again appear, as in the western part of 

the village, in long extended masses, standing in billowy rows of 

knobs. From these exposures northward into and entirely through the 

city of Granite Falls to the banks of the river in sec. 13, T. 116 N., K. 

40 W., these rocks can be followed by a succession of outcrops. The 

prevailing schistose character, the KE.-SW. strike, the dip alternating 

between NW. and SE. 5 the dark color, the predominance of hornblende, 

and the generally fresh, undecomposed condition of the rocks very 

generally prevail. Near the center of the city of Granite Falls, one or 

two blocks north of the court-house, an interesting curve in the strata 

was noted, and to the west, near the house of E. C. Shannon, lies an 

exposure of a highly porphyritic modification of the gabbro (sp. 5438). 

This spot has already been mentioned (Chap. Ill, p. 35). The porphyritic character of the rock is shown in PI. XXIV, A.

The texture of the groundmass is rather fine. Its color is a gray 

black. A certain granular habit is due to the shape of the pyroxenes, 

augiteand hypersthene, and their relations to the feldspars and subordinate constituents of the rock. The numerous cleaved surfaces of the 

several minerals give a decidedly glistening appearance to the surfaces
88 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

of fresh fractures. A good pocket leiis reveals on some of these surfaces the twinning lamellae of plagioclase feldspars, and on others 

the strong markings of hornblende. Where the surface has been 

exposed for a long time it becomes a dull black color. The process 

of color change follows steadily the removal of the feldspars, the first 

step being contemporaneous with the incipient kaolinization of these 

individuals. The kaolinization process continues until the feldspar 

grains and crystals are completely altered and the soluble products are 

removed. Broken specimens show upon their fresh fractures every 

stage of the alteration process between the surface phase, where the 

feldspars are wholly removed, and that other extreme, only about onefourth, of an inch below the surface, where scarcely a trace of kaolinization can be seen.

Quite thickly studding this dark, medium-grained, foliated groundmass are the porphyritic feldspar crystals before mentioned. These 

crystals vary in size from a quarter of an inch to 3 inches in length, 

nearly all of them exceeding 1 inch (PI. XXIV, A). Through-the 

same kaolinization process characterizing the matrix the surfaces of 

these crystals exposed to the weather become white in color, dull in 

luster, and have been washed away until they lie in depressions below 

the general surface of the rock. Within they are of a greenish hue, 

owing to the presence of mineral accessories. Fresh surfaces show 

the perfect cleavage of feldspar, and when the cleavage is on a plane 

parallel with OP the stria3 are very clear. These porphyritic feldspars are anorthite, as is seen from the extinction angle, 36°-43°, and 

the chemical composition given below. Besides the cleavage, there is 

a conspicuous fracturing of the crystals corresponding to the fractured 

condition of the matrix. There are many inclusions in these anbrthite 

crystals. Among those identified are magnetite, easily separated from 

the powdered mineral, augite, which extinguishes at angles varying 

between 45° and 55°, and, in fewer numbers, slender, minute crystals of 

hornblende. The specific gravity of anorthite is 2.715.

Within the matrix the usual minerals of a hornblende-hypersthenegabbro are seen, viz, labradorite-anorthite feldspars, hornblende, 

augite, diallage and hyperstheue with the accessories, magnetite, hematite, and chlorite. The hypersthene has the usual pleochroism; it 

occurs in grains wholly devoid of crystal planes and usually completely 

enwrapped with the pyroxene and its decomposition product, hornblende. The pyroxene is only to a small extent diallage. This is indicated by the almost universal absence of the characteristic diallagic 

parting, as well as in the high extinction angle shown by the individual grains. Indeed, the prevalent prismatic cleavage and an extinction angle generally ranging between 42°'and 48° point to augite as 

the predominant pyroxenic specigs.

The hornblende is well distributed. It is partly uralitic and partly 

in well-cleaved, brown individuals. There is little doubt of its secondary origin in both modifications.
HALL.] HYPERSTHENE-BEARING GABBRO-SCHISTS. 89

The specific gravity of the matrix is 3.015.

The chemical composition of the porphyritic hornblendic hypersthenegabbro is of interest. Analyses were made of both the matrix and the 

labradorite-anorthite crystals by Prof. E. J. Babcock, of the University 

of North Dakota. The following are his results:

Analyses of porpliyritic Uornllendic hypersthene-gabbro from Granite Falls.

Constituent.

Qin /ailipQ^

CaO (lime) .................................................

K2 0 (potash) ..............................................

Total ..........................................:.....

I.

Per cent. 

47.43

23. GO

13.00

11.21

3.15

0.204

0.146

0.00

90.76

11.

Per cent. 

46.40

33.87

0.34

14.73

0.04

1.06

1.88

no qo

I. Porphyritic hornblendic hypersthene-gabbro (sp. 5438); the matrix rock, from 

which the anorthite crystals had been removed. Granite Falls.

II. Crystals of anorthite from the foregoing matrix.

From these results it is seen that the material agrees very closely in 

chemical composition with its physical characters as outlined on the 

preceding pages and determined by quite different methods. Attention 

may be called to the nearness in composition of the foregoing anorthite 

and that of the great gabbro masses of northeastern Minnesota, 

reported by Professor Irving,1 and the Hammerfest (Norway) anorthite, 

quoted by Dana 2 from Pisani, first published in 1876.

Northwest of Granite Falls the gabbro lies in a gentle fold constituting the northern anticline of this locality. Along the south side of 

the river, from a little above the bridge and railway station, at present 

practically the middle of the city north and south, the road passes northwesterly in sight of almost continuous exposures of gabbro-schists for 

more than a mile. They are broken through in several places by the 

dikes which cross the valley in a NE.-SW. direction.

In this upper portion of the area the veins and feldspathic segregations are not so frequent as they are south of Minnesota Falls. Where 

they do appear they are a coarse mixture of green hornblende and 

labradorite or a closely related feldspar, with little, if any, quartz. 

The thin sections made show no appreciable quantity of this lastnamed mineral in the coarser segregations (sp. 5442). It is sufficient 

to say that they exhibit the lithological characters of the schists 

already noted lying along the south side of Granite Falls. The feld-

'The copper-bearing rocks of Lake.Superior, by R. D. Irving: Mon. TJ. S. Geol. Survey, Vol. V, 

1883, p. 438. 

2 A System of Mineralogy, by E. S. Dana, 6th ed., 1892, p. 339.
90 GNEISSES, GABBRO-SCHIiSTS, ETC., OF SW. MINNESOTA. [BULL. 157.

spar has the high extinction angle of anorthite. This mineral is little 

altered. Frequently the individuals carry minute crystals of pyroxene 

and hornblende.

The pyroxenic constituents are hypersthene and augite. The former 

is more generally altered than the latter. The hypersthene grains are 

frequently surrounded by borders of hornblende. It is probable that 

it is the source of the greater portion of the latter mineral. The 

monoclinic pyroxene has generally the green color already noted for 

other exposures in this neighborhood. Yery rarely is the diallagic 

cleavage oo P o5 (100) seen. Besides, the optical orientation determines the mineral to be augite rather than diallage, the extinction on 

the prismatic cleavage being between 47° and 50°. Hornblende is so 

abundant that the hand specimens were identified as hornblendeschist. The gabbroid character of the rock was not seen until the thin 

sections revealed the pyroxenic constituents.

On the north side of the river the gabbro-schists are finely developed at the bend of the river in Granite Falls, near the corner of sees. 

33 and 34, T. 116 N. f li. 39 W., and sees. 3 and 4, T. 116 K, B. 39 W., 

and thence northward to the Chicago, Milwaukee and St. Paul Bailway station and the grain elevators of the city. Beyond these points 

gneissic rocks prevail to the northwest. They are of medium coarseness, distinct lamination, a prevailing dark color (which often, however, 

becomes somewhat lighter through the predominance of the feldspars), 

and a very fresh, vitreous surface along fractures. The strike of the 

laminse is NE.-SW., with a dip of 37° SE. Several dikes break through 

these rocks. One, near the Pillsbury elevator, is of considerable 

width.

As will be seen in the list of literature references (pp. 12-19), these 

rocks have long been known to the geologists and explorers of the 

Northwest, yet their general external characters have been cited in the 

vaguest and most cursory way. Featherstonhaugh briefly describes 1 

these exposures as "resembling granite in every particular except its 

stratification," and in a subsequent work 2 he mentions masses of 

quasi-stratified granite, most of which "were red quartzose granite 

with a slight quantity of mica, but some were gneissoid." N. H. 

Wiuchell says 3 that "At Granite Falls, as at Minnesota Falls, and 

all the way between, the rock in the valley is a schistose granite, 

almost a mica-schist, but it varies to a hard, gray granite that 

resembles that at St. Cloud both in color and composition." Upham 4 

gives a much more precise description of the prevailing rocks, " the 

gneisses," as he designates them, of this locality.

'Eeport of a geological reconnoissance made in 1835, etc., by G. W. Featherstonhaugh: Doc. 333, 

printed by order of the Senate, Washington, 1836, p. 28.

2 A Canoe Voyage up the Minnay Sotor, by G. W. Featherstonhaugh, 2 vols., Vol. I, London, 1847, pp. 

333,334.

3 The geology of the Minnesota Valley, by N. H. Winchell, State geologist, and S. F. Peckham, State 

chemist: Second Ann. Kept. Geol. Nat. Hist. Survey Minnesota, 1873, p. 170.

4 The geology of Yellow Medicine, Lyou, and Lincoln counties, by "Warren TJpham: Geol. Nat. Hist. 

Survey Minnesota, Vol. 1,1884, pp. 596,597.
HALL.] HYPERSTHENE-BEARING GABBRO-SCHISTS. 91

In microscopical characters they present a series from those exhibiting all the constituents in very fresh and apparently unchanged con- 

( dition to those where many of the original constituents are completely 

obliterated through the metamorphic changes undergone. Such a 

series may be seen by comparing several of the accompanying illustrations. PI. XVII, B (sp. 5335, si. 2263), shows an unusually fresh hyper - 

sthene-gabbro taken from one of the small and somewhat isolated knobs 

southeast of Odessa. The feldspars are fresh and strongly polarizing, 

yet the pyroxenic constituent shows some traces of alteration. This 

specimen was taken to illustrate the freshest rock to be found at these 

exposures. PI. XXI, A, shows (sp. 5278) a specimen taken south of 

the old darn in Minnesota Falls. The hypersthene at this locality is 

partly- altered to a light-brown hornblende. The other constituent, a 

labradorite feldspar, is quite as fresh and unaltered as in the Odessa 

gabbro just mentioned. Magnetite is in the same physical condition 

and the same proportion in both. Again, in PI. XXI, JB, we see a 

gabbroid rock (sp. 5418), also from the southern part of Minnesota Falls, 

in which the pyroxenic constituents have entirely disappeared, having 

almost wholly altered into hornblende, with some addition to the 

amount of magnetite. The feldspars have also suffered corrosion to a 

very perceptible extent. The rock has thus become a diorite, a rock 

species which appears in several localities and in all cases under 

essentially the same physical conditions.

Everywhere about Granite Falls and Minnesota Falls the feldspar is 

sufficiently fresh to be identified, and with but two or three exceptions 

it is clear and brightly polarizing. The grains are of the typical hypidiornorphic structure. They contain few inclusions of any kind. The 

feldspar is of the labradorite type, but many variations from the normal extinction angle of this mineral were noted.

Augite, with its typical characters, is seen in almost every slide. 

Oftentimes areas occur which, both in general characters and in the 

more special ones, are hard to distinguish from diallage. In all the green 

color prevails, which has already been mentioned as characteristic of 

the pyroxenes in the gabbro-schists, while in the diabases of this same 

neighborhood the augite, where unaltered, is brown. In the matter 

of extinction, some areas present the normal angle of augite 45° 

and more in prismatic sections' while others vary toward diallage. 

Diallage, with the typical characters, is also common. It presents some 

interesting illustrations of internal molecular changes, since almost 

everywhere in the valley the mineral is from circumference to center 

undergoing alteration into hornblende. A green color pervades the 

whole diallage individual, which some authors consider a mark of 

incipient alteration from diallage to hornblende, and here and there, in 

some cases thickly scattered through the host, are plates and fibers of

'Hiilfstabelleu zur Mikroskopischen Miueral-bestirumungin Gesteine, by H. Rosenbuscb, Stuttgart, 

1888, Tab. Ill, c.
92 GNEISSES, GAHBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

dichroic hornblende. Barely the diallage is fresh that is, brown in 

color or slightly colored green ; rarely, too, an area of clear hornblende 

appears which bears undoubted evidence of being derived entirely 

from a diallage individual; therefore, the great mass of material shows 

the intermediate condition above described, which is also illustrated in 

PI. XIX, A.

Orthorhombic pyroxene appears in all stages of alteration, from the 

clear, fresh grains seen in specimens 5335 and 5445 (PI. XVII, B] 

to the wholly altered pyroxene material lying in the long belt of knobs 

extending from the bend in the river, near the corner post of sees, 3 

and 4, T. 115 K, E. 39 W., and sees. 33 and 34, T. 118 N., E. 39 W., westward along the north side of sec. 4, as well as northeastward through 

sec. 34. In the former specimens the fresh grains are strongly outlined 

against the plagioclastic feldspars, and exhibit scarcely any traces of 

decomposition, even along the cleavage lines. Other areas are partly 

altered (PI. XXI, A), and still others are wholly altered (PI. XXI, B]. 

Thus the change can be clearly and undoubtedly traced. Much of the 

resultant mineral extinguishes parallel with that from which it springs, 

but occasionally it assumes a radial arrangement and lies in diverging 

fibers about the host. Hornblende is scattered everywhere through 

the whole series of slides examined. Many of the areas are brown and 

sharply marked off with cleavage lines, while others are green and ill 

denned. There is a strong dichroism in nearly all these individuals. 

In addition, many localities show a fibrous hornblende. This is always 

so associated with some pyroxenic mineral, usually hypersthene, as to 

give the strongest proof of derivation directly from it. Slide 9472 

shows some areas of hypersthene fresh and almost unaltered, other 

areas partly altered to a fairly fibrous green hornblende, and still other 

areas, composed wholly of fibrous green hornblende, which bear evidence in their situation of having been formed at the expense of 

hypersthene individuals now totally obliterated.

Biotite, while occurring in many places in apparently independent 

areas, is noticed to be, as a rule, in such intimate association with the 

hornblende as only a descendant from that mineral can exhibit. Elsewhere it evidently springs directly from the alteration of the pyroxene. 

Earely it is brown; as a rule it is green, and occasionally it is brown 

and green when rotated in polarized (not analyzed) light. Locally this 

mineral can be seen more frequently in sections from the Minnesota 

Falls samples than from those around Granite Falls. In some of those 

taken from near the road skirting the south side of the river to the 

north of Granite Falls no biotite whatever was seen. Much secondary 

hornblende is, however, present. Augite and hypersthene are both 

noted at these localities, and no doubt they both contributed to the 

formation of the hornblende.

Quartzite also occurs, but in every instance where seen it bears 

undoubted evidence of being a secondary mineral. The small, brightly
HALL.] HYPERSTHENE -BEARING- GABBBO-SCHISTS. 93

polarizing grains are free from inclusions. They occupy positions 

between the feldspars, even, within them, and near the decomposing 

diallage areas. Also, wherever hornblende and biotite have been 

formed granules of quartz may be expected. The grains generally 

occur in clusters. Where quartz is present in isolated grains it is 

usually included in the other minerals.

Opaque minerals are frequent. Sometimes the areas can be proved 

to be pyrite, and at other times magnetite or a titaniferous magnetite 

must be assumed. It is common to see, microscopically, a partially or 

wholly altered pyroxene individual thickly strewn with spherical globules of some opaque mineral.

Apatite as an accessory is rare in all the Minnesota Falls and Granite 

Falls gabbro-schists j so, too, is sphene. Epidote occurs more frequently, seldom, however, in good-sized or well-formed individuals, but 

rather as a secondary product disseminated in small and clustered 

grains in the midst of other minerals, which thus play the part of host. 

It is thus found nestled in feldspars, in hornblendes, and in biotites.

MONTEVIDEO AREA.

Passing to Montevideo (see PI. XI), we find a mound of the gabbroschist on the line between sees. 18 and 19, T. 117 E"., E. 40 W. The 

mound stands 40 or 50 feet above the flood plain of the river, but its 

size is not great. The foliation of the rock is clear, with a NE.-SW. 

strike and a dip of 30° to 50° SE. The foliation is apparently due to 

the alternating preponderance of the feldspathic and ferromagnesian 

minerals. The feldspathic portions resist weathering longer than the 

basic, thus forming a corrugation upon the exposed surfaces. Numerous 

lenticular quartz veins occur scattered over the exposure. A slightly 

smoky appearance is noticed, as well as the skiinmed-milk opalescence 

which was seen so markedly developed in the gneisses at La Framboise's, below old Fort Eidgely. The opalescence here is probably 

due to the same cause as at La Framboise's, namely, to needles of rutile 

scattered through the quartz grains, since these needles are seen in 

large numbers. Many cavities occur in the quartz, mostly gas filled. 

