[From The American Journal of Science, Vol. XXIX, June, 1910.] 
 
 CONTRIBUTIONS to the GEOLOGY of the 
 GRAND CANYON, ARIZONA.— The 
 GEOLOGY of the SHINUMO 
 
 AREA. 
 
 % 
 
 S' A ^ 
 
 By L. F. Noble. 
 (With Plate VJ 
 
 (Contributions from the Geological Department, 
 Yale University, New Haven, Conn*, U* S. A*) 
 
b b I 
 
 N &<o c. 
 
 THE 
 
 AMERICAN JOURNAL OF SCIENCE 
 
 [FOURTH SERIES.] 
 
 
 Art. XXXII.— Contributions to the Geology of the Grand 
 Canyon , Arizona.—The Geology of the Shinumo Area • 
 by L. F. Noble. 
 
 Part I. 
 
 Preface. 
 
 Introduction. 
 
 Location. 
 
 Field Work. 
 
 Literature. 
 
 Acknowledgments. 
 
 Geologic Nomenclature. 
 
 Topography, Climate and Vegetation. 
 General Geology. 
 
 Archean — Vishnu Schist. 
 
 Name. 
 
 Distribution in the Kaibab Division. 
 
 Occurrence and Distribution in the Shinumo Area. 
 Lithology. 
 
 Origin. 
 
 Age and Correlation. 
 
 Bibliography. 
 
 Preface. 
 
 Exposures of unaltered pre-Cambrian sediments are of great 
 interest to the geologist in any part of the world. Especially 
 is this true in the Grand Canyon of Arizona, where the cer¬ 
 tainty of stratigraphic position and completeness of exposure 
 are almost without a parallel, and where the spectacular nature 
 of the occurrence partakes of the stupendous and fantastic 
 scenery of that mighty gorge, from the spell of which the 
 observer is never entirely free. During a pleasure trip in the 
 Grand Canyon region in January, 1908, in company with 
 Professor H. E. Gregory of Yale and others, the writer chanced 
 to visit a hitherto undescribed exposure of these rocks on the 
 
 Am. Jour. Sci.— Fourth Series, Vol. XXIX, No. 173 .—May, 1910. 
 
 25 
 
370 Noble—Geology of the Grand Canyon , Arizona. 
 
 Colorado River at the foot of Bass Trail. Although only a 
 few hours were spent in the vicinity, the diagrammatic sim¬ 
 plicity with which the structural relations were revealed was 
 such that the interest could be immediately realized. In the 
 summer of that year the writer returned with the intention 
 of making a detailed study of these pre-Cambrian rocks. He 
 speedily found that the interest in this part of the Grand 
 Canyon did not end with the study of the pre-Cambrian alone: 
 it was found that the line of displacement of the West Kaibab 
 fault which runs into the Grand Canyon at this point was itself a 
 remarkable structural feature and worthy of considerable atten¬ 
 tion ; that the entire Paleozoic section of the canyon wall was 
 here, as elsewhere, undescribed in absolute detail; and finally 
 that there were problems of physiographic interest in the Grand 
 Canyon region the ultimate solution of which could be greatly 
 helped by a detailed study at this point. It was therefore de¬ 
 cided to extend the work to a complete areal study of the 
 region, including for this purpose the greater part of the new 
 Sliinumo topographic sheet of the United States Geological 
 Survey. This report, accompanied by an atlas containing the 
 geologic maps, sections, and plates, was presented to the faculty 
 of the Graduate Department of Yale University in June, 1909, 
 as a thesis in partial fulfillment of the requirements for the 
 degree of Ph.D., under the title of “ The Geology of the 
 Sliinumo Area, Grand Canyon, Arizona.” 
 
 It is intended to present the substance of the thesis in two 
 or more articles in this Journal. The entire report is to be 
 published in extenso at a later date as a Bulletin of the United 
 States Geological Survey. For this reason, as well as because 
 of the. large scale and elaborate nature, the geologic map, sec¬ 
 tions, and plates will not be published here. 
 
 The first article will be presented in two parts. Part I, now 
 published, comprises a general introductory description of the 
 Sliinumo area and deals with the basement rocks of the Archean. 
 Part II, to follow in a later number, will be devoted to the 
 Algonkian rocks of the Grand Canyon series and will include 
 a map showing the distribution of the Vishnu and Grand 
 Canyon series in the Grand Canyon. 
 
 It is also proposed to devote a later article or articles to the 
 Paleozoic section of the canyon wall; to a structural study of 
 the area, including both the pre-Cambrian and later structure 
 and a study of the displacements on the line of the West 
 Kaibab fault; and finally to features of physiographic interest 
 in the area,—such as the drainage system of the plateaus and 
 the origin of the Esplanade and Tonto platforms. 
 
The Shinurno Area. 
 
 371 
 
 Introduction. 
 
 Location and geography .—The area studied lies in Coconino 
 County in the northern part of Arizona in what is known as 
 the Plateau Province of the Territory. It is inclosed between 
 meridians 112° 25 r and 112° 15 r and parallels 36° 20' and 36° 
 05', and includes about 150 square miles. It comprises rather 
 more than the eastern half of the area of the Shinurno topo¬ 
 graphic sheet of the United States Geological Survey. 
 
 The greater part of the area lies within the depths of the 
 Grand Canyon and includes the Shinurno Amphitheater, which 
 is the largest of the great amphitheaters in the Kaibab divi¬ 
 sion. The eastern and northern parts of the area include that 
 part of the “ mainland ” north of the canyon which forms the 
 immediate rim of the Shinurno Amphitheater, reaching from 
 the promotory of Point Sublime on the east to the Muav 
 Saddle on the west; this northern mainland is a part of the 
 Kaibab Plateau. The northwestern part of the area includes 
 the surface of Powell Plateau, a great butte, or u island,” lying 
 athwart the course of the Grand Canyon. The southwest cor¬ 
 ner includes a part of the southern “ mainland ” which cul¬ 
 minates in the great promontory known as Havasupai Point; 
 this southern mainland belongs to the Coconino Plateau. 
 
 About a mile west of Havasupai Point is situated Bass Camp, 
 the only habitation in the area. The camp was established by 
 Mr. W. W. Bass some twenty-five years ago to accommodate 
 the tourist traffic. From this point a trail was constructed to 
 the river, which was crossed by boat at times of low water. 
 A permanent camp was established in the bottom of the can¬ 
 yon on the north side of the river about a mile up Shinurno 
 Creek, where an irrigated garden was made upon the site of an 
 ancient one cultivated by the prehistoric inhabitants of the 
 region. Later a trail was constructed up Muav Canyon to the 
 north rim, by means of which access may be had to points on 
 the Kaibab Plateau and the settlements in Southern Utah. 
 Recently deposits of copper and asbestos have been discovered 
 by Mr. Bass in the depths of the canyon, to which he has con¬ 
 structed additional trails. In March, 1908, three wire cables 
 were suspended across the granite gorge at the foot of Bass 
 Trail. On these cables travels a wire car, so that at the present 
 time the Colorado River may be crossed at this point regard¬ 
 less of high water. Bass Camp is most easily reached by 
 wagon road from Bass Station on the Grand Canyon branch of 
 the Sante Fe Railroad, a distance of 20 miles southeast; or 
 from the El Tovar Hotel at the terminus of the railroad 25 
 miles east. Another road leads 20 miles southwest to the rim 
 of Cataract Canyon, into which a trail leads down to the Supai 
 
372 Noble—Geology of the Grand Canyon , Arizona. 
 
 Indian village. Roads also lead to the towns of Williams and 
 Ash Fork, about 60 miles respectively south and southwest. 
 
 Field Work. —The geological field work upon which the 
 thesis is based was begun upon the 23d of August, 1908, and 
 ended upon the 12th day of December of the same year. The 
 greater part of the work was done from a permanent camp on 
 the Shinumo, which served as a base of operations for studying 
 all the region in the greater depths of the canyon. Two trips 
 were made to the north rim. The first was made in September 
 and the second in November. Each trip lasted a week. Camp 
 was established in the Muav Saddle and excursions were made 
 from there over the surface of Powell Plateau and over the 
 Kaibab mainland as far as the head of the Shinumo Amphi¬ 
 theater. The writer was accompanied on these occasions, as 
 well as during his stay in camp on the Shinumo, by Mr. John 
 Walthenberg as guide, whose detailed knowledge of the region 
 and whose assistance in other ways were invaluable. The work 
 in the southern part of the area was done from Bass Camp, 
 from which point excursions were made over the Coconino 
 Plateau and into the upper part of the canyon. 
 
 Literature. —Two references to the geology of the area are 
 to be found in geological literature: 
 
 Captain Dutton in his monograph entitled “The Tertiary 
 History of the Grand Canyon District” (Dutton, a) describes 
 most fully and charmingly the geology of the north rim in 
 this section of the Grand Canyon. In Chapter YII he describes 
 the surface features and scenery of the Ivaibab Plateau in the 
 vicinity of Point Sublime. Chapter VIII is devoted to the 
 panorama disclosed from Point Sublime, while Chapter IX 
 describes the walls of the amphitheaters of the north side in 
 detail. The Muav Saddle and Powell Plateau are described 
 on pp. 162-167, and the Shinumo Amphitheater on pp. 167-174. 
 His work, however, did not extend into the depths of the 
 canyon. 
 
 Mr. J. S. Diller of the United States Geological Survey 
 describes in his report on the production of asbestos in u Min¬ 
 eral Resources for 1907” (Diller, a) the deposits of asbestos 
 occurring in the Algonkian sediments of this area near Bass 
 Ferry. This is the only reference in the literature to the 
 presence of Algonkian strata in this part of the Grand Canyon. 
 
 Acknowledgments. —In 1901 Mr. Charles D. Walcott and 
 Mr. G. K. Gilbert spent several days at Mr. Bass’s camp on the 
 81iinumo and at that time worked out the structure of the 
 pre-Cambrian sediments, which Mr. Walcott correlated with 
 the section described by him in Unkar Valley (Walcott, a, b , 
 c, d , and e). His notes, however, are unpublished, and it is 
 due to his kindness and courtesy that the writer is enabled to 
 present the first description of the area. To Mr. Walcott the 
 
The Shinumo Area. 
 
 373 
 
 writer is also indebted for the identification of Cambrian 
 fossils, for a list of the Cambrian fossils found in the region, 
 and for assistance in interpreting the stratigraphy. 
 
 To Professor Joseph Barrell, to Professor Charles Schu- 
 chert, and to Professor Louis V. Pirsson, all of Yale University, 
 the most sincere thanks are due for continued interest and 
 advice during all stages of the work. To Mr. W. W. Bass the 
 writer is indebted for material assistance and guidance during 
 his work in the field. 
 
 Geologic Nomenclature .—The geologic names employed in 
 this article are revised names recently authorized by the 
 United States Geological Survey, with the exception of those of 
 the Cambrian, which as yet have not been formally revised. 
 The first table below gives the rock formations of the Shinumo 
 area in terms of the revised terminology, while the second 
 table gives the equivalent of the newer in the older nomen¬ 
 clature. 
 
 System 
 
 I. 
 
 Geologic column of the Shinumo Area. 
 
 Series Group Formation 
 
 r 
 
 Carboniferous 
 
 f Kaibab limestone 
 
 Pennsylvanian ... Aubrey .. Coconino sandstone 
 
 (_ Supai formation 
 
 Pennsylvanian and Redwa]1 __ Redwall liraestone 
 Mississippian 
 
 (Unconformity of erosion without uniformity of dip) 
 
 “ Marbled limestone” 
 
 Cambrian . 
 
 Acadian 
 
 (and Saratogan?) 
 
 . Tonto _. 
 
 “ Tonto shale ” 
 
 “ Tonto sandstone ” 
 
 (Great angular unconformity) 
 
 Algonkian_Grand Canyon .. T Unkar 
 
 (Greatest angular unconformity) 
 
 Archean.. Vishnu 
 
 IT. 
 
 Classic Nomenclature of the reports of 
 Revised Nomenclature Dutton , Gilbert, and Walcott. 
 
 Formation ' Formation Group 
 
 Kaibab limestone_Cherty limestone- } tt Anhrev 
 
 Coconino sandstone_Cross-bedded sandstone .. \ ' * ^ 
 
 Supai formation. .Lower Aubrey sandstone._ Lower Aubrey 
 
 Redwall limestone._Redwall limestone—.— Redwall 
 
 “Marbled limestone” .. Marbled limestone --| t T T t 
 
 •‘Tonto shale”.. Tonto shale .j Uppel i0 10 
 
 a Tonto sandstone ”_Tonto sandstone.— Lower Tonto 
 
374 Noble—Geology of the Grand Canyon , Arizona. 
 
 Topography , Climate , Vegetation. 
 
 The surfaces of the plateaus in the Shinumo area through 
 which the pathway of the Grand Canyon is trenched are every¬ 
 where developed at the same horizon on the highest member 
 of the Paleozoic series occurring in the Canyon wall and known 
 as the Kaibab limestone, the Mesozoic and Tertiary formations 
 having been stripped back to the terraces of southern Utah. 
 
 The surface of the southern, or Coconino, plateau slopes to 
 the southwest away from the canyon rim at the rate of about 
 200 feet to the mile. The drainage system of the plateau 
 surface consists of a series of mature, open-floored valleys with 
 gently sloping sides, which contain no living streams. These 
 valleys trend southwesterly away from the canyon rim with 
 the slope of the plateau surface and drain into the broad, shal¬ 
 low synclinal basin occupied by Cataract Canyon. In tracing 
 one of these mature valleys toward its head, it is a common 
 thing to find, it truncated as a hanging valley by the wall of 
 the Grand Canyon. So general is this phenomenon, that the 
 stranger who loses his way on the southern plateau has only to 
 keep in mind that if he follow any valley far enough headward 
 he will come out upon the rim of the Grand Canyon. 
 
 The surface of the northern, or Kaibab, plateau is in every 
 respect similar to that of the Coconino. The same system of 
 mature valleys covers its surface, which slopes southwesterly to 
 the rim of the canyon. The only difference is that the surface 
 drainage runs into the Grand Canyon instead of away from it. 
 Neither plateau surface contains a living stream. 
 
 Powell Plateau may be regarded as a dismembered part of 
 the Kaibab. Its surface is developed at the same horizon on 
 the Kaibab limestone, slopes southwesterly at the rate of 900 
 feet in five miles, and contains the same mature drainage 
 system. It is substantially a great island, surrounded on three 
 sides by mile-deep canyons, and isolated from the mainland by 
 erosion in the line of displacement of the West Kaibab fault. 
 The gap that separates Powell Plateau from the mainland is 
 calied the Muav Saddle, and is notched 800 feet below the 
 surface of the plateaus. No more striking topographic con¬ 
 trast can be imagined than that between the mature drainage 
 systems of the plateaus and the youthful topography within 
 the deep-trenched canyon. 
 
 The strata of the Paleozoic rock system within the area dip 
 gently and almost uniformly to the southwest at the rate of 
 about 200 feet to the mile, and the surfaces of the plateaus are 
 everywhere accordant with the rock structure. A slight local 
 interruption occurs where the line of the West Kaibab fault 
 crosses the area, along which the strata are flexed into a mono- 
 
The Shinumo Area. 
 
 375 
 
 cline clipping sliarply northeastward. A few minor flexures 
 cross the area in a northwesterly direction; all dip to the 
 southwest and are hardly more than gentle swells upon the 
 general warping. 
 
 The Shinumo Area is a critical area for the study of the 
 topography within the canyon itself. It is here that the topog¬ 
 raphy undergoes a transition from the profile which is char¬ 
 acteristic of the Ivaibab division to that which is characteristic 
 of the Kanab. 
 
