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 Ton to 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 Sante Fe Railroad. 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 

 Ransome (Ransome, 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 

 EW.-S.E. and dipping about 15° KE. This locality is 