As a rule, they are arranged in planes passing through all the constituents of the rock alike, and suggesting conditions of origin similar to 

those pointed out by Julien in the gneiss of New Kochelle1 . These 

quartz veins contain some feldspar, usually plagioclastic, and fibrous 

secondary hornblende. The gabbro-schist is medium to fine grained, 

gray in color, and somewhat weather stained. .The feldspar is chiefly of 

the labradorite type. Some quartz is found. The proportion of basic 

constituents is unusually small. Some hornblende is present, as are 

also occasional folia of biotite. Both mouoclinic and orthorhornbic 

pyroxenes are present, showing the usual characters of these minerals 

as they appear elsewhere in the valley.

1 On the fissure-Inclusions in the fibrolitic gneiss of New Kochelle, by A. A. Julien: Am. Quart. 

Micros. Jour., Vol. 1,1878, p. 103.
94 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

Northwest of Montevideo, in the SW. J sec. 31, T. 118 K, E. 40 W., 

stands a small exposure, only 60 paces in length, rising just above the 

level of the prairie. The strike at this exposure is N. 30° E., and the 

dip is 55° SE. On the northwest side the rock is a gneiss of a reddish-gray color, considerably altered, and highly siliceous, dipping 

beneath the gabbro, which constitutes the larger part of the rock in 

sight (see fig. 6, p. 37). The gabbro is a dark-colored, highly hornblendic rock, carrying lenses of coarsely crystalline hornblende. Its 

feldspar is of the labradorite-anorthite type. Quartz is extremely rare, 

as are also magnetite and biotite. The hornblende is so strongly individualized that it can not be regarded as secondary, yet analogy and 

the evidence of other localities in the valley and in Wisconsin, 1 as 

shown by the writer, all tend toward the view that it is secondary to 

pyroxenes. So long ago as 1882 Irving, basing his conclusions on the 

combined investigation of the rocks of Wisconsin by himself and Professor Van Hise, stated the conviction "that all of the hornblende of 

the rocks of this region is but altered augite." 2 Those words are of 

significance here, since the rocks of the two States are very similar in 

general characters, and at the time the report cited was published the 

schistose gabbros were for the most part classed as gneisses.

The pyroxene is in part monoclinic, doubtless three-fourths diallage. 

Its color is green, and there are evidences of alteration from the incipient stage to that where individuals are completely changed to other 

mineral species. The orthorhombic pyroxene, hypersthene, is like 

the same mineral in the many localities to the southeast, already 

described.

ODESSA AREA. /

The last point up the river which discloses the presence of gabbroschists. is a moderate-sized exposure south of the Odessa station, on 

the Chicago, Milwaukee and St. Paul Eailway, 1,200 paces K. and 1,500 

paces W. of the southeast corner of sec. 9, T. 120 K, E. 45 W (sp. 

5334, 5335). A knob some 20 feet in height and not more than 250 

paces in length, largely turfed over, stands in the bottoms. The 

strike is N. 45° E. and the dip is 45° N W., although in places the schists 

seem to dip toward the southeast. The rock is dark colored, considerably weather stained at the surface, and very badly shattered. The 

texture is fine, for this type of rock, and the lamination is quite distinct. 

Both color and texture, aside from this character, are very even. Owing 

to the shattered condition of the rock, the effects of weathering extend 

rather deep. A rusted condition is seen along the sides of the fractures, 

penetrating the rock an inch or more. Where the fracturing is fine 

this condition induces a concentric weathering, which is conspicuous 

on the loose bowlders lying about on the outskirts of the quarry.

'.Notes of a geological excursion into central Wisconsin, by C. W. Hall: Bull. Minn. Acad.Nat. 

Sci., Vol. Ill, No. 2,1891, pp. 256, 257. 

3 Crystalline rocks of the Wisconsin Valley, by K. D. Irving: Geol. Wisconsin, Vol. IV, 1882, p. 714.
HALL.] HYPERSTHENE-BEARING GABBRO-SCHISTS. 95

Quarrying has disclosed the rock to a depth of several feet. From 

this depth the freshest specimens show an unusually fresh and unaltered condition of the rock. It is a typical hypersthene-gabbro-schist 

or norite-schist (sp. 5335, si. 2263). (See PI. XVII, B.)

It was from this quarry that a sample of an augite-schist was secured 

for the Educational Series of Rock Specimens of the United States 

Geological Survey (No. 140). The following is summarized from a 

description by W. S. Bayley: l

The rock is a very dark gray crystalline mass, with an obscure 

schistose structure and a sugary texture. Its grain is so fine that but 

one of its constitutents biotite may be recognized with any degree 

of certainty. This is uniformly scattered through the rock. In addition to this, there can also be detected a granular white mineral whose 

particles are so small that their nature can not be determined. The 

specific gravity of the rock is 2.770.

In thin section the rock is seen to consist chiefly of quartz, plagioclase, and pyroxene, with biotite, garnet, pyrite, and magnetite as 

accessory constituents. In natural light there appears an apparently 

homogeneous, colorless groundmass, in which lie large, irregular grains 

of a very highly refractive mineral with well-marked cleavage, smaller 

dark-brown flakes, small irregular or rounded grains or aggregates of 

grains of an opaque substance, and occasional isolated, highly refractive grains of a pinkish tinge. When examined with low power a 

certain parallelism may be detected in the arrangement of the various 

minerals. The majority of the brown flakes and of the highly refractive grains lie with their long axes in approximately the same direction. 

This supports the evidence of the unaided eye that the rock is schistose, 

and also the conclusion that the schistosity is not well marked.

When examined under crossed nicols the apparently homogeneous 

groundmass is seen to be an aggregate composed of very irregularly 

shaped interlocking grains of quartz and feldspar. Both are perfectly 

pellucid in natural light, and both contain the same kinds of inclusions. 

They may be distinguished by the brighter polarization colors of the 

quartz and by the twinning striations of the feldspar. From the intricate 

manner in which they interlock, it is impossible to determine which is 

the older. Both include small particles of. all the other components, 

and therefore they must be younger than these. They contain also 

tiny liquid inclusions a few with movable bubbles large quantities of 

black, red, and green dust, and irregularly outlined green and reddish 

inclosures that appear to be either decomposition products of some 

substance whose nature can no longer be determined, or secondary 

infiltration products. The latter supposition seems the more plausible, 

on account of the varied character of the inclusions, which are at the 

same time undoubtedly such as are produced by secondary infiltration.

The feldspar, like the quartz, is in irregular grains, colorless, and

1 See Bull. IT. S. Geol. Survey No. 150,1898, pp. 358-362.
96 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

pellucid. It is a plagioclase. The twinning striae usually occur in a 

single series of parallel lines, but sometimes they appear in two series, 

crossing each other at nearly right angles. The maximum symmetrical 

extinction of two contiguous lamellae, measured against their line of 

contact, is about 26°, indicating a plagioclase somewhere in the labra. 

dorite series. The specific gravity of the feldspar separated from the 

rock powder was 2.C73. In any given position of the section between the 

nicols, one portion only of the lamellae of any series of most grains is 

dark, the other ends of the larnellse remaining bright. As the section 

is revolved the darker portions become bright, the dark zone gradually 

moving toward what were the bright ends in the first position of the 

section, thus showing fine undulatory extinction.

The irregular, highly refractive grains have a slight pleochroism in 

pale-greenish and pinkish tints, the latter when the most prominent 

cleavage runs perpendicular to the vibration plane of the lower nicol. 

The extinction is parallel to this cleavage where only one set of cleavage lines is observed, and on sections where two cleavages are seen it 

bisects, the angle between them. The axis of elasticity parallel to the 

single cleavage is smaller than the one at right angles to it, and the 

double refraction is low. Since the long axes of nearly all the grains 

run about parallel to the plane of schistosity of the rock, and the sections are made parallel to this plane, it is not surprising that so few 

basal sections of the mineral can be found. In the one or two that may 

be discovered in each section there are two series of cleavage lines, 

which make with each other angles-of about 88° and 92°. No inclined 

extinctions were seen in any section where only one set of cleavage 

lines appeared. The mineral is probably an orthorhoinbic pyroxene. 

It is usually fresh and free from inclusions. In sections cut from the 

weathered portions of the rock, however, an interesting alteration is 

observed to be in progress. The cleavage lines of the pyroxene are 

filled with a brown pleochroic substance without definite morphological 

characters. This brown substance gradually spreads into the surrounding pyroxenic material, replacing it in part, so that often what at 

one end is a perfectly fresh, almost colorless pyroxene is at the other 

end a mass of brown, pleochroic substance. As the amount of the 

brown substance increases its character becomes more pronounced, until 

finally the substance takes the form of biotite. The biotite often lies 

embedded in pyroxenic material, often it borders large grains of that 

material, and sometimes it occurs in the spaces between neighboring 

grains. There can be no doubt that it is an alteration product of the 

pyroxene.

The greater portion of the biotite in the section is in isolated grains 

between the colorless constituents quartz and plagioclase. Its long 

axes are nearly always approximately parallel to the long axes of the 

pyroxene. The mineral shows the ordinary pleochroism of mica. It 

is, of course, impossible to state positively that all of the biotite in the,
HALL.] HYPERSTHENE-BEARING GABBRO-SCUIbTS. 97

rock has the same origin as that found in and near the pyroxene, but 

since it has exactly the same properties of the secondary mica we may 

say that it is probably of the same origin. 1

The only constituents remaining to be described are garnet, pyrite, 

and magnetite. The first mentioned is in almost colorless, very highly 

refractive grains, without cleavages and without inclusions. They are 

irregular in cross section, but in outline approximate more or less 

closely to circles. They may be easily distinguished from all the other 

constituents, since they are perfectly isotropic between crossed nicols. 

The large grains are crossed by irregular fracture lines, distinguishable 

from cleavage cracks by their lack of parallelism. The garnets are 

found usually as inclusions in quartz. Keasons are given later (p. 102) 

for believing that the more abundant garnets of a gabbro-schist are of 

secondary origin. If this conclusion is true, it is highly probable that 

the garnet of this rock is also secondary, although the evidence is 

not so clear as in the case of the gabbro-schist referred to. The pyrite 

and magnetite both occur in irregular opaque grains that may be distinguished from one another by the brassy luster of the former in 

reflected light.

In an attempt to name the rock, we are met with the difficulty always 

experienced in discussing the classification of schistose rocks. No satisfactory method of classifying them has yet been proposed. It would 

seem best to confine the-"schists" to aggregates of quartz and some 

bisilicate, and the "gneisses" to aggregates of quartz, orthoclase, and 

some bisilicate. According to these distinctions, our rock is neither a 

schist nor a gneiss, since it consists essentially of quartz, plagioclase, 

and pyroxene. It is a schistose rock with the composition of gabbro. 

Eoth designates such rocks as zobtenites, and Dathe calls them llasergabbros. The flaser-gabbros, however, have a larger amount of hornblende ; consequently our rock is excluded from that class. If we accept 

zobtenite as a general name covering all rocks produced from gabbros 

by pressure, it may safely be applied to the rock under discussion, for 

its composition is the same as that of the gabbros, and it has evidently 

been subjected to great pressure. Professor Hall, who has made a 

careful study of this rock in the field, thinks there can be no doubt 

that it was once a typical gabbro. If, however, we disregard the plagioclase as a classifieatory element, we may call the rock a biotite-augiteschist.

In either case it is extremely interesting. Augite-schists are very 

rare, having been described only a few times in the literature of American 

petrography.2 Zobteuite without hornblende, in which the pyroxene

'Preliminary description of the peridotytes, gabbros, diabases, and andesytes of Minnesota, by 

M. E. Wadsworth : Bull. Geol. Nat. Hist. Survey Minnesota, No. 2,1887, p. 65.

"Microscopical observations of the iron-bearing (Huronian) rocks from the region south of Lake 

Superior, by Arthur Wichmann: Geol. Wisconsin, Vol. IH, 1880, p. G45. Hooks of the Wisconsin 

Valley in the vicinity of Wausau, by K. D. Irving: ibid., Vol. IV, pp. 66!) and 694^6. Preliminary 

paper on an investigation of the Archeaii formations of the North western States, by E. D. Irving: 

Fifth Ann. Kept. U. S. Geol. Survey, 1885, p. 211.

Bull. 157 7
98 GNEISSES, GABBKO-SCHISTS, ETC., OF SW. MINNESOTA [BULL. 157.

and the plagioclase are perfectly fresh, are even more exceptional in 

occurrence than augite-schists, for the pressure that so squeezes a 

massive rock as to flatten its constituent grains produces a condition 

very favorable to the alteration of pyroxene into hornblende. Schistose gabbros, in which hornblende exists in large quantities, have 

been described in many foreign publications and in several in this 

country. It is not necessary to enumerate them here. 1

-HYPERSTHENE-FREE GABBRO-SCHISTS.

In several localities in the valley the gabbro-schists show no traces of 

hypersthene. In all other respects they are identical with the hyper - 

sthene-bearing masses just considered. Indeed, in several instances a 

series of samples was taken, one of which would exhibit well the 

characteristic hyperstheue, while another, from the same belt and only 

a few feet away, would prove to be an ordinary iionhypersthene-bearing 

gabbro. At no place was there noted any geological change in general 

direction and situation, and the most careful examination failed to show 

any break whatever in the arrangement of the rock series. Further, 

the examination of samples in the study served to show no differences 

whatever between the several rocks, save the presence of hyperstheue 

in one and the absence of it in another; every scale of proportion 

between these two extremes was seen.

At 800 paces BT. and 400 paces W., sec. 34, T. 112 K., E. 33 W., there 

stands an exposure of a somewhat altered gabbro. It is a narrow belt, 

not more than 175 or 200 paces long, elevated only a few feet above the 

level of the flood plain of the river. Some contortion can be seen in 

the lamina}, but the general direction is NW.-SE. The laminae stands 

about vertical. The rock is dark green in color, and under the hammer 

is extremely tough. Lighter-colored bands alternate with the darker 

and more basic. Pyrite is frequently seen in compact nests of crystals. 

A fine texture generally prevails.

Quartz has entered to no little extent into the mineral composition of 

this rock. It has assumed the pegmatoid character, and in many places 

affords a matrix to the other and older constituents, particularly to 

the kaolinized feldspars. Only occasionally is the feldspar sufficiently 

preserved to disclose its labradoritic character. The pyroxenic contents are augite and diallage in about equal proportions. The normal 

extinction of these two minerals is readily shown.

While it is more or less true of both pyroxenes, it is particularly so 

of the diallage, that it is in large areas and that the feldspars and other 

minerals occupy vermicular channels and spherical spaces within the 

diallage matrix. PI. XXIII, B (from sp. 5199), shows a field of this 

kind in which feldspar, quartz, and hornblende lie in canals and open1 The greenstone-schist areas of the Menomiuee and Marquette regions of Michigan, l>y G. H. 

Williams: Bull. TJ. S. Geol. Survey No. 62, 1890, pp. 52, 216.
HALL.] HYPERSTHENE-FREE GABBRO-SCHISTS. 99

ings in a single area of diallage. The boundary between the diallage 

and the included minerals is partly sharp and clear and partly broken 

up into narrow reaction rims, along which there seem to lie several 

minerals.

In the freshest specimens the augite and diallage are almost unaltered. In others they are both altered to hornblende. In places 

whole areas of these minerals are altered, and in other places rims of 

hornblende surround cores of the pyroxene. A decidedly fibrous 

character is in some places assumed by the secondary hornblende, 

and in other places numberless plates of green hornblende of minute 

size thickly stud the areas of pyroxene and give a green color to 

entire individuals. In the more weathered portions of the rock probably more than one-half of the bulk of the basic constituents is secondary hornblende, and there is in addition a considerable sprinkling of 

biotite. Between the folia of biotite are wedged many lenses of quartz 

and epidote. (See PI. XX, C.)

Around Granite Falls several localities show no appreciable quantity 

of hypersthene in the gabbro-schists. South of Granite Falls, in sec. 

4, T. 115 N,, K. 39 W., lie some exposures of these rocks. While the 

great mass of the gabbro which stretches from this section northward, 

extending, with occasional interruptions, 2 or 3 miles, is hypersthenebearing, several localities may be cited where no hypersthene whatever 

was seen.

In sec. 4, at 1,500 paces north of the southeast corner, some exca- 

. vating has been done in a search for gold. The veinlike matter which 

has already been mentioned as occurring here was followed about 

20 feet down. In the neighborhood of the vein the rock is decidedly 

massive, but at the distance of a few feet from the contact it has a 

distinctly schistose character. The pieces secured show admirably 

the shattering of the feldspars and the anastomosing through them 

of the veins of hornblendic material. A strongly marked diallage is 

the other leading constituent. No hypersthene Avas seen.

