REVIEWS 99 



of the exposures, and the absence of a vegetal mask reveal the geologic 

 history in great detail. 



The rocks in the quadrangle range from Archean to late Paleozoic in 

 age. The pre-Cambrian portion of the section follows: 



Proterozoic 



Grand Canon series (Unkar group) 

 Great unconformity 



Dox sandstone 2,297 feet. 



Shinumo quartzite 1,564 



Hakatai shale 580 



Bass limestone 335 



Hotauta cong o to 6 " 



Archeozoic 



Great unconformity 

 Vishnu schist 



The Proterozoic sediments were deposited on a surface that repre- 

 sented almost perfect peneplanation. At the close of the period of 

 deposition, uplift and great normal faulting inset these beds deeply into 

 the Archean. This led to their preservation during the next period 

 of great erosion, which again resulted in peneplanation by the close of the 

 pre-Cambrian. Where not protected by faulting the Proterozoic beds 

 were removed. The remnants are in great wedge-shaped masses, each 

 bounded by a fault plane, and the two great erosion surfaces. In no 

 other known region do two profound peneplains meet in a line. 



Cambrian and Carboniferous sediments exist throughout the quad- 

 rangle. A disconformity represents the intervening systems. Mesozoic 

 and Tertiary rocks ranging up to 6,000 feet in thickness formerly covered 

 this area. In early Quaternary times a cycle of erosion, known as the 

 "great denudation," drove their outcrops many miles to the north. 



The writer follows Davis and others in recognizing but two cycles 

 of erosion in the formation of present physiographic features. The first, 

 the great denudation, developed a virtual peneplain, and the second, 

 during the latter part of the Quaternary, resulted in cutting the Grand 

 Canyon. The Esplanade and Tonto platforms, explained by Dutton as 

 temporary base-levels, are held to be structural benches. 



The writer also follows Davis in holding that the present course of 

 the Colorado River was established before the beginning of the uplift 

 that resulted in the canyon cycle of erosion. It is a superposed stream, 

 let down from the surface of the peneplain of the great denudation. 



W. B. W. 



