CHAPTER XIV 



THE PROTEROZOIC ERA 1 



FORMATIONS AND PHYSICAL HISTORY 2 



The time between the Archeozoic era and the deposition of the 

 oldest system (the Cambrian) of rocks containing abundant fossils 

 constitutes the Proterozoic era. It was during this era that sedi- 

 mentation first became the leading process in the formation of the 

 geological record. The composition of the sediments, now indura- 

 ted, implies mature weathering, and their extent and thickness 

 imply the prolonged deposition on low lands or in the sea, of the 

 sediments which were the products of mature weathering. During 

 the era several great systems of sedimentary formations were 

 formed. With the sedimentary formations there is much igneous 

 rock, some of which is intrusive and some extrusive. 



Stratigraphic relations of the Proterozoic rocks. Great uncon- 

 formities separate the Proterozoic rocks from the Archean below 

 iind the Paleozoic above. Great unconformities usually involve 

 three elements: (i) a change in the attitude of the lower formation, 

 as the result of w r hich it is subject to erosion; (2) a long period dur- 

 ing which its surface is eroded; and (3) the deposition of the over- 

 lying rocks on the eroded surface. 



A sequence of events which might have given rise to the uncon- 

 formable relations of the Archean and Proterozoic is illustrated by 

 Figs. 296 and 297. Fig. 296 represents an area of land composed 

 of Archean rock, subject to erosion. The sediments derived from 

 it are deposited in the sea (at a). In Fig. 297, the land is represented 

 as having sunk so as to be mostly submerged. Sediments (Al) 

 washed down from the remaining land are being deposited uncon- 

 formably on the eroded surface of &. Though widespread, the 



1 Proterozoic, as here used, is a synonym for Algonkian as used by the U. S. 

 Geol. Surv. 



2 A review of the pre-Cambrian geology of North America, by Van Hise and 

 Lritli, is found in Bull. 360, U. S. Geol. Surv., 1909. This Bulletin suggests probable 

 correlations of the pre-Cambrian of different regions, so far as now warranted. 



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