The rock at this locality shows some features of so much lithologic 

interest, particularly in the presence of a generous proportion of 

garnet (see PI. XXII, B), that a set was collected for the United 

States Geological Survey's Educational Series of Rock Specimens (No. 

109). Prof. W. S. Bayley prepared the description for this series, and 

from his description 1 the following summary is taken:

The prevailing color of the rock is dark green, speckled with large 

patches of dark red, and small areas of greenish yellow or white. 

Upon close inspection the yellow and white areas are seen to be the 

glistening cleavage surface of a striated plagioclase. The red areas 

are the surfaces of a dark-red, very hard, transparent mineral. It has 

no distinct cleavage, and is insoluble in acids. The properties are 

those of garnet. The nature of the dark-green matrix in which the

1 See Bull. U. S. Gcol. Survey No. 150,1898, pp. 282-286.
100 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BOLL. 157.

garnet and plagioclase are embedded can not be determined, though, 

from its dark color it may be assumed to be very basic. The specific 

gravity of the rock is 3.105.

Under the microscope the thin section shows a granular aggregate 

of plagioclase, green augite, garnet, and magnetite, with small quantities of green hornblende, a few grains of quartz, and tiny crystals of 

apatite.

The plagioclase is in irregular allotriomorphic grains, crossed by welldefined cleavage cracks and irregular fissures filled with decomposed 

products and stained by iron oxides. The feldspar is clear and colorless, except where rendered cloudy by inclusions. The most abundant 

of these are tiny flakes of chlorite and irregular masses of an opaque 

earthy substance, besides dustlike particles of magnetite and little 

nests o'f a brightly polarizing calcite. All these are usually abundant 

in the neighborhood of the fissures but are rare in the other parts of 

the grains. Other plentiful inclusions are long, narrow, apatite crystals 

with the ordinary characters. The average length of these apatite 

crystals is about O.I mm., and their thickness some 0.05 mm., though 

a few have cross sections measuring 0.2 mm. in diameter. Augite, 

hornblende, and large masses of magnetite are also included in the 

plagioclase, but these will be spoken of later.

Under crossed uicols the plagioclase twinning becomes very apparent 

as mediumly wide bands, usually running entirely across the grains. 

Some curve slightly, others wedge out as they pass toward the interiors 

of the grains, and still others spring from the sides of cracks, etc. These 

phenomena indicate that the rock has been subjected to pressure since 

it solidified. The schistosity observed in the field is probably a result 

of this. In certain restricted areas in the section, notably in the 

neighborhood of large garnets, there is often a second series of twinning lamellae, with more or less undulous extinction, cutting the first 

series at some acute angle. The second series comprises numerous 

lamellte, not so distinct as those of the first set and not so sharply 

marked off from one another. The specific gravity of the plagioclase 

is somewhere about 2.68, or near the andesine division in the plagioclase 

series, though probably a more basic member is also present, with a 

density of about 2.72.

The next most abundant component of the rock is a pyroxene, in 

green allotriomorphic grains and older than the plagioclase. This 

augite, where fresh, is marked by two series of cleavage lines, crossing 

each other at angles of about 90° in basal sections, and in other sections 

by a single series of parallel lines. The extinction is usually inclined to 

these. The augite in these sections may easily be distinguished from 

hornblende by the lack of strong pleochroism and by the large angles 

of extinction against the cleavage lines in a single direction, which 

often reach as high as 43°, whereas those of hornblende rarely approach 

24°. Occasionally a slight difference in absorption may be detected 

in the mineral, but this is so slight that it may easily be overlooked.
HALL.] HYPERSTHENE-FREE GABBRO-SCHISTS. 101

In addition to the two cleavages mentioned as prominent in basal 

sections there is often another, presenting itself as a series of closely 

crowded parallel lines, bisecting the larger of the two nearly rectangular intersections of the cleavage lines. Since the shorter of the two 

lateral axes in pyroxene is the orthoaxis, this cleavage must be parallel 

either to the orthozone or to the orthopinacoid. It is determined to 

be an orthopinacoidal cleavage, which is characteristic of diallage. 

At one time this cleavage of diallage was regarded as original, and the 

mineral was considered as a distinct species of pyroxene. Now, as 

shown by Professor Judd, 1 it is known, that the cleavage is secondary, 

and that its origin is often the direct consequence of pressure to 

which the rock containing it was subjected.2

Though much of the diallage is fresh, a still larger proportion is 

altered. The interior is often stained brown, or yellowish brown, and 

lacks the power of polarizing brilliantly. A little magnetite, in rounded 

grains, has formed around the edges of the altered portion, and a more 

or less fibrous cleavage has developed in it. In places the yellowish 

substance occurs in little plates or needles, arranged in parallel lines 

inclined to the cleavage, thus giving rise to the appearance of fine 

cleavage. In some cases the lines are so straight and narrow that they 

are with great difficulty distinguishable from true cleavage even under 

a high power. The origin of somewhat similar inclusions, so common 

a feature in diallage and hypersthene, has been the subject of much 

discussion, Judd considering them secondary infiltration products along 

planes of easy solution, 3 and others regarding them as original inclusions taken up by the mineral during its growth.4

Around the outside edges of the pyroxene another alteration is 

observed, it being surrounded by large plates or small granules of a 

bright-green substance with strong pleochroism in yellowish-brown and 

dark bluish-green tints and an extinction inclined to the cleavage. 

This substance, which is a hornblende, surrounds the pyroxene as a 

narrow rim, which sometimes extends into the pyroxene grain and at 

other times is formed of granules that seem to have been added to the 

grain after its formation. In either case the substance is to be regarded 

as secondary in origin.5 In a few cases, where the pyroxene grains 

were small, the entire substance has changed, and in its place are now 

areas of hornblende that might be regarded as original were it not for 

the fact that so much of the hornblende of the rock is undoubtedly 

secondary. Another product, in some places formed by the alteration 

of pyroxene, is biotite. This is in small, reddish-brown flakes mingled 

with hornblende on the periphery of the areas.

1 On the Tertiary and other peridotites of Scotland, by Prof. J. AV. Judd: Quart. Jour. Geol. Soo. 

London, 1885, pp. 378-379.

2 Preliminary descriptiou of the peridoty tes, gabbros, diabases, ami audesy tea of Minnesota, by M. E. 

Wadaworth: Geol. 'Nat. Hist. Survey Minnesota, Bull. No. 2,1887, p. 55 et scq.

3 Loc. cit.,p. 354.

4 Discussions of the peridotites and norites of the Cortlandt series, by G. H. AVilliams: Am. Jour. 

Sci., 3d series, Vol. XXXI, Jan., 1886, p. 33, and Vol. XXXIII, Fob., 1887, p. 143.

0 Note on the enlargement of hornblendes and augites in fragmental and eruptive rocks, by C. 11. Van 

Hise: Am. Jour. Sci., 3d series, Vol. XXXIII, May, 1887, p. 385.
102 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

The magnetite and pyrite appear as large irregular grains scattered 

more frequently near the pyroxene and garnet than elsewhere in the 

rock. Both have resisted alteration and both are equally opaque.

The garnet is the characteristic mineral of the rock. It is in large, 

cellular masses, sometimes measuring a half inch in diameter. In 

the section it appears as a highly refractive, isotropic, deep-pink substance filled with inclusions and crossed by many irregular cracks, 

along the sides of which are stains of yellow iron oxides. So large 

and so numerous are the inclusions that the garnet substance, in its 

arrangement, reminds one of the section of a coarse sponge saturated 

with various colorless products^ The largest and most striking of the 

inclusions are quartz grains. These are colorless and without cleavage 

traces. They inclose little mica plates, apatite crystals, dust particles, 

and thousands of small liquid-filled.pores arranged in lines. Under 

crossed nicols most of the larger grains break up into aggregates, with 

the lines of inclusions passing from one grain into another without interruption. Occasionally some of the clear inclusions in the garnet are 

discovered to be plagioclase, but these are comparatively rare. The 

other substances inclosed by the garnet are small pieces of biotite, crystals and particles of magnetite, crystals of apatite, and thousands of tiny 

cavities filled with liquid. As in the quartz, these are arranged in lines, 

and the lines are sometimes continuous in both substances. This 

latter phenomenon would indicate that the inclusions are of secondary 

origin, and that they were formed after the quartz and garnet had 

assumed their present positions.

The associations of the garnet show it to be younger than the other 

constituents. Its cellular nature is evidence that it did not separate 

from the magma whose cooling gave rise to the main portion of the 

rock, because garnet formed in this way is usually among the oldest 

products, whereas in the present case the mineral is younger than' even 

most of the quartz.

Another form in which the garnet exists is in small granules surrounding the magnetite. Here the mineral has the same properties as 

when in large pieces, except that it contains no large inclusions of 

quartz and feldspar.

The quartz is in colorless masses surrounded by garnet, as already 

mentioned, and also in larger pieces associated with the garnet but not 

included in it. It usually occurs most abundantly near the garnet, in 

the sections containing a great deal of that mineral, and is almost if 

not totally absent from the sections in which there is none.

Since the garnet is probably secondary, i. e., since it was probably 

formed after the main portion of the rock had solidified, it is probable 

that the quartz also is of secondary origin, and that the original components of the rock were essentially diallage and plagioclase, with 

magnetite, pyrite, and apatite as accessory constituents.

The rock is a gabbro. The principal mass under study is schistose,, 

but it is evident that this is an imposed structure, since even in the'
HALL.] HYPERSTHENE-FREE GABBRO-SCHISTS. 103

apparently massive variety represented by the hand specimen abundant indications of pressure are present. The massive phase, since 

it is characterized by the garnet, must be denominated garnetiferous 

gabbro. The schistose variety, being a squeezed gabbro, may be called 

a gabbro-schist.

Another rock, on the Eussell farm, south of Minnesota Falls, possesses 

so much lithological interest that a set of specimens was selected to 

constitute one'number of the Educational Series collected and distributed by the Survey (No. 141). The exact location is 800 paces N., 100 

paces W., sec. 11, T. 155 1ST., E. 39 W. In the field, at this particular 

spot, the rock is distinctly foliated; it strikes nearly E.-W. and dips 

N. at 30°. The hand specimen at first shows little evidence of schistosity, but closer examination discloses the long axis of a majority of 

the grains lying approximately in the same direction, although not 

necessarily in the same plane. The following summary also is from 

the description prepared by W. S. Bayley.1

The rock is a medium-grained, distinctly crystalline aggregate of 

a white, striated plagioclase and a lustrous black hornblende showing 

a good cleavage. The cleavage surface is less even than that of mica.

The thin section of the rock reveals a general parallelism in the 

arrangement of its darker constituents. Most areas show only a coarsegrained aggregate of plagioclase and light-green hornblende. Other 

areas show in addition a very light-colored, highly refractive augite.

The plagioclase is in very irregular 'grains, clear and colorless except 

for tiny dustlike inclusions scattered through them. lii addition to 

these it contains also tiny specks of magnetite and small flakes of 

green hornblende. Under crossed nicols the twinning lamella^ in the 

majority of cases, form a single series of parallel lines. In other 

cases the lamella? form two series, cutting each other at angles of about 

78°. Some of the lamellas run entirely across the grains, others spring 

from their edges and wedge oat toward their centers, while still others 

spring from the sides of cleavage cracks and extend only a short distance into the feldspathic substance. In many grains the lamellae 

have an uudulatory extinction, different portions of the same lamella 

extinguishing in different positions of the stage as it is revolved. All 

these phenomena indicate that the feldspars have been subjected to 

pressure, which is probably the same as that producing foliation in the 

rock mass. The maximum symmetrical extinction of contiguous 

lamella is somewhere in the neighborhood of 33°, and the density of 

the mineral is 2.731. It is thus a basic bytownite.

Though the main mass of the feldspar is fresh, as has been said, the 

corners of grains and the small areas between neighboring ones are 

often filled with a micaceous, brightly polarizing substance, in tiny 

flakes, accumulating in little cloudy masses in and between the fresh 

plagioclase grains. The substance has the peculiarities of kaolin, a

1 See Bull. U. S. Geol. Survey No. 150,1898, pp. 369-372.
104 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

common alteration product of all feldspars, and is undoubtedly that 

material.

The hornblende is in large masses, of green color and of moderate 

pleochroism. Many of the grains have but one set of cleavage lines. 

The extinction against these lines varies between parallelism and an 

inclination of 28°, according as the section is parallel to the orthopinacoid or to the clinopinacoid, and the pleochroism is in clear-green and 

yellowish-green tints. Other grains have two series of cleavage lines. 

Where the lines are most distinct and most sharply cut, the angles 

made by the two series are 124° and 56°. The pleochroism is dark 

green and light greenish yellow; <x=greeuish yellow, ft and y=greQu. 

Absorption, fi=y>a. The different individuals of the mineral are 

very irregular in shape, and usually four or five aggregate into groups. 

Most of these are simple individuals, but others have two or more parts 

in twinued relation to each other. The only inclusions noted in the 

hornblende were little grains of magnetite, small dust particles, and, in 

certain sections, masses of a light-colored, highly refractive mineral 

augite.

This augite is found only in the interior of masses of green hornblende. It has a very high index of refraction, and is not pleochroic. 

It is either colorless or a very light shade of green. Under favorable 

conditions it may be seen to be crossed by two series of cleavage cracks 

making angles of nearly 90° with each other. The extinction bisects 

these angles, and the polarization colors are very brilliant. This 

augite is not always sharply separated from the surrounding hornblende. It gradually assumes a greenish tinge on its edges. This 

color deepens as the distance from the interior of the augite increases, 

and the substance acquires more or less pleochroism, until it finally 

becomes indistinguishable from the surrounding hornblende. This 

relation of the two minerals indicates that the hornblende has been 

derived from the augite.

Occasionally, in the edges of hornblende grains, and especially where 

these are in contact with feldspar, small, reddish-brown flakes of 

strongly pleochroic biotite may be found. These flakes sometimes 

extend, with frayed ends, far into the hornblende. Biotite is present 

in but small quantities, either between plagioclase and hornblende, as 

described, or between neighboring hornblende grains. It is probably 

secondary, being due to a reaction between the hornblendic and the 

plagioclastic materials. The alteration of hornblende into biotite is 

by no means unknown. 1

The chief components of the rock as it at present exists are plagioclase and hornblende. The latter is evidently an alteration product of 

augite, and the plagioclase is bytownite. Both minerals are allotriomorphic, but the rock has been given a schistose structure by pressure.

'The greenstone-schist areas of the Menominee and Marqnette regions of Michigan, by G. H. 

Williams: Bull. TJ. S. Geol. Survey No. 62,1890, p. 182.
HALT,.] HYPERSTHENE-FREE GABBRO-SCHISTS. 105

If we reconstruct the predecessor of the schist, we shall have a granular 

aggregate of plagioclase and augite i. e., a gabbro. From the presence of Tiyperstheue in the fresher rocks in the immediate neighborhood of this specimen, it is fair to presume that it was originally a 

hypersthene gabbro. Its composition is now that of diorite, and its 

structure is schistose. Hence the rock may be termed a gabbro-diorite. 1

Near Eussell's farm, sec. 12, T. 115 K, E. 39 W., a sample was taken 

which lay almost alongside the gabbro just described a typical representative of an altered gabbro. In this sample (sp. 5279) the pyroxene 

is almost completely altered to hornblende, yet enough remains to show 

the monoclinic habit of the constituent. The hornblende in this specimen exhibits in a very beautiful manner the characteristic granular 

condition which the secondary phase of this mineral assumes in nearly 

all localities and among all rock types of the Minnesota Kiver Yalley 

and of central Minnesota. Quartz is here an abundant accessory; so, 

too, is biotite in those areas within and around the space formerly occupied by the pyroxenes (PI. XXII, A).

In the SW. £ of the SW. J sec. 2, T. 113 N., E. 36 W., there stands a 

long, low belt of rather coarse hornblendic schist (sp. 5260, 5739, 5740, 

5741). Everywhere over the surface course light-colored granitic veins. 

While these veins run in every conceivable direction, a general parallelism with the laminiE of the schist can be clearly traced. Their field 

relations to the ordinary gneisses can not be made out, further than that 

they lie near them, direct contact being thoroughly concealed.

The feldspars are well preserved in their general characters, few 

areas showing a breaking up into kaolin amd here and there a staining by ferric oxide. Only a very few individuals of pyroxene are left 

from the alteration into dark-green hornblende, and these are green in 

color and augitic in optical characters. A few accessory minerals were 

also noted in the thin sections. So far as studied there seems to be 

little difference, in the degree of preservation shown by the individual 

minerals, between the finer, compact rock and the coarser, crumbling 

phases.