 In the eastern, or Ivaibab, division of the canyon, the de¬ 
 scent of the wall is unusually abrupt throughout the entire 
 Paleozoic rock series. The only bench or terrace of any 
 extent or definition is that which is developed near the bottom 
 of the canyon upon the summit of the basal sandstone of the 
 Tonto group. The bench is known as the Tonto platform ; in 
 it is cut an inner gorge, in which the river flows upon the base¬ 
 ment schists of the Archean. The platform averages a mile 
 in width on either side of the canyon, and is so well defined 
 that one may travel upon it throughout the length of the 
 Ivaibab division. In the next western, or Kanab, division of 
 the canyon the lower terrace has disappeared. Instead, a 
 wide bench is opened out upon the summit of the hard red 
 sandstones of the Supai formation of the Aubrey group, about 
 a thousand feet below the level of the canyon rim. This 
 bench forms a broad level platform averaging two miles in 
 width on either side of the canyon, and has been named by 
 Dutton the ‘‘Esplanade.” Through it is cut a deep and narrow 
 inner gorge, at the bottom of which flows the river. 
 
 The topography of the Ivaibab division is characterized by 
 a much greater dissection than that of the Ivanab; great 
 amphitheaters are eroded back into the north wall, thronged 
 with buttes and outliers fashioned out of every formation of the 
 Paleozoic rock series and trenched by a multitude of side 
 gorges. The topography of the Kanab division is very simple, 
 consisting only of a broad outer canyon in which is cut the 
 inner gorge; the great dissection which makes the fantastic 
 scenery of the Ivaibab division is entirely lacking. 
 
 Eastward from Havasupai Point in the Shinumo area the 
 topography in the canyon is that which is characteristic of the 
 Ivaibab division. Westward, however, the upper platform or 
 “Esplanade” begins to appear. The Shinumo amphitheater, 
 which occupies the greater part of the area to be described, 
 presents a combination of both types of topography ; the great 
 dissection and the presence of the Tonto platform are features 
 characteristic of the Ivaibab division, but the Esplanade charac¬ 
 teristic of the Kanab division is developed to quite an equal 
 extent upon the upper surface of the Supai sandstone. In the 
 
376 Noble—Geology of the Grand Canyon , Arizona. 
 
 western part of the area the Tonto platform fades out, the 
 river closes in a deep inner gorge in the Esplanade, and the 
 profile of the canyon becomes that of the Kanab division,— 
 the view of the canyon westward from Bass Camp discloses a 
 broad expanse of Esplanade for fifteen miles, through which 
 the inner gorge sweeps in two great meanders almost from wall 
 to wall. In the center of the Shinumo area the Esplanade is 
 rather more dissected than in the Kanab division farther west, 
 but its identity as a topographic feature is already well 
 established. 
 
 The floors of the Esplanade and Tonto platforms, like the 
 surfaces of the plateaus, are structural surfaces developed 
 everywhere at the same horizon on the underlying formation. 
 In conformity with the dip of the rock system they likewise 
 slope to the southwest at the rate of about 200 feet to the 
 mile. 
 
 Another topographic contrast is presented in the difference 
 in dissection of the two sides of the Grand Canyon, a feature 
 which is characteristic of the entire pathway through the 
 Kaibab. The north rim lies three times as far back from the 
 river as the south rim ; the great amphitheaters with their 
 limiting promonotories extending far into the canyon, the 
 buttes and temples, and the deep lateral gorges all belong to 
 the north side of the canyon. The south wall presents a 
 simple aspect: the side gorges rarely extend back into the rim 
 of the canyon, there are few buttes and outliers, and the great 
 amphitheaters are wholly lacking. Compared with the fan¬ 
 tastic topography of the north side, the scenic effect of the 
 south wall is precipitous and somber. 
 
 The Colorado Biver enters the area in the southeast corner, 
 flows northwestwardly to the center of the area, and turns 
 sharply west at the point where it is joined by the Shinumo. 
 The Shinumo is the only living tributary in the area, and is 
 the master stream that drains the Shinumo amphitheater of the 
 north wall. It is a stream of clear water of the same order of 
 magnitude as Bright Angel Creek twenty miles east, and is in 
 striking contrast to the muddy Colorado. 
 
 The topography of the Shinumo amphitheater, besides pre¬ 
 senting an equal development of both the Esplanade and Tonto 
 platforms, is remarkable in another way. In the other great 
 amphitheaters of the Kaibab division the master gorges trend 
 to the southwest; the tributary gorges trend in the same general 
 direction and lateral gorges perpendicular to the main axes of 
 the amphitheaters are of minor development. In the Shinumo 
 amphitheater the lateral gorges have become the dominant 
 feature, so that the main axis of the amphitheater trends to the 
 northwest at right angles to the course of the master stream 
 
The Shinumo Area. 
 
 377 
 
 and parallel to the course of the Colorado River. The greatest 
 lateral gorge extends entirely across the Shinumo amphitheater 
 from Point Sublime on the southeast to the Muav Saddle on 
 the northwest. Only a mile of this lateral gorge is occupied 
 by the master stream of the Shinumo; the remainder is occupied 
 by two small intermittent streams. The western part of the 
 gorge which extends from the Muav Saddle to Shinumo Creek 
 is drained by Muav Creek and is called the “ Muav Canyon ” ; 
 the eastern part extending from the Shinumo to Point Sublime 
 is drained by Flint Creek and has no local name. The entire 
 lateral gorge will be referred to as the “ Muav-Flint Creek 
 Canyon.” This remarkable linear depression is situated upon 
 the line of the West Kaibab fault and has been conditioned bv 
 erosion in that line of fracture. Two smaller lateral gorges 
 cross the heart of the amphitheater parallel to the “ Muav- 
 Flint Creek Canyon ”; these are situated upon minor lines of 
 fracture. 
 
 It may be noted here that Tapeats amphitheater to the west¬ 
 ward, in the Kanab division of the canyon, is characterized by 
 similar topographic features where it is 'crossed by the West 
 Kaibab fault. 
 
 In general the surface of the Kaibab plateau at the rim of 
 the canyon is about 1000 feet higher than that of the Coconino 
 plateau directly opposite. But since the surfaces of the plateaus 
 and of the platforms within the canyon are structural surfaces, 
 elevations diminish progressively southwestward in all parts of 
 the area in accordance with the dip of the underlying rock 
 structure. For example : the elevation of the Kaibab Plateau 
 at the head of the Shinumo amphitheater is 8000 feet; at the 
 end of the promontory of Point Sublime, four miles southward, 
 it is 7500 feet. The elevation of the Coconino Plateau at the 
 end of Havasupai Point is 6750 feet; at Bass Camp, a mile 
 southwestward, 6652 feet. The eastern end of Powell Plateau 
 at Dutton Point is 7555 feet; the western end is 6600 feet. 
 Within the canyon the elevation of the Esplanade platform on 
 the north side of the river below Point Sublime is 6250 feet; 
 on the south side of the river below Bass Camp, 5400 feet. » 
 In the eastern part of the area the Ton to platform is 3500 feet; 
 in the western part, 2700 feet. 
 
 The elevation of the surface of the Colorado River where it 
 enters the area on the east is 2250 feet; where it leaves the 
 area on the west, 2150 feet,—a drop of 100 feet in 10 miles. 
 The river at Bass Ferry is 300 feet wide and 50 feet deep, with 
 a rise of 40 feet at time of high water. 
 
 The greatest drops in the shortest linear distance in the area 
 are at Dutton Point, where a drop of 5355 feet to the river is 
 accomplished in a distance of three miles; and at Havasupai 
 
378 Noble—Geology of the Grand Canyon , Arizona. 
 
 Point, where a drop of 4500 feet is accomplished in a distance 
 of a mile and a half. The latter drop is not equaled elsewdiere 
 in the Grand Canyon. 
 
 The width of the Grand Canyon in the Shinumo area from 
 Bass Camp to Dutton Point is seven miles; from Bass Camp 
 to the head of the Shinumo Amphitheater it is twelve miles; 
 from Havasupai Point to Point Sublime it is five and one-half 
 miles. The latter is the minimum width of the Grand Canyon 
 in the Kaibab division. Even at this, the narrowest point, the 
 width is over five times the depth. 
 
 The differences of climate to be found within the Shinumo 
 area are remarkable. The range in climate between the Kai¬ 
 bab Plateau on the north rim and the bottom of the canyon is 
 as great as that between the mountains of Colorado and the 
 Mojave Desert. The winters on the Kaibab are extremely 
 severe : from November to April the snow lies deep in the 
 woods, often accumulating to a depth of ten feet; even in mid¬ 
 summer the nights are chilly and days delightfully cool. Within 
 the canyon, however, the snow rarely falls below the level of 
 the Esplanade, while on the Tonto platform a fall of snow is 
 practically unknown. The winters in the depths of the can¬ 
 yon are mild, and freezing temperatures are rare. From April 
 to October the entire canyon below the Bed Wall concentrates 
 the solar heat, transforming it into a veritable oven; all day 
 the bare rocks absorb the heat of the sun, becoming so hot as 
 to burn the hand ; by nightfall the wind becomes a furnace 
 blast, and until after midnight the rocks radiate their heat, 
 feeding the hot wind, which blows without ceasing. The 
 effect of the heat, however, is not enervating, for the dryness 
 of the warm wind evaporates all moisture from the body and 
 keeps it cool. The climate of the southern, or Coconino, Pla¬ 
 teau at Bass Camp is characterized by more open winters than 
 the Kaibab, as well as by warmer summers : the snow in winter 
 rarely accumulates on the surface to a great depth, and as a 
 rule vanishes entirely within three days after a storm ; in sum¬ 
 mer the days are unpleasantly hot. 
 
 The climate of the Kaibab Plateau is decidedly moist, the 
 precipitation being probably twice as great as that received 
 upon the Coconino Plateau across the canyon ; this is chiefly 
 due to its greater altitude. In winter the precipitation takes 
 the form of snow ; in summer it comes in the form of showers, 
 which occur through the afternoon and evening. Looking 
 across the canyon from Bass Camp on the south rim, almost 
 any summer evening one may see storm after storm sweeping 
 over the Kaibab surface, usually accompanied by violent elec¬ 
 trical display, while the sky overhead and to the westward 
 over the Kanab Desert remains as clear as crystal. Another 
 
The Shinumo Area. 
 
 379 
 
 phenomenon that contributes to the greater rainfall of the 
 north rim seems to be the presence of the Grand Canyon itself: 
 after every general storm that visits both sides of the canyon 
 alike, there follows on the south rim a day of clearing; but the 
 clouds that rise out of the canyon after the storm sweep back 
 over the north wall and re-precipitate on the Kaibab surface; 
 these secondary storms almost never return over the south rim. 
 The climate of the Coconino Plateau in the area is semi-arid ; 
 often no rain will fall for a month at a time. The precipi¬ 
 tation is greatest in the winter months and in the month of 
 July. Within the canyon much of the rainfall evaporates 
 before it reaches the greater depths, so that the climate of the 
 lower part of the canyon is more arid than that of the south¬ 
 ern plateau. 
 
 Powell Plateau, whose higher eastern portion has an altitude 
 equal to that of the Kaibab rim, and wliese lower western por¬ 
 tion has an altitude equal to that of the Coconino, has an inter¬ 
 mediate climate. Its situation as an island in the canyon serves 
 to moderate the cold in winter, for the warm air, rising out of 
 the deep canyons that surround it, acts as a radiator: the snow 
 does not accumulate so deeply on the eastern end as on the 
 Kaibab; and on the western end does not accumulate at all. 
 Its surface is a resort in winter for the game and wild horses 
 that are driven out of the Kaibab by the snow. Its exposed 
 position subjects it to violent gales of wind at all times of the 
 year. The higher eastern end receives an abundant rainfall, 
 while the lower western end is semi-arid. 
 
 The variation of the flora in the Shinumo area is as great as 
 that of the climate. The surface of the Kaibab Plateau is 
 covered with a magnificent open forest of yellow pine ; the 
 trees grow large and far apart and the ground is free from 
 undergrowth, giving its surface the aspect of a great park ; 
 Englemann spruces grow on the north slopes of the washes, 
 and cottonwoods, aspens, and scrub oaks in their bottoms; a 
 minor flora of flowering plants, exceedingly rich in species, 
 covers the floor of the forest. The flora of the plateau surface 
 of the south rim of the canyon differs completely from that 
 of the Kaibab ; it is covered with a forest of gnarled and 
 stunted trees of juniper and pinon, with here and there a buck¬ 
 brush bush ; the trees never form thickets, but grow wide 
 apart; while the open stretches are covered with sage brush 
 and mormon tea, with occasional cactus, mescal, and plants of 
 the century family. This difference between the floras of the 
 north and south rim is due to the differences in precipitation 
 and temperature, which vary directly with the altitude. For 
 this reason the floras of the plateaus furnish an almost unfail¬ 
 ing index of the elevation. This is beautifully shown on the 
 
380 Noble—Geology of the Grand Canyon , Arizona. 
 
 southwestward-sloping surface of Powell Plateau, the whole 
 eastern half of which lies at an elevation of from 7000 to 7500 
 feet and is covered with the open pine forest characteristic of 
 the Ivaibab. At about 7000 feet the character of the flora 
 changes, and passes into the gnarled and stunted forests of juni¬ 
 per and pinon characteristic of the southern plateau across the 
 canyon. 
 
 Within the canyon itself the variation in the flora is just as 
 great, and is again an index of the elevation. 
 
 The flora of the Esplanade platform, a thousand feet below 
 the south rim, consists of stunted bushes of juniper and pinon 
 with grease wood as the ground bush in place of the sagebrush 
 of the Coconino Plateau. The cactus, mescal, and plants of the 
 century family are present in greater abundance than on the 
 plateau, but in less abundance and in more stunted develop¬ 
 ment than in the bottom of the canyon. This is due to the fact 
 that the Esplanade level is within reach of the winter snows 
 and frosts. 
 
 The flora of the Tonto platform, three thousand feet below 
 the south rim, and of all the interior of the canyon below the 
 Red Wall, is the flora of a hot and arid desert in its most 
 characteristic form. The dominant plants are the greasewood 
 bush, the mormon tea, and the cactus. The mescal and the 
 plants of the century family here attain their greatest develop¬ 
 ment and size. The cacti are particularly rich in species. 
 Every plant in the flora is either prickly or aromatic ; leaf sur¬ 
 faces are reduced to a minimum; devices for storing water 
 attain the greatest perfection; and the dominant color is a 
 somber gray. The somber colors and the reduction of leaf 
 surface are apt to deceive the observer, both in regard to the 
 richness of the flora in species and the abundance of plant 
 life, which is far greater than one would suspect. The only 
 tree is the screw-mesquite, which grows in the beds of those 
 washes that contain living or intermittent streams. 
 
 The vegetation in the bottoms of those canyons of the north 
 side in the Shinuino Amphitheater which contain living streams 
 is beautiful beyond description, and in refreshing contrast to 
 the desert flora of the Tonto platform. Tall cottonwoods 
 grow in the lower canyons; the walls are hung with maiden¬ 
 hair fern in the shady places; and willow thickets border the 
 stream. Grass grows on the banks where there is soil. 
 Higher up in the canyons, oaks, maples, and other deciduous 
 trees come in, and often beds of tall rushes. The most 
 characteristic bush of these upper north-side canyons is the 
 manzanita, which does not grow on the south side of the 
 Grand Canyon. 
 
The Shinumo Area. 
 
 381 
 
 General Geology. 
 
 Archean — Vishnu Schist. 
 
