A RECONNAISSANCE OF THE ARCHEAN COMPLEX OF THE GRANITE 

GORGE, GRAND CANYON, ARIZONA. 

By L. F. NoBLE and J. FRED. HuNTER. 1 

INTRODUCTION. of Bright Angel shale on which the trail passes 

The field work upon which this article is around the head of the canyon. Except Bass . 

based was done in March and April, 1914. trail and the trail built by Mr. Bass between 

From Garnet Canyon, near the west end of the Bass Canyon and Copper Canyon, there is now 

Granite Gorge, the route followed the Tonto very little west of Hermit Creek that can proptrail along the so-called lower plateau, or erly be called a trail. The traveler will find 

Tonto platform, to Red Canyon, at the east the Geological Survey's topographic maps of 

end of the Granite Gorge. The distance cov- the greatest value in making this part of the 

ered was roughly 100 miles measured along the trip, because, although nearly all traces of a 

trail, or about 40 miles measured along the trail have disappeared, the route marked on 

course of Colorado River. Supplies were car- the maps as the Tonto trail still shows the most 

ried by pack burros, and Mr. John w althen- feasible way along the Tonto platform. 

berg acted as packer and guide. Stops of sev- It was thought that there would be difficulty 

eral days each were made at Garnet Canyon, in finding water in many of the side canyons 

Copper Canyon, Bass Canyon, Serpentine Can- where it was proposed to camp, because most 

yon, Ruby Canyon, Turquoise Canyon, and of them do not contain living streams, and 

Slate Creek, in the Shinumo quadrangle ; the intermittent underground flow derived 

Boucher Creek, Hermit Creek, Salt Creek, Pipe from the melting snows on the sheltered slopes 

Creek, Lone Tree Canyon, and Grapevine high under the south rim rarely lasts long 

Creek, in the Bright Angel quadrangle ; and after the opening of spring. vVhile the beds 

Cottonwood Creek, Hance Creek, and Red Can- of most canyons proved to be dry for the 

yon, in the Vishnu quadrangle.2 : greater parts of their courses, whether on bedFrom Hermit Creek eastward the Tonto trail . rock or on gravelly waste, water was always 

is often traveled by tourists and was known to found flowing for a few hundred yards where 

be in fairly good shape. The western part of the stream courses cross the upper layers of the 

the trail, however, between Hermit Creek and cliff-making Tapeats sandstone. Most of the 

Bass Canyon, was not known to have been used camps were made at this level and proved to 

by any party with pack animals since 1905 be equally convenient for the examination of 

when the Shinumo quadrangle was topographi~ the Archean rocks below the camps and the 

cally surveyed, but progress over it was made Paleozoic rocks above, up to the base of the 

with little difficulty. At only one place was Redwall limestone. During April the small 

it impossible to pass without unloading the ~ow of water was constantly diminishing, and 

pack animals. This was at the head of the 1t seemed unlikely that there would be any 

small canyon just east of Serpentine Canyon. flow whatever by June. It was evident that 

i_n the Shinumo quadrangle, where a flood and in any part of the year except early in spring 

landslide had carried away the narrow ledges camps at intervals so short and in places so 

conveniently situated would be impossible. 1 Field work by Mr. Noble ; microscopic study of rocks by 

Mr. Hunter. 2 These three quadrangles have been mapped by the United 

States Geological Survey. 

The chief object of the excursion was the 

study and areal mapping of the Algonkian and 

Paleozoic formations of the southern wall of 

95 
96 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

the Grand Canyon in the Kaibab division. Detailed sections of the Tonto group were made 

in seven places at intervals of 5 or 6 miles, and 

particular attention was given to the interesting unconformity at the summit of the Tonto 

group, where the Devonian is present in some 

places and lacking in others. Three sections 

of the Carboniferous formations were made, 

one in each quadrangle. Parts of the three 

quadrangles were mapped in detail, including 

all of the Bright Angel quadrangle south of 

the Granite Gorge, all of the Shinumo quadrangle south of the Granite Gorge as far west 

as Garnet Canyon, and all of the Vishnu quadrangle south of the Granite Gorge and west of 

Red Canyon. 

' are unknown. It was realized that if the 

Archean· could be examined at short intervals 

through the Granite Gorge it would at least 

be possible to determine definitely the petrographic character of many of the rocks, and a 

foundation would be laid for future detailed 

study. Something might also be learned of 

their origin and structure. It was therefore 

decided to make this reconnaissance, and the 

results are set forth in the pre8ent paper. A 

second paper will be devoted to the detailed 

discussion of the Paleozoic rocks, and another 

to a description of the faults in the Grand 

Canyon on the south side of the river. The 

location of the known Archean rocks in the 

Grand Canyon is shown in figure 9. 

FIGURE 9.-Key map of northwestern Arizona showing location of exposures of Archean rocks in the Grand Canyon. I , 

Granite Gorge of Kaibab division (described in this paper) ; II, Lower Granite Gorge; III, Granite Gorge of Shivwits 

division. 

It had not been intended originally to make 1 

any examination of the Archean rocks, because 

an adequate study of their character and structural relations would take at least as much 

time as the writer had planned to devote to 

the entire trip. On arrival in the canyon, however, it was found that it would not be difficult 

to get down to the Archean from camp in each 

of the larger side canyons and devote a part of 

the time spent in each camp to the study of 

these rocks. Geologic literature concerning 

the Archean complex of the Grand Canyon is 

very meager, and even the fundamental facts 

of the lithology and distribution of the rocks 

GEOGRAPHY. 

Three elements in the topography of the 

Grand Canyon below the Red wall in the Kaibah division that are important to the geologist 

who wishes to study the Archean are the Tonto 

platform, the Tapeats cliff, and the Granite 

Gorge. All are intimately related to geologic 

structure. 

The Tonto platform, locally misnamed the 

" lower plateau," is the wide ledg~ left in the 

interior of the canyon by the wasting back of 

the relatively weak Bright Angel shale from 

the summit of the underlying resistant Tapeats 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 98 PLATE XIX 

GRANITE GORGE OF THE GRAND CANYON, ARIZ., LOOKING NORTHWEST. 

AR.CHEAN COMPLEX OF GRANITE GORGE, GRAND CAlrYON, ARIZ. 97 

sandstone.1 It affords the only possible route 

of travel through the interior of the canyon on 

land. 

The Tapeats cliff is the outcropping edge 

of the resistant Tapeats sandstone below the 

Tonto platform. It is wonderfully persistent, 

averages 300 feet in height, and is the chief 

obstacle that bars the way to the Archean 

rocks. 

The Granite Gorge (see Pl. XIX) is the 

deep, narrow V-shaped canyon which Colorado 

River has cut in crystalline Archean rocks below the Tapeats sandstone. Descending into 

it, the traveler leaves the wide ·outer canyon, 

whose walls are carved in horizontal Paleozoic 

strata, and enters what is geologically and 

scenically another world. At the base of the 

Tapeats cliff a great unconformity of erosion 

separates the Paleozoic beds from the two 

underlying systems of rock-the Archean crystalline rocks and the Algonkian Grand Canyon 

sediments, which are inset in the Archean in 

places by block faulting. The change in scenery is no less abrupt than the change in geologic structure. The sides of the Granite 

Gorge, carved in Archean rocks, descend to 

the river in steep, bare slopes whose surface 

is jagged and chaotic in the extreme. 

The Granite Gorge is about 40 miles long, 

measured along the course of Colorado River, 

and runs from southeast to northwest. The 

average .width between the inclosing cliffs of 

Tapeats sandstone is perhaps 2,000 feet, and 

the average depth, measured from the base of 

the Tapeats cliff to the river, about 800 feet. 

At irregular interv~ls of a mile or so tributary gorges are cut back into the Tonto platform. Most of these do not expose the Archean 

rocks far back from the main gorge of the 

river, but some of the larger ones expose them 

for several miles. The upper end of the Granite Gorge begins abruptly in the Vishnu quadrangle near the mouth of Red Canyon, where 

the river flow~ southwestward out of great 

masses of Algonkian strata that dip steeply 

northeastward upstream. It ends gradually in 

the Shinumo quadrangle below Garnet Canyon, where the river flows southward across a 

broad zone in which the Paleozoic strata dip 

1 The Tapeats sandstone is the basal formation of the 

Tonto group of strata, of Cambrian age, which comprises the 

Tapeats sandstone, Bright Angel shale, and Muav limestone. 

southwestward; as the dip of the strata is 

greater than the grade of the stream, the river 

bed is gradually carried out of the Archean 

rocks into the overlying Tapeats sandstone. 

The inaccessibility of the exposures of Archean rocks and the extreme ruggedness o£ the 

topography in the Granite Gorge are the 

chief obstacles to geologic study. The sheer, 

high cliff of Tapeats sandstone along the rim 

of the gorge is, in most places, an impassable 

barrier to descent _from the Tonto platform, 

and there are only three situations in which it 

has broken down enough to allow the traveler 

to reach the Archean-( 1) at the heads of 

tributary gorges, where it is cut by stream 

erosion; (2) in places where a hill or" monadnock " of Archean or Algonkian rock rises 

through the sandstone and excludes it; (3) 

where a line of faulting crosses the sandstone 

and shatters it. The most numerous breaks are 

those caused by stream erosion at the heads of 

tributary gorges. Once the Archean rocks are 

reached progress is exceedingly difficult and 

slow, except in the beds of the larger tributary canyons, and it is impossible to travel 

very far along the edge of the river, because 

the walls rise steeply from the water. 

The exposure of Archean rocks in the Granite Gorge is so small a part of the vast areas 

buried under Paleozoic and later strata north 

and south of the canyon that the chance of deciphering much of the structure from it would 

be remote were it not for the fact that the 

northwest course of Colorado River crosses at 

right angles the dominant trend of nearly all 

the structural features in the Archean. Thus 

the Granite Gorge is a magnificent 800-foot 

cross section 40 miles in length. The difficulties incidental to the roughness of the topography are largely compensated, for the geologist, by the clearness with · which the structure 

is revealed in the bare slopes. 

PLAN OF INVESTIGATION. 

The method of study followed was to descend to the Archean rocks in every side canyon where a camp was made and in any other 

place that could be reached by detour when 

moving from one camp to another. Twenty 

localities we.re visited in traveling the length 

of the Granite Gorge. The distance between 

them ranged from three-tenths of a mile .to 
98 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

4 miles, but the average interval was about 2 

miles. .At these localities 67 hand specimens 

were collected.:~ and thin sections were cut from 

48 of them. Eight distinct groups of rocks 

were crossed in exploring the Granite Gorge, 

as set forth in the accompanying table. (See 

also fig. 10.) In this table and the following 

discussion the localities and groups of rocks 

are numbered in the order of their occurrence 

from west to east. 

vailing strike of the gneissoid banding is N. 