Save in the presence of augite at this locality there is almost no 

difference perceptible between this schist and that in sec. 4, T. 114 N"., 

E. 37 W. (sp. 5376).

Three miles up the river from La Fraioboise's, in sec. 34, T. 112 N., 

E. 33 W., there stands a series of small knobs of a schistose rock, 

extremely tough under the hammer and almost unweathered. On macroscopic inspection there appears a contortion so marked that the true 

direction of the schistosity can not be accurately stated, but an average 

of measurements shows it to be nearly NW.-SE. The exposures stand 

about vertical. The contorted bands exhibit frequent nests of pyrite 

and many lenticular feldspathic streaks. The color of the rock is dark 

green, the texture fine.

1 The gabbros ami associated hornblende roclcs occurring in the neighborhood of Baltimore, Md., 

by G. H. Williams: Bull. U. S. G-eol. Survey No. 28, 1880, pp. 17, 27-32.
106 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

The three principal minerals present are a plagioclastic feldspar, 

augite, and magnetite. The feldspars are badly decomposed. Their 

alteration has proceeded so far that only occasional individuals are 

still sufficiently fresh to show the twin lamellae. The two replacing 

minerals of the feldspathic constituent are kaolin and quartz, the lastnamed mineral appearing in very numerous but scattered granules.

While the feldspars are thus largely obliterated, the augite areas are 

unusually fresh. A green color predominates, and cleavage markings 

can be seen almost everywhere. The extinction angle is around 55°, 

thus indicating a normal augite. Not infrequently the individuals are 

of so large size that they include many other minerals, as feldspars 

and quartz. Some twinned forms occur. The hornblende, which for 

the most part seems to be secondary to the augite, is in small areas, 

either as narrow borders around the areas of augite or as small 

detached areas within the grains. In places biotite is present, and a 

characteristic of this mineral seems to be the intercalation of lenticular 

segregations of epidote and quartz between the folia. Either this is 

due to the alteration of some mineral which in its change produces 

both biotite and epidote, or the biotite has since its maturity yielded, 

as an alteration product, the material which has developed into the 

interfoliar epidote.

PYROXENE-FREE GABBRO-SCHISTS OR GABBRO-DIORITES.

In their alteration the gabbro-schists in several localities show a 

total disappearance of the pyroxenic constituent so characteristic of 

the fresher members of the series and its replacement by hornblende 

or by hornblende and biotite. In all their field relations and they have 

been studied from one end of the valley to the other no conditions of 

any sort were detected which might be regarded as peculiar to one 

member of the series and not to be found in another. The more or less 

schistose structure could be seen almost everywhere. The localities 

where this structure is lacking have already been indicated. The dark 

color, modified by the proportion of feldspar or the degree of hornblendization, was repeatedly seen. A medium texture always prevailed, frequently modified, however, by the occurrence of coarser 

segregations or laminae.

While veins or dikes of a granitic character are abundant throughout the gneissic series, they are generally rare in the gabbro-schist.

In the west half of sec. 21, T. 114 tf., E. 37 W. (PI. VIII), there stands 

a mass of highly contorted hornblende-schist which is dioritic in type 

(sp. 5376). Some bands are biotitic. It is surrounded by several other 

knobs, at no great distance, which are gneissic. No well-defined direction of schistosity can be stated, yet the general strike is E.-W., with 

a northward dip.

Well-individualized hornblende occupies about two-thirds of the bulk 

of the rock. Frequently twinned individuals occur. The feldspar is
HALL.] PYROXENE-FREE GABBRO-SCHISTS. 107

of the labradorite-anorthite type, and, while very fresh and strongly 

twinned, shows great unevenness in composition. In places a cloudiness, due to decomposition, completely hides the feldspathic characters, 

and in other places a ferric oxide is disseminated to a very considerable 

extent through the feldspars and hornblendes alike. Some rather large 

crystals of apatite were seen. This exposure seemed to be quite isolated 

from that of any other rock of similar type. Those which most resemble it are the dark-colored schists in sec. 2, T. 113 N.; E. 36 W. (PL VIII), 

and in sec. 31, T.11S N., E. 40 W. (PI. XI), both of which have already 

been noted, the former still carrying some portion of pyroxene and the 

latter containing hypersthene.

Among the high, bold knobs of gneissic rocks which stand scattered 

throughout sees. 3, 4,5, 10, and 11, T. 113 K, E. 36 W. (PI. VI), are several interlaniinated belts of hornblende-schist. In addition to these 

alternating bauds, there are many lenticular nests, large and small. 

The texture is medium; as a rule, the broader, thicker bands are the 

coarser. The schistosity lies in a very regular direction, striking E. 

10° S., and dipping southward from a low angle to one as high in places 

as 75°. In mineral composition this rock varies little from that just 

mentioned. The feldspars are more altered here than is the usual 

condition.

Coming again to Granite Falls and Minnesota Falls (PI. X), several 

localities may be noted where the diorite-schists occur. In the south 

part of the village of Minnesota Falls, at 1,775 paces N. and 75 paces 

W., in sec. 11, T. 115 N., E. 39 W., is a finely crystalline schist (sp. 

5418) which is badly shattered. It lies in the vicinity of a granitic 

vein of considerable size and only a short distance from a very prominent dike (sp. 5419). Examination indicates that this rock was originally a gabbro-schist, in which, at the present time, the pyroxenic 

constituents are completely obliterated. In spite of this fact the rock 

is fresh, and the existing mineral constituents can be diagnosed with 

ease. (See PI. XXI, B.)

Starting southwestward from the corner post between sees. 33 and 

34, T. 116 K, E. 39 W., and sees. 3 and 4, T. 115 N., E. 39 W., crossing 

the Minnesota Elver and proceeding a few degrees south of west for 

a mile or more through sec. 4, one passes a succession of knobs which 

have a schistosity striking westerly and dipping southerly. The prevailing color is dark, due to the predominance of hornblende. The 

texture is medium, and the whole aspect of the rock is bright and 

fresh. The mounds are not high; their linear arrangement and shape 

are due to the strike and inclination of the rock^belt. There is little 

veiniug to be seen, and both structure and texture are very uniform 

throughout.

Still another locality is in the heart of Granite Falls, just north of 

the court-house and the several churches of the city. A belt of rather 

coarse gabbro-gneiss extends down from the northwest, swings round
108 GNEISSES, GABBRO-SCHLSTS, ETC., OF SW. MINNESOTA.

to the right, is interrupted by a narrow dike, and disappears toward 

the southwest. While the rock here is as fresh as along the belt just 

mentioned, it has a lighter color and a more gneissoid aspect, due to a 

smaller amount of hornblende. The term u gneissoid" is applied to 

indicate that the proportion of feldspar at this locality is much larger 

than in the typical exposures. Everywhere the feldspar individuals 

are fresh, brightly polarizing, and affected by alteration to a comparatively slight extent. Twinning is everywhere very pronounced, and 

the extinction angles point to the labradorite-anorthite type. The 

granular structure is very uniform, and all the feldspars are apparently 

of a single generation. Wherever alteration has crept in, it results in 

segregations of finely crystalline granules in the central portions of the 

individuals affected. Very little quartz was noticed, and that appeared 

to be secondary.

The leading basic constituent in all the specimens examined is.hornblende. As a rule it is the green variety, but in one or two instances 

brown hornblende is present as the center or core around which the 

green is arranged. In a few instances a highly fibrous condition is 

seen, with a rim of deep green surrounding the areas packed with 

fibers. Since in this same group of rocks (sp. 5285, 5238, and especially 5445) the mineral hypersthene is seen changing into this fibrous 

hornblende, we can assume that origin for this portion of the hornblende. There is certainly every indication in the external and internal characters of these rocks to warrant such conclusion. If this view 

of the origin of the rocks carrying the fibrous hornblende be tenable, 

it must also be so of those in which this mineral is expanded into folia 

and grains, whether the folia and grains be green or brown in color. 

The same reasoning can be extended also to include masses in which a 

portion of the alteration product is chlorite and those in which a part 

of the hornblende has been further changed into biotite.

If lithologic proof of the character cited be insufficient to show the 

relationship of the hypersthene-bearing, the hypersthene-free, and the 

pyroxene-free rocks of this locality, the field relations may be cited. 

The belts of hypersthene-bearing rocks which occur in fine exposures 

in sec. 34, T. 116 N., E. 39 W. (PI. X, sp. 5445, 5293), can be followed 

step by step across the river in a direction a little south of west, through 

sec. 4, T. 115 N., E. 39 W., until all the rocks of the valley finally 

disappear under the glacial drift which rises as the southwestern 

boundary of the river plain.

In the exposures north of the court-house, where the hornblende is 

scarce, the feldspars-are apparently identical with those to the south in 

the long belt stretching from the neighborhood of the railway station 

entirely across the valley. But between the feldspar individuals there 

are bands and clusters of finely granular quartz. These lie, as a rule, 

along the sides of the feldspars which show most evidence of corrosion, and they bear every aspect of secondary origin. The hornblende
HALL.] PYROXENE-FREE GABBRO-SCHISTS. " 109

areas are usually clusters of individuals of small size lying compactly 

together, nearly all of them having substantially the same crystallographic orientation. They are undoubtedly secondary. The minerals 

adjacent to a dike are in no way different from, the individuals of the 

same species lying many feet away. The rock is undoubtedly secondary to some phase of preexisting gabbro. It may have become somewhat more silicified than the other diorites and many of the gabbroschists of the valley during the metamorphism of the rocks.

On the top of the hill in the western part of the city of Granite 

Falls is a rock (sp. 5441) of rather peculiar mineral and textural composition. In only one other locality in the whole valley has such a rock 

been found, namely, at Montevideo. It is a somewhat porphyritic diorite. No augite is present, hornblende abounds, and biotite is plentiful. 

The hornblende is green and somewhat granular, with occasionally wellformed crystals extending into the old and partly kaolinized feldspars. 

The porphyritic feldspars are plagioclases, with the twinning striae 

largely obliterated through the kaolinizatiou. Around them is fresh, 

transparent feldspathic material, apparently orthoclase. The apatite 

needles, which are thickly strewn in every section, are of varying sizes 

and rarely entire. Many of them are not only broken and displaced, 

but are shoved until they overlap one. another. (PI. XXIV, B.) 

Aside from these minor characters the rock shows little effect of 

pressure.
OHAPTEE VI.

PERIDOTITE AND SERPENTINE. 

PERIDOTITE.

Peridotite occurs in only one definitely located spot in the Minnesota 

Kiver Valley so far as is at present known. That spot is 1,300 paces 

N., 600 paces W., sec. 9, T. 112 ST., E. 34 W., being 3 miles in a direct 

line southeast of Morton. As was stated in Chapter III, bowlders are 

found within a few miles of this place, in sees. 32 and 33, T. 112 N., E. 

33 W., which are of essentially the same material.

The exposure in sec. 9 shows only a small area. The rock stands 

beside one of the many small lakes formed through the abandonment 

of a former channel by the river. Its entire length measures 300 paces, 

and a portion of this is now .covered. There are no other outcrops of 

the crystalline rocks within a half mile, and in this exposure itself 

there is no visible evidence of other rock species than peridotite and 

its alteration product, an impure serpentine; therefore its relations to 

the nearest neighboring rocks, the chlorite-gneiss of sec. 10 and the 

ordinary hornblendic biotite-gneiss in the south half of sec. 4, could in 

no way be determined.

The present shape of these peridotite knobs is undoubtedly due to 

the erosion of the Ei ver Warren and its successor, the present Minnesota 

Eiver. The strike of the knobs is N. 75° W., yet this can not be accepted 

as determining the direction of the eruptive material; it rather marks 

the direction of the currents of the eroding stream. Trees and turf 

cover the surface of the rock, save where the slopes are too steep to 

retain soil.

The rock is unusually dark colored. It exhibits many spots of a 

bronzelike hue, which are probably enstatite cleavages, and many dark 

olive-green markings of irregular contour. In more decomposed places 

the fresh fractures are spotted with bright green, pointing to the alteration of one or more of its constituents into chlorite. In still other 

places fresh fractures show that the color of the more massive portion 

varies to a mixture of dirty, light-brown patches set in a groundmass 

of dull-green, finely crystalline material, while at the surface the color 

becomes brown to black on account of the presence of organic matter. 

Beneath this superficial coating of foreign material the dull-brown 

color of the abundant limonite results from the proneuess to alteration, 

one of the marked characters of the rock. ,

The exposures everywhere are badly fractured. Along every seam 

no
HALL.] PERIDOTITE AND SERPENTINE. Ill

aud parting plane alteration has extended into the rock. Percolation 

channels are thus afforded, along which waters have corroded the rock, 

usually leaving a spongy, cavernous surface. Locally, however, there 

has been deposited from infiltration calcite in small masses, together 

with crystalline and crystallized quartz in much greater quantity. The 

quartz is occasionally amethystine, and clusters of rather small amethyst crystals were seen. The calcite was almost everywhere characterized by a waxy luster and by undulous cleavage planes.

In mineral composition the peridotite is varying. The freshest specimens are composed of enstatite and oliviue, with their alteration products, serpentine, chlorite, limonite, and chromite. From this composition 

it is seen that the original rock was that variety of peridotite called by 

Wadsworth1 saxonite, and described by Dathe2 some years ago under 

the name eustatite-olivine rocks from Kussdorf, Saxony. The rock of 

the Minnesota Valley differs from the type described by Dathe chiefly 

in the parallel (gneissic?) structure of the latter; both rocks are characterized by the predominance of enstatite over olivine and all other 

constituents, both accessory and secondary. The freshest specimens at 

hand (sp. 5215,5731) show in several thin sections an average eustatite 

content near 40 per cent, while there is less than 10 per cent of olivine. 

Taking into consideration the secondary contents of the rock, and 

assuming their origin from their color and textural characters, the 

following estimate is made of the original mineral composition of this 

saxonite: Enstatite, 60 per cent; oliviue, 35 per cent; all other constituents, 5 per cent; total, 100 per cent. The specific gravity of the rock, 

the average of four determinations, is 2.827.

Microscopically the enstatite grains are fresh in their central portions 

aud disintegrated around their borders. Barely are well-defined boundaries seen. Many areas are still large, indicating an originally coarsegrained lock. The color is light brown, the cleavage parallel to the 

axis c is conspicuous, and the separation of the grains is into thin 

plates, in many places so marked as to give a fibrous habit when the 

section is cut across the planes. There is no dichroism and the extinction is parallel. This is probably the oldest mineral in the rock.

Hext in interest is olivine. This mineral is everywhere surrounded 

by secondary products. Its position indicates that it was formed subsequent to the enstatite. Many of the grains are included within the 

enstatite individuals, as Wadsworth showed for the Minnesota enstatite 

(hypersthene?)-bearing gabbros.3 In every case there is between the 

two minerals a band of decomposition products partaking of the

1 Lithological Studies; a Description and Classification of the Rocks of the Cordilleras, by M. E. 

AVadsworth, Cambridge, 1884, p. 125.

2 Neues Jahrbucb. fiir Mineralogie, Geologie imd Palseontologie, by E. Dathe, Stuttgart, 1876, pp. 

233-235.

3 Preliminary description of the peridotytes, gabbros, and uudesytos of Minnesota, by M. E. Wails- worth: Bull. Geol. Nat. Hist. Survey Minnesota, No. 2, 1887, pp. 59, 62.
112 GNEISSES, GABBEO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

nature of a reaction rim. Thus they are never seen in contact. This 

band resembles in color and textural characters the decomposition, 

products of olivine, rather than of enstatite, as those products have 

been grouped in the examination of the thin sections. Owing to some 

local differentiation of which the rock seems to bear indications, olivine 

might originally have constituted the major portion within small areas, 

although it now comprises only a small per cent in the very freshest 

specimens. The grains are rounded and elliptical in shape. Generally 

several small areas lie clustered in the same crystallographic position, 

thus indicating that they are all parts of the same original crystal grain. 

Occasionally the existing grains are penetrated by veinlets of secondary 

material. This comparative freshness of certain grains may indicate, 

not any ability to resist decomposition, but rather that the prismatic 

zone of these individuals lies in the field. 1 The surfaces of the olivine 

grains generally show that rough appearance due to a high refraction index. Inclusions are rare, and the mineral is unusually free from 

impurities. This may be due in part to the fact that the portions of 

the several grains which were once studded with impurities were the 

first to succumb to alteration and have completely disappeared. (See 

PI. XXVII, A.)

The remaining constituents of the rock seem to be secondary. The 

first among them in importance is serpentine. This mineral flows around 

the others as a matrix in which the remains of the primary constituents 

and the crystals of magnetite and other secondary minerals lie 

embedded. It constitutes nearly half of the entire rock mass in the 

freshest specimens, and an increasing percentage in the more decomposed, until the whole hand specimen is frequently serpentine. It is 

everywhere fibrous in structure. Locally it appears as plates across 

which the extinction shades play in wavelike succession as the field is 

rotated between crossed nicols.