 Name. —The name “ Vishnu Terrane ” has been given by 
 Walcott (Walcott, c) to the fundamental crystalline com¬ 
 plex of the Grand Canyon region that underlies the unaltered 
 sedimentary rocks of Algonkian age, and is separated from 
 them, as well as from the overlying Cambrian, by a profound 
 unconformity. The type locality is situated on the Colorado 
 River thirty miles east of the Shinumo area at the base of one 
 of the great buttes called “Vishnu’s Temple,” from which lie 
 derives the name. 
 
 I)istrihution in the Kaibab division. —The rocks of the 
 Vishnu series are exposed continuously for a distance of over 
 forty miles within the Kaibab division of the Grand Canyon in 
 the bed of the Colorado River and in the walls of its gorge 
 which is cut beneath the Tonto platform. The Shinumo area 
 is at the western end of this long exposure. The eastern limit 
 of the exposure is determined by the appearance of great 
 thicknesses of overlying strata of the Grand Canyon series 
 dipping northeastward into the bed of the river, in the classic 
 locality described by Walcott. The western limit of the 
 exposure is likewise defined by a structural cause : three miles 
 west of the Shinumo area a southward bend of the river 
 causes it to flow parallel to the gentle southwestward dip of 
 the Paleozoic rock system of the canyon wall, carrying the bed 
 of the stream up out of the schists and into the basal Tonto 
 sandstone of the Cambrian. 
 
 Occurrence and distribution in the Shinumo area. —The 
 Vishnu schists are exposed for a distance of three miles in 
 the gorge of the Muav-Flint Creek Canyon. They are exposed 
 in the bed of the Colorado river throughout its course across 
 the Shinumo area. Alone; the south bank of the river for a 
 distance of four miles, and along the north bank for a distance 
 of seven miles, in the eastern half of the area, sediments of 
 the Grand Canyon series intervene between the Vishnu schists 
 and the Tonto sandstone of the basal Cambrian. Elsewhere 
 the schists are exposed without break in the walls of the river 
 gorge beneath the Tonto sandstone. 
 
 So detailed study of these rocks was made in the Shinumo 
 area beyond the general location and determination of the vari¬ 
 ous types represented, since the limited extent of the expo¬ 
 sures precludes the possibility of unraveling any general 
 Archean structure from the study of this area alone. They 
 are here a metamoiqfiiic complex of quartz, mica, and horn¬ 
 blende schists, invaded by a batholithic mass of quartz-diorite, 
 and injected by veins of pegmatite and aplite. 
 
382 Noble—Geology of the Grand Canyon , Arizona. 
 
 Five main types of rock are found in this complex within 
 the area: 
 
 The first type is a quartz schist, with graduations into mica 
 schist. It comprises the greater part of the Vishnu schists 
 that are exposed in the gorge of the Colorado River west of 
 the cable crossing at Bass Ferry. It is also exposed in the 
 bed of Muav Canyon, and in the bed of Flint Creek above the 
 junction with the Shinumo. 
 
 The second type is a quartz schist with gradations into 
 quartz-hornblende schist. It is exposed in that part of the 
 Muav-Flint Creek Canyon that is occupied by the living stream 
 of the Shinumo, grading both eastward and westward into the 
 quartz-mica schist. 
 
 The third type is a hornblende schist. It occurs in one 
 small outcrop about 200 feet wide, on the east side of “Fault 
 wash,’’ a dry wash which joins the Shinumo just below the 
 mouth of Muav Canyon, and is sharply bounded on both sides 
 by the quartz-mica schist. 
 
 The fourth type of rock is a quartz-diorite. So far as 
 observed, it constitutes all of the Vishnu series of the Shinumo 
 area that is exposed in the “ granite gorge ” of the Colorado 
 River east of the cable crossing. Its western contact is well 
 defined. The eastern limit of the exposure was not located. 
 
 The fifth type of rock is a granitic pegmatite which occurs 
 in dikes that cut all the rock types of the Vishnu. These 
 dikes occur in two generations. The older generation is folded 
 with the schists. The younger generation occurs in a great 
 network or mesh of dikes cutting both the quartz-diorite and 
 the schists. 
 
 The Vishnu schists have a typical schistose structure. The 
 planes of scliistosity have a dip that seldom departs greatly 
 from the vertical. The dominant trend of the scliistosity is 
 northeasterly, but varies, from place to place. Locally, the 
 schists are much twisted and contorted. 
 
 Lithology .—The less micaceous phase of the quartz-mica 
 schist has a dark grey color with a greenish tinge. The cleav¬ 
 age is imperfect and the surfaces have a satin-like luster. 
 The texture is fine-grained and the unaided eye can distinguish 
 no mineral constituents except quartz. The microscope shows 
 the rock to be composed almost entirely of fine interlocking 
 grains of quartz, with occasional small flakes of white mica, 
 arranged in parallel lines. In the extreme quartzose phase 
 the amount of mica is almost negligible, just enough being 
 present to impart a satin-like luster to the rock. 
 
 The micaceous phase of the schist has a slight grey color 
 with either a pinkish or greenish tinge on the fresh fracture. 
 Where the rock is weathered, the color becomes red. The 
 
The Shinumo Area. 
 
 383 
 
 cleavage is rather distinct. The texture ranges from fine to 
 coarse. To the unaided eye both the quartz and mica are visi¬ 
 ble. Under the microscope the rock is seen to be composed 
 of interlocking grains of quartz, with an almost equal amount 
 of mica. The mica flakes are arranged in parallel lines. The 
 greater part of the mica is muscovite, with occasional flakes of 
 brown biotite. 
 
 Locally the schist is garnetiferous, and in one locality tour¬ 
 maline was observed. All gradations between the quartzose 
 and the micaceous phases occur, but in no instance does the 
 mica exceed the quartz in quantity. 
 
 The quartz-hornblende schist is dark green in color. The 
 cleavage is imperfect, and the texture fine-grained and uni¬ 
 form. The rock is dense and hard. The mineral constituents 
 cannot usually be distinguished in the hand specimen without 
 the aid of the lens. Under the microscope the rock is seen to 
 consist of about equal proportions of quartz and green horn¬ 
 blende. The quartz is present in interlocking grains. The 
 hornblende tends to form automorphic crystals whose longer 
 axes have a roughly parallel arrangement. No other minerals 
 were present in the slides examined. In some phases of the 
 rock the quartz exceeds the hornblende in amount. 
 
 The hornblende schist is dark green in color. It is a soft 
 rock, considerably disintegrated, and crumbling under the 
 hammer. The texture is coarse granular. Megascopically the 
 rock appears to consist almost entirely of dark green horn¬ 
 blende. No schistose structure is observable in the mass. 
 The microscope shows the rock to consist almost wholly of 
 large crystals of green hornblende in all stages of alteration. 
 A small amount of interstitial quartz occurs. The quartz 
 granules are strung out in a roughly parallel fashion. The 
 rock is badly altered and the thin section is unsatisfactory. 
 
 The quartz-diorite found in the river gorge eastward from 
 the cable crossing is a coarse-granular rock of typical granitic 
 texture. It is a hard, resistant rock, which tends to weather 
 into roughly angular blocks, a feature that distinguishes it in 
 the mass from the exposures of the schists. The minerals visi¬ 
 ble to the unaided eye are white striated feldspar, dark horn¬ 
 blende, and glistening black biotite. The color of the rock is 
 dark grey and the appearance is remarkably fresh. The tex¬ 
 ture is uniform throughout the exposures observed, and the 
 rock is apparently without contact modifications. No ten¬ 
 dency to gneissoid banding was observed. 
 
 Under the microscope it is seen to be a coarse-granular rock 
 of granitic texture. The dominant mineral constituents are 
 plagioclase and common hornblende. The plagioclase ranges 
 from oligoclase to labradorite. Microcline, orthoclase, and 
 
384 Noble—Geology of the Grand Canyon, Arizona. 
 
 quartz are present in about equal proportions, but their total 
 amount does not equal that of the plagioclase. Brown biotite 
 is present in somewhat less quantity than the hornblende. 
 Titanite and magnetite occur as accessories. Occasionally the 
 quartz is poikilitic in the orthoclase. The feldspathic and fer- 
 romagnesian minerals are present in about equal proportions. 
 If it were not for the preponderance of the plagioclase the 
 rock might be classed as a quartz-monzonite or grano-diorite. 
 It is probably best classed as a quartz-diorite with a monzo- 
 nitic aspect. The microscope reveals no cataclastic structure 
 nor other evidence of dynamic action, and the minerals are 
 fresh and unaltered. 
 
 Th e pegmatites are pink in color and usually very coarse in 
 texture. They are composed chiefly of quartz and pink ortho¬ 
 clase. Some of the dikes carry large crystals of silvery-white 
 mica. No other minerals were noted. The dikes usually 
 exhibit the typical comb-structure inward from the walls and 
 the graphic arrangement of the quartz and feldspar. Along 
 the walls of some of the dikes the texture becomes aplitic. 
 
 Origin .—There is no clear evidence in the Shinumo area by 
 which the original character of the schists of the Vishnu 
 series can be determined. There is no evidence of a banding 
 that can be clearly referred to original sedimentary bedding, 
 nor is there evidence of any original clastic texture. The 
 mineralogical composition, however, suggests a sedimentary 
 origin for the quartz schists of the mica and hornblende type : 
 either might have resulted from the regional metamorphism of 
 an arkose sandstone or shale. According to the lesser or 
 greater abundance of iron in the original sediments the rock 
 would assume the mica or hornblende type of quartz schist. 
 Certainly the great preponderance of quartz as a constituent 
 of these rocks would seem to weigh against an igneous origin 
 in the balance of probability. The causes operating to pro¬ 
 duce the schists in their present aspect are conceived to be 
 the processes connected with regional metamorphism, namely, 
 subsidence and deep burial, subsequent folding and mashing, 
 and a slow recrystallization in process of time. 
 
 The original character of the hornblende schist described as 
 occurring in a narrow outcrop between the schists is not clear. 
 The fact that the rock consists of little else than hornblende 
 suggests that it originally constituted an igneous rock of a 
 basic type. The fact that the microscope discloses a schistose 
 structure is evidence that the rock is at least earlier in age 
 than the period of regional metamorphism that was responsible 
 for the present structural and mineralogical character of the 
 enclosing schists. 
 
 The origin of the quartz-diorite is reasonably clear. Since 
 
The Shinumo Area. 
 
 385 
 
 it is a coarse-granular, igneous rock of plutonic aspect occur¬ 
 ring over a large area, clearly cutting the schists, and showing 
 no textural modifications at the contact, it is concluded that 
 it represents a deep-seated igneous invasion of considerable 
 size, of the type known as a “ batholith.” Because of the 
 unaltered character of the rock and the absence of any gneis- 
 soid or cataclastic structure, it is argued that the batholithic 
 invasion took place at a time later than the period of regional 
 metamorpliism that produced the recrystallization and schistos- 
 ity of the enclosing schists. The question may arise as to 
 whether the invasion of the batholith was not of itself a par¬ 
 tially operative cause in producing this recrystallization and 
 schistosity. The field evidence is against such a conclusion : 
 there is no gradation in the schist away from the contact, 
 either in texture or in mineral composition, nor are the planes 
 of schistosity parallel to the contact. 
 
 The older pegmatite dikes are folded intimately with the 
 schists. Their injection may have either preceded or accom¬ 
 panied the regional metamorpliism. 
 
 The younger pegmatite dikes cut both the schists and the 
 quartz-diorite. Where they cut the schists they break cleanly 
 across the schistosity. The injection of these dikes is the latest 
 visible event in the igneous activity of Archean time within 
 the area. 
 
 Age and correlation. —The rocks of the Yishnu series are 
 rocks which, in the light of present knowledge, can only be 
 conceived to have acquired their present character at great 
 depths beneath the earth’s surface in what is technically known 
 as the u zone of towage.’’ It is therefore evident that the 
 unconformity which separates them from the overlying Algon- 
 kian sediments of the Grand Canyon series represents a vast 
 amount of erosion and the consequent lapse of an enormous 
 interval of time,—an interval vastly greater even in events 
 than that represented by the profound unconformity which 
 separates the succeeding Grand Canyon series from the over- 
 lying Paleozoic. The Yishnu schists are therefore assigned to 
 the Archean in the usage of the United States Geological 
 Survey; but whether as a group or a complex is as yet unde¬ 
 cided. It seems likely, as stated by Bansome ( a , p. 21), that 
 they are to be correlated with the Pinal Schist of the Globe 
 and Bisbee regions, and “ present somewhat different aspects 
 of the fundamental crystalline complex of Arizona/ 1 
 
 Up to the present time no detailed study of these rocks has 
 been made in the Grand Canyon region, and their internal 
 structural relations are unknown. It is not unlikely when 
 such a study is made of their exposures in the Kaibab division 
 of the canyon, in the Shiv wits division to the west, and along 
 
 Am. Jour. Sci.—Fourth Series, Vol. XXIX, No. 173. —May, 1910. 
 
 26 
 
498 Noble—Geology of the Grand Canyon , Arizona. 
 
 Colorado in the eastern end of the Kaibab division and the 
 mouth of Tapeats Creek some 80 miles below in the eastern 
 end of the Kanab division, where the strata of the Grand 
 Canyon Series are exposed between the crystalline schists of 
 the Archean and the basal Tonto sandstone of the Cambrian. 
 The location of these exposures is shown on the map accom¬ 
 panying this article. Five of the localities are within the 
 Kaibab division; the sixth is within the Kanab. 
 
 The first of these localities is the classic area below the 
 mouth of the Little Colorado described by Walcott. This is 
 the largest areal exposure of these rocks in the Grand Canyon, 
 and includes both the Unkar and Chuar groups. 
 
 The second locality lies five miles west of the first at the 
 head of the inner gorge of Clear Creek on the north side of 
 the Colorado river within the depths of the Ottoman Amphi¬ 
 theater. The exposure is limited to less than a square mile. 
 It comprises a small portion of the basal Unkar and is struc¬ 
 turally a unit with the first locality. 
 
 The third locality lies along the north side of the Colorado 
 river at the mouth of Bright Angel Creek opposite the railroad 
 terminus and hotels of the Saute Fe Kailroad. About 1000 
 feet of the basal portion of the Unkar group are there repre¬ 
 sented and the areal extent of the exposure is about three 
 square miles. This locality has been briefly described by 
 Kan so me (Kansome, a). It lies about 10 miles west of the 
 type locality. 
 
 The fourth locality comprises a limited exposure of basal 
 Unkar strata which lies in the depths of the Hindu Amphi¬ 
 theater on the north side of the Colorado river about three 
 miles up Crystal Creek from its mouth. It is situated some 
 20 miles west of the type locality. The areal extent of the 
 exposure is about one square mile. It is as yet undescribed. 
 
 The fifth locality lies about the month of Shinumo Creek 
 about 30 miles west of the exposures of the type area. It com¬ 
 prises about 12 square miles in areal exposure and represents 
 nearly the entire thickness of the Unkar group. It is hitherto 
 undescribed in geological literature and is the subject of the 
 succeeding pages of this article. 
 
 The sixth locality is situated on the Colorado river just 
 above the mouth of Tapeats Creek in the eastern end of the 
 Kanab division of the Canyon, about 12 miles northwest of 
 the mouth of the Shinumo in a direct line and about 25 miles 
 down the river in its actual course. The length of the expo¬ 
 sure is about three miles in the bed of the river and in its 
 narrow gorge beneath the Tonto sandstone. About 4,000 feet 
 of the basal portion of the Unkar group are exposed, striking 
 H.W.-S.E. and dipping about 15° N.E. This locality is 
 
The Shinumo Area. 
 
 499 
 
 structurally a unit with the exposures of the Shinumo area. 
 It is unmentioned in geological literature. 
 