60° E. and the dip nearly vertical. In places 

between Garnet Canyon and W althenberg Canyon the strike becomes northwesterly. Here 

and there the gneiss is greatly mashed and contorted. The commonest type of gneiss is hornblende gneiss (metamorphosed quartz diorite, 

specimen No. 3) , and there are some bands 

of a very dark hornblende schist (amphibolite, 

No. 4). .A quarter of a mile below the mouth 

KAIBAB PLATEAU 

LEGEND 

~ Archean rocks 

•.......... ~ Rim of Grand Canyon 

+-- Arrows point to approximate 

locations of boundaries between the groups of Archean rocks 

9'-----'---:--....Lif> __ ___,'j' MII .. £S 

FIGURE 10.-Map of Kaibab division of Grand Canyon, Ariz., showing distribution of Archean rocks and localities where 

the rocks were examined. Quadrangles mapped by United States Geological Survey are indicated by ·outline letters as 

follows : A, Shinumo; B, Bright Angel ; C, Vishnu. Localities examined: A, Garnet Canyon ; 8, Walthenberg Canyon; C, Copper Canyon; D, Shinumo Creek; E, White Creek; F, Flint Creek; G, Cable Crossing; H, Bass Canyon; 

I, Serpentine Canyon; J, Ruby Canyon; K, Turquoise Canyon; L, Slate Creek; M, Crystal Creek; N, Boucher Creek; 

0, Hermit Creek; P, Salt Creek; Q, Pipe Creek; R, Bright Angel Creek; S, Clear Creek; T, Lonetree Canyon; U, 

Boulder Creek; V, Grapevine Creek; W, Cottonwood Creek; X, Hance Creek; Y; Mineral Canyon; Z, Red Canyon. 

THE ROCKS. 

GROUP I. GNEISSES, AMPHIBOLITES, AND 

GRANITE. 

From the west end of Granite Gorge to the 

mouth of W althenberg Canyon, a distance of 

about 6 miles, the rocks are banded gneisses 

cut by pink intrusives. The gneisses are of 

several types and occur in parallel bands which 

range in width from a few feet to over 300 

feet. From Garnet Canyon westward the preof Garnet Canyon there is a 50-foot band of 

biotite-chlorite gneiss (No. 5) containing numerous garnets about the size of a pea, frorn 

which Garnet Canyon was named. Within 

Garnet Canyon an intrusive body of pink 

granite (No.6) is the prevailing rock. 

The pink intrusive rocks that are associated 

with the gneisses are granite, aplite, and pegmatite. The granite is a medium-grained eventextured rock which consists essentially of 

quartz, microcline, soda plagioclase, muscovite, 
Archean rocks examined in Granite Gorge, Grand Canyon, Ariz. 

Distance Nos. of specimens · Localities where the Archean rocks were examined between Quadrangle. and specimens collected (reading from west to east). localities from which thin Group. 

(miles). sections were cut. 

1. Gorge of Colorado River from mouth of Garnet 

Canyon to a point 1 mile south ................ .. ~ ................... 3,4,5 I 2. Garnet Canyon (0.2 mile east of mouth) .......... 1.0 6 

3. Gorge of Colorado River 0.5 mile southwest of · mouth of Copper Canyon ...................... 2.9 (b) 

4. Copper Canyon (near mine) .............. _ ...... . 3 1,2 II 

5. Gorge of Colorado River at Cable Crossing ... _ .. _ 1.9 8 

Shinumo. 

6. Bass Canyon (near mouth) .................... _ .4 (c) 

7. Serpentine Canyon (head of inner canyon to river). 1.6 9,10,11 III 

8. Ruby Canyon (0.8 mile southwest of mouth) ..... 1.7 13 

9. Turquoise Canyon (0.6 mile southwest of mouth). 2.4 15,16,17,18,20 IV 

10. Slate Creek (1 mile southwest of mouth) ........ 2.9 22,23,24,25,26,27 

11. Boucher Creek (0. 7 mile southwest of mouth) •.. 1.5 31,32,33,34 v 

12. Hermit Creek (head of inner canyon to river) .. _ 1.9 35,38,39,40 

Bright Angel. 13. Salt Creek (0.7 mile southeast of mouth) ........ 2.7 45,46 VI 

14. Pipe Creek (0.8 mile south of mouth) .......... 2. 7 49,50 

15. Lonetree Canyon (0.5 mile southwest of mouth). 4.0 53,54 

16. Boulder Creek (0.7 mile southwest of mouth) .... 1.2 (b) VII 

17. Grapevine Creek (1.6 miles southwest of mouth). 1.5 56,57 

18. Canyon 0.7 mile southeast of mouth of Cottonwood Creek ..... _ ... _, ..................... 2. 7 60,61 

Vishnu. 19. Hance Creek (1.5 miles southwest of mouth) ... _ 1.4 62,63 VIII 

20. Gorge of Colorado River from mouth of Mineral 

Canyon to end of Granite Gorge .•............ 1. 7 64,65,66,67 

a Pistance in strai~ht line between east and west ends of Granite Gorge, 31 miles; by course of river, 40 miles, b No thin sections. 

Approximate 

length of exposure, 

Predominant type of rock. measured roughly 

by the course of the 

river (miles).a 

Hornblende gneiss, granite 

gneiss, and amphibolite 

(much injected granite and 6 

pegmatite). 

Mica schist (some injected 

granite and pegmatite). 4.2 

Massive basic intrusives (some 

injected pegmatite). 4.5 

Metabasite and metadiorite 

(much injected granite and 2 

pegmatite). 

Mica schist (little injected 

granite and pegmatite in 

eastern part; much in west5.7 

ern part). 

Granite gneiss and amphibolite (much injected granite 10.4 

and pegmatite). 

Mica schist (little injeGted granite and pegmatite in western 

part; much in eastern part). 

3.5 

Granite gneiss (much injected 

granite and pegmatite). 3. 7 

c See U. S. Geol. Survey Bull. 549, pp. 32-37, 1914. 

> 

~ 

0 

~ t:;j 

> 

~ 

0 

0 

~ 

"'d 

~ ~ 

0 

I'Xj 

Q 

~ ~ 1--( 

H 

t:;j 

Q 

0 

~ Q 

~ ~ 

t:::l 

0 

> 

z 

Kj 

0 

"'~ 

;:.. 

~ 

et:> 

~ 
100 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

and biotite, named in the order of decreasing 

abundance. The aplites and pegmatites consist as a rule of quartz, microcline, and muscovite, but in places the muscovite is absent. 

Apparently the pink intrusives comprise 

from one-fourth to one-third of the rocks of 

Group I, but the proportion varies considerably from place to place and may be actually 

much less. Some of them are in sheets parallel 

with the bands of gneiss. Others, chiefly granites, are in irregular bodies that cut the gneisses. 

In places the granite in these bodies grades 

into pegmatite. Still others, chiefly pegmatites, are in dikes that cut both the gneisses and 

granites. 

Petrography of specimens.1 

No.3. Hornblende gneiss (probably a metamorphosed 

quartz diorite).-The rock in hand specimen is gray, 

well banded, and medium grained and contains a considerable. amount of quartz in crystals larger than the 

other mineral constituents. In thin section the 

gneissoid character is less conspicuous, the rock having a seriate porphyroid fabric in which the individual crystals average about 1 millimeter in the 

longest direction, although the quartz crystals may 

be as much as 4 m~llimeters. Quartz, plagioclase, 

hornblende, and orthoclase are the essential mineral 

constituents named in the order of decreasing abundance. The quartz occurs in large irregularly shaped 

crystals which interlock with and in places include 

the other constituents. It also contains numerous 

microscopic inclusions whose character has not been 

determined. The quartz crystals have no uniformity 

of orientation, although some of them, particularly 

those cut at a small angle to the principal optic axis, 

show undulatory extinction as an. evidence of straining. The plagioclase has the composition of andesine 

(about Ab~na:~), is usually twinned after the albite 

law, and is partly altered to sericite. Orthoclase is 

thought to be present in small amounts but is very 

much altered. The hornblende crystals are of deep 

color, showing pleochroism in greens and yellows, and 

for the most part are arranged with their c' axes 

parallel. There is a small amount of leached biotite 

which is altering to epidote and a reddish mineral 

thought to be chlorite. The section also contains .iron 

oxides, apatite, zircon, epidote, and titanite in accessory amounts. The rock may have been metamorphosed from a granodiorite or a quartz diorite. 

No.4. Hornblende schist or amphibolite.-The specimen is fine grained, black, and schistose. Little else 

but hornblende can be discerned with the unaided 

eye. Under the microscope_ a thin section cut nearly 

parallel to the schistosity exhibits a seriate fabric in 

1 The thin sections were studied microscopically by J. 

Fred. Hunter in April, 1915. Only the chips from which 

the thin sections were cut were available for megascopic 

study. 

which very few of the individual crystals a~e as 

much as 1 millimeter in longer dimension. Mineralogically the rock consists essentially of hornblende, 

quartz, and feldspar (altering to sericite) , named in 

about the order of decreasing abundance. The feldspar is almost entirely changed to sericite, which is 

nearly as abundant as the quartz. Epidote and sericite 

occur in narrow veinlets cutting the other minerals 

and in small grains associated with hornblende. 

Titanite is the commonest accessory, but iron oxides, 

apatite, and zircon, in microscopic size, are also 

present. 

No. 5. B ·iotite-chlorite gneiss, or schist.-This is 

a dark-green medium-grained micaceous rock, not 

well lamellated. Its degree of metamorphism is not 

apparent from the small hand specimen examined. 

Mineralogically the rock consists essentially of quartz, 

biotite, chlorite, epidote, sericite, feldspar, and garnet, 

displaying seriate fabric in thin section in which the 

grains are as much as 3 millimeters in longer dimension and average about 1 millimeter. The thin section shows little evidence of schistosity or metamorphism of the deeper and region&:- type. However, 

the rock has suffered certain profound changes which 

probably are due more to thermal agencies than to 

dynamic metamorphism. Thus the feldspars, which 

apparently were in large part plagioclase, are altered 

to sericite and chlorite. Some of the chlorite, particularly that which has replaced feldspar, is greenish, 

but other areas of it are yellow. The yellow variety 

occurs as pseudomorphs, the original nature of which 

can not be determined, although the shape of the 

individuals suggests that they may have resulted from 

rhombic pyroxene. The epidote occurs in subhedral 

to euhedral grains and shows noticeable pleochroism 

in pale yellows to light browns. Fine grains of magnetite and even larger crystals of apatite are abundant. This band may represent a relatively late intrusion into the metamorphic complex. 

No. 6. Granite gneiss.-Pink medium-grained granite the small hand specimen of which suggests a 

m~re or less gneissoid character. Mineralogically the 

rock consists chiefly of quartz, microcline, orthoclase, soda plagioclase, muscovite, biotite, and iron 

oxides, named in order or decreasing abundance. 

The individual crystals are allotriomorphic, average 

about 1 millimeter in diameter, and are arranged in 

a seriate porphyroid fabric as seen in thin section. 