An opaque mineral is present, which frequently presents the rectangular sections indicating cubic crystallization. In size the individuals 

seldom exceed 0.025 mm., although clusters are much larger. When 

crushed the rock affords a considerable quantity of a jet-black, 

strongly magnetic powder. Careful qualitative tests fail to give 

reactions for chromium or titanium. The mineral is therefore magnetite. These magnetite crystals occur in the most completely 

altered portions of the rock. Wherever there are traces of the enstatite or olivine the opaque constituent lies in clusters and dendrites of 

granular habit. These clusters are crowded to the walls of the percolation canals, and often afford boundaries between the primary constituents and the serpentine. So far as observed, magnetite does not 

occur within the unaltered primary minerals.

It has already been noted that quartz, in crystallized coatings and 

clusters of amethysts, is frequently seen along the joints and in the

1 See Peridotite of Elliott County, Kentucky, by J. S. Diller: Bull. II. S. Geol. Survey No. 38, 

1887, p. 12.
HALL.] PERIDOTITE AND SERPENTINE. 113

more-altered portions of the rock. Thin sections cut from the mostaltered hand specimens show that quartz is steadily replacing the 

more soluble constituents. It occurs in finely crystalline aggregations 

as veins and tonguelike processes extending into the very interiors of 

the serpentinous areas. Locally it has replaced all other constituents. 

It is free from inclusions, and the grains are mutually penetrating. 

Some very beautiful specimens of vein quartz were seen in which the 

pyramids of opposite crystals interlocked and the concentric structure 

of the pyramids was a marked feature of the mineral.

Oalcite is very seldom seen in the fresher hand specimens; in the more 

decomposed it occurs as incrustations of a resinous, yellowish-white 

mineral upon the surfaces of the joint planes, and microscopically within 

the rock mass as veins filling the spaces formed by the removal of the 

olivine material. It is probable that much of the carbonate is dolomite, 

since magnesia occurs so abundantly in both original constituents, 

enstatite and olivine, and the pulverized rock is more extensively dissolved in heated than in cold chlorhydric acid. It is in the partially 

altered rock that the carbonates are found; in those most completely 

changed quartz seems to be taking the place of all other minerals, both 

primary and derived.

Chlorite is present in small quantity in the more-altered portions. It 

presents the usual characters of this mineral developed as a secondary 

product in ultrabasic rocks, being fibrous in texture and sometimes 

arranged in rosettelike shapes of radiating needles.

Biotite is present in a few scattered folia, which were too minute for 

the complete demonstration of chemical or physical characters.

The chemical composition of the freshest specimens of saxonite 

(sp. 5215) was determined (I) by Mr. Alonzo D. Meeds, in the chemical 

laboratory of the University of Minnesota. A separate determination 

(ID of the silica and alumina was made by Mr. F. E. Smith.

Analyses of saxonite from the Minnesota River Valley.

Constituent.

Total................................................

I.

Per cent. 

43.65

5.14

6.81

4.86

12.91

0.43

1.12

7.46

98.84

II.

Per cent. 

43.52

6.81

I. Peridotite, var. saxonite. Sp. 5215. Locality, seer 4, T. 112 N., R. 34 W.

II. Same specimen and same locality as I. A separate determination of the silica 

and alumina.

Bull. 157 8
114 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

By comparison it is seen that the silica content of this saxonite is 

very near that of the olivine-enstatite rock (saxonite 1?) found on St. 

Pauls Bocks, 1 although in its total chemical characters it is close to 

the picrite of Schriesheim of Baden, analyzed by Fuchs.2 The specific 

gravity of the two is very near that of Fuchs's Schillerfels, being 2.82.

SERPENTINE.

The serpentine is an alteration product of the peridotite, and as the 

alteration of the latter has already been considered, little need be said 

of the serpentine. It is, of course, the chief resultant product of the 

decomposition of the original minerals of the rock. Chlorite, magnetite, finely crystalline quartz, and one or more carbonates are also 

readily distinguished (sp. 5216, 5733). (See PI. XXVII, B.) Serpentine varies considerably in color, the variation being due to the changing proportions of the accessory minerals. The dark-colored portions 

are the more compact and heavier. Calcite in unusually resinous 

habit is a frequent vein mineral in the serpentinous phases of this 

locality. The specific gravity of. one of the bowlders collected in sees. 

32 and 33, T. 112 K, E. 33 W., is 3.206.

1 British Petrography, by J. J. H. Teall, London, 1888, p. 103.

2 Schillerfels bei Schriesheim, by C. W. C. Fuchs: Neues Jahrbuch fiir Min., Geol., und Pet., Stuttgart, 1864, pp. 326-332.
OH APT BE VII.

DIKES.

OCCURRENCE.

From one end of the Minnesota Valley to the other dikes of a heavy, 

dark-colored basic rock diabase cut the gneisses and schists. These 

dikes, "the igneous filling of fissures," 1 are usually in vertical position. 

Exceptionally they come to the surface by following the lamina? of 

the rocks through which they break, as at Montevideo. Every thickness, from a fraction of an inch to 165 or 175 feet, is shown. As a rule 

these rock bodies are nearly straight in their course, and they hold 

their thickness with surprising uniformity. It is extremely rare that 

one "pinches out" or that it is interrupted by the cutting across it of 

veins. Intrusions and veins of a granitic type are not here considered.

These dikes, with two or three exceptions, are of finer texture than 

the gneisses and gabbro-schists in which they occur. Structurally they 

are considerably shattered, and it is only in the broadest of them that 

blocks of any size free from joints can be seen. A basaltic structure is 

frequently evident, with the horizontal columns at right angles to the 

walls of the inclosing rock. Weathering has not yet attacked them 

to any marked extent, either on the exposed tops or where the country 

rock has been broken away on the edges of the dikes.

No products of disintegration are to be seen among the rocks, either 

in the shape of concentrically weathered blocks on the surface of 

masses or in comminuted rock debris in depressions. This is due in 

part, perhaps, to the comparatively recent removal of the loosened 

blocks, through glacial agencies, as fast as they became loosened 

in their places in the natural walls. It must be granted that a fraction 

only of the dikes actually occurring in the valley can be seen in the 

comparatively small area of rock surfaces in sight above the glacial 

drift and river alluvium. Those that were.seen, and from nearly all of 

which specimens were taken, in the exploration of the valley are cited 

on the following page.

1 Geology of Wisconsin, by T. C. ChaTuberlin, Vol. I, 1883, p. 43.

115
116 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157. 

Dikes in Minnesota Eiver Valley from ivhich specimens were taken.

Location.

T.lllN.,E.32W.,sec.22 ..........................

T. 112 N., B. 33 W., NE.J SE. J, sec. 34....;. ........ 

T. 112 N.,E.34W.,SE.JNE.;J, sec. 9........ .......

T. 115 N., E. 39 W., SW.JSW.J, sec. 1.. ............

T. 115 N., E. 39 W., SWi N W. J, sec. 12 .............

T.116N..B.39W., SE.iNE.£,sec.29.............. 

T.116N., E. 39 W.,SW.JSW.i sec. 34. ............

T. 116 N., E. 39 W., NW-. i SE. $, sec. 34. ............

T. 117 N., E. 40 W..NW.JNE.J, sec. 34.. ...........

T. 117 N., E. 40 W., N W. J NE. J, sec. 34 .............

T. 117 N., E. 40 W., SW. i S W. J, sec. 21 .............

T.114N.,E.38W.,NW.jNE.J,sec.l3.............

T.112N.,E.34W.,NE.iNW.i,sec.24.............

T. 115 N., B. 39 W., NE. £ NE. £, sec. 24. ..............

T.115N.,E.39W.,NE.iSE.J,sec.4 ...............

T. 116 N., E. 39 W., NE. J NE. J, sec. 32 .............

T. 116 N., E. 39 W.,SE.JNE.£, sec. 33. .............

T. 116 N., E. 39 W., NW. i NW. J, sec. 33 ............

T. 116 N., E. 39 W., N W. J NW. i, sec. 33. ...........

T. 116 N., E. 39 W..SW.JSW.J, sec. 34.. ........... 

T. 115 N., B. 39 W., NE. J NE. J, sec. 4 ..............

T. 115 N., E. 39 W.,NE.iNW.J, sec. 4. .............

T.116N.,E.39W.,NW.4SE.isec.34.............

T. 116 N., E. 39 W., NW. £ SE. £, sec. 34 .............

T. 115 N., E. 39 WM NW.JNW.J, sec. 3...... .......

T.115N.,E.39W.,NW.4NW.J,sec.3.............

T. 115 N., E.39W.,NE.JSW.J, sec. 4 ..............

Strike.

NE. and SW . .

N.GO°E. ......

N.75°E. ......

N.65°E. ......

N.30°E. ...... 

N.60°E. ......

N.60°E. ......

N.65°E. ......

N.65°E. ......

NE.-SW......

N.30°E. ......

N.85°E. ......

N.55°E. ......

N.45°E. ......

N.40°E. ......

N.40°E. ......

N.82°E. ......

N.80°E. ......

N.65°E. ......

N.65°E. ......

N.80°E. ......

N.80°E. ......

N.80°E. ......

Specimen 

number.

5191, 5724

5185, 5199 

5215-5216

5238

5280

5286 

5294

5295

5299

5300

5303

5368

5403, 5763

5419

5423

5432

5440

5441

5442 

5443

5446

............

............

5769

............

5446

The average direction of these dikes is as nearly NE.-SW. as can 

well be estimated, thus indicating the Lake Superior Basin as the 

point to the northeast toward which they, as a group, seem to trend. 

Between the valley and Lake Superior lies the great central Minnesota 

area of hornblende-biotite-granites, with smaller areas of gneisses and 

schists. Throughout this area, also, there are numerous dikes which, 

in many of their mineral and chemical characters and in their contact 

phenomena, are similar to those under consideration. In general direction, too, there is the same NE.-SW. trend in central Minnesota as was 

noted throughout the length of the Minnesota Yalley.

The walls of the dikes present a sharp mineralogical contrast with 

the contact surfaces of the gneisses and gabbro-schists through which 

they break. Many slides were examined to discover whether any perceptible effect was produced by these intrusions of plastic basic matter 

on the containing rocks, but no very considerable effect was seen. 

The normal freshness of the gneisses and gabbro-schists is maintained 

throughout the entire zone of contact. There is a deposition of microcrystalline quartz along the immediate contact, and perhaps a slight 

kaolinization of the feldspars. Aside from the points noted, the 

gneisses and gabbro-schists were as greatly altered far away from any
HALL.] DIKES. 117

known dikes as they were upon the very zone of contact. The two 

rocks, the dike and the country rock, are so firmly united that thin 

sections extending across the contact zone could frequently be made, 

as PI. XXVI, A, will show (sp. 5294). Constant watch was kept, in 

passing these dikes, for some evidence of dynamic action suggested in 

their contact with other rocks and in their mineral structures. Nothing 

was seen that pointed to any profound disturbance subsequent to their 

intrusion. Nowhere was there seen any foliation of the rock itself or 

any development of foliated minerals, both of which, according to 

Williams.1 are produced by dynamic action. Their position is always 

vertical, save where, as below Montevideo, they break through the 

gneisses along the lines of least resistance and thus follow, for short 

distances, the foliation of the rock. Nowhere was the foliated gabbro 

of such a diabasic type as to be regarded as a formation of similar 

origin with the contents of the dikes. Everywhere that a contact 

between the two was seen it proved the structural deformation of the

former before the appearance of the latter.

From the foregoing considerations it will be seen that the lithological 

distinction between the gabbros and the diabases is as clear and 

undoubted as that between the gabbros and the gneisses.

Around Minnesota Falls and Granite Falls, as will be seen from the 

list on the preceding page, dikes are rather numerous. Many were 

seen which were not sampled and are not enumerated in the table. The 

general direction is NE.-SW. Their breadth varies from a few inches 

to 40 or 50 feet. They are all harder than the surrounding rocks, and 

resist weathering much better, and thus, in the low ground where erosion has been accomplished, these rock masses stand in ridges, either 

continuous or interrupted, which stretch for long distances across the 

valley and add in no small degree to the billowy aspect due to unequal 

erosion of the rock masses extending over several square miles in 

this neighborhood. All the dikes are much alike in color, in horizontal columnar structure, and in tendency to conchoidal fracture. There 

is a variation in texture connected with the breadth of the dike, and 

considerable difference in the degree of preservation of the original 

mineral constituents. In some of the dikes the basic constituents have 

been entirely obliterated, while in others alteration has obtained to only 

a slight degree.

AGE.

The nonlaminated character of all the dikes seen by the writer in 

Minnesota suggests that the crustal movements, local displacements, 

and foldings which were undoubtedly the chief agency in the formation 

of the schistose character of the other rocks of the region, took place 

long before the intrusion of the diabases. Another fact, already empha-

'Tho greenestone schist areas of the Menomiueo and Marqnette regions of Michigan, by G. H.. 

Williams: Bull. U. S. Geol. Survey No. 62,1890, p. 206.
118 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BOLL. 157.

sized, that no dikes have thus far been found among the horizontal 

sandstones and dolomites constituting the widely distributed Cambrian 

beds of the Northwestern States, places the period of their formation 

near the time of volcanic activity in the great Lake Superior syncline 

designated Keweenawan.

GENERAL PETROGRAPHICAL CHARACTERS.

MacroscopicaL These dikes vary in texture more than in structure or 

in chemical or mineralogical composition. This variation can often be 

seen as well in the central and marginal portions of the same dike as 

in two different dikes from extreme portions of the valley. As a rule, 

the narrow dikes are of firm, compact, and even texture, while the 

broader ones are correspondingly coarser and more porphyritic in 

habit. Everywhere the broken surface shows a firm, fresh rock, which 

rings under the hammer and separates with a conchoidal fracture into 

sharply angular fragments.

In color all the dikes around Minnesota Falls are dark, some of them 

almost black. Others take on a greenish hue, as, for instance, the 

great dike in the lower valley in sees. 23 and 24, T. 112 N., R. 34 W., 

which, according to the degree of alteration the rock has undergone, 

becomes in places of a decidedly pea-green color. Other dikes, as in 

western Granite Falls (sp. 5441), exhibit a color alteration toward 

brown. This is attained, however, by only a part of the constituents. 

In the field the darkness and evenness of color and the uniformity 

of texture enable one to distinguish at sight the dikes from the darkcolored laminated rocks through which they break.

The porphyritic habit of the broader dikes is very clearly shown to 

the unaided eye. The coarser-textured samples frequently exhibit 

cleaved surfaces of polysynthetically twinned feldspars from a half to 

three-quarters of an inch in length, and a general lathlike habit for

all the individuals of these minerals. The fresher and more firmly 

textured dikes show in a less degree this same porphyritic habit.

Now and then are to be seen nests of feldspathic material due to 

the segregation of the feldspars. A greenish tinge is always seen 

in the midst of and around these segregations, produced by the alteration to hornblende of the augitic constituent in their immediate 

neighborhood.

It is true that many phases of texture and mineral composition 

can be seen. There is a wide range in textural characters. In mineral composition a variation has been traced as wide as the difference between an almost fresh and perfect condition of one of the 

essential minerals, augite, on the one extreme (PI. XXV, J5), and the 

utter destruction of this mineral and the building up on its remains of 

clear, fresh individuals of hornblende on the other (PI. XXI, B). 

Yet there is no evidence at hand to show that these degrees of preservation are in any way due to a difference in the age of the dikes or
HALL.] PETROGRAPHICAL CHARACTERS OF THE DIKES. 119

to any local conditions surrounding their intrusion. 1 The difference in 

mineral composition is probably due to unequal weathering. Nowhere 

in the valley has quarrying enabled one to gather evidence of any more 

deeply seated alteration.

Microscopical. In thin section, as well as macroscopically, these 

dikes present the diabasic type. In places, as in the large dike south 

of Granite Falls (sp. 5423), a beautiful ophitic structure is seen. In 

some instances a porphyritic habit, with marked absence of the ophitic 

structure, is very pronounced. PI. XXI shows very clearly the two 

types of diabasic structure exhibited by the rocks in the valley. 

Nowhere are these rocks entirely free from alteration, although some 

hand specimens freshly broken appear exceedingly fresh. The feldspars are assuming a more or less distinct kaolinization. The changes 

of the pyroxene to hornblende and biotite are as pronounced as in the 

gabbro-schists.

