CHAPTER XV 

 THE PROTEROZOIC ERA 1 



FORMATIONS AND PHYSICAL HISTORY 



The time between the Archeozoic era and the deposition of the 

 oldest system of rocks containing abundant fossils (the Cambrian) 

 constitutes the Proterozoic era. It was the era when sedimenta- 

 tion first became the leading process in the formation of the geolog- 

 ical record. During the era several great systems of sedimentary 

 formations, unconformable with one another, were formed. With 

 these sedimentary formations there is much igneous rock, some 

 of which was intruded as sills and bosses, and some of which was 

 extruded. 



The Proterozoic rocks include the first great series of sedimen- 

 tary strata which imply mature weathering, and the prolonged and 

 continuous deposition, on low lands or in the sea, of weathered 

 material derived from the adjacent lands. It is important to 

 emphasize the inauguration of the dominance of these processes, 

 for they have been the most conspicuous ones ever since. Taken 

 as a whole, the era was marked by more igneous activity than any 

 later one, and may therefore be regarded as a transition-time from 

 the profoundly igneous era that preceded, to the markedly sedi- 

 mentary eras that followed. 



Stratigraphic relations of the Proterozoic rocks. Great uncon- 

 formities separate the Proterozoic formations from the Archean 

 below and from the Paleozoic above. Great unconformities 

 usually involve three elements: First, a change in the attitude of 

 the lower formation, as the result of which it is subject to erosion; 

 second, a long period of erosion during which \\< surface is much 



1 Proterozoic, as here used, is a synonym for AI</tiL-/>m :\t usr.l l>y the 

 U. S. Geol. Surv. 



448 