The rock is relatively fresh, although the feldspars, 

particularly the orthoclase and soda plagioclase, contain a large amount of finely divided ferritic material, 

which gives them a dirty reddish-brown appearance 

in transmitted light. · The muscovite is largely secondary, as are also small quantities of sericite, chlorite, 

epidote, and most of the iron oxides. Apatite occurs 

as an acce~sory. 

GROUP II. MICA SCHIST. 

From the mouth of W althenberg Canyon 

to a point about half a mile east of the mouth 

of Shinumo Creek, a distance of 4i miles, the 

rocks are rather fine grained dull reddish or 
ARCHEAN COMPLEX OF GRANITE GORGE, GRAND CANYON, ARIZ. 101 

pinkish mica schists which are cut in places 

by dikes of pink pegmatite. One of these dikes 

in Copper Canyon is over 200 feet wide. In 

general, however, the amount of intrusive rock 

is small and the schist is free from it over 

large areas, particularly in the western part 

of the exposures. 

The schist is rather homogeneous in composition. Specimen No. 1, from Copper Canyon, 

represents the prevailing type of rock. In 

places where the schist is not too intensely 

crumpled and mashed, there are numerous 

alternating bands a few feet in width in which 

the rock becomes denser, finer grained, and 

more quartzose in appearance and changes in 

color from pinkish to gray. In the field the 

character of this banding strongly suggests an 

original sedimentary bedding. Specimen No. 

2, from Copper Canyon, is the schist from one 

of these bands of denser rock. 

The areas of schist that exhibit this obscure 

banding are in the western part of the exposures, extending from Copper Canyon to 

W althenberg Canyon. Here the prevailing 

strike of the banding is northwesterly and the 

dip from 45 ° to 75 ° NE., both corresponding 

in a general way with the strike and dip of the 

schistosity. If the banding represents an original sedimentary bedding and if there is no 

duplication by folding or faulting, the exposed 

thickness of the series can not be less than 5,000 

feet and is probably much greater. 

East of Copper Canyon, near the contact of · 

the schist with the quartz diorite of Group III, 

there is a great deal of local twisting and contortion in the schists and they are injected with 

pink feldspar and milky quartz in veins and 

lenses. In this area the strike of the schistosity 

is approximately N. 20° E. and the dip nearly 

vertical. The schist contains a great deal of 

pink feldspar and some black tourmaline and 

in places is rather coarse in texture. Specimen 

No. 8 is a piece of this schist obtained near the 

Cable Crossing. 

Petrography of specimens. 

No. 1. Mica schist.-Gray, with pinkish tones; yellow on weathered surface. Mica and quartz individuals are easily distinguishable with the unaided 

eye. The rock has doubtless been entirely recrystallized during the process of metamorphism, so that no 

trace of the original structure remains. In a thin 

of the micas, which average 0.5 millimeter in length, 

whereas the other constituents average approximately 

0.3 millimeter across and a few individuals are as 

much as 1 millimeter. The mineral composition of the 

rock may be represented approximately as follows : 

Quartz~ biotite zmuscovite ~feldspar> iron oxides, 

with accessory apatite, zircon, and garnets. The 

quartz occurs in subequant anhedral grains and is 

commonly filled with numerous inclusions of mica, 

apatite, and minute specks whose character has not 

been determined. The feldspar is likewise anhedral, 

is altered to sericite and in some places to kaolin, 

and generally has a lower index than that of quartz. 

It probably makes less than 15 per cent of the rock. 

So far as the mineral content is concerned the schist 

may have been derived from the metamorphism of a 

shaly arkosic sandstone or equally well from an 

igneous rock having the general composition of 

granite. 

No. 2. Mica schist.-Gray, more compact, and less 

micaceous than No. 1, but otherwise differing very 

little, though it is finer grained and the micas are 

smaller and less conspicuous, giving it a more 

quartzose appearance. Its mineral constitution is the 

same, and apparently it has about the same amount 

of feldspar, which appears to be orthoclase or soda 

plagioclase. 

These rocks resemble pre-Cambrian rocks from the 

Black Canyon of Gunnison River, in Colorado.1 The 

microscopic study does not give conclusive .evidence as 

to their origin. 

No. 8. Muscovite schist.-Pinkish gray, with silvery 

sheen in certain light ; very micaceous and fissile. The 

specimen indicates more or less weathering. In a 

section cut transverse to the cleavage of the rock 

the parallel arrangement of the micas, which average 

0.6 millimeter in length, brings out the schistosity 

strikingly. The other constituents are allotriomorphic, are equant to subequant, and average 0.2 millimeter in diameter. The mineral composition of the 

schist may be represented approximately as follows: 

Quartz> feldspar ~muscovite~biotite (ferritized) > 

iron oxides, with accessory apatite, zircon, and 

garnets. 

The feldspar, being badly stained with iron oxide, 

which gives it a reddish-brown color, is easily distinguishable from the clear quartz and makes up 

approximately 20 per cent of the rock. It has a low 

index and appears to have been in part orthoclase, 

which was somewhat altered to sericite before it wp.s 

stained by weathering. The biotite likewise is ironstained and nearly opaque. This rock differs from 

Nos. 1 and 2 in texture, but is similar in composition, although the degree of weathering has emphasized the abundance of the feldspars. 

In addition to the exposures in the · Granite 

Gorge there is an area of the schists in the 

Shinumo Amphitheater, on the northeast side 

section, cut oblique to the cleavage of the rock, the 1 A report on the Archean rocks of the Black canyon 

schistosity is indicated by the parallel arrangement . region, by J. F. Hunter, is in preparation. 
102 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

of the 'Vest Kaibab fault, 2 miles northeast of 

the mouth of Shinumo Creek. It extends about 

3 miles southeastward in the inner canyons of 

White, Shinumo, and Flint creeks and a mile 

northeastward up the canyon of Shinumo 

Creek. This area of schists was visited in 

1908, in the course of a study of the Algonkian 

strata of the Shinumo quadrangle, and a few 

specimens of the chief types of rock were collected from which thin sections were made.1 

· Three types 9f schist were recognized-(1) 

quartz-mica schists which, in general aspect, 

resemble the mica schists of the Granite Gorge, 

above described, and which may contain feldspar, though the two thin sections studied 

show little or none; (2) fine-grained quartzhornblende schist, consisting of about equal 

proportions of quartz and green hornblende; 

(3) hornblende schist, consisting almost entirely of green hornblende, which occurs in a 

narrow · outcrop with sharp boundaries and 

apparently represents a body of metamorphosed igneous rock of a basic type. 

No definite hypothesis as to the origin of the 

quartz-mica and quartz-hornblende schists was 

entertained, because no detailed study was 

made of the Archean rocks in the field, but the 

large amount of quartz in the schists suggested 

a sedimentary origin. 

The contact between the gneisses of Group I 

and the schists of Group II near W althenberg 

Canyon was not examined on the present trip, 

because there was no break in the Tapeats cliff 

that would permit descent to the Archean 

within several miles of the contact. Seen from 

above it appeared to be a rather sharp line 

crossing the Granite Gorge from northwest 

to southeast and was thought to be a fault. 

GROUP III. MASSIVE :BASIC INTRUSIVE ROCKS. 

From a point near the Cable Crossing to a 

point about 1 mile southeast of the mouth of 

Ruby Canyon, a distance of 4! miles, the rocks 

are all coarse granular massive igneous rocks 

in which no gneissoid banding or schistosity 

was. observed in the localities visited. For 

at least a mile up the river above the Cable 

Crossing the rock is a coarse-grained quartz 

diorite which is intrusive 1n the schist of 

Group II. This quartz diorite was examined 

1 Noble, L. F., The Shinumo quadrangle, Grand Canyon district, Ariz,: U. S. Geol. Surv-ey Bull. 549, pp. 32-35, 1914. 

in 1908 near the mouth of Bass Canyon aad 

has been described as follows: 2 · 

The quartz diorite in the river gorge east of Cable 

Crossing is a coarse-grained, dense, resistant rock of 

typical granitic texture, which tends to weather into 

roughly angular blocks and thus to assume forms that 

distinguish it in the mass from the Vishnu schist. 

It is dark gray, looks remarkably fresh, and contains 

visible particles of white striated feldspar, dark hornblende, and glistening black biotite. The rock is uniform in texture throughout the exposures observed, 

is apparently without contact modifications, and 

shows no gneissoid banding. 

Under the microscope it is seen to be a coarsegranular rock of granitic texture. Its dominant 

mineral constituents are plagioclase and common hornblende, the plagioclase ranging from oligoclase to 

labradorite. Microcline, orthoclase, and quartz are 

present in ·about equal proportions, but their total 

amount does not equal that of the plagioclase. 

Brown biotite appears in somewhat less quantity 

than the hornblende, and titanite and magnetite are 

accessories. Occasionally the quartz is poikilitic in 

the orthoclase. The feldspathic and ferromagnesian 

minerals occur in about equal proportion. The microscope reveals no cataclastic structure nor other evidence of dynamic action, and the minerals are fresh 

and unaltered. 

In 1908 the rocks of the Granite Gorge 

were not examined beyond a point 1 mile up 

the river from the Cable Crossing. It could 

be seen clearly, however, that they are massive 

igneous rocks at least as far as the mouth of 

Ruby Canyon. All the rock in the Granite 

Gorge between Bass Canyon and Ruby Canyon 

was therefore mapped as intrusive quartz 

diorite, but the statement was made in the 

text of Bulletin 549 that the eastern limit of 

the diorite had not been located, and the eastern part of the exposure was indicated on the 

geologic map as outlines by a reconnaissance 

survey. 

On the present trip, when what was supposed 

to be the quartz diorite was examined in Serpentine Canyon, the next canyon up the river 

from Bass Canyon where access to the Archean 

was possible, the rock was found to be a gray 

porphyritic hypersthene gabbro. Specimens 

Nos. 9 and 11 came from this locality. No. 10 

is a specimen containing one of the phenocrysts. 

No. 13 is hypersthene gabbro from Ruby Canyon, 2 miles southeast of Serpentine Canyon. 

The hypersthene gabbro is exposed over an 

area of about 4 square miles and is the largest 

single body of massive igneous rock in the 

2 Idem, pp. 35, 36. 
ARCHEAN COMPLEX OF GRANITE GORGE, GRAND CANYON, ARIZ. 103 

Granite Gorge. It is characterized throughout 

by a well-marked sheeting or jointing that 

strikes northwest and dips 20°-30° NE. It 

decays more readily than any other rock in the 

Archean, so the sides of the Granite Gorge in 

the area occupied by the gabbro have a much 

gentler slope than elsewhere and the Tapeats 

cliff has retreated nearly twice as far from the 

river as it has in the area between Bass Canyon · 

and Serpentine Canyon where the diorite is 

exposed. The boundary between the diorite 

8.nd the gabbro must be at the point where this I 

change in the topography takes place and is 1 

therefore not far down the river from the 

mouth of Serpentine Canyon. It was not possible to visit the locality on the present trip, 

and the nature of the contact is unknown. 

