THE PROTEROZOIC ERA 51 



Grand Canyon of the Colorado. — In the Grand Canyon of the 

 Colorado River, there are excellent exposures of Algonkian rocks 

 with their relations to the Archeozoic and and Paleozoic well 

 exhibited (Fig. 24). The Archean complex comprising "granites, 

 gneisses, and schists was almost completely leveled before the 

 deposition of the next overlying series. The unconformably over- 

 lying series is the Grand Canyon (Algonkian) series, 11,950 feet 

 thick, consisting of two groups — the Unkar, 6,830 feet thick, 

 and the Chuar, 5,120 feet thick. The Unkar group consists of 

 sandstones and limestones interst ratified with basalts, and has 

 at its base a conglomerate. . . . Resting upon the Unkar group 

 with a slight unconformity is the Chuar group. It consists of 

 shales, sandstones, shaly limestones, and limestones. " 1 



Correlation of Algonkian Rocks. — The statements made regard- 

 ing the difficulties of correlating the Archean rocks apply almost 

 equally well here. Algonkian rocks being, however, more largely 

 distinctly sedimentary, and usually not so severely metamor- 

 phosed; usually separated into series by well-defined unconform- 

 ities; and with fossils gradually coming to light in certain of the 

 uppermost series, afford a much more satisfactory basis for apply- 

 ing ordinary stratigraphic methods of correlation than do the 

 Archean rocks. Remarkable similarities such as exist between 

 the Superior and Grand Canyon Algonkian series are highly sug- 

 gestive of correlation, though far from actually demonstrable at 

 present. Lithologic and structural similarities alone are not safe 

 methods of correlation. Future studies, however, are quite likely 

 to yield satisfactory results in some cases at least. 



Not only the lack of fossils, but also the vast length of time of 

 the Proterozoic era, are great difficulties in the way of correlation. 

 Considering the time necessary for the deposition of the tremendous 

 thickness of Algonkian rocks, and the several long unrecorded 

 time intervals, it seems reasonable to believe that the Proterozoic 

 era was fully as long as the Paleozoic. Hence two similar series of 

 Algonkian rocks resting directly upon the eroded surface of the 

 Archean in widely separated regions may in reality be far different 

 in age because the Archean in one region may have remained 

 unsubmerged very much longer than in the other. Or again, an 

 Algonkian series of one district may actually have been deposited 

 during a time represented by an unconformity in another district. 

 1 Van Hise and Leith: U. S. Geol. Survey Bull. 360, 1909, p. 778. 