 In the western end of the Kanab division in a section across 
 the Grand Canyon at the foot of Toroweap valley, 50 miles 
 west of the Shinumo area, Dutton figures “ rocks of Silurian 
 and Archean unconformable” in the bed of the river beneath 
 the basal Tonto sandstone of the Cambrian (Dutton, a , p. 88). 
 It is probable that these u Silurian” rocks there represent the 
 Grand Canyon series. 
 
 Whether these rocks appear at places in the Sliivwits 
 division between the Vishnu schist and the Tonto sandstone 
 is not at present known. 
 
 Stratigraphic position , structiire , and distribution in the 
 Shinumo Area. —Two unconformities determine the strati¬ 
 graphic position of the sediments of the Grand Canyon series 
 in the Shinumo Area : they are separated from the underlying 
 Vishnu schists of the Archean by a profound unconformity 
 which represents a base-leveled surface of erosion, and from 
 the overlying Tonto sandstone of the Cambrian by an uncon¬ 
 formity which represents a similar base-leveled surface above. 
 
 The strata of the Grand Canyon series here constitute a 
 wedge-shaped mass whose apex lies along the south side of 
 the Colorado river parallel to its northward course in this 
 part of the area. The mass as a whole constitutes one great 
 tilted block, which in turn consists of a great number of minor 
 tilted and rotated blocks pitching at successively greater angles 
 northeast away from the apex of the w r edge, until at a distance 
 of three miles from the Colorado river the whole mass is 
 dropped by a profound fault which brings up the under¬ 
 lying Vishnu schist from a great depth,—a structure which 
 strikingly resembles that of the area of similarly faulted 
 Triassic blocks of the Connecticut valley. The strike of the 
 strata is K. 40° W. The dips are variable: in general the 
 strata of the fault blocks near the apex of the wedge dip 
 10°-15° N.E.; near the center of the wedge the dips average 
 25° N.E.; while in proximity to the line of the great limiting 
 fault on the northeast they are completely reversed by the 
 “drag” along the fault plane. The truncation of this pre- 
 Cambrian structure by the unconformity at the base of the 
 Tonto sandstone is absolute. 
 
 The great pre-Cambrian fault that limits the wedge upon 
 the northeast represents the exposure in the basement rocks of 
 the line of displacement of the West Kaibab fault and displays 
 in the most spectacular manner a phenomenon analogous to that 
 described by Walcott upon the line of the East Kaibab mono¬ 
 cline (Walcott, a). Upon the line of the ancient fault in the 
 Shinumo area two later displacements have taken place after 
 
500 Noble—Geology of the Grand Canyon , Arizona. 
 
 the deposition of the entire Paleozoic series of the Canyon 
 wall and probably later strata. The first of these is a mono- 
 clinal flexure which reverses the throw of the' pre-Cambrian 
 fault, while the second is a still more recent fault super¬ 
 imposed upon the line of the monoclinal flexure. 
 
 The strata of the Grand Canyon series are exposed beneath 
 the Tonto sandstone in all that part of the inner gorge of the 
 Muav-Flint Creek canyon which is on the south side of the great 
 pre-Cambrian fault,—a distance of about three miles. They 
 are exposed for three miles in the gorge of the Shinumo 
 Canyon; for seven miles along the north side of the Colorado 
 river; and in all the inter-canyon valleys within that distance 
 which are eroded below the base of the Tonto sandstone. The 
 two largest of these inter-canyon valleys are the u East Wash,” 
 a mile east of the Shinumo, and the “ Asbestos Canyon,” three 
 miles to the west. 
 
 The gorge of the Colorado river has everywhere been 
 trenched to a depth sufficient to expose the Vishnu schists 
 along the river beneath the overlying strata of the Grand 
 Canyon series. This is due to the fact that the course of the 
 river lies close along the southern apex of the wedge. 
 
 The exposures on the south side of the river are more lim¬ 
 ited, due to the thinning out of the wedge in that direction 
 and the lack of inter-canyon valleys trenched beneath the Tonto 
 sandstone. The strata are exposed for two miles above and 
 one mile below a point opposite the mouth of the Shinumo. 
 
 A southwestward bend in the river in the western part of 
 the area carries it beyond the apex of the wedge, below which 
 point the Tonto sandstone caps the Vishnu schists which lie in 
 the river gorge. Southeastward up the river, in the eastern 
 part of the area, a similar relation obtains. 
 
 The hard middle members of the Unkar resisted the erosion 
 which preceded the deposition of the Cambrian sandstone and 
 stood as an island in the Tonto sea. 
 
 This long monadnock of quartzite runs across the area in a 
 JVW.-S.E. direction parallel to the general strike of the strata 
 of the wedge, and a narrow outcrop of these quartzites is ex¬ 
 posed along the Tonto platform on the north side of the river 
 just at the base of the Kedwall clifT, running for a distance of 
 about five miles beyond the main areal exposures about the 
 mouth of the Shinumo and uniting with the limestones of the 
 upper Tonto group to form the lower part of the great cliff of 
 Redwall limestone. In the eastern part of this exposure the 
 quartzite monadnock projects 700 feet above the base of the 
 Tonto sandstone. Westward from the exposures about the 
 Shinumo the prolongation of the monadnock along the strike 
 of the quartzites exposes them in a narrow outcrop upon the 
 Tonto platform one mile west of the Shinumo. 
 
The Shinumo Area. 
 
 501 
 
 The total areal exposure of the Grand Canyon series in the 
 Shinumo area is about 12 square miles. 
 
 In following up the Shinumo from its mouth to the point 
 where it leaves the lateral gorge of the Muav-Flint Creek can¬ 
 yon, a traverse is made of the total exposed thickness of the 
 Unkar group from the unconformity at the base to the highest 
 member that is limited by the profound fault on the northeast. 
 There can hardly be a more magnificent illustration of details 
 of geological structure than is here revealed. Along the entire 
 western side of the Shinumo canyon, in cliff faces a thousand 
 feet above the bed of the stream, is displayed every detail of 
 the structure beneath the basal Tonto sandstone. Westward 
 down the Colorado river the intersection of the two great 
 unconformities forming the apex of the wedge is seen in the 
 cliff face above the* river bank, below which point the river 
 narrows in its somber gorge in the Vishnu schists. From here 
 northeastward, in the cliff faces along the western wall of the 
 Shinumo canyon, bed after bed of the Unkar strata appears, 
 wedging out southwestward beneath the plane of the uncon¬ 
 formity beneath the Tonto sandstone. Every detail of the 
 successive fault blocks of the great wedge is clearly shown,— 
 their increasing tilt northeastward, the dips of the fault planes 
 that bound them, and the occasional down-dropped wedges. 
 Above runs the plane of the pre-Tonto unconformity, revealing 
 in cross section the monadnock in this peneplain which existed 
 as a rocky island during the inroads of the Tonto sea, the debris 
 from its wave-cut cliffs being incorporated and preserved to the 
 minutest detail in the Tonto sandstone. In the background, 
 bed above bed in conformable succession, lies the horizontal 
 Paleozoic section in the wall of the mile-deep Canyon. After 
 traversing a thickness of 5800 feet of Unkar strata in a distance 
 of three miles dipping northeastward into the bed of the stream, 
 the traveler crosses the line of the great pre-Cambrian fault of 
 more than 5800 feet and comes once more into the Vishnu 
 schists on the farther side of the Muav-Flint Creek canyon. 
 
 Here again is an instance of the simplicity with which the 
 geological structure is revealed in this wonderful country. 
 Along the wTiole northern wall of the lateral gorge lie the 
 Vishnu schists below the Tonto sandstone cliff. On the south¬ 
 ern side, at the same level, lie the upper sandstones of the 
 Unkar, their beds dragged up sharply against the fault line, 
 which lies in the bed of the stream. The whole Paleozoic 
 system on the northern side of the gorge has been dropped 
 500 feet by the torn monocline of the West-Kaibab fault, 
 reversing the throw of the pre-Cambrian fault on the same 
 line in the basement rocks. Looking westward up the Muav 
 . Canyon, the beds of the Paleozoic are seen bending down 
 
502 Noble—Geology of the Grand Canyon , Arizona. 
 
 against the fault line in a graceful arc. Far up the canyon 
 at its head under the Muav Saddle, the more recent fault has 
 reversed its throw and restored the throw of the monoclinal 
 flexure. There is not a detail of this structure that may not 
 he seen at a glance. 
 
 Sediments of the Unkae Group.—Lithology. 
 
 Preliminary Outline .—The pre-Cambrian sedimentary rocks 
 of the Shinumo area represent the greater part of the Unbar 
 group of the Grand Canyon series of Algonkian age. The upper, 
 or Clmar, group is not represented in the area. Although 
 these sediments present no more evidence of alteration or meta- 
 morpliism, aside from local igneous contact phenomena, than 
 the overlying beds of the Paleozoic, they are destitute of fos¬ 
 sils or decisive evidence of life. In the absence of fossils the 
 natural basis for dividing the group into component mem¬ 
 bers is the lithology. On this basis the Unkar group in the 
 Shinumo area is divisible into five members, succeeding each 
 other in conformable stratigraphic succession. It is not in¬ 
 tended that the importance of this division should be greatly 
 emphasized. Its chief value lies in the fact that it furnishes a 
 basis for comparing the lithological succession of the Unkar 
 group in this area with that in the type locality described by 
 Walcott 30 miles to the east, as well as a basis for distinguish¬ 
 ing in a broad way the main changes in the physical conditions 
 under which these sediments were laid down. 
 
 At the base, resting upon the profoundly eroded and base- 
 leveled surface of the metamorpliic rocks of the Vishnu series, 
 
 Tonto Group 
 Unconformity. 
 
 4. 
 
 3. 
 
 Unkar Group. 
 
 Synopsis. 
 
 Micaceous shaly sandstone (exclusive 
 
 of 36 feet of intrusive rock).. 2297 
 
 Sandstone and quartzite_ 1564 
 
 Argillaceous and arenaceous shale 
 (exclusive of 950 feet of intrusive 
 rock)__ 580 
 
 I 2. Calcareous shale and limestone__ 335 
 
 1. Basal conglomerate__ 6 
 
 i 
 
 i 
 
 Unconformity 
 Vishnu series. 
 
 T otal.. 
 
 4782 ft. 
 
The Shinumo Area. 
 
 503 
 
 is a thin conglomerate. This constitutes the basal member of 
 the Unkar. Overlying the conglomerate is a series of lime¬ 
 stones and calcareous shales. These grade upward into argil¬ 
 laceous and arenaceous shales which are intruded by a thick 
 sill of diabase, and are succeeded in turn by great thicknesses 
 of sandstone and quartzite. The uppermost exposed member 
 of the group in the area is a thick series of micaceous slialy 
 sandstones. 
 
 It has been shown that these strata lie in a wedge-shaped 
 mass inset in the Vishnu schists, and that this wedge is com¬ 
 posed of a great number of smaller titled fault blocks. It is 
 apparent from this relation that nowhere in the Shinumo area 
 can the thickness be measured in one unbroken section. Since, 
 however, the lithological characters of the strata are constant 
 and easily recognized, and since the throw of the faults that 
 bound the titled blocks seldom exceeds 100 feet, the restoration 
 of a section showing the unbroken sequence is not a matter of 
 great difficulty. 
 
 Detailed sections were made upward from the base of the 
 Unkar through each fault block until its limiting fault was 
 reached. The highest bed measured was then located in the 
 next block to the northeast, and the measurement resumed at 
 that point. Except in the fifth, or highest member of the 
 group, all sections were measured with a tape along the nearly 
 vertical wall faces of the box canyons of the Shinumo and 
 other w T ashes that cut across the strike of the strata. In the 
 fifth, or highest member of the group, the strong drag of the 
 great fault on the northeast has flexed and contorted these 
 slialy sandstones in such a manner that accurate measurement 
 with the tape alone was impossible. Their thickness was 
 computed by the aid of trigonometric formulae, using the com¬ 
 bined data afforded by the use of the tape, the topographic 
 map, and observations of the varying strike and dip. 
 
 The section incorporated in this article was made in two 
 places. The greater part of the total thickness was measured 
 in a traverse up the Shinumo. This section includes all the 
 strata above the diabase sill which is intruded midway in the 
 “arenaceous and argillaceous shales” which comprise the third 
 member of the Unkar. It would have been perfectly possible 
 to make a complete section of the group in a traverse of the 
 entire course of the Shinumo from the basal unconformity at 
 the mouth of the creek to the great fault three miles above, 
 although four faults cross the creek between its mouth and the 
 place where the diabase sill dips beneath the bed of the stream. 
 But a place was found in the canyon of the East Wash where 
 all the strata between the basal unconformity and the diabase 
 sill lie in a continuous unfaulted section, in a fault block that 
 
504 Noble—Geology of the Grand Canyon, Arizona. 
 
 is tilted about 10° N.E. The section of the basal members of 
 the group was measured in this locality, where the sequence is 
 unbroken by faulting. 
 
 Detailed Section. 
 
 First Member.—Basal Conglomerate. The surface rep¬ 
 resented by the upper unconformity that separates the Unkar 
 group from the basal Cambrian is a striking enough example 
 of a base-leveled surface, although monadnocks rise in places to 
 a height of 700 feet above the base of the Tonto sandstone. 
 But the surface represented by the lower unconformity that 
 separates the Unkar group from the Vishnu is an almost per¬ 
 fect plane : nowhere in the seven linear miles exposed in the 
 Shinumo area can a difference in relief be observed that 
 exceeds 20 feet. The depth of weathering below this surface 
 appears to be slight, in spite of the enormous amount of rock 
 that has been removed, and the weathering appears to be the 
 result of physical disintegration rather than of chemical 
 decomposition. 
 
 The basal conglomerate is an arkose conglomerate varying 
 in thickness from 1 to 6 feet in the Shinumo area. It is com¬ 
 posed of angular or subangular fragments of the rocks of the 
 underlying Vishnu series, cemented by a matrix of red arkose 
 mud which usually contains small fragments of pink feldspar. 
 Occasionally the matrix contains small rounded grains of 
 quartz. 
 
 The degree of induration of this conglomerate presents all 
 variations from a hard, dense, siliceous rock, which fractures 
 across matrix and enclosed rock fragments alike, to an easily 
 disintegrated rock in which the matrix crumbles away from 
 the enclosed fragments. This phenomenon, however, does not 
 depend upon original cementation, but upon metamorphic 
 effects produced by the diabase sill that is intruded in the over- 
 lying rocks, the degree of induration depending upon how far 
 the conglomerate lies below the contact of the sill. 
 
 The matrix is usually of the same composition everywhere 
 in the area. The character of the enclosed fragments, how¬ 
 ever, is sharply localized by the character of the underlying 
 rock. The rock which underlies the conglomerate in the East 
 Wash is the quartz-diorite of the batholith previously 
 described. For three feet below the conglomerate the diorite 
 is weathered along the joints into roughly angular blocks. 
 These joints are filled with the red arkose material of the 
 matrix. Above follows a layer of the conglomerate one foot 
 in thickness, composed of weathered fragments of the diorite 
 cemented with the red arkose material. Then follows a layer 
 six inches thick composed of small rounded quartz pebbles and 
 
The Shinumo Area. 
 
 505 
 
 fragments of cliert of the same character as that contained in 
 the overlying limestones. The whole is cemented with the red 
 mud. The conglomerate is very little indurated in this local¬ 
 ity. Although the contact of the diorite with the mica-scliists 
 in the underlying Vishnu is not two hundred yards distant, 
 there is not a fragment of the mica-schist to be observed in the 
 conglomerate. 
 