Petrography ot specimens. 

No. 11. Hypersthene gabbro.-Gray, with greenish 

tones when viewed at an angle to the surface, porphyritic with large poikilitic crystals of hornblende. 

Under the microscope the rock is found to have a 

poikilitic structure, large crystals of plagioclase and 

hornblende serving as hosts for hypersthene and 

.augite, which are subhedral to euhedral and 3 millimeters in maximum length. The mineral composition 

of the rock may be represented approximately as follows :Plagioclase> augite~hypersthene> hornblende> 

biotite>iron oxides>chlorite>epidote, apatite, zircon. 

The plagioclase is extensively altered to sericite, chlorite, and epidote but appears to have been at least as 

calcic as labradorite. The rock is granular and with-

{)Ut schistosity or cataclastic structure and does not 

seem to have suffered any considerable metamorphism 

{)f the regional type. 

No. 9. Hypersthene gabbro.-Similar to No. 11, but 

the feldspars are considerably more altered and the 

s ection shows hypersthene. 

No. 13. Hypersthene gabbro.-This rock is very 

similar to No. 11, is medium grained and porphyritic, 

and contains large poikilitic phenocrysts of hornblende 

more than three-fourths of an inch long. Microscopic 

study shows the rock to be hypidiomorphic and granular and, save for the few large phenocrysts, to have 

a seriate fabric in which the individual crystals average between 1 and 2 millimeters in diameter, although 

the plagioclase may be as much as 4 millimeters in 

length. The composition of the rock may be represented as follows : Plagioclase> hypersthene> hornblende ~augite> actinolite> biotite> chlorite> epidote, 

with accessory magnetite, pyrite, and apatite. The 

plagioclase has the composition of labradorite ( approximately Ab45An55) and is altering to sericite and 

epidote. The hornblende is changing to a mixture 

{)f actinolite and magnetite. 

No. 10. Altered gabbro.-The hand specimen contains a large phenocryst of hornblende 15 millimeters 

long and 7 millimeters in diameter. In section the 

rock is found to consist of large euhedral crystals of 

hornblende and areas of chlorite aggregates, both of 

which include anhedral crystals of epidote. The section also contains considerable sericite (after plagioclase), iron oxides, and a small amount of biotite. 

Both the diorite and the gabbro are cut by 

a great mesh of ramifying dikes of pink pegmatite, which is very conspicuous in contrast 

with the prevailing darker rock and may be 

distinguished many miles away from points 

along the rim of the canyon. These dikes, 

however, though widely distributed, form only 

a small part of the whole group of roGk. In 

the gabbro many of them have been injected 

parallel to the sheeted structure. The pegmatite is very coarse and consists of quartz, 

microcline, and muscovite. In Ruby Canyon 

the gabbro is altered to a dark hornblende or 

biotite schist for a few inches away from the 

contacts of the pegmatite dikes. 

GROUP IV. METABASITE AND METADIORITE. 

At a point about a mile southeast of the 

mouth of Ruby Canyon the Granite Gorge narrows abruptly in response to a change in the 

character of the rock. There is a tremendous 

increase in the amount of pink intrusive material, and from this point to an undetermined 

point near the mouth of Turquoise Canyon, a 

distance of nearly 2 miles, the rocks are chiefly 

dark-green metabasites and metadiorites, cut 

by pink intrusives that in some places constitute at least half of the rock. The metabasites and metadiorites are in bands that have 

a more or less gneissoid structure. Metadiorite, 

or quartz diorite that h~~s suffered little or no 

dynamic metamorphism1 is the prevailing rock. 

Specimens Nos. 15, 16, 17, and 18 were obtained 

in Turquoise Canyon from separate bands, each 

of which has a width of several hundred feet. 

The strike is about due east or a little north of 

east and the dip from 45° to 60° S. Some of 

the bodies of pink intrusive rock are here more 

than 200 feet wide, and many of them send off 

numerous pegmatite veins and stringers into 

the metabasites. Graphic granite, fine-grained 

pink granite (No. 20), and coarse pink pegmatite consisting of quartz, microcline, and 

muscovite are the types present. Crystals of 

muscovite as much as 4 inches in diameter and 
104 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

crystals of microcline measuring as much as 6 

inches were observed in the coarser pegmatite. 

Petrogra.phy of specimens. 

No. 15. Metabasite.-Dark- green coarse- grained, 

rather granular hornblende rock in which poikilitic 

crystals of hornblende 10 millimeters or more in length 

predominate over smaller crystals of biotite. In thin 

section the rock appears to be 80 per cent hornblende, 

which occurs in large crystals, including biotite, apatite, magnetite, epidote (possibly allanite), and garnet ( ? ) , named in the order of decreasing abundance. 

The constituent minerals are usually very ragged in 

outline, vary in size, and show no regularity of arrangement. Although the rock exhibits no schistosity 

or other evidence of dynamic metamorphism it has 

probably been entirely recrystallized, primarily through 

thermal agencies. It may have been derived from a 

basic rock such as gabbro, diabase, or pyroxenite. 

Rock of this type is not uncommon. In the Lake Superior area it has been called a greenstone, or hornblendite. Other workers have called it amphibolite, 

and still others metabasite. If ~ definite name is desired it seems best to use the term metabasite for these 

nonschistose, thermally metamorphosed rocks·, reserving the terms amphibole or ho~nblende schist and amphibolite for the schistose phase of kindred metamorphic rocks. The term metabasite is applied to 

metamorphosed basic rocks ranging from metadiorites 

to amphibolitic schist.1 

No. 16. Metadiorite.-Fine-grained dark-green granular rock. In section the rock has a seriate homeoid 

fabric in which the individual crystals are allotriomor-

. phic, many of them nearly equant, and average about 

0.5 millimeter in diameter. Plagioclase having the 

composition of calcic andesine is the predominant constituent, and plagioclase and hornblende are the essential minerals of the rock. The feldspar is altering to. 

sericite, and some epidote has been developed. The 

hornblende crystals have a deep-green color, are of 

irregular shape, and may not be original. Quartz, if 

present, is subordinate. The rock also contains apatite, titanite, and iron oxides in accessory quantities. 

This rock shows no schistosity in thin section or in 

the small hand specimen. The character of the hornblende suggests that the rock has suffered some metamorphism. It was probably derived from a diorite, 

although it may have been originally a somewhat potassic gabbro. · 

No. 17. Metabasite.-Dark-green rock similar to No. 

15, but finer grained. In thin section hornblende is 

found to constitute 90 to 95 per cent of the rock. Biotite, iron oxides, apatite, epidote, and possibly quartz 

are present in subordinate or accessory amounts. The 

hornblendes are anhedral and of ragged outline, average less than 1 millimeter in diameter, and show no 

regularity of arrangement. This rock is probably 

of the same origin as No. 15. 

No. 18. Metadiorite.-Dark- green hornblende- rich 

rock resembling No. 16 but of slightly coarser grain. 

1 It has been used for similar rocks in Finland by G. V. 

Hackman and J. ·J. Sederholm. 

Under the microscope this rock is found to have ·a 

seriate fabric in which the individual crystals are 

anhedral and range from 0.5 to 2 millimeters in 

longer dimensions. It consists essentially of plagioclase, hornblende, biotite, and iron oxides, named in 

order of decreasing abundance. The plagioclase is 

altering to sericite but has the composition of andesine to labradorite. The rock contains a small amount 

of quartz and an accessory amount of apatite. 

No. 20. Granite.-Medium- grained even- textured 

pink granite. In thin · section this specimen has a 

seriate porphyroid fabric in which the individual 

crystals are anhedral and attain 4 millimeters in 

longer dimension but average about 1 millimeter. 

Mineralogically the rock consists essentially of quartz, 

microcline, soda plagioclase (ranging from oligoclase 

to albite), muscovite, and biotite named in the order 

of decreasing abundance. A small amount of orthoclase may also be present, and apatite and iron oxides 

occur in accessory amounts. Many of the feldspar 

crystals, particularly plagioclase, have a turbid, reddish appearance due to the presence of very finely 

divided iron oxides. They are also in small part 

altered to sericite~ 

The rocks of Group IV are very similar in 

character to those of the Gunnison River area 

in Colorado, as well as to certain pre-Cambrian rocks of the Lake Superior region. 2 

It was not possible to examine the boundary 

between the rocks of Group IV and the massive 

gabbro of Group III, but from a distance it 

had the appearance of an intrusive contact. 

GROUP V. MICA SCHIST. 

From a point near the mouth of Turquoise 

Canyon to a point about . a mile below the 

mouth of Hermit Creek, a distance of 5l miles, 

the rocks are a series of mica, quartz-mica, 

and chlorite schists in numerous roughly parallel alternating bands which vary in composition and texture and range from a few 

inches to several hundred feet in width. In 

a few places there are bands of hornblende 

gneiss and mica gneiss and a small amount 

of hornblende schist. All these rocks are very 

schistose. The prevailing strike is N. 40° E., 

and the dip ranges from steep northwest 

through vertical to steep southeast. There is 

a great deal of local faulting and some crumpling and mashing, so that there are many variations from the prevailing structure, which appears to be, roughly, that of a series of sharply 

compressed folds. The thickness of the series 

2 Williams, G, H., The greenstone schists of the Menominee and Marquette regions of Michigan : U. S. Geol. Survey 

Bull. 62, p. 146, 1890. 
AR.CHEAN COMPLEX OF GRANITE GORGE, GRAND CANYON, ARIZ. 105 

of banded schists can not be estimated, even 

approximately, but it is probably at least 5,000 

feet. All the rocks of this group are cut by 

dikes of pink pegmatite of the character . 

already described, but east of Agate Canyon 

the relative amount of this intrusive material 

is small. 

Slate Creek occupies a long, straight canyon 

which runs about N. 40° E. Across Colorado 

River and directly in line with the canyon of 

Slate Creek is the still longer gorge of Crystal 

Creek, which drains the great Hindu Amphi- 1 

theater. The rectilinear course of these two I 

canyons is determined by a line of faulting in I 

the Paleozoic strata which has guided their 

erosion. All the way down the inner gorge of 

Slate Creek and all the way up the inner gorge 

of Crystal Creek the strike of the bands o:f 

schist js parallel with the course of these 

gorges, and the dip is vertical or else steep to 

the northwest or southeast. The same bands 

of schist that are exposed in Slate Creek a pparently continue across the river and on up 

Crysta1 Creek to the depths of the Hindu 

Amphitheater, where a small mass of strata of 

the Algonkian Unkar group overlies them, so 

that the total length of the exposure of these 

schist beds along the strike must be nearly 5 

miles. A traverse was made across the upper 

part of the inner gorge of Slate Creek, in order 

to ascertain the gene.ral succession of the schist 

series in this locality. Here the structure is 

fairly simple, the dip being 75° SE. throughout the exposure examined. The section given 

below reads from west to east : 

1. Beds of greenish-gray mica schist (specimens Nos. 

22, 24, and 27), alternating with one another 

and with beds · of pinkish-brown quartz-mica 

schist (No. 23). In some places the separate 

beds have a thickness of only a few inches; in 

others they are many feet thick. The contrast 

between the different types of schist is greater 

in the field than in thin section, owing to differences in color and in degree of weathering. In 

the field some of the layers, notably those represented by specimen No. 27, have a well-developed slaty cleavage and might be called slates. 