In its ordinary occurrence the feldspar appears in slender, lathlike 

individuals, either simply or polysynthetically twinned. They were 

developed very early in the crystallization of the dikes. Great diversity in the position of the individuals exists. Occasionally clusters 

of the feldspar individuals may be noted, which give to the naked eye 

the appearance mentioned as porphyritic. These clusters show at 

times a beautiful twinning of the crystals, as well as series of parallelplaced individuals. The twinning most frequently follows the Carlsbad law. The extinction varies somewhat, thus showing variation in 

the relations of the axis, as well as modifications in the chemical composition of the mineral constituent. As a rule the smaller feldspar 

individuals are rather free from inclusions, and in a majority of 

the specimens gathered even the larger ones contained few foreign 

bodies. Where inclusions do occur there are, in certain localities, 

abundant crystals and granules of hornblende gathered in the centers 

or cores of the larger feldspar areas. This segregation shows that a 

molecular change of no little importance and significance is going on 

among the crystalline constituents of these rocks. In other localities 

the feldspars are loaded with minute granules of a brown color. It is 

difficult to determine just what these inclusions are; in places they are 

granular, translucent, and, when taken alone, almost without color; elsewhere, when the granules are larger, they resemble minute particles of 

the ferric oxide, hematite, scattered through the field. In places the 

feldspars are partly altered into kaolin or saussurite. Occasionally the 

former seems to be the mineral formed by the alteration process, and 

again the latter, thus differing somewhat from the rocks of like nature 

in the Menominee region of Wisconsin and Michigan.2 In a few cases 

microcline was observed to play a leading role. Aside from these

»Cf. M. E. Wadsworth: Proc. Boston Soc. Nat. Hist., Vol. XIX, p. 2; also Geol. Nat. Hist. Survey 

Minnesota, Bull. 2,1887, p. 6.

2 Consult further, The greenstone-schist area of the Menominee and Marquette regions of Michigan, 

by G. H. Williams: Bull. U. S. Geol. Survey No. 02,1890, p. 138.
120 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

instances, where seen in subordinate quantity, its condition and environment are such as to lead to the opinion that it is a secondary product, as has been shown to be the case in the gneisses with which these 

dikes are associated.

The pyroxenic constituent is largely ordinary augite. Seldom, however, is this mineral fresh and unaltered save in the central portions of 

the areas. In the few instances where an unaltered grain is to be seen, 

a green or a pink color prevails in place of the ordinary light brown so 

characteristic of the mineral elsewhere. In places the individuals are 

large and constitute a matrix for the porphyritic lath-shaped feldspars, 

thus forming admirable examples of the ophitic structure. Where 

the proportion of augite to feldspar is small, the areas are minute 

and serve simply to fill up the interstices between the feldspar individuals. In the alteration of this mineral the change begins at the margin 

or along the numerous cracks which in the vicissitudes of time have 

been produced in the larger individuals. As a rule, hornblende is the 

direct result of the change, but more rarely biotite is one of the alteration products. In most cases the change, whether to hornblende or 

biotite, or to both, has introduced other minerals of secondary origin, 

the first of which is quartz in minute and clustered granules. Paramorphic hornblende is not produced by the alteration ; neither is 

pseudomorphic hornblende a conspicuous feature of the change. To 

all appearances the resulting minerals encroach on the feldspars as 

well as on the pyroxenic constituents. Only rarely is the axial position 

of the original grains perpetuated in the secondary products a phenomenon found in some instances in the associated gabbro-schists.

In the various stages of alteration from the rocks containing the 

clear and nearly fresh augite to those bearing fresh-looking green 

hornblende only, the whole cycle of changes from diabase to diorite is 

clearly illustrated. Only a step is lacking to show the evolution of 

the hornblende-schists. That step is the change brought about by 

mashing.

Quartz is found in nearly every dike. Nowhere was it seen as a porphyritic separation, as is the case in a few localities in Stearns County, 

Minnesota, but always as a secondary product accompanying the metarnorphic changes in the rocks. The grains are very minute; they 

polarize brightly and are almost free from inclusions. In places they are 

clustered, suggesting miniature lenticular or gash veins of quartz. The 

vermicular quartz so abundant in all the gneissic rocks of this valley 

is usually absent in the dikes. In those samples where hornblende 

and biotite as alteration products occur in particularly large and pronounced individuals, quartz grains appear as a prominent portion of 

the intercrystalline material.

Accessory minerals, aside from those of a secondary origin, are comparatively obscure and unobtrusive. Both magnetite and pyrite are 

present, usually in rounded grains or clustered masses. Menaccanite
HALL.] DIKES IN MORTON DISTRICT. 121

(ilmenite) also appears at times, surrounded by an encircling baud of 

titanite. Occasionally numberless needles of apatite are to be seen.

The chemical constitution of the dikes has not been carefully studied. 

A determination of the silica in a specimen (5421) from Minnesota 

Falls showed 52.01 per cent. This is probably an average silica content of these basic rocks. It differs little from the silica content 

of similar dikes in central and northeastern Minnesota and northern 

Wisconsin and Michigan.

Passing to matters of detail and of a certain local interest, we note 

the following more special characters of the dikes already discussed 

in a general way.

MORTON DISTRICT.

In sec. 19, T. 112 K, E. 33 W., and sees. 23 and 24, T. 112 K, E. 34 W., 

lies a series of knobs stretching nearly east and west (see PI. V). These 

knobs constitute a part of by far the largest dike seen in central and 

southwestern Minnesota. The width is from 165 to 175 feet, and the 

height of the knobs varies from 10 to 30 feet above the flood plain. 

The country rock is the hornblende-biotite-gneiss so characteristic of 

the Fort Eidgely district. So far as could be noted at the time of the 

visit, neither the texture nor the mineral composition of the gneiss was 

modified by contact with the intrusive. The dike stands vertical. 

Several cases of marked slickensides were noted where a readjustment 

had been effected between the dike and the gneiss. Texturally the 

dike is of medium grain, finer at the borders than toward the central 

portion, but occasionally carrying porphyritic feldspars. Little tendency to amygdaloidal structure was noted, and this near the contact 

of the gneiss.

The color of the rock is greenish black. The fracture is sharply conchoidal, and the rock rings under the hammer. Weathering corrodes 

it but slowly, the most perceptible result being a yellowing of the surface through the change of the iron pyrites to a hydrous ferric oxide.

In mineral composition this rock is a partially altered diabase or 

diabase-porphyrite. The feldspar is plagioclastic, lying in lathlike 

forms. The porphyritic areas resolve into more or less altered segregations of the lathlike forms instead of exposing broad, poly synthetically 

twinned plagioclases, as in the gabbro-schists. The individuals are 

comparatively free from primary impurities, but kaolinization has crept ° 

in to a considerable extent in some cases. The segregations mentioned 

decompose into a fibrous mineral of a slightly green color, with parallel 

extinction, probably saussuritic in composition.

The augite is more or less altered, and everywhere lies between the 

lathlike feldspar individuals. Where fresh a light-brown color prevails. The angle c varies from 37° to 43°. Alteration has nearly everywhere set in, resulting, first, in green hornblende, partly massive and 

partly fibrous, and, second, in biotite. The process has not proceeded 

far enough to give distinct and individualized forms to the resultant
122 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

minerals, except near the surface, where the alteration has gone so far 

as almost to obliterate the augite individuals.

Magnetite and pyrite occur in grains of varying size and shape, not 

as a rule of great size nor of any regularity of form. Skeletonlike 

aggregations of pyrite are sometimes very noticeable. Epidote and 

apatite are both disclosed by an inspection of thin sections. The latter 

is present in some portions of the dike in numerous long, slender, 

needle-shaped crystals, frequently broken.

Quartz as a resultant of the alteration processes rather frequently 

appears in brightly polarizing areas. As a rule these areas are not 

large, and they lie in the most-altered portions of the rock. Surface 

weathering is especially effective in developing this mineral, which is 

deposited in the contact areas between the inclusions and the normal 

rock. It extends into both portions, filling interstices in the diabase 

as well as forming the outer zones of the inclusions themselves, and by 

degrees gives place in the feldspars to the kaolinic alteration product. 

In these situations the quartz throughout large masses of the rock has 

arranged itself in the same crystallographic directions, thus extinguishing simultaneously in large areas, even though, as appears in the 

section, the different portions of quartz be entirely separated by other 

minerals. PI. XXVI, J9, is an effort to show this intercalated quartz 

as it is exhibited in specimen 5400.

VICKSBURG DISTRICT.

In the Vicksburg district (see PI. VIII), in the area designated 

"rock barrens" by the laud surveyors, there are several dikes. One 

of them, in places 40 feet wide and having a direction JS". 30° E., lies 

exposed at several points in sees. 12 and 13, T. 114 N., E. 38 W. The 

color is a mixture of green and white. Weathering has changed the 

surface to a dull, dirty brown or reddish brown. This dike is of 

medium coarseness. The texture is distinctly diabasic. This and the 

narrower neighboring dikes are considerably shattered.

The leading mineral constituents are a plagioclase feldspar and one 

or two pyroxenes. At the surface the feldspar is so greatly altered 

that the extinction angle can be read in only a portion of the individuals. This extinction points to the auorthite end of the feldspar 

series. The usual twinning phenomena were noted. The product 

resulting from the alteration of the feldspar seems to be kaolin. The 

pyroxene is so completely altered to hornblende that the optical characters of this constituent could not be well determined; 39° marked 

the extinction of one small core, which placed that individual in the 

category of ordinary augite. Green hornblende is the leading product 

resulting from the breaking up of the pyroxene. It is in places developed into well-formed individuals, some of which are strongly twinned. 

In places the hornblende individuals, as well-developed plates, lie in a 

matrix of clear secondary quartz, and again a decidedly fibrous condi-
HALL.] . DIKES IN VICKSBURG DISTRICT. 123

tion prevails, and. quartz may or may not be present. A few small 

areas of biotite seem to spring from the alteration of the hornblende. 

Quartz is scattered throughout the rock as interstitial matter, generally 

taking the place of the feldspars. Occasional small areas are pegmatitic. Magnetite is present in small, scattered grains and crystals. 

Apatite needles are rather numerous, although small.

GRANITE FALLS DISTRICT.

In the Granite Falls district (PI. X) there are a large number of 

dikes. Among them are those which are ophitic and very fresh, and 

others which have lost all their pyroxenic constituents through alteration into hornblende. These last are simply diorite dikes. N o geographical classification is possible, since the degree of alteration seems to 

depend in large measure on the width of the dike and the conditions of 

rock association under which it occurs.

The two dikes which represent most perfectly the ophitic structure 

lie the one on the north side of Granite Falls (sp. 5286), 1,100 paces N., 

100 paces W., sec. 29, T. 116 N., B. 39 W., and the other to the south of 

Granite Falls (sp. 5423), 1,000 paces N., paces W., sec. 4, T. 11.5 N., E. 

39 W. The former has a width of 40 feet and a course N. 30° B. The 

latter is at least 25 feet wide and runs F. 45° E. While the texture of 

these two dikes is medium it is coarser than that of others in this district. 

The color is dark green to black. On the borders of the dikes there are 

traces of an amygdaloidal structure in the occurrence of vesicles. Barely 

porphyritic crystals of plagioclastic feldspar are found. The augite 

occurs in large areas from 2 to 5 mm. across (see PI. XXV, 73, from sp. 

5286, si. 2241). Their borders are somewhat roughened by alteration 

and the intrusion of secondary horn blende. Freshly fractured surfaces 

of the rock show numerous cleavage planes of the augite thickly interwoven with the cleaved lathlike crystals of the feldspar. The.spaces 

between these augite surfaces are filled with a finely granular matrix, 

which lacks the bright reflection just mentioned and is of a green color. 

These reflecting surfaces seem to be identical with those described by 

B.Pumpelly in the melaphyr or " greenstone" extending alongKeweenaw 

Point and stretching southwest ward. 1 The structure has been seen by 

the writer in many rocks of the Northwest, including diabases, gabbros, 

and even granites. Aside from the lathlike labradorite crystals which 

penetrate them in every direction, the augite crystal areas are singularly free from inclusions. There are fractures which to a greater or 

less extent serve as alteration zones and facilitate the progress of 

mineral changes.

The feldspar crystals, which are labradorite or very near it in 

optical characters, are filled with brown, dustlike inclusions. Their 

distribution through the feldspar crystal is not uniform, but they are

1 Metasomatic development of the copper-bearing rocks of Lake Superior,by B.Pumpelly. Proc. 

Am. Acad. Arts Sci., Vol. XIII, 1878, pp. 260-261; also Geol. of Wisconsin, Vol. Ill, 1880, p. 33.
124 GNEISSES, GABBRO-SCHISTS, ETC., OP SW. MINNESOTA. [BULL. 157.

clustered in certain portions, where they impart a decided brown color 

to the mineral. Another inclusion which seems to be quite common in 

the feldspars is finely granular augite, which was apparently caught 

up in the crystallization of the rock. The boundaries of the feldspars 

are sharp and smooth planes. The shreds and prismatic blocks of 

augite everywhere, seem to penetrate them. This is more particularly 

true of the ends and diagonal edges of the feldspars than of their 

prismatic surfaces.

Olivine is present, but only a few central cores remain of what was 

once no inconsiderable constituent. The rough surface, bright polarization colors, aud optical extinction show the nature of these cores. 

They are bounded by a bright-green hornblendic alteration product 

which is radially developed around the cores. Associated with the 

hornblendic product is a black, opaque, granular material, probably 

an oxide of iron, which in places is developed in festivated forms, due 

to the contact of the granules. Magnetite in large crystals and granular areas is a comparatively frequent mineral. Biotite was seen in all 

the slides examined, doubtless occurring as a secondary product.

Entering the neighborhood of Minnesota Falls and Granite Falls 

from the south, one meets the first dike to the south of Minnesota Falls, 

in sec. 12, T. 115 N., K. 39 W. This dike (sp. 5280), about 20 feet in 

width, breaks vertically through a gabbro-schist. The feldspars were 

lath-shaped individuals; the original boundaries of many are now 

obliterated, and the polysynthetic twinning can no longer be seen. 

Nowhere is augite visible, but its place has been taken by hornblende 

and biotite as secondary products. The hornblende occurs in fine 

granules, lying sometimes in parallel position, but usually scattered 

in all directions. The proofs of the secondary nature of the hornblende do not lie in these slides only; they can be seen in the rocks 

of the locality taken as a whole. At this place the gabbro-schist cut 

contains much fresh pyroxene, and yet this mineral has utterly disappeared in an intersecting dike. Magnetite, epidote, and pyrite are 

present. The last two, and doubtless much of the first named, are 

secondary minerals.

Just across the river, in sees. 2 and 11, and almost due north from 

the dike just described, extend other dikes, somewhat broader and 

coarser textured (sp. 5419). The direction is N. 44° E. Mineralogically 

they are almost precisely like the dike south of the river (sp. 5280), 

differing chiefly in their coarser texture, in the larger proportion of 

secondary biotite, and in the skeletonlike segregations of pyrite. The 

alteration of the pyroxene into hornblende has proceeded far enough 

to change to some extent the sharp boundaries between the feldspar 

individuals and their basic neighbors.

In sec. 34, T. 116 N., E. 39 W., East Granite Falls, around the railway 

station and the grain elevators, and stretching south west ward to the
HALL.] DIKES IN GRANITE FALLS DISTRICT. 125

river, are several occurrences of dikes. One of them appears to be of 

considerable width, at least 30 or 40 feet. (Sp. 5294.) Its direction 

is W. 30° S. The central part is rather coarse textured, firm, and 

fresh, while along the contact with the gabbroid rock it is rather finely 

crystalline. Another dike (sp. 5295), exposed in several places near 

the Pillsbury grain elevator, is of nearly equal extent. An examination of the rocks constituting these dikes shows the feldspars to be 

somewhat altered. They carry some small grains of hornblende, and 

are literally colored brown by the numberless granules, which could 

not be resolved with an immersion lens. Their color and the general 

aspect of the rock lead to the view that they are the same substance 

which has been mentioned as occurring in the fresh, ophitic, diabase 

dikes. The augite has entirely disappeared, and its place is now 

occupied by hornblende lying in fine and irregularly distributed 

grains. In places these grains of hornblende include numberless 

minute black particles. Where biotite is seen, it lies near the corroded 

particles of iron pyrites.

In the south side of this same section (34) lie other dikes having the 

same general macroscopic characters as those named. A slide made of 

the contact material between one of these dikes and the country gabbro-schist shows a very sharp contact (sp. 5294). The gabbro-schist 

and the dike show an entire alteration of the pyroxenic constituents 

into finely granular hornblende and into an abundance of fresh-looking 

plagioclastic feldspars. The dike material is finely crystalline and the 

plagioclase is in part porphyritic. A very fine texture is noticeable 

along this contact, which does not appear in the middle of the dike, 

and a belt of finely granular quartz forms an almost complete zone of 

separation between the dike and the country rock (see PI. XXVI, A).

Along the river's edge and near the highway which leads out of 

Granite Falls toward the northwest are several dikes, some of considerable width. They are seen in that portion of sec. 28 lying south of 

the Minnesota River, and along the northern border of sec. 33. In the 

last-named section three of four dikes are nearly parallel as they stretch 

across the brow of a hill. Their general direction seems to be S. 40° W. 