 In the Asbestos Canyon, four miles to the west of the East 
 Wash, the underlying rocks are mica schists and veins of 
 quartz and pegmatite. Here the Vishnu schist is scarcely 
 weathered at all below the unconformity. The overlying 
 conglomerate is 6 feet thick and consists of angular fragments 
 of the underlying mica-schists, fragments of pegmatitic feldspar 
 and vein quartz, and the arkose cement described above. , The 
 degree of induration is here very great, and the rock fractures 
 across the grains like a hard, dense quartzite. This is due to 
 the fact that the lower contact of the diabase sill lies only 150 
 feet above this basal conglomerate in the Asbestos Canyon, 
 while in the locality in the East Wash it lies 550 feet above. 
 
 Two important features characterize the basal conglomerate 
 of the Unkar in the Shinumo area; the arkose nature, and the 
 lack of sorting and transportation of the component frag¬ 
 ments. 
 
 Second Member.—Calcareous Shales and Limestone. The 
 section was measured on the west side of the canyon of 
 the East Wash. This and all the following sections read from 
 the base upward, a. 1 being the bottom bed, overlain by a. 2, 
 etc. 
 
 Charactek Thickness 
 
 a. Basal white limestone. 
 
 1. White, nodular, cherty limestone. The chert 
 occurs in nodules with a roughly concentric 
 structure somewhat suggestive of the structure 
 
 of Cryptozoon___— 1' 6" 
 
 2. White, cherty limestone carrying the chert in 
 thin parallel bands which are etched out by the 
 weather on the cross sections. The surface of 
 each chert layer shows polygonal cracks sugges¬ 
 tive of sun-cracks in shale. This structure 
 belongs to each separate chert layer and is not 
 a columnar structure. The weathered surfaces 
 of these chert layers are dotted with small cubic 
 depressions which were apparently formed by 
 the leaching out of some mineral of a cubic 
 
 habit__—. . 4' 6" 
 
 Total.......- 6' 
 
506 Noble—Geology of the Grand Canyon , Arizona. 
 
 Character 
 
 Thickness 
 
 b. Argillaceous and calcareous red shale and limestone. 
 
 1. Soft, purple shale...... 
 
 2. Purple, cherty limestone.... 
 
 3. Purple shale with occasional bands of purple 
 
 calcite...... 
 
 4. Purple, crystalline limestone___ 
 
 5. Alternating layers of buff and chocolate-red 
 
 shale with a splintery habit of weathering and 
 a roughly concretionary structure. Like all the 
 succeeding shales and sandstones of the Unkar 
 they are mottled with light-colored spots which 
 are usually circular or elliptical in form and of 
 all sizes_____ 
 
 6. Blue limestone, white for 1" at the base and 
 
 showing dendritic markings.. 
 
 7. Red shale____ 
 
 8. Blue limestone... 
 
 9. Red shale ......... 
 
 10. Purple limestone ...... 
 
 11. Calcareous, red shale with three thin bands of 
 
 purple limestone____ 
 
 12. Red, crystalline limestone .... 
 
 13. Red shale_____ 
 
 14. Red, crystalline limestone.... 
 
 15. Red shale....... 
 
 16. Blue limestone____ 
 
 17. Red shale ....._ _. 
 
 18. Blue limestone__ 
 
 19. Pink limestone ____ 
 
 20. Red shale.... 
 
 21. Cherty, white limestone.. 
 
 22. Compact, red shale forming a cliff.... 
 
 23. Alternating layers of buff and red shale. 
 
 24. Dense, purple, calcareous shale carrying bands 
 
 of pink calcite and forming a cliff. Contains 
 occasional thin bands of chert .. 
 
 25. Blue, calcareous shale with an onion-like concre¬ 
 tionary structure on a large scale... 
 
 4' 
 
 1 ' 
 
 5' 6" 
 
 5" 
 1' 5" 
 4" 
 3 ' 10 " 
 
 a » 
 
 9' 10" 
 1 ' 
 
 P 
 
 1 ' 
 
 11 ' 
 
 3" 
 
 1 ' 
 
 2 " 
 
 1' 
 
 4' 4" 
 4" 
 
 8 ' 
 
 13' 6" 
 
 9' 6" 
 5' 
 
 Total___ _ 85' 5" 
 
 c. White limestone. 
 
 1. Thin-bedded, white, cherty limestone carrying 
 
 the chert in parallel bands, and containing three 
 paper-thin layers of purple shale. Weathers 
 to a white powder. Dendritic markings are 
 present_____ 2' 
 
 2. Nodular, white, cherty limestone. The chert is 
 present in irregular nodules of no definite shape. 
 
The Shinumo Area. 
 
 507 
 
 Character Thickness 
 
 The upper part of the stratum has a paper-thin 
 bedding giving it the aspect of a calcareous shale. 
 
 The limestone weathers to a white powder. 
 
 Dendritic markings are present... 2' 
 
 3. Dense, homogeneous, white, crystalline lime¬ 
 
 stone, forming a cliff. The upper part is thin- 
 bedded __....... 3' 8" 
 
 4. Homogeneous, thin-bedded, white, crystalline 
 
 limestone containing occasional thin bands of 
 chert and nodules whose character suggest the 
 structure of Cryptozoon_ 8' 6" 
 
 5. White limestone carrying a large amount of 
 
 chert in undulatory and gnarled bands_.. 2' 4" 
 
 6. Lumpy and gnarly, white limestone carrying 
 chert in large irregular nodules. Crumbles to 
 
 a white powder... 2' 6" 
 
 V. Purple shale ...... 
 
 8. Layers of undulatory-banded, bluish chert_ 1' 10" 
 
 9. Soft, purple shale __ 5 " 
 
 10. Layer of gnarled and twisted chert nodules in a 
 matrix of white talc. The surface of the talc 
 
 is covered with dendritic markings .. 2' 
 
 11. Thin-bedded, white limestone crumbling to white 
 
 powder or weathering into thin plates like a 
 shale. 2 ' 
 
 12. Soft, purple shale.... 1' 
 
 13. Thin-bedded, crystalline, white limestone. 3' 10" 
 
 14. Soft, purple shale.... 1' 10" 
 
 15. Dense, blue, crystalline limestone forming a 
 
 small cliff. 3' 
 
 16. Homogeneous, thin-bedded, white limestone, 
 crumbling to a white powder and weathering 
 
 into plates like a shale. 15' 
 
 17. Dense, crystalline, blue limestone.. 4" 
 
 18. Purple shale...... 3' 3" 
 
 19. Layers of undulatory-banded, nodular chert in a 
 
 matrix of earthy white limestone. 3' 10" 
 
 20. Thick-bedded layers of pure, homogeneous, 
 white marble, forming the strongest cliff in the 
 
 second member of the Unkar__ 6' 8" 
 
 21. Same in thinner beds___ 6' 6" 
 
 22. Undulatory-banded, cherty limestone becoming 
 
 crystalline above ... 10' 
 
 23. Red shale below and purple shale above, sepa¬ 
 rated by a thin layer of chert._. 5' 
 
 24. Thick-bedded, crystalline, white limestone of 
 the same character as 20, forming a strong 
 
 cliff...... 3' 6" 
 
508 Noble—Geology of the Grand Canyon , Arizona. 
 
 Character Thickness 
 
 25. Thin-bedded, crystalline, white limestone_ 5' 6" 
 
 26. Gnarled and nodular, white, cherty limestone.. 7' 
 
 27. Very hard, dense layer of flint forming small 
 
 cliff...-. 5" 
 
 Total______ 
 
 d. Blue slate and white limestone. 
 
 1. Soft, purple shale..... 
 
 2. Hard, blue slate forming small cliff.. 
 
 3. Fissile, blue slates with fine partings.. 
 
 4. Thin-bedded, purple, crystalline limestone _ 
 
 5. Dense, purple, crystalline limestone forming a 
 
 small cliff______ 
 
 6. Same as 4_____ 
 
 7. Same as 5, forming small cliff .... 
 
 8. Thin-lamellar, spotted, blue slate_ 
 
 9. Gnarly layers of fine-lamellar, blue calcareous 
 
 slate with a very coarse concretionary struc¬ 
 ture. Irregular nodules of chert occur in the 
 middle portion...... 
 
 10. Dense, blue, crystalline limestone forming small 
 
 cliff___ 
 
 11. Thin-bedded, platy, white limestone. 
 
 12. Very thin-lamellar, fissile, blue slate__ 
 
 13. Calcareous, blue slate forming a cliff__ 
 
 14. Thin-bedded T platy white limestone_ 
 
 15. Fissile, blue slate__ 
 
 16. Dense, lumpy, white crystalline limestone _ 
 
 17. Pinkish-green, fissile, siliceous slate of a jaspery 
 
 appearance forming a cliff .. ..... 
 
 18. Layers of dense, white, crystalline limestone 
 
 separated by thin bands of pale-green, talcose 
 material____ 
 
 19. Dense, red and black-banded jasper, weathering 
 
 green between the layers and forming a cliff. 
 The layers contain shrinkage cracks and ripple 
 marks ____ 
 
 20. Layers of dense, white, crystalline limestone 
 
 separated by bands of pale green, talcose 
 material.... 
 
 Total _____ 
 
 Synopsis of second member of the TTnkar. 
 
 a. Basal white limestone____ 
 
 b. Argillaceous and calcareous red shale and limestone 
 
 c. White limestone .... 
 
 d. Blue slate and white limestone.. 
 
 To 
 
 o 
 
 rH 
 
 2" 
 
 3' 
 
 6" 
 
 1' 
 
 
 7' 
 
 
 1' 
 
 2" 
 
 
 7" 
 
 2' 
 
 2" 
 
 
 S" 
 
 6' 
 
 6" 
 
 33' 
 
 
 2' 
 
 3" 
 
 2' 
 
 6" 
 
 3' 
 
 Q f 
 
 
 o 
 
 4' 
 
 6" 
 
 7' 
 
 
 2' 
 
 
 5' 
 
 
 10' 
 
 
 11' 
 
 4" 
 
 2' 
 
 
 108' 
 
 2" 
 
 6' 
 
 
 85' 
 
 5" 
 
 105' 
 
 2" 
 
 108' 
 
 2" 
 
 Total thickness 
 
 304' 9" 
 
The Shinumo Area. 
 
 509 
 
 When the specimens of these rocks were examined in the 
 laboratory it was found that all the limestones were more or 
 less dolomitic. The limestones of division “c” were found to 
 be entirely dolomites. 
 
 Thin sections were cut from specimens from twenty separate 
 beds in the second member. Eighteen of these slides were cut 
 from the limestone strata and two from the red shales. The 
 sections of the limestones were cut both from the chert 
 bands and the nodules and from the limestone itself, for 
 the purpose of ascertaining the exact mineralogical char¬ 
 acter of these rocks, and in the forlorn hope that they 
 might reveal traces of a structure that could be referred 
 to something organic. No minerals other than calcite and 
 quartz were revealed by the microscope in any of the slides. 
 The silica of the chert bands and nodules was found to exist 
 in the form of interlocking grains of quartz. None of the 
 grains were rounded and there was no suggestion that the 
 quartz grains of the chert bands represented an inwashed sand. 
 Nor was there any trace of an organic structure revealed, 
 either in the chert or in the limestone. The purer limestones 
 were found to consist of calcite (or dolomite) alone, the crys¬ 
 talline forms having the typical structure of marble. The 
 impure varieties were found to consist of mixtures of quartz 
 and calcite in all proportions. The greater part of the lime¬ 
 stone w T as of this impure character. The shales were found to 
 consist of a fine impalpable ferruginous or calcareous mud, 
 containing occasionally a minute grain of quartz. 
 
 Several features of interest are shown in the lithologic sec¬ 
 tion of the second member of the Unkar as a whole. 
 
 Nipple marks and sun cracks appear for the first time in the 
 shales in stratum No. 18 of division' just below the highest 
 limestone stratum at the summit of the member. 
 
 The increasing intensity of metamorphic phenomena in the 
 section from the base upward may also be noted. This is due 
 to the approaching proximity to the lower contact of the dia¬ 
 base sill which is intruded in the member above. The 
 metamorphic action is manifested in the shales by their change 
 upward both in color and in degree of induration: the shales 
 of division “ b ” are almost entirely red ; from the summit of 
 this division upward the color changes to purple and blue. 
 Below division ic d” the shales are soft and crumbly; within 
 this division, however, they become dark blue slates, while in 
 the upper part they become extremely hard, siliceous jaspers. 
 
 The vertical succession of the strata is seen to be broadly 
 characterized by continual and rapid alterations of- limestone 
 and shale. According to the dominance of either type of rock 
 the four divisions a , b , <?, aud cl are separated : division “ a ” is 
 
510 Noble—Geology of the Grand Canyon , Arizona. 
 
 entirely limestone, u b ” is alternating limestone, and u d” 
 predominantly sliale. Thus there are four major cycles of 
 oscillation upon which the minor cycles are superimposed. 
 
 A comparison of the above section in the East Wash with a 
 section measured in the Asbestos Canyon, four miles to the 
 west, is of interest. In the Asbestos Canyon the “ basal white 
 limestone ” (a) has a thickness of 30 feet contrasted with a 
 thickness of 6 feet in the East Wash. The lower stratum of 
 “nodular, cherty limestone ” is there 7' 9", contrasted with 
 V 3" in the East Wash. The upper stratum of “parallel- 
 banded, cherty limestone ” is 22' 3" in the Asbestos Canyon, 
 containing in the middle an intercalated layer of purple shale, 
 and near the top a thin layer of rather tine arkose conglomerate. 
 The “argillaceous and calcareous red shales and limestones ” of 
 division “ b have a thickness of 88 feet in the Asbestos 
 Canyon contrasted with 85' 5" in the East Wash. They do 
 not have the red color that characterizes them in the East Wash, 
 but are purple and blue, and much indurated. This change of 
 color and difference in degree of induration is due to their 
 closer proximity to the diabase sill in the Asbestos Canyon. 
 
 The correspondence in lithographical character and vertical 
 succession of these two sections four miles apart is so close that 
 the individual strata of the sections can be matched bed for 
 bed. The only marked contrast in thickness occurs in the 
 basal white limestone (a). 
 
 Third Member.—Argillaceous and Arenaceous Shale. 
 
 Section measured at East Wash in continuation of the pre¬ 
 ceding section. 
 
 a. Cliff-forming gasper. 
 
 1. Dense, hard layer of blue-black jasper mottled 
 with red spots and showing no banding in the 
 mass, forming with the three following layers a 
 strong perpendicular cliff. This is the most 
 resistant rock in the Unkar. Where the under 
 surface shows beneath the overhang of the cliff 
 it is sun-cracked on a large scale and in several 
 
 generations.____ 28' 
 
 2. Same general character as 1, but showing a 
 
 banded structure. The lower 2 ft. are slaty and 
 weather out, giving the cliff an overhang_ 19' 
 
 3. Same as 2 with a soft slaty layer at the base... 14' 
 
 4. Same as 2 and 3 with a soft layer at the base. 12' 
 
 Total 73' 
 
The Shinumo Area. 
 
 511 
 
 b. Calcareous blue slate. 
 
 1. Slaty, black jasper, sun-cracked throughout... 12' 
 
 2. Pink, crystalline limestone. p q" 
 
 3. Slaty, blue jasper with small red spots. 4' 6" 
 
 Total 18' 
 
 c. Cliff-forming jasper. 
 
 1. Dense, hard layer of blue-black jasper mottled 
 
 with red spots, forming with the following 
 layer a strong cliff__ 14 ' 
 
 2. Same as 1 with a soft slaty layer at the base 
 
 which weathers out, giving the cliff an overhang. 3' 6" 
 
 Total 17' 6" 
 
 cl. Sanclg quartzitic jasper. 
 