The name Slate Creek is derived from the occurrence of this rock. Many of the rocks were supposed in the field to be typical chlorite schists. 

The quartz-mica schists, owing to their density 

and fineness of grain, were supposed to be quartzites until the thin sections were examined. In 

the field there is a vague yet persistent suggestion in these layers of an original sedimentary 

crosslbedded structure which recrystallization and 

shearing had not been able entirely to obliterate. 

This feature and the variation in the schists 

across ·the strike suggested that they might be 

metamorphosed. sediments. ·Thickness, -400 feet 

or more. 

2. Hornblende gneiss (No. 26), apparently representing a metamorphosed quartz diorite that was 

intruded into the schists. In the field this rock 

resembles some of the metadiorite of Group IV. 

Thickness, 300 feet, more or less. 

3. Fine-grained, finely banded granite gneiss, occurring, like the hornblende gneiss, in a band parallel with the schist bands. Thickness, 100 feet, 

more or less. 

4. Mica schists of the same character as those of No. 

1. Thicknesl?, 200 feet or more. 

Although the dominant strike is very persistent, there is a great deal o:f local folding 

and crumpling in the schists, and they are 

injected in places with pink feldspar and milky 

quartz~ 

In Boucher Canyon the rocks are essentially 

the same as on Slate Creek, but the structure 

is much more complicated by :faulting and folding, so that it is difficult to pick out a dominant 

strike that is characteristic of the area. The 

dips, as elsewhere in the Archean, are very 

steep or vertical. The rocks are in alternating 

bands of varying composition; they are, however, rather more crumpled and mashed than 

those on Slate Creek. No. 31 is chlorite schist 

from this locality; No. 32 is quartz-mica schist 

which is hard to distinguish from quartzite in 

the field; No. 34 is a very fine grained fissile 

schist or slate. A bed of hornblende schist 

(No. 33) occupies a band 200 feet in width 

between the mica schists. 

Petrography of specimens. 

No. 22. Mica schist (chlorite schist).-Very fissile 

greenish-gray rock, with pronounced silvery sheen. 

In the thin section, cut parallel to the schistosity, the 

rock has a nearly equigranular fabric in which the 

individual grains range from 0.1 to 0.4 millimeter in 

diameter and, save for the micas, are equant to subequant. This particular section consisted essentially 

of quartz, muscovite, biotite, chlorite, altered feldspar, and epidote, named in about the order of decreasing abundance. A section cut transverse to the 

cleavage would give a better idea of the relatiye 

abundance of the mica. Zircon, apatite, garnet, magnetite, and other iron oxides are present in accessory 

amounts. The feldspars are sericitized, somewhat 

kaolinized, and stained with ferritic material, so that 

their character is not determinable. The section 

studied contains less chlorite than would be suspected 
106 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

from the hand specimen, but this is in part due to 

the softness of the chlorite and its consequent loss 

in grinding. These schists may well have been derived 

from sedimentary rock. 

No. 23. Quartz-mica schist.- Dense pinkish-brown, 

very fine textured, banded quartzose rock. In thin 

section the rock manifests considerable lamellation 

and is nearly equigranular. The individual crystals 

average less than 0.1 millimeter in diameter and, 

save for the- micas, are subequant to equant. The 

section consists of quartz, altered feldspar, muscovite, 

chlorite, and biotite, named in about the order of decreasing abundance. Curiously enough, the section 

shows more chlorite than that of specimen No. 22. 

Like No. 22 the schist contains a rather unusual 

amount of zircon and considerable magnetite, apatite, 

epidote, and garnet, all in accessory quantities. The 

feldspar is more abundant than in No. 22 and is very 

much altered and stained with iron oxides. Both orthoclase and plagioclase are present. The micas occur 

in minute individuals, a fact which accounts for the 

quartzose appearance of the schist in hand specimen. 

No. 24. Mica schist.-Gray micaceous, very schistose rock with silvery sheen on cleavage surface. In 

thin section the rock is well lamellated and is made 

up of allotriomorphic grains mostly 0.3 to 0.5 millimeter in diameter. The essential mineral constituents are quartz, muscovite, and biotite, and the accessory minerals observed are zircon, magnetite, apatite, garnet, and chlorite. The schist contains very 

little if any feldspar but is richer in mica than 

No. 23. 

No. 25. Mica gneiss (or granite gneiss).-Pink, very 

fine grained, delicately banded gneiss. In section the 

rock has a pronounced banded structure in which the 

individual crystals are allotriomorphic and average 

0.15 millimeter in diameter. The rock consists of 

quartz, feldspar (microcline, soda plagioclase, orthoclase?), biotite, and chlorite, named in about the 

order of decreasing abundance. Epidote, apatite, zircon, magnetite, and other iron oxides occur in accessory amounts. The rock has the composition of a 

granite and might well be called a granite gneiss. 

No. 26. Hornblende gneiss.-Dark-green mediumgrained hornblendic rock with a strong suggestion of 

lamellation in the hand specimen. The thin section 

studied, probably cut parallel to the cl~avage of the 

rock, displays little evidence of lamellation. It shows 

a seriate fabric in which the individual crystals are 

allotriomorphic and are 3 millimeters or less in longer 

dimension. The rock consists essentially of feldspar, 

qua:rtz, .hornblende, chlorite, and biotite, named in 

order of decreasing abundance. Epidote, zircon, apatite, iron oxides, and garnet are present in accessory 

amounts. The plagioclase is altered to sericite and is 

stained with iron oxides but probably has the composition of oligoclase. The hornblende and micas have 

very irregular and ragged outlines and are entirely 

without crystal boundaries. The rock represented by 

this specimen was probably derived from a rock having the composition of a quartz diorite. 

No. 27. Mica schist.-Very fissile, slaty silver-gray 

schist. In a thin section cut nearly parallel to the 

schistosity the rock has a nearly equigranular fabric 

in which the constituent minerals are subequant 

to equant and average less than 0.2 millimeter in 

diameter. The dominant mineral constituents are 

quartz, muscovite, and biotite. The feldspar has 

nearly all been altered to sericite but may have constituted as much as 15 or 20 per cent of the rock. 

Zircon, apatite, epidote, and iron oxides are the commonest accessory minerals. The rock is a mica schist 

similar in composition to Nos. 22 and 24. 

No. 31. Chlorite schist.-Silver-gray, micaceous, and 

very schistose. In a thin section cut nearly parallel 

to the schistosity the rock has a seriate fabric in 

which the individual grains are allotriomorphic and 

range in size up to 0.4 millimeter in length. Quartz, 

chlorite, biotite, and muscovite are the dominant mineral constituents, and iron oxides, zircon, apatite, 

tourmaline, epidote, and garnet occur in accessory 

amounts. Feldspar, in large part altered to sericite, 

is present in subordinate amount. The chlorite is 

probably slightly more abundant than either of the 

micas, so the rock may be called either mica schist or 

chlorite schist. 

No. 32. Quartz-mica schist.-Gray compact quartzose 

schist with inconspicuous micas. In thin section the 

rock is found to be very fine grained, few of the largest crystals being as much as 0.2 millimeter across 

and the micas usually being measured in hundredths 

of a millimeter. It consists essentially of quartz, muscovite, biotite, feldspar, and chlorite, named in about 

the order of decreasing abundance. The feldspar is 

subordinate in amount and is in part plagioclase 

(about oligoclase) in fresh individuals, showing good 

. albite twinning. Epidote, zircon, apatite, calcite, and 

iron oxides are the commonest accessory minerals. 

No. 33. Hornblende schist.-Dark-green, moderately 

schistose hornblende rock. The thin section exhibits 

a schistose structure in which hornblende crystals 

making up 90 per cent of the rock, are as a rule. from 

0.5 to 1 millimeter in length and are entirely without 

crystal boundaries. Quartz, feldspar, and epidote 

occur in subordinate amounts ; iron oxides, calcite, 

and apatite are accessory. The feldspar is probably 

all plagioclase, but is extensively altered to sericite. 

No. 34. Mica schist.-Very fine fissile, gray schist 

with thin slaty cleavage. The thin section exhibits 

fine lamellation, in which the banding ranges from 

0.02 to 0.06 millimeter in width, although the individual micas are commonly 0.3 to 0.5 millimeter in 

length. The principal constituents of the rock are 

quartz, muscovite, biotite, chlorite, iron oxides, and 

epidote. Feldspar is present in subordinate amounts, 

but owing to the fineness of grain and the abundance 

of dark minerals its relative proportion is difficult 

to determine. 

The mica schists o:f Group V strongly suggest metamorphosed sediments, but, as is usually the case, microscopic evidence can be considered only auxiliary to the field evidence. 

The rocks o:f this group likewise resemble the 

quartz-mica schists of the Gunnison Oanyon 
ARCHEAN COMPLEX OF GRANITE GORGE, GRAND CANYON, ARIZ. 107 

area, which have been separated from the more 

prevalent biotite schist type of that region. 

The character of the boundaries between 

Groups IV and V and between Groups V and 

VI could not be determined on the present trip. 

GROUP VI. GRANITE GNEISS AND AMPHIBOLITE. 

From a point about a mile west of the mouth 

of Hermit Creek to a point about a mile west 

of the mouth of Clear Creek, a distance of 

about 91 miles, the rocks are chiefly granite 

gneiss and amphibolite. 

On Hermit Creek the prevailing rock 1s a 

coarse pinkish granite gneiss (No. 35). The 

gneissoid character is strongly developed, and 

the banding is very wavy and crumpled in 

detail. The strike of the gneissoid banding is 

N. 20° E. and the dip 80° SE. In places 

the mica in the gneiss is partly segregated 

in curious porphyritic lumps. No. 40 is a 

specimen of this phase of the rock. There 

are areas of a finer-grained gneiss (No. 38), 

and here and there narrow bands of chlorite 

schist (No. 39), probably inclusions. 

The gneiss is everywhere crowded with pink 

pegmatitic injections. The pegmatite consists 

of quartz, microcline, muscovite, and biotite. 

Some of the bodies of pegmatite are several 

hundred feet wide and cuntain crystals of 

muscovite as much as 6 inches in diameter. 

The gneiss in the canyon of Salt Creek 

(No. 45) is somewhat less micaceous than that. 

on Hermit Creek but differs little in general 

appearance. It suggests a sheared granite in 

the field. The strike of the gneissoid banding 

is N. 20° E. and the dip 75° SE. The gneiss 

contains some bands of amphibolite (No. 46), 

which occur at intervals of about 100 feet and 

range from 1 foot to 6 feet in width. They 

have the same strike and dip as the gneisses. 

