158 W. O. CROSBY — ARCITEAN-CAMBRIAN CONTACT IX COLORADO 



there a harder layer of lava or quartzite forms a low ridge, but as a whole the basal 

 layei's of the Cambrian were deposited upon a nearly level surface." 



The accompanying figure, the more essential part of which is repro- 

 duced here, is in perfect agreement with the text, and shows a like un- 

 conformity between the Grand Canyon group (Unkar and Chuar) and 



Figure \U. — Section of North Side of Grand Canyon (Gilbert). 

 Showing same uuconformities as figure 33 and a pre-Tonto fault. 



the Vishnu group of the Algonkian. Both of these unconformities were 

 also noted b}^ Gilbert* (figure 34). 



According to Walcott,t Packsaddle mountain, in Texas, exhibits a 

 similar relation of the Algonkian and Cambrian strata (figure 35). 



Ci)per Cambrian, Ibsails. 

 , . j-L .^^perOaml>ria7uIbs3Us. 



Zilano fbrmatCon- of the u'Ll^onktartf 



Figure 35. — Section of Packsaddle Monntain, Texas (Waleott). 

 Showing Cambrian strata resting unconforniably on the Algonkian. 



Irving J describes the pre-Potsdam (pre-Cambrian) land surface of 

 Wisconsin as exhibiting, in the main, but gentle undulations. 



"Looked at in greater detail, hoM^ever, it is seen to have numerous minor and 



*14th kxm. Rep. U. S. Geol. Survey, p. 507. 

 t Am. Jour. Sci., 3d ser., 28, pp. 431-132. 

 nth Ann. Rep. U. S. Geol. Survey, p. 401. 