 1. Slaty, blue, spotted jasper.. 4' 
 
 2. Fine-grained, pink quartzite, ripple-marked_ 5' 
 
 3. Slaty, blue, spotted jasper..... 12' 
 
 * 4. Fine-grained, pink quartzite_ 4' 
 
 5. Slaty, blue, spotted jasper with sun-cracks. 6' 
 
 6. Fine-grained, pink quartzite, ripple-marked... l' 
 
 7. Pink, quartzitic jasper. 5' 
 
 8. Fine-grained, pink quartzite.... . 4' 
 
 9. Fine-grained, pink, sandy jasper, sun-cracked.. 11' 
 
 Total 52' 
 
 e. Red and blue jasper. 
 
 1. Banded, blue jasper with curious spots. Sun- 
 
 cracked throughout .. 22' 
 
 2. Red and black banded jasper__ 9' 
 
 Total 31' 
 
 f. At this horizon is intruded a sill of diabase, whose 
 thickness varies from 650 feet on the Shinumo 
 to 950 feet or more in the Asbestos Canyon. 
 
 The remainder of the section was measured in a traverse up 
 the Shinumo, starting with the upper contact of the diabase 
 
 sill. 
 
 g. Blue slate and quartzite forming a cliff. 20' 
 
 h. Blue slate forming a slope____ 100' 
 
 i. Red, argillaceous shale — sun-cracked throughout. 
 
 The rock is very soft and forms a slope together 
 • with the underlying blue slate.... 91' 
 
 j. Alternating, vermilion, argillaceous shale and sand¬ 
 
 stone. 
 
 The alternations in this series occur with remarkable reg¬ 
 ularity. The sandstone is white in color and is compact and 
 
512 Noble—Geology of the Grand Canyon , Arizona. 
 
 fine-grained. It is cross-bedded and ripple-marked through¬ 
 out. The shales of the alternating beds are very soft and 
 weather out, leaving etched-out bands between the sandstones 
 which are very conspicuous in the cliff-faces. On the under¬ 
 surface of each sandstone layer are beautifully preserved sun- 
 cracks. The shales are tine-grained, fissile, and argillaceous. 
 
 The succession in this alternating series is as follows: 
 
 No. 
 
 Bed 
 
 Thickness 
 
 S. S.—Sh. cycle 
 
 1. 
 
 Sandstone _ 
 
 4' 2" 
 
 
 2. 
 
 Shale. __ 
 
 ... 3To" ... 
 
 .. 8' 
 
 3. 
 
 S. S_ 
 
 ...2' 
 
 
 4. 
 
 Sh_ 
 
 ... 3' 
 
 _ 5' 
 
 5. 
 
 s. s_ 
 
 ... 1' 
 
 
 6. 
 
 Sh_ 
 
 ...2' 
 
 _ 3' 
 
 7. 
 
 s. s_ 
 
 ... V' 
 
 
 8. 
 
 Sh_ 
 
 . . . 10' 6" ... 
 
 _17' 6" 
 
 9. 
 
 s. s_ 
 
 ... 5' 
 
 
 10. 
 
 Sh_ 
 
 ...3' 
 
 .. 8' 
 
 11. 
 
 s. s_ 
 
 ... 6' 
 
 
 12. 
 
 Sh_ 
 
 ... 2' 6" ... 
 
 . ... 8' 6" 
 
 13. 
 
 s. s_ 
 
 ... 6' 
 
 
 14. 
 
 Sh_ 
 
 ... 3' 6" ... 
 
 .. 9' 6" 
 
 15. 
 
 s. s_ 
 
 ... 6' 
 
 
 16. 
 
 Sh_ 
 
 .. . 3' 6" ... 
 
 _ 9' 6" 
 
 IV. 
 
 s. s_ 
 
 ... V' 6" 
 
 
 18. 
 
 Sh_ 
 
 ... 4' 
 
 _11' 6" 
 
 19. 
 
 s. s _ 
 
 . . . 3' 6" 
 
 
 20. 
 
 Sh .. 
 
 6" .... 
 
 _ _ 4' 
 
 21. 
 
 s. s_ 
 
 . . . 2' 
 
 
 22. 
 
 Sh_ 
 
 ... r _ 
 
 . .. 3' 
 
 23. 
 
 s. s _ 
 
 ... 3 To" 
 
 
 24. 
 
 Sh _ 
 
 ... 2' 6" ... 
 
 __ 6' 4" 
 
 25. 
 
 s. s _ 
 
 2' 
 
 
 26. 
 
 Sh _ 
 
 ... 6' 6" ... 
 
 . .. 8' 6" 
 
 2V. 
 
 s. s _ 
 
 ... 3' 
 
 
 28. 
 
 Sh _ 
 
 ... 4' 
 
 .. 7' 
 
 
 Total, 
 
 109' 4" 
 
 Ave., 7 TO" 
 
 Jc. Alternating, vermilion, arenaceous shale and sandstone. 
 
 The sandstone is white, compact, and fine-grained. It is 
 cross-bedded and ripple-marked throughout. The under¬ 
 surface of each sandstone layer is sun-cracked where it rests 
 upon the arenaceous shale. The shale is vermilion in color, 
 soft, and very sandy. Sun-cracks occur throughout. 
 
 The succession is as follows: 
 
The Shinumo Area. 
 
 513 
 
 1. Sandstone. 9' 4 
 
 2. Arenaceous shale. 21' 1" 
 
 3. Sandstone. 9' 
 
 4. Arenaceous shale... 11' 8" 
 
 5. Sandstone... 2' 
 
 6. Arenaceous shale... 24' 
 
 7. Sandstone. 1' 
 
 Total 78' 1" 
 
 Synopsis of third member of the Unkar. 
 
 a. Cliff-forming jasper.. 73' 
 
 b. Blue slate with calcareous band_ 18' 
 
 c. Cliff-forming jasper.. 17' 6" 
 
 d. Sandy quartzitic jasper.. 52' 
 
 e. Red and blue jasper_. . 31' 
 
 f (Intrusive diabase) 
 
 g. Blue slate and quartzite..._ 20' 
 
 h. Blue slate. .100' 
 
 i. Red argillaceous shale.. 81' 
 
 j. Alternating vermilion argillaceous 
 
 shale and S. S..109' 4" 
 
 k. Alternating vermilion arenaceous 
 
 shale and S. S... 78' l" 
 
 Total thickness 579'11" 
 
 Thin sections were cut from several specimens of the 
 jaspers. The slides were unsatisfactory, however, because of 
 the exceedingly fine grain of the rock. The highest power of 
 the microscope revealed nothing more than an impalpable 
 silicified mud. A slide of the “ quartzitic jasper ” showed it 
 to have been a somewhat arkose sandstone indurated to a 
 siliceous quartzite. It was seen to be composed chiefly of 
 small rounded quartz grains about which secondary silica had 
 been deposited, lying in a fine arkose matrix made up of small 
 fragments of pink feldspar. A thin section w r as also made 
 from a specimen of one of the sandstone layers in the “ alter¬ 
 nating argillaceous shale and sandstone” of division u j . ” 
 The rock proved to consist of small, well-rounded grains of 
 quartz, cemented by silica in the form of secondary quartz. It 
 is a pure, fine-grained sandstone. 
 
 The metamorphic effects produced by the diabase sill 
 intruded at the horizon u f” are seen in the lithologic section of 
 the third member given above. This metamorphic action is 
 manifested in three ways : 
 
 1. Induration by silicification,—jaspers. 
 
 2. Induration by baking,—slates. 
 
 3. Decoloration,—red to blue and black. 
 
 Am. Jour. Sci. —Fourth Series, Vol. XXIX, No. 174. —June, 1910. 
 
 34 
 
514: Noble—Geology of the Grand Canyon , Arizona. 
 
 It was noted in the summary of the features of the second 
 member that the shales became successively slates and jaspers 
 above, while their color changed from red to blue. In the 
 third member the shales are represented entirely by jaspers 
 and quartzites. Just below the contact the induration is very 
 great, and the jaspers are tough and vitreous; the prevailing 
 color is blue or black. Above the contact the overlying rocks 
 are hard blue slates for 20 feet, succeeded by 100 feet of less 
 indurated slate, grading upward into the original red shale. 
 
 The metamorphic effects above and below the contact differ 
 in degree of intensity as well as in kind: above the contact 
 the induration and decoloration characterize only about 100 
 feet of strata; below the contact this action extends through 
 300 feet. Above the contact the strata are characterized by 
 baking and decoloration only, changing the red shale into a 
 blue slate ; below the contact there has been a considerable 
 addition of silica, transforming the red shales into blue and 
 black jaspers; added to this are the effects of baking and 
 decoloration. 
 
 It may be said in summary that the third member is charac¬ 
 terized by argillaceous shales in the lower portion, which grade 
 upward into arenaceous shales and sandstones through the 
 interesting series of alternations described in division u j.” 
 There is hardly a stratum in the entire member that does not 
 bear marks of shallow water origin, manifested by either sun- 
 cracks, ripple-marks, or cross-bedding. 
 
 Fourth Member.—Sandstone and Quartzite. 
 
 Section on the Shinumo. 
 
 a. Purple-brown sandstone of fine grain containing locally. 
 
 an occasional lense of fine conglomerate and sometimes 
 a thin local bed of red or purple shale. The sand¬ 
 stone is cross-bedded throughout. 406' 
 
 b. Compact, white quartzite of fine and uniform grain, 
 
 displaying a faint cross-bedded structure. This quartz¬ 
 ite is the most resistant rock in the fourth member. It 
 is exposed everywhere in one massive perpendicular 
 cliff face, which does not display the slightest break 
 except where it is cut by faults. Wherever its base 
 rests upon a shaly lense the under surface displays 
 well preserved mud-cracks. The face of the cliff is 
 stained magenta by the ferruginous cement of the 
 shale lenses in the overlying sandstones, which washes 
 
 down from above...... 119' 
 
 c. Purple-brown sandstone of the same character as a _ 353' 
 
 d. Banded, white quartzite, stained magenta on the expo¬ 
 
 sures, and forming a cliff... 120' 
 
The Sfiinumo Area. 
 
 515 
 
 e. Fine-grained, purple sandstone with a white band in the 
 
 middle. The white band is constant and presents a 
 conspicuous feature by which this purple sandstone 
 can be distinguished at a distance of several miles. 
 
 The rock is cross-bedded and sometimes displays a 
 “ twisted and gnarled ” structure.__ 150' 
 
 f. Compact, cliff-forming, white quartzite of the same 
 
 character as b , though not so massive in structure_ 250' 
 
 g. Banded white quartzite____ 20' 
 
 h. Irregularly bedded sandstone .... ... 
 
 1. “ Curiously twisted and gnarled layers,” of fine¬ 
 grained white sandstone containing large red spots of 
 a circular and elliptical form. The upper part of the 
 bed is more massive. The twisted and gnarled struc¬ 
 ture seems to have been a phenomenon of the original 
 deposition. It gives the rock an appearance which 
 suggests that the original sand was moist and plastic 
 and once flowed by rolling over and over in the form 
 of a quicksand. 105' 
 
 (It may be noted that a bed of this character is 
 described by Walcott in his section in Unkar Val¬ 
 ley. (Walcott 5, page 511.) It occurs at the same 
 horizon as the bed described above, and contains the 
 
 same red spots.) 
 
 2. Banded, purple sandstones.. 20' 
 
 3. Green, cross-bedded sandstone ... 21' 
 
 146' 
 
 The total thickness of the fourth member of the Unkar is 
 1564 feet. 
 
 Slides were cut from several specimens of the sandstones 
 and quartzites. All were found to consist of small rounded 
 quartz grains, the size of which seldom exceeds 0*7 mm . This 
 extreme fineness and roundness of the grains, as well as the 
 cleanness of the sorting, is remarkable. The cement is usually 
 siliceous, sometimes slightly ferruginous. A slide was made 
 from a specimen taken from one of the small conglomerate 
 lenses in division a. The conglomerate was found to consist 
 of small rounded quartz pebbles, lying in a fine arkose matrix. 
 Occasional large angular fragments of ortlioclase and micro- 
 cline were revealed. 
 
 In summary it may be said that the fourth member is char¬ 
 acterized by great thicknesses of pure, fine-grained sandstone 
 of a uniform character. All divisions of the member are 
 resistant to the weather and form cliffs. Bipple marks and 
 cross-bedding occur throughout. 
 
 Fifth Member.—Micaceous Shaly Sandstone. 
 
 Section measured in Fault Wash, west side of Shinumo 
 Canyon. 
 
516 Noble—Geology of the Grand Canyon , Arizona. 
 
 a. Gray-green, pinkish-green, and brown micaceous shaly 
 
 sandstones, cross-bedded and ripple-marked, varying 
 in character only through gradations in color. Fre¬ 
 quent arenaceous and argillaceous shaly partings occur, 
 causing the rock to weather like a soft sandy shale. 
 
 The shale partings are usually green. Some of the 
 sandy layers near the base show the “gnarled and 
 twisted structure.” ...... 1100' 
 
 b. The green, gray, and brown beds of a pass upward 
 
 through change of color into red and vermilion beds 
 of the same general character. These are likewise 
 micaceous shaly sandstones, cross-bedded and ripple- 
 marked, with arenaceous and argillaceous shaly part¬ 
 ings, which display well preserved mud-cracks. The 
 
 shaly partings are either green or red... 1197' 
 
 Total... 2297' 
 
 The summit of the Unkar section is obliterated in the beds 
 of division b by the plane of the pre-Tonto unconformity. 
 The highest beds of this division lie at the upper end of the 
 Fault Wash, dragged against the Vishnu schist on the north 
 by the great pre-Cambrian fault previously described, and the 
 whole overlain by the Tonto sandstone. 
 
 In the uppermost part of division b are four thin sills of 
 rotten basaltic rock, weathering green and crumbling to small 
 fragments. They occur between the beds of vermilion sand¬ 
 stone and shale. Their intrusive character is shown by the 
 fact that they have baked and decolorized the vermilion 
 beds for a few inches both above and below the contacts, 
 changing the color to purple. The rock is too badly weathered 
 to allow a petrographic determination in thin section. 
 
 In summary the fifth member of the Unkar may be charac¬ 
 terized as a series of micaceous shaly sandstones of uniform 
 character, varying only in color, and bearing marks of shallow 
 w r ater origin throughout. 
 
 Comparison with the type section in TJnkar Valley .— 
 A comparison of the above section of the Unkar group with 
 that described by Walcott (d) in the type locality 30 miles to 
 the east reveals the fact that the correspondence between the 
 two sections is very close, both in thickness and in lithological 
 succession ; only in the lower portions do they differ materi¬ 
 ally. The type section in Unkar valley is characterized by a 
 greater thickness of the basal conglomerate, and by only a third 
 as much limestone in the members which correspond to the sec¬ 
 ond member on the Shinumo; the deficiency in limestone is made 
 up by greater thickness of arenaceous and argillaceous shale. 
 Proceeding upward in the section there is a somewhat greater 
 
The Shinumo Area. 
 
 517 
 
 proportion of sand in that division of Walcott’s section which 
 corresponds to the third member on the Shinumo. The succeed¬ 
 ing members correspond closely in character and thickness 
 even to the minor divisions ; an example of this is the 
 “gnarled and twisted layers” previously cited. 
 
 The writer had the privilege of examining Mr. Walcott’s 
 field specimens in the National Museum in Washington and 
 was particularly impressed by their absolute lithological iden¬ 
 tity with the series collected by himself from corresponding 
 horizons on the Shinumo. The two series of specimens, with 
 the exception of those rocks altered by local metamorpliic phe¬ 
 nomena, might have come from the same locality. 
 