The Archean was next visited at the " corkscrew " on the Cameron trail, where the trail 

descends to Pipe Creek in the line of the 

Bright Angel fault. The rock west of the fault 

is a pink medium-grained granite (No.49) that 

is somewhat less gneissoid than the gneisses 

already described but otherwise resembles them 

in appearance. The rock on the east side of the 

fault and on the east side of Pipe Creek is a 

much-contorted greenish hornblende schist 

(No. 50), which is much injected with pink 

pegmatitic material. The strike is northeast308300-17-8 

erly and the dip nearly vertical. This hornblende schist was also found underneath the 

tiny cap of Unkar strata that lies below the 

end of the long promontory of Tapeats sandstone jutting out east of Pipe Creek, and 

it appears to be exposed over a considerable 

area to the southeast up Colorado River. 

Petrogmphy of specimens. 

No. 35. Granite gneiss.-Ooarse, lamellated micaceous rock with large crystals ot" feldspar and quartz. 

In thin section the rock exhibits evidence of crushing 

and partial granulation. The individual crystals have 

very ragged outlines and are entirely without crystal 

boundaries. .·They range in size from a fraction of a 

millimeter to 3 millimeters. The rock consists chiefly 

of quartz, biotite, muscovite, sillimanite, and feldspar, 

with accessory apatite, zircon, garnet, and magnetite. 

The larger quartz crystals show undulatory extinction. The feldspar is altered and iron stained, but is 

determinable as orthoclase and soda plagioclase. The 

sillimanite occurs in myriads of long, slender needles, 

which give it a fibrous appearance, and is probably 

of the variety called fibrolite. 

No. 38. Granite gneiss.-Pinkish-gray, rather fine 

grained. The gneissoid structure of the rock is well 

shown in the thin section by the parallel arrangement 

of the micas. The individual crystals are of irregular 

outline, and the largest are 1.5 millimeters in longer 

dimension. Quartz is the most abundant constituent, 

and with altered feldspar, muscovite, biotite, and 

chlorite makes up most of the rock. Garnet, iron 

oxides, apatite, and zircon are the commonest accessory minerals. The feldspar is sericitized and stained 

with iron oxides, but is in part determinable as .soda 

plagioclase. The gneiss was probably derived from a 

granite and may be called either a granite gneiss or 

mica gneiss. 

No. 39. Chlorite schist.- Greenish-gray schistose 

rock. In thin section the schistosity of the rock is 

very pronounced. The bands of chlorite alternate 

with quartz and sericitic muscovite, which has apparently been derived from the alteration of feldspar. 

The rock is rather fine grained, the quartz crystals 

and pseudomorphs after feldspar being, as a rule, not 

more than 0.3 millimeter in longer dimension, although 

the chlorite crystals may be as much as 3 millimeters 

in length. Quartz, chlorite, and sericitic muscovite are 

the chief mineral constituents; subordinate amounts 

of feldspar and iron oxides, with accessory apatite, 

zircon, garnet, and epidote, are also present. 

No. 40. Segregation in gneiss.-The chip is largely 

muscovite in a well-lamellated, contorted structure, 

with feldspar and quartz. The microscope reveals a 

mixture of quartz, altered feldspar, muscovite, chlorite, and iron oxides. Some of the muscovite crystals 

are very large and poikilitically include the other constituents. The feldspar is altered to sericite, somewhat kaolinized and iron stained, but apparently was 

originally orthoclase and soda plagioclase. Zircon 

and apatite were observed in accessory amounts. The 
108 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

mineral constituents of the rock are entirely without 

crystal boundaries, and many of the mica crystals are 

contorted and broken. 

No. 45. Granite gneiss.-Pinkish gray and well 

banded. The thin section exhibits gneissoid structure with · a seriate fabric in which the individual 

crystals are allotriomorphic and range from a fraction of a millimeter to .5 millimeters in longer dimenhighest monadnock of Archean rock in the 

Kaibab division of the canyon, although it is 

several hundred feet lower than a number of 

the monadnocks of Algonkian quartzite. 

GROUP VII. MICA SCHIST. 

sion. Quartz, feldspar, and mica are the dominant From a point half a mile west of the mouth 

minerals; iron oxides, . apatite, and zircon occur in of Clear Creek to a point near the mouth of 

accessory amounts. The feldspar consists of micro- Grapevine Creek, a distance of 3! miles, the 

cline, soda plagioclase, and orthoclase and is slightly rocks are a series of mica schists and mica altered to kaolin and, in a less degree, to sericite. 

No. 46. Hornblende schist or amphibolite~-Dark quartzites which strike northeast and dip vertigreen, medium grained, and very schistose. In a . cally or steep northwest. These rocks, exposed 

section cut nearly parallel to the cleavage the schis- southwest of Vishnu Temple, were named the 

tosity, which is well displayed in the hand specimen, Vishnu terrane by Walcott,z who recognized is not at all evident. _Hornblende, soda plagioclase, 

and quartz are the principal constituents; apatite, theirsedimentaryorigin. In the exposure across 

iron oxides, and zircon are present in accessory the strike there is probably some duplication 

amounts. The plagioclase is largely altered to seri- by folding or faulting, but the thickness must 

cite but apparently had the composition of oligoclase be at least 5,000 feet. The same rocks are ex~ 

to andesine. This schist is similar to No. 4, of posed for a distance of 2 miles along their 

Group I. 

No. 49. Granite.-Pink medium-grained biotite gran- strike in the gorge of Clear Creek, on the 

ite. In thin section the rock has a seriate porphyroid· north side of Colorado River. 

fabric in which the individual crystals are allotrio- In Lone Tree Canyon the prevailing rock is 

morphic and range in size from a fraction of a milli- a fine-grained pinkish to greenish gray quartz..: 

meter to ])].ore than 2 millimeters. Quartz, feldspar, sericite schist (No. 53), which is the least meta-· and biotite are the principal constituents ; iron oxides, . h ed k f d ed · t · · zircon, and apatite occur in accessory quantities. i, morp os roc ? suppose s Imen ary ?rig:tn 

The feldspar is somewhat kaolinized and consists of that was found In the Archean complex In the 

microcline and plagioclase having the approximate Grand Canyon. The original bedding can be 

composition of oligoclase. made out in places and corresponds in the main 

. No. 50. Hornblende schist.-:-Dark- green medium- ' with the direction of the. schistose banding· 

grained schistose rock, with reddish feldspar. In I th t b d d' f g . h d by sr ht 

section the rock shows a schistose structure produced e separa ~ e s are IS In UIS e Ig 

by the parallel arrangement of the hornblende crys- differences In color and texture. Nearly all 

tals. · The individual minerals are allotriomorphic and the rocks have a very fine, almost paper-thin 

range fr~m a fraction of a millimeter to more than slaty lamination. Some beds display a faint 

2 millimeters in longer dimension. The rock consists cross-bedded structure. The prevailing strike 

essenth;tlly of hornblende, plagioclase, quartz, and is between N. 20o E. and N. 7oo E., and the 

biotite, named in about the order of decreasing abundance. The plagioclase, which has approximately dip between 45° and 80° NW. They are cut 

the composition of andesine, has a reddish color due by numerous faults of. small throw. It seemed 

to inclusions of numerous minute specks of iron oxide. probable in the field that the rock was origiApatite, titanite, epidote, iron oxides, and calcite were nally a micaceous shaly sandstone like the 

observed in accessory amounts. Dox sandstone of the Algonkian U nkar group 

About a mile west of the mouth of Clear or like many beds in the Cambrian Tonto 

Creek, on the north side of the river, there is group. Curiously, many of the beds show. 

a huge' irregular body of what appears to be numerous small circular spots of a magenta· 

massive granite in the gneisses. Owing prob- color which are similar to the spots found all 

ably to its massive character this rock resisted through the strata of the U nkar and Tonto 

the pre-Cambrian erosion and now projects groups. 

well above the_ plane of the unconformity at the Specimen No. 54 is a piece of sandstone from. 

base ·of ·.the Tapeats sandstone and excludes the overlying unmetamorphosed Tonto group 

the sandstone entirely . . This monadnock was taken for comparison with the Archean speci-: 

noted by Davis.1 It is, so far as known, the men No . . 53. When the labels are removed it 

1 Davis, W. M., An excursion to the Grand Canyon of the 

Colorado .:. Harvard Coll. Mus. Co.mp. Zoology Bull. 38, geol. 

l!lei'., vol. 5; No. 4, ti'g. 15, p. 174, i901. 

2 Walcott, C. D., Pre-Cambrian igneous rocks of the Unkar 

terrane, Grand Canyon of the . Colorado, Ariz.: U. S. GeoL 

Survey Fourteenth Ann. Rept., pt. 2, p. 506, 1894. 
ARCHEAN COMPLEX OF GRANITE GORGE, GRAND CANYON, ARIZ. 109 

is impossible to tell with the unaided eye which 

is the unmetamorphosed rock, and in thin sections the resemblance is almost as close. 

The rocks of Lone Tree Canyon greatly resemble in the field those of Group V in Slate 

Creek, in the Shinumo quadrangle, but are 

less metamorphosed. There are no pink intrusions in the schists of Lone Tree Canyon, 

but there is a small amount of milky quartz in 

thin veins and stringers. 

In the canyon of Boulder Creek the rocks 

and structure are practically the same as in 

Lone Tree Canyon. 

In the canyon of Grapevine Creek, near the 

east end of the exposures of Group VII, the 

rocks are mica schists and qu.artz-mica schists 

in alternating bands, injected by dikes of coarse 

pink pegmatite. There is a great deal of 

milky quartz in the schists, in veins and 

stringers. The pink intrusive rocks are of the 

same character as those in other groups of the 

Archean. In this locality they occur for the 

most part in very irregular masses, some of 

which appear to be mashed and folded with the 

schists. Specimen No. 56 is mica schist from 

the canyon of Grapevine Creek; No. 57 is 

quartz-mica schist. There is a great deal more 

evidence of metamorphism here than in Lone 

Tree Canyon. The schists are greatly mashed 

and contorted and no prevailing strikes and 

dips are obtainable. These rocks in the canyon 

of Grapevine Creek resemble in the field those 

near the eastern boundary of Group II, at the 

Cable Crossing, in the Shinumo quadrangle. 

Petrography of specimens. 

No. 53. Quartz-sericite schist._;_White, with pink 

tones on cleavage surface. Thin-bedded, platy, with 

fine sandy texture on surface transverse to cleavage. 

In thin section quartz is the most abundant mineral 

and occurs in allotriomorphic grains usually less than 

0.2 millimeter in diameter, although there are coarser 

segregations in which the crystals may be as much as 

0.5 . millimeter across; These coarser areas show less 

interstitial material and may represent pebbles in the 

original sediment. The quartz grains are cemented 

bgether by a finely comminuted aggregate composed 

chiefly of sericitic muscovite, chlorite, and epidote, 

which may have been derived from the cementing 

material of the original rock. Considerable feldspar 

may have originally been present, but it is now almost 

entirely altered to sericite and in minor degree to 

kaolin. Muscovite, biotite, and epidote occur in subordinate amounts; zircon, tourmaline, apatite, and 

iron oxides are the commonest accessory minerals. 