Several minor dikes show considerable variation from this course. In 

mineral composition they are all essentially the same.

In Granite Falls a narrow dike 5 inches wide was sampled (sp. 5432), 

which macroscopically shows a distinct porphyritic structure. A microscopic examination shows that this appearance is due to nests of hornblende granules within a matrix of plagioclase, hornblende, and biotite. 

In their distribution these granules mark the form of some preexisting 

crystals which might have been olivine, thus presenting a rock like 

that at Sauk Eapids, where olivine crystals are still clear and distinct 

in a porphyritic rock occurring as a narrow dike in the hornblendebiotite-granite of the Sauk Eapids granite quarry.

Between the fresh dikes and the much-altered ones above described
126 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

are many intermediate stages, in which the feldspars are tinged with 

alteration and the augites show hornblende around their borders and 

along cracks within the grains. The phases of alteration, while varying slightly in some details, have their broader characters in common, 

and represent every stage from the fresh augite to the clear hornblende.

MONTEVIDEO DISTRICT.

Around Montevideo (see PI. XI) there are several dikes of some 

lithologic interest. They lie in the bottoms south of the railroad track, 

in sees. 20,21, and 34, T. 117 N., E. 40 W. They are of diverse widths 

and vary in degree of preservation. With a single exception, so far as 

observed, they stand in vertical positions in the rocks. This exception is 

in a railway cut in sec. 20, where a dike a few inches in width follows 

the bedding of the red acidic gneiss of the district.

Where these dikes are narrow, as in sec. 34 (sp. 5299, 5300), they 

show the total disappearance of augite and the plentiful occurrence of 

needles of apatite in every part of the section. The alteration products 

of the augite are hornblende and biotite, nearly in the proportion of 2 

to 1 in the slides examined. While many localities show the direct 

formation of hornblende alone from the augite, these rocks exhibit a 

remarkably even distribution of the biotite, not only around the hornblende areas but within them as well.

In the broader dikes (sp. 5303) the individuals of all the mineral 

constituents are much larger. The feldspars, which frequently lie in 

nests of twinned forms, are loaded with minute granules of hornblende 

scattered as an impurity through their central portions. These hornblende granules all seem to lie in the same crystallographic direction 

in any given feldspar. A characteristic feature of these feldspars is 

the brown color which appears in almost every section. In places 

for instance, where the hornblende granules appear in their cores 

the brown color is exceedingly slight, and as a rule the feldspars are 

more mashed around the borders of the areas, although this condition can not be clearly defined. Twinning in very striking forms is 

shown in the larger feldspars, combinations after the pericliue and 

albite law being frequent. A zonal structure is conspicuous in many 

of them. It appears to be of two kinds: first, a structure due to interruptions in the growth of the crystal grains; and, second, where 

additions of a secondary nature were made to the original feldspar 

individuals after the consolidation of the rock and during its alteration. 

Occasionally the feldspar areas show the presence of kaolinic material. 

The augites have begun to change to hornblende, as their rims of finely 

crystalline, green material clearly show. In places the alteration is 

complete. (See Pl.XXY, A.) Olivine is not present, and no coresof that 

mineral were noticed, although many areas of secondary material occur, 

particularly in that part of the dike near the contact. These areas are
HALL.] DIKES IN MONTEVIDEO DISTRICT. . 127

small, and in all cases are composed of a radiating viriditic mineral 

having the general characters of hornblende and in every respect identical with the riuis around the olivine cores of the two ophitic dikes of 

Granite Falls. Little biotite is seen. Pyrite is present, but not in 

abundance.

EXCEPTIONAL DIKES.

Besides the diabases described in the foregoing paragraphs, there 

were noted two or three areas of somewhat different rocks. Although 

these exposures are of the diabasic type, they are so different from the 

normal dikes of the valley that they will be considered separately.

On the south side of the river, in sees. 14 and 15, T. 112 N., E. 34 W., 

there are large masses of this rather coarse-grained rock. A gneissic 

structure is pronounced. This structure is developed, not through any 

noticeable parallelism in the position of the constituent minerals, but 

by a distinct segregation of these minerals into bands or Iamina3 extending considerable distances through the rock.

The field relations of this belt of rocks are very obscure. No contact with other rocks on either side could be seen. At the surface the 

rock is considerably decomposed, and the reddish hue of much of the 

feldspars is imparted to it. The weathering produces a rough, uneven 

surface. While some contortion is noticeable, the general strike of 

these masses is NE.-SW., with a dip frequently as high as 80° S. 

Scattered through this rock are inclusions of material more or less 

feldspathic than the average. To the naked eye the rocks seem to be 

unusually free from accessory minerals. Here and there small nests of 

pyrite can be seen, and occasionally a porous condition near the surface points to the dissolving and carrying away of some mineral more 

readily acted upon than most of the constituents.

While the texture of this rock is medium, there is a peculiar and 

appareutl-y secondary structure running through it, seen in every expoure. This consists chiefly in the presence of broad, thin leaves of 

the hornblende. This mineral in its formation seems to have been 

arranged with many of its individuals having their axes, c, in a common direction within considerable areas, 1 and 2 inches across, thus 

giving the impression of large individuals. This impression is intensified by the reflection of the light at the same angle from a large number 

of small grains breaking and cleaving in the same plane. Their composite character can be best seen where the feldspars predominate, 

since color contrasts serve to bring out more sharply the outlines of 

the areas. The fresh fracture, when studied with low power, has a 

decidedly granular appearance, with the grains all presenting their 

cleavage planes at precisely the same angle. Between these grains can 

be detected the feldspars, the epidote grains, and, in the more weathered portions, biotite folia. While the length and breadth of these 

segregations are considerable, they are very thin. Looked at on edge,
128 GNEISSES, GABBRO-SCHISTS, ETC., OF SW. MINNESOTA. [BULL. 157.

they are mere dark lines through the field of light-colored constituents. 

This condition of itself would give a lamination to the rock, did all these 

tabular segregations lie in one general direction, which is not the case.

While these are not porphyritic individuals of hornblende, they seem 

to have a relation to the arrangement of the augite peculiar to certain 

eruptive rocks. This arrangement suggested to Pumpelly, 1 as he 

studied the melaphyre of Lake Superior, the significant term "lustermottling." This structure has since been observed in contemporary 

diabases and gabbros, and in gneissic rocks of still greater age. 2

A slide made from the lighter-colored bauds would show probably 

75 per cent of feldspar and epidote, the chief alteration product of the 

feldspar, and one from the darker band an equal proportion of hornblende. A greenish color pervades many of the feldspar individuals, 

especially those of the laminae poor in hornblende. This is due, doubtless, to the epidote present. In places, too, the hornblende itself has 

this pale-green tone. Where there are variations toward a veinlike 

character, the feldspars are red in color and the hornblende exhibits 

well-formed crystals.

The only exposure of these exceptional dikes noted on the north side 

of the river is in the bottoms in sec. 34, T. 112 N., R.33 W. The area is 

small, lying beside a small circular lake and not more .than 15 feet 

above the surface of the water. While the rock is generally massive, 

it shows in places a distinct lamination, due to the segregation of the 

darker-colored constituents into bands of varying thickness and varying distances apart. In other places segregations of spherical shape 

occur; these are of a lighter color and somewhat harder than the body 

of the rock, so that they resist weathering better and thus stand out 

in- knobs. While the mass is somewhat shattered, it is much less so 

than are the normal dikes of the valley.

The microscopic characters of the exposures are those of very much 

altered rocks. The feldspars are so far altered to kaolin aud epidote 

that near the edges of three or four individuals only could the extinction 

angle of the labradorite group be made out. The diabasic structure is 

seen, although in some sections a greater breadth is noticed than in 

the feldspar individuals of the ordinary dikes of the valley. In many 

cases crystal outlines occur, or even perfect porphyritic crystals showing the prismatic, pyramidal, and pinacoidal faces. In the gneisses, 

as has been stated, there are frequently secondary feldspars, doubtless 

resulting from the degradation of the primary ones. Here, however, 

the secondary minerals are in no observed instances feldspars. As a 

rule the borders of the feldspar crystals are in a much better state of

1 Metasomatic development of the copper-bearing rocks of Lake Superior, by R. Pumpelly: Proc. Am. 

A cad. Arts Sci., Vol. X1LL, 1878, p. 260. Lithology of the Keweenawan system, by E. Pumpelly: 

Geol. Wisconsin, Vol. Ill, 1880, p. 33. Preliminary description of the peridotytes, gabbros, diabases, 

and andesytes of Minnesota, by M. E. Wadsworth: Bull. Geol. Nat. Hist. Survey Minnesota, No. 2, 

1887, p. 107.

2 The copper-bearing rocka of Lake Superior, by E. D. Irving: Mon. U. S. Geol. Survey, Vol. V, 

1883, p. 73. Ibid., p. 42. Notes of a geologic excursion into central Wisconsin, by 0. W. Hall: Bull. 

Minn. Acad. Nat. Sci. Vol. Ill, No. 2, p. 25.
HALL.] EXCEPTIONAL DIKES. 129

preservation than are their centers, a fact of very wide application 

among the crystalline rocks of the Northwest.

Quartz is reasonably abundant. Everywhere it is a secondary mineral, free from inclusions and very limpid. It is in pegmatitic forms, 

occupying interstices between the feldspars, or in granular condition 

it forms the lenticular fillings seen between the biotite folia. In the 

first-named position it exhibits sharply marked bpundaries, and all the 

different areas over a considerable surface lie in the same crystallographic position and extinguish together. This is illustrated in PI. 

XXVI, J5, from a thin section cut from specimen 5400. The lenticular fillings are almost as frequent as the areas of biotite. They are 

wedged in between the folia and oftentimes stretch them to a considerable extent. In places other minerals serve as such filling, particularly epidote and calcite, but these minerals in this situation are 

rare as compared with quartz. While these fillings are granular, the 

grains very generally extinguish parallel with the biotite.

Of the basic constituents in the slides examined, augite remains in 

one or two small cores only, surrounded by secondary hornblende. Many 

areas of this last-named mineral containing no augite show by the 

granular and impure condition of their central portions that they are 

of secondary origin, and the evidence of scores of specimens taken 

throughout the valley points to preexisting augite or diallage individuals from which the hornblende was derived. In addition to the larger 

areas mentioned, many small individuals of hornblende lie scattered 

about. These may arise from, the transfer of material during the alteration of the rock.

Besides the hornblende mentioned as an alteration product, chlorite 

..is rather abundant. It occurs in clustered areas of radial, fibrous 

spherules, which lie in proximity to the hornblende areas and the 

epidote masses alike. Other areas are found in the midst of the 

decomposed feldspar individuals. Both in its occurrence here and in 

its arrangement in the rock, the chlorite seems to unite this rock closely 

to the gneissic rocks figured from Tracy's place, only 3 or 4 miles away 

on an air line. (PI. XVII, A.)

Epidote is an alteration product of considerable importance in these 

rocks. It is probably derived in large part from the decomposition of the 

feldspars. These, as has just been stated, belong chiefly to the labradorite type, thus containing sufficient lime for the formation of the 

epidote and the small quantity of calcite present. While much of the 

epidote lies within and around the changed feldspar areas in a way 

to suggest its origin from that mineral, as maintained by Chester in 

his description of the Delaware gabbros, 1 and by Williams in his 

work on the gabbros in the neighborhood of Baltimore,2 that portion

1 The gabbros and associated rocks in Delaware, by F. D. Chester: Bull. TJ. S. Geol. Survey No. 59, 

1890, p. 35.

2 The gabbros and associated hornblende rocks occurring in the neighborhood of Baltimore, Md., 

by G. H. Williams: Bull. U. S. Geol. Survey No. 28,1886, pp. 31-32.

Bull. 157 9
130 GNEISSES, GABBRO-SCHISTS, ETC.,.OF SW. MINNESOTA. [BULL. 157.

of the epidote which occurs in largest individuals and in strongest 

color is more intimately associated with the hornblende in situation 

and possibly in derivation.

No analysis of this rock as a whole, or of any of its constituent minerals, was made, so that no considerations based on its composition can be 

stated. However, taking the analyses of many samples from the tables 

in Dana's System of Mineralogy, 1 we glean the following facts: The 

average per cent of calcium oxide, OaO, in the augite of metamorphic 

and eruptive rocks, computed from the twenty-three analyses given, is 

nearly 22 per cent. The average of seven analyses of hornblende gave 

slightly over 13 per cent of calcium oxide, which leaves an excess to 

supply the epidote, after furnishing all necessary for the hornblende in 

the change of augite into hornblende, which is assumed. Nowhere are 

the individuals of epidote of large size, but everywhere they are granular. Twinning striae were not seen. In no other rock of the valley 

was so large a proportion of the mineral epidote observed, and in no 

other examined was the feldspathic constituent so completely obliterated as in this, save, perhaps, in those gneisses of Redwood Falls 

and Birch Cooley where entire rock masses have been converted into 

an impure kaolin.

Calcite also occurs in this rock. Its characters are those usually 

seen in this mineral in the altered crystalline rocks. While sometimes 

it may be a filling for primary or secondary cavities 2 in normal eruptive rocks of even very ancient age, as in the Keweenawan and associated rocks of the Lake Superior region, in this case it plainly fills 

interstices between the existing minerals which have been formed by 

the alteration of the original plagioclases and pyroxenes. It most 

frequently occurs around the segregated hornblende, chlorite, and 

epidote areas, a circumstance which strongly suggests that the larger 

part of it was formed by the alteration of the augite. Such a change 

Puuipelly noted among the melaphyres of Lake Superior as long ago 

as 1878.3 Occasionally twinning striae appear, but they are comparatively rare. The mineral, as a rule, is very finely granular.

Calcite, as a decomposition product in these rocks, occurs also in the 

midst of the feldspar areas. Its characters there are precisely those 

named in interstitial situations. As a result of alteration this mineral 

has been seen almost nowhere else in the valley except in rocks of the 

same general character south of the river. Its rarity confirms the 

assertion of Eosenbusch, who regards it as an alteration product from 

lime silicates.4 A few small crystals of apatite are present.

The light-colored spherical nodules mentioned as included in places 

in this rock appear to carry a much smaller proportion of hornblende

>A System of Mineralogy, by J.D. Dana, 5th ed., 1868, pp. 218, 238.

^Preliminary description of [the peridotytes, gabbros, etc., of Minnesota, by M. E. Wadsworth: 

Bull. Geol. Nat. Hist. Survey Minnesota, No. 2,1887, p. 82.

3 Metasomatic development of the copper-bearing rocks of Lake Superior, by E. Pumpelly: Proc. 

Am. Acad. Arts Sci., Vol. XIII, p. 268.

4 Mikroskopische Physiographic der Minoralien und Gesteine, by H. Eosenbusch, 3d ed., 1892, Vol. 

I, p. 404.
HALL.] EXCEPTIONAL DIKES. 131

and biotite than the main mass of the rock. The feldspar in these 

nodules is considerably fresher than elsewhere, frequently showing the 

twin striation with sufficient clearness for measurement, and the epidote 

is much more thickly scattered throughout the mass. Calcite is very 

infrequent in these nodules. Magnetite and pyrite are scarcely seen.

In the village of Cotton wood, T. 113 N., K, 40 W., the boring of an 

artesian well was attempted in 1895. At a depth of 280 feet granite 

rocks were reached. These were penetrated 190 feet. At some point 

within this distance a band of dark-green, finely textured rock was 

bored through. One piece, 3£ inches long, was brought up by the slush 

bucket (sp. 5768). Its surface has a decidedly greasy feel,but a rough 

fracture is disclosed by breaking. Its specific gravity is 2.85.

Microscopically the rock is an altered diabase. The two principal 

constituents, labradorite and augite, still remain in fragments sufficiently large and fresh for identification. The augite has a light-brown 

color, extinguishes at an uuusaally low angle, and is largely altered

into chlorite, hornblende, and associated products. The feldspars are 

labradorites, with a prevailing low extinction, indicating the presence 

of soda, an indication borne out by the presence of a suassuritic 

mineral among the products of alteration. Magnetite is plentiful, 

with its clearly defined crystal contours.

PLATE XVI.

133
PLATE XVI.

Fig. A. Granite-gneiss, a block 3| by 4 inches, from the Ortonville quarries.

The light pink represents quartz with the color of underlying feldspar seen through the transparent grains; the dark pink the second feldspars ; the black the few scattering folia of biotite with, rarely, grains of 

secondary hornblende.

Fig. B. Contorted hornblende-biotite-gneiss, a block 3| by 4 inches, from the quarries at Morton.

The white represents the quartz and a portion of the feldspars; the pink 

that portion of the feldspars slightly stained, and the black biotite and 

hornblende in segregations and bands, with smaller grains in places 

more evenly distributed among the quartz and feldspars.

See p. 73. 