 Diabase Intrusive in the Unkar. 
 
 Occurrence .—The diabase occurs in the form of an intrusive 
 sheet or sill which occupies three separate stratigraphic hori¬ 
 zons in the TJukar sediments in different parts of the Shinumo 
 area. On the west side of the Asbestos Canyon three miles 
 west of the Shinumo it is intrusive in the limestones of division 
 c of the second member of the Unkar at a horizon 15 feet above 
 their base. On the east side of the Asbestos Canvon it ascends 
 out of this horizon and breaks across the overlying limestones 
 and shales in an eruptive contact. Most of the lower, and part 
 of the upper, sections of the eruptive contact are beautifully 
 displayed in the walls of this canyon, but the complete expo¬ 
 sure is obliterated above by the truncating unconformity 
 at the base of the Tonto sandstone. Between the exposures in 
 the Asbestos Canyon and the main areal exposures about the 
 mouth of the Shinumo, the pre-Cambrian structure is hidden 
 beneath the sandstone of the Tonto platform. Where the dia¬ 
 base reappears in the exposures about the Shinumo it is found 
 
 to lie intruded midwav within the shales of the third mem- 
 
 «/ 
 
 her of the Unkar at a horizon 400 feet above that which 
 it occupies in the Asbestos Canyon. This is the stratigraphic 
 position which it holds in all the exposures of the central part 
 of the Shinumo area. Eastward up the river from the East 
 Wash the diabase again disappears beneath the pre-Tonto un¬ 
 conformity. But directly under Iiavasupai Point, about three 
 miles farther east, limited outcrops of the basal portion are 
 exposed in small inter-canyon valleys on both sides of the river. 
 Here the lower contact of the diabase lies just at the top of 
 the u white limestones” of division c of the second member 
 of the Unkar. 
 
 The thickness of the sill is approximately 650 feet on the Shi¬ 
 numo and 950 feet on the west side of the Asbestos Canyon. 
 Eastward from the Shinumo the outcrop is considerably 
 
518 Noble—Geology of the Grand Canyon , Arizona. 
 
 thinned by a strike fault, precluding the possibility of measur¬ 
 ing the thickness in that direction. 
 
 ij 
 
 Petrography. 
 
 Megascopic .—The fresh specimens characteristic of the 
 greater part of the mass show it to be a tough, heavy, holo- 
 crystalline rock of medium to coarse grain and of a grey color. 
 The minerals visible to the unaided eye are plagioclase, olivine, 
 augite, and an occasional grain of magnetite. Although the 
 olivine exceeds the augite in amount, it is less conspicuous to 
 the eye. Aside from a somewhat waxy luster of the feldspars, 
 the rock is remarkably fresh. The weathered surface has a 
 characteristic warty appearance, imparted by the presence of 
 lumps or balls which are of a coarser grain and different tex¬ 
 ture than that of the mass of the rock and are more resistant 
 to the processes of disintegration. These lumps and balls can 
 be seen to consist of coarse ophitic intergrowths of augite and 
 plagioclase. The diabase weathers by mechanical disintegra¬ 
 tion to a greenish-olive sand in which are innumerable lumpy 
 kernels of all sizes derived from the ophitic masses described 
 above. The rock has no typical columnar structure, but usu¬ 
 ally displays a rough vertical jointing such as is characteristic 
 of granite. 
 
 Microscopic .—The slides examined show the typical rock to 
 consist primarily of plagioclase feldspar (near labradorite) and 
 olivine in about equal amounts, with a subordinate quantity of 
 augite and brown biotite. A very little magnetite is present. 
 The feldspar is somewhat altered, but all the other minerals 
 are fresh. The olivine occurs characteristically in rather large, 
 rounded crystals of automorphic habit. The augite is chiefly 
 confined to the globular masses which weather out as lumps 
 and kernels and does not characterize the rock as a whole. 
 Slides cut from these kernels show them to be conqDosed 
 entirely of augite and feldspar. The augite is enclosed within 
 the feldspar, displaying beautiful examples of ophitic texture. 
 Several of the magnetite crystals were observed to have rims 
 of brown biotite. The small amount of magnetite is rather 
 remarkable, and it seems likely on this account that the olivine 
 is rich in magnesia. Because of the predominance of olivine 
 and plagioclase in the greater part of the rock, the diabase is 
 classified as an olivine-diabase with a troctolitic aspect. 
 
 Variations in character .—All parts of the mass are subject 
 to variations in texture and composition toward a coarser grain. 
 These are of two kinds. The first type occurs in the ophitic 
 intergrowths of augite and plagioclase in the lumps and balls 
 described above, and is a segregation phenomenon character- 
 
The Shinumo Area. 
 
 519 
 
 izing the mass as a whole; sometimes this texture becomes 
 very coarse, the separate crystals of augite or plagioclase attain¬ 
 ing an inch in length. The second type occurs in typical peg¬ 
 matite dikes cutting the diabase in many places and varying in 
 width from a few inches to several feet. - The minerals are 
 plagioclase and augite and the texture is usually ophitic, 
 although not always so. In some of these dikes crystals of 
 plagioclase exceeding three inches in length were observed. 
 
 The contact facies of the diabase are frequently line-grained 
 or glassy, but never for more than a few inches from the con¬ 
 tact. The slides typical of this zone reveal a hyalopilitic 
 arrangement of glass with skeleton crystals of magnetite 
 between badly altered crystals of feldspar. 
 
 For about a half-mile east and west of the Shinumo there 
 occurs in the upper part of the diabase sill along the upper 
 contact a pink holocrystalline rock of medium grain. The con¬ 
 tact of this rock with the overlying blue slates is sharp and 
 well-defined. Downward it appears to grade into the normal 
 diabase, and no definite line of contact can anywhere be 
 observed. Unfortunately the writer did not collect transition 
 specimens, but took only one specimen from the middle of the 
 pink mass. The slide from this specimen when examined 
 under the microscope showed it to be a granular rock of 
 medium texture, consisting of rather fresh crystals of ortho- 
 clase, with subordinate quartz and a somewhat altered ferro- 
 magnesian mineral which was made out to have been originally 
 a hornblende. Some of the quartz displayed a micrographic 
 arrangement within the feldspar. The rock is a typical horn- 
 blende-syenite, and is apparently an interesting example of 
 differentiation in place within the diabase sill. But a more 
 complete set of specimens across the apparent transition zone 
 must be collected before such a conclusion can definitely be 
 established. 
 
 In the Asbestos Canyon both the lower and upper eruptive 
 contacts of the diabase are ragged and considerably injected. 
 Many small dikes penetrate the country rock from the main 
 mass. They are glassy in texture and badly altered. 
 
 Bansome, in his report upon the u Geology of the Globe 
 Copper District, Arizona ” (Bansome b , p. 80 ff.) describes a 
 diabase of post-Carboniferous age occurring in thick sills in 
 the pre-Carboniferous sedimentary rocks of that region. This 
 diabase closely resembles the Algonkian diabase described 
 above, both in mineralogical character and in the presence of 
 the ophitic balls of plagioclase and augite. The analogy is 
 made the more striking by the fact that several small masses 
 of pink hornblende-syenite are described occurring within 
 the diabase sills of the Globe district as a possible segregation 
 phenomenon within the diabasic magma. 
 
520 Noble—Geology of the Grand Canyon, Arizona. 
 
 Contact metamorphism .—Since the diabase sill occupies 
 relatively different horizons in the Unkar strata upon the 
 Shinmno and in the Asbestos Canyon, and since in the former 
 locality the strata between which the sill is intruded lie in 
 undisturbed sedimentary contact in the latter locality, and 
 vice versa, the study of the respective contact effects upon the 
 invaded strata could be made with great facility. 
 
 The contact effect upon the shales which lie above and 
 below the diabase along the Shinumo has already been 
 described in the detailed section of the Unkar: there the 
 shales were shown to be altered to jaspers by baking and silici- 
 fication. It was shown that the intensity of metamorphic 
 action was much greater below the sill than above, extending 
 through 300 feet of strata below the lower contact and through 
 only 100 feet above the upper contact. In the Asbestos 
 Canyon these rocks lie in undisturbed sedimentary contact 
 and are there unaltered red shales. 
 
 The contact effect upon the limestones can be studied in the 
 Asbestos Canyon, where the diabase sill lies intruded within 
 them. Immediately below the lower contact of the diabase, 
 which is sharp and well defined, is a thin layer of green ser¬ 
 pentine. Below lie layers of pure crystalline limestone (dolo¬ 
 mite) alternating with layers of the same character containing 
 bands and nodules of serpentine. Within one of the layers 
 containing the bands and nodules of serpentine are cross¬ 
 fiber veins of golden-yellow chrysotile asbestos which are 
 parallel in general trend to the bedding of the limestone. 
 These limestones are the layers at the base of division c of the 
 
 second member of the Unkar. They overlie the red shales of 
 
 •/ 
 
 division b , which are here baked to blue slates. 
 
 Section below diabase contact (reading upward from the base) 
 
 Dense, purple, calcareous slate (5, 24 of second member 
 
 of Unkar). 9' 
 
 Soft, blue slate ( b , 25)______ 3' 
 
 Nodular, cherty limestone (c, 1, 2) .... 4' 
 
 Banded, crystalline limestone with bands and nodules of 
 
 serpentine (c, 3, 4,) . . ___ 10' 
 
 Serpentinous, nodular and banded layer carrying veins 
 
 of asbestos. T 
 
 White, crystalline limestone with bands and nodules of 
 
 serpentine... 2' 
 
 Pure, white, crystalline limestone.. ]' 6" 
 
 Layer of green serpentine_ 2' 
 
 Diabase. 
 
 Above the upper contact of the diabase the limestones con¬ 
 tain several alternating layers of green serpentine and narrow 
 
The Shinumo Area. 
 
 521 
 
 veins of asbestos occur at several horizons in proximity to the 
 contact. 
 
 The geological occurrence of the asbestos is fully described 
 by Diller in “ Mineral Resources of the U. S. for 1907” (Diller, 
 a\ and again by the same writer in “ Mineral Resources for 
 1908 ” (Diller, h). The contribution of the writer of the 
 present article consists of the further data afforded by a micro¬ 
 scopic study of the rocks associated with the asbestos in this 
 locality. A microscopic study was made of 25 thin sections 
 cut from the limestones, the bands and nodules of serpentine, 
 and across the veins of asbestos. Aside from the serpentine 
 and asbestos no other minerals were revealed in the limestones 
 beyond the dolomitic calcite and interlocking grains of quartz 
 already described in the slides cut from the limestones of the 
 same horizon in the section on the East Wash, where the same 
 strata lie in undisturbed sedimentarv contact. The texture of 
 the limestones is entirely that of marble. The serpentine of 
 the bands and nodules shows no trace of an alteration structure 
 which might indicate a derivation from pyroxene, hornblende, 
 or olivine. The slides cut across the veins of asbestos showed 
 them to be later than the serpentine in which they are usually 
 enclosed. A great number of veins of asbestos of microscopic 
 size was revealed in some of the slides where their presence 
 was unsuspected. Some of these veins were observed cutting 
 across both the serpentine and the limestone in the same slide. 
 
 The asbestos which occurs in the larger veins is commercially 
 of high grade and the liber is of great tensile strength. It is 
 pronounced by Diller to be the best in quality yet found any¬ 
 where in the United States. (Diller, 5, p. 11.) Locally the 
 crossfiber is 4 inches in length. The horizon of the larger 
 veins is confined, so far as is known, to the limestones which 
 lie beneath the lower contact of the diabase sill. The veins 
 above the contact, although more widely distributed through 
 the limestones, are usually of smaller size. The horizon below 
 the contact is not absolutely constant in stratigraphic position 
 and may lie anywhere from 3 to 15 feet below the contact. 
 The width of the veins within this horizon varies greatly from 
 place to place, so that a vein of three inches in width in one 
 locality may be represented by a zone of innumerable small 
 veins in another, but the actual continuity of the zone that 
 carries the asbestos is rarely broken. 
 
 Conclusions .—The serpentine and asbestos occur in the 
 limestones only where these strata are invaded bv the dia¬ 
 base sill; where the diabase lies between shales there is no 
 development of these minerals within the invaded strata. In 
 no place in the area are they developed within the diabase 
 itself. It is therefore clear that they are a contact metamorphic 
 
522 Noble—Geology of the Grand Canyon , Arizona. 
 
 phenomenon conditioned by the invasion of the limestones by 
 the diabase. It seens probable, as suggested by Diller (a, p. 
 72), that the serpentine which encloses the veins of asbestos is 
 derived from some mineral in the limestones and not from the 
 diabase. The limestones themselves are magnesian, and locally 
 siliceous in the form of chert bands and nodules. In another 
 part of the area the conversion of the shales to jaspers where 
 they are in contact with the diabase is evidence that the fuma- 
 rolic action accompanying the injection of diabasic magma 
 was manifested by aqueous and probably siliceous emanations 
 and was fairly intense. It seems possible that the operation 
 of the fumarolic action upon the elements already present in 
 the magnesian limestones might have been sufficient to convert 
 the more siliceous portions into serpentine. The occurrence 
 of the asbestos in veins that cut both the nodules of serpentine 
 and the limestones is evidence that the formation of the cross¬ 
 fiber asbestos was itself a somewhat later phenomenon. 
 
 Age and Correlation. 
 
 The Grand Canyon series is referred to the Algonkian in 
 the usage of the United States Geological Survey and is ten¬ 
 tatively correlated with the Keweenawan series of the Lake 
 Superior region and with the Llano series of Texas, following 
 the conclusions established by Walcott (J, p. 518). In the 
 Shinumo area the profound nature of the unconformity which 
 separates these strata from the basal Tonto sandstone is even 
 more striking than in the area described by Walcott, while 
 the certainty of their stratigraphic position is in its clearness 
 and spectacular character probably unparalleled in the world. 
 
 Geologic History. 
 
 Outline. 
 
 The following sequence of events may be distinguished : 
 
 1. Deposition of the Vishnu sediments, source unknown. 
 
 2. Regional metamorphism. 
 
 a. Subsidence and profound burial. 
 
 b. Orographic movement of folding and compression 
 
 resulting in recrystallization and schistosity, accom¬ 
 panied or preceded by injections of pegmatite. 
 
 c. Elevation, accompanying or succeeding the orographic 
 
 movement. 
 
 3. Batholithic invasion of quartz diorite. 
 
 4. Later pegmatitic injections. 
 
 5. Long cycle of erosion carried through to a featureless pene¬ 
 
 plain of no relief, which truncates the verv roots of the 
 
 Vishnu structure. 
 
The Shinumo Area. 
 
 523 
 
 6. Sudden incoming of the shallow Unkar sea. 
 
 7. Deposition of the Unkar sediments and succeeding strata of 
 
 the Grand Canyon series in shallow water (or upon land ?) 
 
 8. Intrusion of diabase sills. 
 
 9. Orographic movement of block faulting and tilting accom¬ 
 
 panied or succeeded by elevation. 
 
 10. Long cycle of erosion carried through to a peneplained sur¬ 
 
 face of small relief. 
 
 11. Incoming of the Tonto sea, succeeded by the deposition of 
 
 the Paleozoic strata of the wall of the Grand Canvon. 
 