The rock has · the appearance of a more or less metamorphosed shaly sandstone or grit, although it does 

not show pronounced evidence of recrystallization and 

pressure. Th~ term quartz-sericite schist is suggested 

as indicating its content and structure without connoting its origin. The terms mica quartzite or sericite 

quartzite might be used equally well. The rock is 

strikingly similar in structure and composition to 

specimen No. 54, from the Cambrian Tapeats sandstone, although it is less even grained and apparently 

contains mor{~ interstitial material. However, a section oblique to the cleavage would have been more 

satisfactory for comparison of the two rocks than 

the one studi·~d. 

No. 54. Tar.•eats sandstone.-Bluish-gray thin, platy 

sandstone. The section, cut obliquely to the cleavage 

of this rock, resembles No. 53 rather closely. More 

than 90 per ·~ent of the rock is quartz in grains of 

irregular shape, averaging about 0.2 millimeter in 

diameter. Mica, chiefly biotite, and sericitic muscovite have b·~en developed and occur in bands about 

0.5 millimeter apart, giving the rock its good cleavage. A few 1·elics of much-altered feldspar are present, and chl•)rite, epidote, zircon, and iron oxides 

were observed in subordinate or accessory quantities. 

No. 56. Mica schist.-Gray, with silvery sheen on 

cleavage surface, well lamellated and fine grained. 

In section the rock has a pronounced schistose. structure in which the individual grains are allotriomorphic 

and are usually less than 0.2 millimeter in diameter, 

although the mica::; may be as much as 1 millimeter in 

longer dimenBion. Quartz, muscovite, and biotite are 

the chief mineral constituents of the rock. The feldspar has been almost entirely altered to sericite and 

kaolin but was probably subordinate to the minerals 

named. Epidote, zircon, apatite, and iron oxides were 

observed in accessory amounts. 

No. 57. Qutwtz-mica schist.-Fine grained, compact, 

dark gray, quartzose. In thin section the rock has a 

vague schistose structure, and the individual crystals 

are allotriOID<)rphic and average about 0.2 millimeter 

in diameter. Quartz, feldspar, biotite, and muscovite 

are the dominant mineral constituents, named in about 

the order of descending abundance. The feldspar is 

altering to sericite and in less degree to kaolin but 

apparently was originally orthoclase and soda plagioclase. The micas are not so abundant as in most of 

the other schists and occur in fine leaves. Zircon, 

apatite, epidote, and iron oxides are the commonest 

accessory minerals. 

Specimens Nos. 56 and 57 resemble Nos. 31 and 32, 

respectively. 

As was the case with most of the other groups 

of Archean rock, neither the eastern nor the 

western boundary of Group VII could be 

reached on the present trip because there was 

no place near them where a break in the Tapeats cliff would permit descent. Viewed from 

a distance, the boundary between Groups VI 

and VII ?peared to be a fault, but the increasing intensity of metamorphism and the 

injected character of the eastern border o:f 
110 SHOHTER. CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

Group VII suggested that the boundary between it and Group VIII may be an intrusive 

contact. 

GROUP VIII. GRANITE GNEISS. 

Group VIII is exposed from a point near 

the mouth of Grapevine Creek to the east end 

of the Granite Gorge, a distance of 3-r"0 miles. 

The rocks were examined in the small canyon 

half a mile east of Cottonwood Creek, in the 

canyon of Hance Creek, and in the Granite 

Gorge between Mineral Canyon and Red Canyon. In the locality east of Cottonwood Creek 

the prevailing rock is a much-contorted pinkish gneiss (No. 60) , which in places grades into 

a more micaceous gneiss (No. 61). The gneiss 

is much injected by pink pegmatite and milky 

quartz, which appear to be folded with it, and 

is cut by younger bodies of pink pegmatite. 

Because of the intense crumpling of the gneiss 

there is no general strike and dip traceable 

over any large area. 

The gneiss in the canyon of Hance Creek 

resembles that just described. There are two 

chief types which grade into each other-a 

pink granite gneiss (No. 62) and a gray micaceous gneiss (No. 63). The amount of pink 

pegmatitic material is even greater than in 

the locality east of Cottonwood Creek, and 

some of it may be folded and mashed with the 

gneiss. In places the gneiss itself grades into 

pegmatite and aplite. The contortion is so 

great that no prevailing strike and dip can be 

made out. 

Between Mineral Canyon and the east end 

of the Granite Gorge the rocks are in gneissoid bands of varying composition, but the 

prevailing rock is pink granite gneiss (Nos. 64 

and 66). There are scattered bands of garnetiferous mica schist (No. 65) and bands of biotite schist (No. 67), also considerable pegmatite. 

In general, the strike of the gneissoid banding 

is northeasterly and the dip nearly vertical. 

Petrography of specimens. 

No. 60. Granite gneiss.-Red, fine-grained, and 

banded. The section, which is cut parallel to the 

banding, exhibits a seriate porphyroid fabric in which 

the individual crystals range from a small fraction 

of a millimeter to more than 1 millimeter in diameter. 

Quartz, feldspar, and biotite are the principal con· 

stituents. The feldspar consists of orthoclase, microcline, and microperthite, in nearly equal amounts, and 

plagioclase having about the composition of oligoclase. 

The plagioclase and orthoclase particularly and the 

microcline to a less degree are weathere(i and stained 

and include numerous fine grains of iron oxides 

which give them a reddish color. The biotite is 

leached out and is altering to chlorite. Muscovite, 

apatite, zircon, and iron oxides occur in subordinate 

or accessory amounts. 

No. 61. Mica gneiss.-Well-banded gneissoid rock 

containing considerable mica. Under the microscope 

the rock exhibits a gneissoid structure and is made 

up of allotriomorphic crystals, the majority of which 

range between 0.5 and 1.5 millimeters in longer 

dimension. Quartz, plagioclase (chiefly oligoclase), 

biotite, and orthoclase are present, named in about 

the order of decreasing abundance. As in No. 60, the 

feldspars are weathered and iron stained and the 

biotite is being leached and altered to chlorite. Zircon, apatite, and magnetite are present as accessory 

minerals; chlorite, iron oxides, and epidote are largely 

products of surface alteration. The composition of 

this rock hardly permits its being considered a metamorphosed granite of the normal microcline type. 

The uneveri fracture, the segregation of the several 

constituents, and the general appearance in the hand 

specimen, together with the predominance of plagioclase over potash feldspar in the thin section suggest 

that it is an injection gneiss. 

No. 62. Granite gneiss (or gneissoid granite).-

Pink medium-grained granitic rock. The thin section 

exhibits a seriate intersertal fabric in which the individual crystals are allotriomorphic and range in diameter from less than 0.5 to 2 millimeters. The rock 

consists of quartz, microcline, soda plagioclase, orthoclase, biotite, and muscovite, named in approximately 

the order of decreasing abundance. The orthoclase 

and plagioclase are extensively altered to sericite 

and kaolin and are colored red by fine inclusions of 

iron oxides. Apatite, zircon, and magnetite occur in 

accessory amounts, and muscovite, epidote, chlorite, 

and iron oxides are for the most part products of 

alteration. Many of the larger individuals, particularly the microcline, show poikilitic structure in thin 

section, and a few of the quartz grains show undulatory extinction. This feature, together wit.h the intersertal fabric, is indicative of the mashing and partial recrystallization suffered by the original granite. 

No. 63. Granite gneiss.-Dark-gray medium-grained 

biotite-rich gneiss. In thin section the gneiss has a 

seriate fabric in which the individual crystals are 

of irregular shape and range from a fraction of a 

millimeter to more than 3 millimeters in longer dimension~ Quartz, altered feldspar, biotite, and muscovite 

are the _- principal constituents. The feldspars- -are 

extensively weathered and iron stained, and kaolin 

is the chief alteration product, although considerable 

sericite has also been developed. Some of the biotite 

is brown and some green, and it is in many places 

leached or altered to chlorite and magnetite. Epidote, 

zircon, apatite, and iron oxides occur in minute 

quantities. 

No. 64. Granite gneiss.-Pinkish fine-grained granite 

gneiss. In section the rock has but a suggestion of 

gneissoid structure and has a seriate intersertal fabric 

in which the individual crystals are allotriomorphic 
ARCHEAN COMPLEX OF GRANITE GORGE, GRAND CANYON, AR.IZ. 111 

and are as a rule not more than 1 millimeter in longer 

dimension. Mineralogically the rock is very similar 

to Nos. 60 and 62 and consists essentially of quartz, 

soda plagioclase, microcline, muscovite, biotite, and 

possibly a small amount of orthoclase, with the usual 

accessory and secondary constituents. The undulatory extinction of the quartz, the poikilitic character 

of the larger crystals of microcline, and the intersertal fabric are evidences of the pressure and partial 

recrystallization suffered by the rock. 

No. 65. Coarse garnetitero'ltS mica schist.-Coarse, 

well-lamellated mica aggregates. The section shows 

a schistose mixture of biotite, muscovite, garnets, and 

iron oxides (chiefly magnetite). The micas are 10 

millimeters or more in length. The garnets are pink, 

many of them include biotite and iron oxides, and a 

few are surrounded by rims of chlorite, to which the 

garnet is apparently altering. A few grains of zircon 

were observed, and a small amount of badly stained 

feldspar may be present in one of the two slides studied. 

No quartz was recognized in the sections, although 

a little may be present in the hand specimen. 

No. 66. Granite gneiss.-Pinkish gray and gneissoid. 

Under the microscope the rock is found to have a 

seriate intersertal fabric in which the individual 

crystals are allotriomorphic and range in size from 1 

a small fraction of a millimeter for the interstitial 1 

crystals to 4 millimeters and more for the larger ones. 

The rock is very similar to the granite gneisses 

already described and consists of quartz, orthoclase, 

microcline, soda plagioclase, biotite, muscovite, and 

iron oxides, named in about the order of decreasing 

abundance. Apatite and magnetite occur in accessory 

amounts, and chlorite, sericite, kaolin, epidote, and 

iron oxides have resulted from weathering and other 

processes of alteration. 

No. 67. Biotite schist.-Dark gray, with obscure 

pinkish tones ; well foliated and reddish brown on 

weathered surfaces. A section cut parallel to the 

schistosity discloses a nearly equigranular fabric in 

which the individual crystals are allotriomorphic and, 

save for micas, are rarely as much as 0.2 millimeter 

in diameter. The micas are of irregular and ragged 

outline and are usually about 0.3 millimeter in longer 

dimension. Feldspar, extensively altered to sericite, 

quartz, and biotite, are the principal mineral constituents, and apatite, iron oxides, epidote, zircon, and 

garnet were observed in accessory amounts. The 

feldspar is too much altered for determination but 

was originally present in quantity equaling or exceeding the quartz. Inclusions of apatite prisms are 

unusually numerous, but muscovite of the sericitic 

variety is rare or absent. This rock has been entirely 

recrystallized and has been rendered completely 

schistose during the process of metamorphism, so 

that no trace of its original structure remains. However, it was most probably derived from a massive 

igneous rock of granitic character. It resembles Nos. 