134
U. S.GEOLOGICAL SURVEY BULLETIN! No 157 PL.XVI

(A) GRANITE - GNEISS

( B) CONTORTED HORNBLENDE -BIOTITE - GNEISS
PLATE XVII.

135
PLATE XVII.

Fig. A. Chloritic gneiss. In the upper part of the figure the radial arrangement of 

the chlorite is seen.

Specimen 5206, slide 1780, X 55, in ordinary light.

The lower part exhibits slender vermiform crystallizations in a field of 

quartz. (1) Chlorite, (2) quartz, (3) sphene. 

Specimen 5729, slide 9478, X 250. 

See p. 59. 

Fig. B. A fresh unaltered hypersthene-gabbro.

(1) Anorthite-labradorite feldspar, very fresh; (2) hypersthene, scarcely 

altered; (3) small, scattering folia of biotite. 

Specimen 5335, slide 2263, X 100. 

See pp. 91,95. 

136
PLXVII.
U. S.GEOLOGICAL SURVEY BULLETIN No. 157 PL.XVII.

'£-'/

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<JV-

(B)

LIUS BlEN a CO. M.V.

( A i CHLOHJTIC GNEISS ( B) HYPERSTHENE - GABBRO
PLATE XVIII.

1SV
PLATE XVIII.

Fig. A. Hornblende-biotite-granite-gueiss. Muscovite, apparently formed by the 

alteration of an albite inclusion in microcline.

(1) A microcline individual much larger than the entire field; (2) an 

albite inclusion entirely surrounded by the microcline individual; (3). 

muscovite within the albite and apparently derived from its decomposition; all portions extinguish together; (4) other areas of muscovite; 

(5) biotite.

Specimen 5338, slide 2265, X 53, in polarized light.

See p. 54.

Fig. li. Hornbleude-biotite-gneiss. Vermicular quartz, within an area of albitic 

feldspar.

(1) Plagioclastic feldspar of low extinction; (2) canals filled with 

quartz in vermicular arrangement within the feldspar.

Specimen 5396, slide 2288, X 250, in polarized light.

See pp. 51,67. 

138
PL.XVIII
U.S.GEOLOGICAL SURVEY BULLETIN No. 157 PL.XVIII.

(B)

JULIUS BIEN ft CO. N

(A )HORNBLENDE -BIOTITE GNEISS { B)'VERMICULAR QUARTZ
PLATE XIX.

139
PLATE XIX.

Fig. A. Hypersthene-free gabbro-schist. Area of diallage in which alteration to 

hornblende is begun. Some areas of hornblende can be seen with a low 

power, but many more can be recognized under higher magnification, 

while a green color shows that incipient alteration pervades the entire 

field of diallage. 

Specimen 5185, slide 1765, X 100. 

See pp. 80,99.

Fig. B. Hornblende-biotite granite-gneiss. Contact zone between microcline and 

' oligoclase where the boundary is not distinct. A finely granular mineral 

not easily identified lies between the two recognizable feldspars. That 

either is secondary to the other does not appear. 

Specimen 5320, slide 2457, X 75. 

See p. 52. 

140
U. S. GEOLOGICAL SURVEY BULLETIN NO. 157 PL. XIX

A, HORNBLENDE IN FINE GRANULES WITHIN A LARGE PLATE OF DIALLAGE. 

B, CONTACT ZONE BETWEEN MICROCLINE AND OLIGOCLASE.
PLATE XX.

141
PLATE XX.

Fig. A. Porphyritic hornblende-biotite-gneiss. Radially arranged individuals of 

biotite within a quartz individual partially filling a minute fissure. 

Specimen 5262, slide 2442, x 350. 

See p. 53.

Fig. B. Hornblende-biotite-gneiss. Microcline occurring in small anhedral individuals .within a field of orthoclase. All the microcline areas extinguish 

top ither, but in this case not in the same direction as the orthoclase. 

.pecimgn 5315, slide 2254, x 150. 

See pp. 52,53.

Fig. C. Hypersthene-free gabbro-schist. Individual of biotite carrying lenses of 

quartz wedged in between the biotite folia. The quartz is secondary and 

undoubtedly the biotite is also. 

Specimen 5186, slide 4432, x 120. 

See pp. 54,99. 

142
U. s. GEOLOGICAL SURVEY BULLETIN NO. 157 PL XX

A. BIOTITE IN RADIAL CLUSTERS WITHIN QUARTZ. 

7?, MICROCLINE WITHIN OLIGOCLASE. 

C, LENSES OF QUARTZ IN BIOTITE.
PLATE XXI.

143
PLATE XXI.

Fig. A. Hyperstheue-gabbro. There has resulted a partial alteration of the pyroxenic constituents into brown hornblende.

(1) Hypersthene, tolerably fresh in the centers of the fragments into 

which it has been fractured; (2) hornblende, apparently resulting from 

the decomposition of the hypersthene: it appears also as a filling in all the 

fractures within the hypersthene individuals; (3) labradorite; (4) augite.

Specimen 5278, slide 2257, X 48.

See pp. 86,91.

Fig. B. Gabbro-diorite. The pyroxenic constituents are completely altered to green 

hornblende.

(1) Hornblende, derived undoubtedly from the decomposition of pyroxenic constituents; (2) feldspar of the anorthite-labradorite type; (3) small 

grains of magnetite.

Specimen 5418, slide 4442, slightly magnified.

See pp. 91,107. 

144
PL XXI.
U. S. GEOLOGICAL SURVEY BULLETIN No. 157 PL.XXI.

/- 'T^'S^Vu- ^ V?

^/>H. v ^SMH , ? /^- 1 -,'_-*rtSV. V>^,-i

f^s^0m^^^^&

(A)HYPERSTHENE GABBRO (B) GABBRO DLORITE
PLATE XXII.

145 

Bull. 157 10
PLATE XXII.

Fig. A. A gabbro in which the monoclinic pyroxenes have been almost entirely 

altered to a granular hornblende-biotite and quart/.

(1) Anorthite-labradorite feldspar; (2) remains of pyroxenic cores; (3) 

hornblende of the uralitic type; (4) biotite arranged in more or less radiating folia; (5) grains of secondary quartz.

Specimen 5279, slide 2444, x 100.

See p. 105.

Fig. B. Garnetiferous gabbro in typical development at the "gold mine" just south 

of Granite Falls.

This gabbro is in Educational Series of Rock Specimens, United States 

Geological Survey, No. 109.

(1) Auorthite-labradorite feldspar; (2) secondary quartz; (3) diallage; 

(4) garnets; (5) magnetite.

Specimen 5421, slide 4444, X 25.

See pp. 99,102. 

146
PL.XXII
U. S.GEOLOGICAL SURVEY BULLETIN No. 157 PL.XXII.

(B)

JULIUS BIEN & CO. M

(A)ALTERED QABBRO (B) GARNETIFEROTJS GABBRO
PLATE XXIII.

147
PLATE XXIII.

Fig A. Altered hypersthene-bearing gabbro-schist. A cluster of minute veins, 

chiefly of hornblende, occupying intergrauular spaces between the individuals of plagioclastic feldspar, and even filling the spaces of fractures 

" pushed open. Vein contents very finely granular. 

Specimen 5399, slide 2289, X 150. 

See p. 85. .

Fig. B. Hypersthene-free gabbro-schist. Diallage developed locally in large individuals containing within them the other mineral constituents of the 

rock. An example of poikilitic texture. 

Specimen 5199, slide 1775, X 75. 

See p. 98.

148
IJ. S. GEOLOGICAL SURVEY BULLETIN NO. 157 PL_ XX

A, ANASTOMOSING OF VEINS OF HORNBLENDIC MATERIAL BETWEEN FELDSPARS. 

£, DIALLAGE AS A MATRIX CONTAINING THE OTHER MINERAL CONSTITUENTS 

AS INCLUSIONS.
PLATE XXIV.

149
PLATE XXIV.

Fig. A. Porphyritic gabbro-schist. Hand specimen. The usual constituents, 

labradorite-anorthite, hornblende, angite, diallage, hypersthene, with 

accessories, lie in the groundmass, within which are developed the large 

and well-defined anorthite crystals. 

Specimen 5438, natural size. 

See ppi 87-89.

Fig. B. Porphyritic diorite. Fresh transparent labradorite grains, in which are 

seen broken apatite crystals so displaced that the fragments are shoved 

until they overlap one another.

Specimen 5441, slide 9470, X 250. 

150
U. S. GEOLOGICAL SURVEY BUL ETIN NO. 157 PL. XXIV

A, HAND SPECIMEN OF PORPHYRITIC GAB3RO, WEST PART OF GRANITE FALLS. 

K, BROKEN APATITE NEEDLES IN LABRADORITE FELDSPARS.
PLATE XXV.

151
PLATE XXV.

Fig. A. A typical diabase of the Minnesota River Valley of the normal granitic 

structure.

(1) Augite, with the borders altered into finely granular hornblende; 

(2) hornblende, in the upper left-hand corner the augite has completely 

altered into such hornblende; (3) broad and polysynthetically twinned 

labradorite-anorthite feldspars; (4) magnetite.

Specimen 5303, slide 4440, X 53.

See p. 126. 

Fig. J). Typical ophitic diabase.

(1) Augite extinguishing simultaneously throughout the entire field; 

(2) labradorite-anorthite of the lathlike type, often included within the 

matrix of the augite; (3) magnetite.

Specimen 5286, slide 2241, X 70.

See p. 123. 

152
PL. XXV
U. S. GEOLOGICAL SURVEY BULLETIN No. 157 PL.XXV.

JS BIEN a CO. N

(A) NON-OPHITIC DIABASE (B)OPHITIC DIABASE
PLATE XXVI.

153
PLATE XXVI.

Fig. A. Contact of finely granular diabase dike and hypersthene-free gabbro. The 

dike is of considerable width and of medium texture away from its contact zones, while near the contact it is porphyritic with labradorite-feldspar individuals. Among the contact minerals occur many clear minute 

grains of quartz.

Specimen 5294, slide 862, x 120.

See pp. 117,125.

Fig. B. Areas of s'econdary quartz extinguishing simultaneously within a corroded 

feldspar. The original identity of the feldspar is obliterated. The quartz 

is very transparent and free from inclusions.

Specimen 5400, slide 2477, X 150.

See pp. 51,122,120.

154
U. S. GEOLOGICAL SURVEY BULLETIN NO. 157 PL. XXVI

A, CONTACT OF FINELY GRANULAR DIKE AND HYPERSTHENE-FREE GABBRO. 

IS, AREAS OF QUARTZ EXTINGUISHING SIMULTANEOUSLY WITHIN A CORRODED 

FELDSPAR.
.trr

PLATE XXYII.

155
PLATE XXVII.

Fig. A. Peridotite, var. saxonite, showing enstatite and olivine, with the alteration 

products chiefly from olivine.

The light-brown portion (1) is a single individual of enstatite, in which 

the arrow marks the position of extinction. The enstatite individual is 

considerably shattered, and the fractures diverge as though they resulted 

from the hydration of the partially altered olivine. Such fracturing seen 

in certain forellensteins (troctolites) ia attributed to that cause. The 

single olivine individual (2) shown in the central portion of the figure is 

at least one-half altered into serpentine and magnetite. The alteration 

hus proceeded from the fractures, and the largest olivine areas occur where 

these fractures were least frequent. The serpentine (3) lies in strings and 

bands between the fresh olivine remnants.

Specimen 5216, slide 1785, X 180.

See p. 112.

Pig. B. Serpentine derived fronl the alteration of saxonite. A portion of the serpentine has been further replaced by quartz. Sec. 9, T. 112 N., R. 34 W.

Specimen 5216, slide 2221, X 180.

See p. 114. 

156
PL.XXVII.
U. S. GEOLOGICAL SURVEY BULLETIN No 157 PLXXVII.

(B)

JULIUS BIF.H & CO.N.Y.

(A)ALTERED PERIDOTITE (B) SERPENTINE
INDEX

Page. 

Alien, J., work of........................... 14

Analyses................... 50,63,68,75,76,89,113

Anderson,C. S., and Clark, Thomas,work of. 15 

Apatite, occurrence and character of....... 58

Area embraced in bulletin................. 9-10

Augite, occurrence and character of........ 57,80

Augite-biotite-granite-gneias, occurrence

and character of ................... 66-72

Augite-schist, description of............... 95-96

exposure of ............................ 26

Babcock, E. J., analyses by................. 89

work of................................ 22

Basic rocks, exposures of................... 26-27

Bayley, TV. S., cited on rock classification .. 54

descriptions by ............ 95,99-100,103-104

Beaver Falls, glacial striae at............... 45

Beaver Falls area, hornblendo-biotite-gneiss

in.................................. 75-76

outcrops in............................. 29-30

Beaver Falls and Morton district, exposures

in.................................. 27-32

Beltrami, J. C., work of..................... 13

Berkey, C. P., cited on St. Croix Dalles..... 22

Bigstone Lake, glacial striae at............. 45

Biotite, occurrence and character of........ 53-54

Biotite-gneiss, exposures of................. 31

Birch Cooley, exposures at................. 29

kaolin from, analysis of................ 76

Carver, Jonathan, cited on explorations in

Minnesota.......................... 11

work of................................ 12

Cathrein, A., cited on saussuritization ..... 85

Chamberlin, T. C., cited on dikes of Minnesota Valley ...... 1..............'. 115

work of................................ 16

Chester, F. D., cited on gabbros of Delaware ........................... 78,79,129

Chlorite, occurrence and character of....... 59-60

Chloritic gneiss, plate showing ............" 136

Clark, Thomas, and Andersen, C. S., work of 15 

Commender, Hans, cited on gneiss.......... 47

Cottonwood, well boring at................. 131

Courtland, exposures of rocks near ........ 20-25

section from Minneopa to.............. 24

section from Pipestone to .............. 16

Dana, E.S., cited on Hammerfest anorthite. 89 

Dana, J. D., cited on analyses ............. 130

cited on rock names.................... 49

Dathe.E.,cited onperidotite............... Ill

Diabase,plate showing..................... 144

Diallage, occurrence and character of...... 80-81

Dikes, age of............................. 117-118

occurrence of ........................ 115-117

petrographic character of .. % ......... 118-121

Diller, J. S,, cited on peridotite............. 112

Dinsmore, A. 0., analysis by................ C8

Dodge, James A., analysis by .............. 50

reference to............................ 22

Dresbach sandstone, figure showing occurrence of................................ 24

East Granite Falls, glacial striae at........ 45

Echo, glacial striae at ...................... 45

Elftman, A. H., cited on rock relations ..... 71

Engineer Corps, work of ................... 17

Epidote, occurrence and character of....... 59

Featherstonhaugh, G. "W., cited on gabbroschists............................. 90

work of................................ 13,14

Feldspars, occurrence and character of.... 51-53,

04-65,80 

Ferrite, occurrence and character of....... 58

Ficus austiniana, occurrence of ............ 44

Folin, 0. K., analysis by.................... 68

Fort Ridgely, character of quartz at....... 50

exposures of gneiss near............... 26

Fort Kidgely area, hornblende-biotite-granite-gneis* in........................ 64-66

Fort Ridgely district, exposures in......... 25-27

Fouque, F.,andL6vy, M., cited on feldspars. 51 

cited on hornblende.................... 55

Fuchs, C. "W. C., cited on picrite ............ 114

Gabbros, plate showing .................... 142

Gabbro-diorite, occurrence and character

of................................ 106-109

plate showing.......................... 140

Gabbro-schist, mineral constituents of...... 80-82

occurrence and character of... 33,34,35,77-80 

hypersthene - bearing, occurrence and

character of........................ 82-98

hypersthene-free, occurrence and character of ........................... 98-106

pyroxene - free, occurrence and character of-.....--................... 106-109

Geikie, A..cited............................ 29

Gilbert, G. K., cited on erosion ............. 21

cited on geology of Henry Mountains,

Utah............................... 18

Glacial striae in Minnesota River Valley ... 45 

Glencoe, well boring at..................... 24

Gneiss, age and character of................ 46-49

character and occurrence of.......... 32-33,34

chloritic, plate showing................ 136

157
160 INDEX.

"Williams, G. H., cited on feldspar in "Wisconsin andMichigan................... 119

cited on gabbros ................ 78,79,98,129

cited on gabbro-dibrite................. 105

cited on hornblende .................... 57

cited on occurrence of muscovite....... 54

cited on schists of Michigan ........... 52

Winchell, N. H., cited 011 occurrence of

liiigula............................. 22

Page. 

"Winchell, N. H., cited on erosion ........... 21

cited i gabbro-schists....'.'............ 90

cited on post-Cambrian deposits ....... 43

work of.......................... 16,17,18-19

Winchell, N. H., and Upham, Tj arren, work

of.................................. lff-17

Wright,G.F., cited on erosion............. 21

Yellowbank Creek, outcrop near............ 39

Zirkel, E., cited on diallage................. 80,81