THE EARTH BEFORE THE CAMBRIAN 



395 



Fig. 368. — Photograph of the wall of the Grand Canyon of the Colorado River, 

 Arizona, showing two unconformities. (See Fig. 369.) 



respects from those of the Lake Superior region. The lower portion 

 of the gorge is sunk into the Archaeozoic gneisses. These are overlain 

 unconformably by a strongly dipping series of sedimentary (Proter- 

 ozoic) strata separated by minor 

 unconformities, and they, in turn, 

 underlie unconformably the Cam- 

 brian strata. Some measure of 

 the length of time represented by 

 the unconformities is shown by 

 the flatness of the floor (Fig. 369) 

 upon which the Proterozoic rests, 

 and also of that above the tilted 

 Proterozoic sediments upon which 

 the Cambrian lies. 



In the Black Hills of South 

 Dakota (Fig. 370, also see Fig. 

 342, p. 356), in the cores of many 

 of the mountain ranges of the 

 west, as well as in the Adiron- 



Fig. 369. — Section of the Grand 

 Canyon of the Colorado River, Arizona. 

 The lower portion shows the complex 

 schists of the Archaeozoic. Upon them, 

 separated by an unconformity CD, rest a 

 series of Proterozoic strata. The Prot- 

 erozoic strata are separated from the 

 overlving Cambrian by the unconformity 

 AB. ' (See Fig. 368.) 