1, 3, and 4 and other mica schists already described, 

although under the microscope it is found to nave a 

higher feldspar content. It is very similar to the 

most prevalent type of biotite schist of the Gunnison 

River area in Colorado. 

CONCLUSIONS. 

CORRELATION. 

It will be apparent to the reader that the 

present study, which was made at points where 

the topography permitted access to the Archean rather than in places where the importance of Archean structure demanded attention, 

has yielded data that are rather too fragmentary to be of much value in solving the 

problems of Archean structure and history in 

the Grand Canyon region. Yet it has afforded 

a fair estimate of the general character of the 

rocks, and a few conclusions may be drawn. 

The three groups of mica schist (Groups II, 

V, and VII) are apparently parts of a single 

series of rocks. They are practically identical 

in mineral composition, are similar in appearance in the field, and are supposed to have had, 

for the most part, a similar origin. 

Groups I, VI, and VIII, chiefly granitic 

gneiss, hornblende gneiss, and amphibolite, 

comprise rocks which, collectively, are very 

similar in lithology and structure. They may 

be included in one series and called gneisses, 

although the rocks within the groups represent 

several periods of geologic time and doubtless 

when a detailed study is made they will be differentiated into more than one series on the 

evidence of their origin and history. 

The relations of the greenstones comprising 

Group IV are uncertain, but the metabasites 

resemble many of those in the gneiss series. It 

seems probable that this group belongs with 

the gneisses, though further investigation may 

show that the metadiorite is a part of the 

diorite of Group III that has suffered thermal 

metamorphism. 

The massive basic rocks are the quartz diorite 

and hypersthene gabbro of Group III. They 

probably represent postdeformation intrusions 

or, at any rate, relatively late intrusions in the 

complex. 

The pink siliceous intrusive rocks comprise 

granite, aplite, and pegmatite and are the 

most widely distributed of the Archean rocks. 

They probably represent several periods of 

igneous activity. The older material is associated chiefly with the gneisses and in places 

appears to be mashed and folded with them. 

It does not appear to be associated with the 

schists except in places near the borders of the 
112 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1916. 

schist groups. The younger material is by far 

the most abundant and widely distributed rock 

of the series. It is chiefly coarse pegmatite, 

which occurs typically in dikes that cut every 

group of rock in the Archean. . . 

Estimated roughly, the gneisses compnse 

perhaps 50 per cent of the rocks exposed in the 

Granite Gorge, the mica schists 30 per cent, 

the basic intrusive rocks 10 per cent, and the 

pink siliceous intrusive rocks 10 per cent. T~e 

relative amount of the pink siliceous rocks IS 

hardest to estimate because of their wide 

though irregular distribution. It may be very 

much more than 10 per cent. 

ORIGIN AND HISTORY. 

The mica schists are clearly the products of 

regional metamorphism, for nearly everywhere they are thoroughly schistose and entirely recrystallized. Those in Lone Tree 

Canyon are the least metamorphosed, and in 

that locality there is evidence that they represent an originally sedimentary series. Inasmuch as the mica schists in all the other 

localities are very similar in mineral character 

to those in Lone Tree Canyon, it seems not 

improbable that they also are in large part, if 

not entirely, of sedimentary origin. 

Both the field relations and the microscopic 

studv indicate that the gneisses are igneous in 

origin, but it is clear that they have had a 

long and complex history. Most of them are 

medi urn to coarse grained rocks with a pronounced gneissoid structure, which is in general parallel in trend with the similar structure 

in the schists. These gneisses, with the schists, 

have gone through at least one period of 

regional metamorphism. Many of the gneisses 

are thoroughly recrystallized; some appear to 

represent metamorphosed quartz diorites; 

others, metamorphosed granites; still others, 

metamorphosed basic rocks. Some of the 

granite gneisses are injection gneisses; others, 

very little altered, are pressed granites. 

Throughout the gneisses granitic rock,t whether 

older and very gneissoid or younger and little 

sheared, is the predominating type. In places 

the gneiss contains bodies of chlorite or mica 

schist that are thought to be inclusions. 

Perhaps the most important problem of the 

Archean geology of the region is the rel:!tion 

1. The term granitic is used here in a broad sense for rocks 

that were originally granite, granodiorite, monzonite, or 

quartz diorite. 

of the gneisses to the schists. If the schists 

are, as the evidence seems to indicate, a ~etamorphosed sedimentary series, are the ~eisses 

the basement upon which these old sediments 

were laid down or are the gneisses wholly .or ' . in part intrusive in the schists~ The solutiOn 

of this problem must depend on future study 

in the field. Unfortunately, on the present 

trip none of the boundaries between the groups 

of ;ocks were near the places where access to 

the Archean was to be had, and so they could 

not be examined. These boundaries are the 

critical localities of the Archean, and the study 

of them will undoubtedly lead to the deciphering of much of the geologic ~is~ory. . . 

Some evidence that may aid In solving this 

problem was obtained on the present trip. ?n 

Slate Creek a broad band of hornblende gneiss 

was found between the prevailing bands of 

mica schist· associated with the hornblende ' . gneiss is a band of mica ~eiss .or ~ranite 

gneiss. The study of these gneisses In t~nn ~ection indicates that the hornblende gneiss 1s a 

metamorphosed quartz diorite and that the 

mica gneiss is a metamorphosed granite. The 

hornblende gneiss and the mica gneiss appear 

to represent quartz diorite and granite intruded into the schists before the close of the 

period of regional metamorphism. Thus it is . 

probab~e that a part, at least, of the rocks of 

the gneiss series are intrusive in the schists. 

The great amount of pegmatitic material in 

the schists near the eastern boundary of Group 

VII suggests that this boundary may prove to 

be an intrusive contact between theschists and 

the granitic gneiss (probably an injection 

gneiss) of Group VIII. 

It is the impression, of the writers that when 

the history of the gneisses and schists is worked 

out it may be found that some of the greatly 

wrinkled and contorted granitic gneisses are 

the oldest rocks of the Archean and are a part 

of the original basement upon which the schist 

series of metamorphosed sediments was laid 

down; that both before and for some time 

after the deposition of the sediments there 

were long periods of complex igneous intrusion 

now represented by amphibolite, granite gneiss, 

and metadiorite; and that some of these intrusives penetrated the sediments after they were 

deposited. 

The most obvious event in the Archean history is the regional metamorphism that in-
ARCHEAN COMPLEX OF GRANITE GORGE, GRAND CANYON, ARIZ. 113 

volved both the gneiss and schist series. As a 

result of it most of the rocks were recrystallized and nearly all of them acquired a gneissaid or schistose banding which has a dominant 

northeast strike and a nearly vertical dip. 

This structure suggests that the compressive 

forces acted either from the northwest or from 

the southeast. It may be noted that a similar 

structure characterizes the Archean complex in 

the Globe region described by Ransome,t and 

the complex on the west and southwest border 

of the Grand Canyon district, described by 

Schrader.2 

The massive basic intrusive rocks of Group 

III may be younger than the period of regional 

metamorphism. The quartz diorite cuts the 

schists of Group II, but the relation of the 

gabbro to the diorite and that of both the 

gahbro and the diorite of Group III to the 

rocks of Group IV are not known. None of 

the earlier pink siliceous intrusives are found 

in the diorite and gabbro, which, however, are 

cut by the later pegmatites of the pink siliceous 

series. 

The age of the earlier granite and pegmatite of the series of pink siliceous intrusives 

is not clear. Some of the material is associated with certain granite gneisses in such 

a way as to suggest that its intrusion accompanied or closely followed that of the material 

from which this gneiss was derived. It was 

rather difficult to determine whether this material has really suffered deformation, whether 

it was intruded during deformation, or 

v1hether it was intruded after deformation, following the contortions of an already deformed 

structure. Detailed study may show, therefore, that some of these rocks should be classed 

with the gneisses, if they prove to have suffered 

deformation. No doubt some of the pegmatitic injections accompanied th.e regional 

metamorphism. Others are thought to have 

been associated with certain injection gneisses 

whose relative age is not yet known. 

There are many bodies of pink granite that 

are clearly later than the regional metamorphism. Perhaps the latest outburst of peg1 Ransome, F. L., Geology of the Globe copper district, 

Ariz.: U. S. Geol. Survey Prof. Paper 12, p. 24, 1903. 2 Schrader, F. C., Mineral deposits of the Cerbat Range, 

Black Mountains, and Grand Wash Clift's, Mohave Cot..nty, 

Ariz.: U. S. Geol. Survey Bull. 397, p. 29, 1909. 

matitic activity accompanied or closely followed the intrusion of these granites. The 

pegmatites of this latest outburst cut every 

group of rock in the Granite Gorge and record 

the latest decipherable event in the igneous 

activity of Archean time. Some of these pink 

granitic intrusives are usually visible, from 

whatever point the visitor may view the inner 

canyon. Because of their wide distribution 

and conspicuous appearance the name Granite 

Gorge is not inappropriate, although pure 

granite is far from being the most abundant 

rock in the gorge. 

The following summary, by Lindgren,3 of 

the igneous activity in the Cordilleran region 

in pre.,.Cambrian time is of interest in comparison with the present study of the Archean 

in the Grand Canyon. The agreement seems 

close enough to show that when the igneous 

history of the pre-Cambrian in the Grand Canyon is unraveled it will be found to differ little 

from that of the great region of which the 

Grand Canyon district is a part. 

Pre-Beltian: 

Granitic gneisses (oldest) ; restricted areas. 

Surface lavas and tuffs; restricted areas. 

Granitic intrusions, now gneisses ; small areas. 

Basic intrusions ; small areas. 

Granitic intrusions ; very large areas. 

Pegmatitic intrusions ; large areas. 

Beltian : Diabase intrusions, probably with basalt 

flows ; small areas. 

The pre-Beltian of Lindgren is the Archean 

o£ the Grand Canyon. The Beltian is the 

Algonkian Grand Canyon series; it is made up 

in the Grand Canyon, as elsewhere, of diabase 

intrusions and basalt flows. 

The Archean complex of the Grand Canyon 

is now known as the Vishnu schist in the usage 

of the United States Geological Survey. It is 

evident, however, from the present study that 

the name includes two or more very different 

series of rocks. Doubtless it will be advisable 

at some future time to restrict the name Vishnu 

schist to the mica schist series and give another 

name or names to the gneisses, but until a more 

detailed study is made a change in the present 

usage would be premature. 

3 Lindgren, Waldemar, The igneous geology of the Cordilleras and its problems : Problems of American geology, p. 

246, Yale University Press, 1915.