 «/ 
 
 The earliest event which is decipherable in the geologic 
 history of the Shinumo area is found in the blurred and 
 mangled record of the Vishnu schists. Far back in the dark 
 ages of geologic time a thick series of more or less arkose 
 kands and muds was accumulating upon a subsiding floor. 
 So much may be reasonably inferred from the mineralogical 
 character of the quartz schists of the mica and hornblende 
 type. So dim and vague is the record that the base of this 
 series, the floor upon which it was laid down, the thickness, 
 and the location of the land mass from which it was derived 
 must, perhaps, remain forever unknown. Following the long 
 accumulation and burial of these sediments came an orographic 
 movement which wrote across the older manuscript in a newer 
 and bolder hand, blurring the ancient writing with the stamp 
 of deep-seated regional metamorphism, and imparting to the 
 manuscript the aspect of a palimpsest. The regional meta¬ 
 morphism is conceived to have been brought about by deep 
 burial of the sediments, followed by folding and compression, 
 which engraved upon them the characters of recrystallization 
 and schistosity in slow process of time, and accompanied by 
 their elevation into lofty mountains. Somewhat later the cores 
 of these mountains were intruded by great masses of igneous 
 rock, here in the form of quartz diorite, followed by peg- 
 matitic injections. Perhaps while this orographic movement 
 was still in progress the forces of erosion were already at work. 
 Then followed a tremendous cycle of erosion carried through 
 to the very end, planing away the ancient mountains to the 
 basal roots, and reducing hard and soft rocks alike to an utterly 
 flat and monotonous level. Such was the completion of the 
 cycle of this vast unknown and unnamed seon of time. 
 
 The next event is the beginning of another great cycle of 
 sedimentation resulting in the deposition of the Grand Canyon 
 series of Unkar and Cliuar strata, ushered in by the sudden 
 invasion of a shallow sea which swept over the featureless 
 desert surface of the Vishnu plain, depositing the basal conglom¬ 
 erate of the Unkar. The clue to the inferences as to the 
 character of this incoming sea, of the rock mantle which it 
 
524 Noble—Geology of the Grand Canyon , Arizona. 
 
 found covering the plain, and of the climate of the time is 
 preserved for us in the basal conglomerate described in a 
 previous part of this article. It was there noted that the 
 weathering below the surface of the peneplain was slight, and 
 the product of physical disintegration rather than of chemical 
 weathering; that the matrix of the conglomerate is red in color 
 and arkose in nature; that the pebbles are angular and show 
 no evidence of transportation and sorting. We may therefore 
 conclude that the conglomerate represents the soil in place 
 which covered the Vishnu plain. All the evidence points 
 to an arid climate,—the lack of chemical weathering, the 
 freshness of the arkose feldspar, and the red color of the matrix, 
 all indicating a lack of vegetation and abundant moisture which 
 could decompose the soil and reduce the iron which imparts 
 the red color. It is therefore not unlikely that the Yishnu 
 plain was a vast desert at the incoming of the sea. The abso¬ 
 lute lack of transportation, sorting, and rounding of pebbles 
 indicates that the incoming sea had little chance to rework 
 the soil mantle by its waves. It seems impossible to account 
 for this phenomenon except by a sudden invasion of the sea 
 across the Yishnu desert. If we interpret the past in the light 
 of the present our only guide is to seek to picture some present 
 condition on the earth which parallels that preserved in the 
 geological record. A possible clue may lie in the conditions 
 about the Caspian Sea to-day. There is in that region a desert 
 about the shores of the Caspian which lies below the present 
 level of the Black Sea; a sudden rise in the level of the ocean 
 might cause the latter to overflow the low barrier which sepa¬ 
 rates it from the Caspian, and in this way a sudden inundation 
 of the desert would be accomplished. It is now thought that 
 wind erosion may carry the surface of an old desert to a level 
 below that of the sea if the c t ycle is continued to extreme 
 maturity. Such forces might have been active in the last 
 stages of the Yishnu peneplain. 
 
 The deposition of the alternating limestones and shales of 
 the second member of the Unkar seems to have taken place in 
 a permanent water body into which mud was frequently 
 washed. It may be noticed from the section of this member 
 that the alternations are almost innumerable. The exact cause 
 of this is speculative. Possibly the alternations are due to 
 climatic oscillation : a movement from an arid to a semi-arid 
 climate would load the rivers with sediment, while arid 
 intervals would retard their flow, if not dry them up entirely, 
 resulting in a temporary clarifying of the sea and a deposition 
 of limestone. Whether the limestones are the result of organic 
 agencies or of purely chemical precipitation is also a matter of 
 speculation, since in regard to decisive evidences of life the 
 
I 
 
 The Shinumo Area. 
 
 525 
 
 geological record is silent. It is believed that the water 
 body was for the most part shallow, since sun-cracks appear in 
 the shales in the upper part of the member below the highest 
 limestone strata. What the polygonal cracks and the small 
 cubic depressions in the banded cherty layers of the basal lime¬ 
 stones mean the writer does not know. The cracks are 
 suggestive of sun-cracks and the depressions stroneflv resemble 
 salt hoppers. 
 
 The most striking feature of the third member is the great 
 abundance of sun-cracks throughout the shaly strata and of 
 ripple-marks and of cross-bedding in the sandstones. The 
 researches of Professor Barrell have made it clear that exten¬ 
 sive sun-cracking is a feature which has a maximum chance of 
 preserval on broad flood plains or deltas in an arid climate. 
 (Barrell, a, pp. 524-568.) In the opinion of the writer of the 
 present paper, the extreme abundance of these cracks in this 
 member of the Unkar is hard to account for except by postu¬ 
 lating wflde delta flats or flood plains. Furthermore, the bright 
 red color of the shaly strata in connection with the mud cracks 
 seems to bespeak an arid climate with little or no vegetation 
 to reduce the iron. It is at least certain that all this part of 
 the Unkar was deposited in very shallow water which often 
 evaporated entirely, leaving broad mud flats exposed to a 
 hot sun. In the upper part of the member is the series of 
 alternating shales and sandstones already described in detail: 
 as may be seen from the sections, the alternations are fre¬ 
 quently as regular as clockwork. The sandstone layers are 
 composed of fine, cleanly sorted and rounded quartz grains, 
 ripple-marked and cross-bedded, while the shales are a fine red 
 mud. It is thought that the clean character of the sandstone 
 layers of this alternating series is a mark of climatic oscilla¬ 
 tion : a climatic movement towards a wetter climate, if increas¬ 
 ing the ratio of run-off to erosion, causes the rivers to flow on 
 a lower grade and sweep seaward the piedmont deposits of 
 sand and gravel; as the clay w r as largely sorted from those 
 deposits when they were first laid down, their redistribution 
 accompanied by a secondary sorting on a delta surface or sea- 
 bottom would be marked by extreme cleanness. 
 
 Great thickness, clear sorting, and extreme fineness and 
 roundness of grain are the characters which distinguish the 
 fourth member, which is composed entirely of sandstones and 
 quartzites. All strata are characterized by cross-bedding and 
 ripple-marks, bespeaking shallow water. The origin of these 
 great thicknesses of sandstone is a puzzle. The clean sorting 
 seems to indicate long transportation : it is not impossible that 
 the rivers carrying this material might have flowed through a 
 great desert of dune sands, picking up and carrying material 
 
526 Noble—Geology of the Grand Canyon , Arizona, 
 
 such as is deposited in the Indus delta to-day from a similar 
 source. Occasional lenses of fine conglomerate within some 
 of the strata suggest scoured and filled stream channels. 
 
 The upper member of the Unkar again bears all the marks 
 of shallow water origin : mud-cracks, ripple-marks, and cross¬ 
 bedding characterize the entire thickness. The addition of 
 micaceous material and of some feldspar gives a slightly arkose 
 character to the rock; possibly a crustal movement rejuve¬ 
 nating the land mass supplying the sediments was responsible 
 for the change in character. Here again are marks of aridity, 
 seen in the vermilion color and the vast development of 
 mud-cracks. 
 
 All subsequent Unkar and Chuar deposits have been removed 
 by the truncation of the pre-Cambrian structure by the plane 
 of the base-leveled surface of erosion beneath the Tonto 
 Sandstone. 
 
 In summary it may be said that the evidence obtainable 
 from the lithologic record of the Unkar sediments in the 
 Shinumo Area points probably to an arid climate, and almost 
 surely to deposition in shallow waters; first in a permanent 
 body of water and later in deltas or on flood plains. Which 
 of the latter conditions prevailed the writer is not competent to 
 say. 
 
 The predominance of clastic sediments instead of limestones 
 in the basal portion of Walcott’s section, 30 miles east, sug¬ 
 gests that that area was nearer to the shore line of the early 
 shallow sea. The close correspondence of the stratigraphic 
 succession and lithology in all higher members in the two 
 areas suggests uniform conditions over at least that distance. 
 
 The next event which can be read from the geologic record 
 is the invasion of the Unkar strata by a thick sill of diabase 
 in the lower members, and by four thin sills of basaltic rock in 
 the upper part of the section. 
 
 Following this came an orographic movement of block 
 faulting and tilting, accompanied or succeeded by elevation, 
 breaking the strata of the Grand Canyon series into great 
 crustal blocks, and throwing them into high ranges of moun¬ 
 tains which in character and aspect were probably not unlike 
 the faulted ranges of the Great Basin or the desert ranges of 
 Arizona. 
 
 Then began a second vast interval of erosion, gnawing 
 slowly but surely into these faulted mountains, reducing them 
 in slow process of time through stages of youth, maturity, and 
 old age, and finally planing away all except the very hardest 
 strata of their cores to form the broad monotonous expanse of 
 a base-leveled surface. The monotony of this surface was 
 broken only by an occasional monadnock of hard Unkar quartz- 
 
The Shinumo Area. 
 
 527 
 
 ite, resisting the forces of erosion in that ancient plain by 
 virtue of the terrific hardness which causes the same strata 
 to-day to wall in the deep box-canyon of the Shinumo. These 
 monadnocks of the Cambrian plain may be compared with the 
 Baraboo ridges of Huronian quartzite which by virtue of their 
 homogeneity and hardness still stand as prominences which 
 have weathered repeated cycles of erosion. It is probable 
 that this cycle of erosion was not finally completed until well 
 along in lower Cambrian time. 
 
 The next event is the incoming of the Tonto sea. Although 
 this chapter belongs to the Paleozoic history recorded in the 
 horizontal strata of the walls of the Grand Canyon, that part 
 which is involved with the distribution of the Unkar strata 
 upon the pre-Tonto surface may properly be anticipated here. 
 
 It is not unlikely that this invading sea transgressed a sur¬ 
 face which strongly resembled the present surface of the 
 great Laurentian peneplain of Canada with its broad areas of 
 crystalline rocks in which are inset occasional blocks of Paleo¬ 
 zoic strata, and above which stand occasional monadnocks of 
 quartzite. The story of the invading sea is written in the 
 Tonto Sandstone, locked up in which is a record of marine 
 transgression which on account of the vertical sections and 
 absence of soil is clear beyond belief. When the Tonto sea 
 came in over the surface of the ancient peneplain the monad¬ 
 nocks stood out as islands which were gradually overwhelmed 
 and buried in the sands of the deepening sea. The long ridge 
 of the quartzites of the middle Unkar in the Shinumo area has 
 already been described. It stood out as a long narrow rocky 
 island whose longer axis extended for an unknown distance 
 northwestward along the strike of the strata. The long south¬ 
 west face of this island was undercut by the marine planation 
 to form a steep cliff. Every detail of the face of this old sea 
 cliff is preserved in the Tonto sandstone in the cross-sections 
 cut by the present canyons across its face: at its base, huge 
 angular blocks of Unkar quartzite as large as houses are pre¬ 
 served in the Tonto sandstone in the exact position where they 
 fell and lodged ; farther out from the base are masses of large 
 bowlders, worn and rounded by the pounding of the waves; 
 these become smaller and smaller and finally run out into 
 lenses of fine pebbly conglomerate, representing the shingle of 
 the ancient beach, dragged out by the undertow. Uo more 
 striking example of a fossil sea cliff can be imagined. 
 
 By far the most impressive feature of this wonderful coun¬ 
 try is to traveller and geologist alike the mile-deep pathway 
 of the Colorado River of the West across the great plateaus. 
 The stupendous and glaring record of erosion revealed to us 
 
528 Noble—Geology of the Grand Canyon , Arizona. 
 
 in the cutting of this mighty gorge has almost blinded ns to 
 the realization of the immensity of the vastly greater record 
 revealed in its walls. But the story told by the two intersect¬ 
 ing unconformities in the bottom of the gorge,—two ancient 
 cycles of sedimentation, uplift, and erosion carried to a finish, 
 separated by hopelessly lost intervals of time, resulting twice 
 in the planing down of lofty mountain ranges to the very core, 
 written vaguely at first on a blurred and time-worn record, and 
 later in an increasingly clearer and bolder hand,—the slow 
 accumulation of the strata of the Canyon wall on the floor of 
 the Paleozoic sea, the subsequently erased record of the accu¬ 
 mulation of vast thicknesses of Mesozoic and Eocene strata, 
 separated in turn by great intervals of erosion, and even the 
 “ great denudation ” which has stripped these later strata back 
 fifty miles to the terraces of Utah—represents a lapse of time 
 compared with which the cutting of the Grand Canyon is but 
 the passing of an afternoon ; for in the light of present knowl¬ 
 edge it is safe to say that the Grand Canyon was entirely cut 
 since the time when, according to the fossil record, the remains 
 of man first appear on earth. 
 
 Bibliography. 
 
 Barrell, J. a. Journal of Geology, vol. xiv, 1906. 
 
 Dfiler, J. S. a. The Production of Asbestos in 1907. Mineral 
 Resources of the U. S. for 1907. U. S. Geol. Surv. 
 
 b. The Production of Asbestos in 1908. Mineral 
 Resources of the U. S. for 1908. U. S. Geol. Surv. 
 Dutton, C. E. a. Tertiary History of the Grand Canyon Dis¬ 
 trict, with Atlas. Monogr. II, U. S. Geol. Surv. 1882. 
 Ransome, F. L. a. Pre-Cambrian Sediments and Faults in the 
 Grand Canyon of the Colorado. Science, vol. xxvii, No. 
 695. April 24th, 1908. 
 
 b. Geology of the Globe Copper District, Ariz. Prof. 
 Paper, No. 12, U. S. Geol. Surv. 
 
 Walcott, C. D. a. Study of a Line of Displacement in the 
 Grand Canyon of the Colorado in Northern Arizona. (Read 
 Aug. 29, 1889.) Bull. Geol. Soc. America, I, 1890 (Feb.), 
 pp. 49-64. 
 
 b. Pre-Cambrian Igneous Rocks of the Unkar Terrane, 
 Grand Canyon of the Colorado, Arizona, by Charles 
 D. Walcott; with notes on the Petrographic Char¬ 
 acter of the Lavas, by Joseph P. Iddings. 14th 
 Ann. Rept. U. S. Geol. Surv. for 1892-1893, Pt. 2, 
 1894, pp. 497-519 (Walcott) and 520-524 (Iddings), 
 pis. lx-lxv. 
 
 Auburn, N. Y., January, 1910. 
 
Am. Jour. Sei., XXIX, 1910. 
 
 Plate V. 
 
 36° 30' 
 
 36° 15' 
 
 112 ° 30 ' 
 
 112 ° 15 
 
 Map showing the Shinumo area (within double ruled lines) and distribution of Grand Canvon and Vi«h™ ^ ,, , 
 
 graphic sheets, U. S. G. S. Contour interval, 250 ft. Scale about 5£ miles per inch. ^ eries. From the Kaibab and Echo Cliffs topo- 
 
 ~T * Grand Canyon Series: Cliuar Group, cross hatched Unkar Groum cnlirl V.iar.u • t ^ n , 
 
 area ; III, Bright Angel area ; IV, Hindu area ; V, Shinumo area ; VI, Tapeats area. Vishnu Series, x x k ’ TjPe ^ described b > T Walcott; II, Ottoman 
 
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