CHAPTER IV. 



THE PROTEROZOIC ERA. 



To the Proterozoic x era is assigned the time between the close of the 

 formation of the igneous complex just described, and the beginning of 

 the lowest system which is now known to contain abundant well-pre- 

 served fossils; or in other words, the time between the close of the 

 Archean and the beginning of the Paleozoic. During the Proterozoic 

 era several great series of sedimentary formations, unconformable 

 with one another, were formed (p. 160). These sedimentary series 

 were rather freely interlarded with igneous sheets, some of which were 

 intruded as sills, and some spread upon the surface while the clastic 

 beds were being formed. It was therefore a time when igneous activity 

 was still rather pronounced, though by no means so overwhelmingly 

 dominant as in Archean times. Sedimentation had become, for the 

 first time, the leading process in the formation of the geological record. 



In contrast with the Archeozoic, the dominant feature of the time, 

 and the feature which was really most significant in the history of the 

 earth, was the effectiveness of the weathering processes and of the 

 hydro-atmospheric work upon the land which furnished the material 

 for sedimentation. These processes were really more significant than 

 the sedimentation, because they were prerequisites to it. They, in 

 turn, were dependent on the deformations of the body of the earth which 

 gave the topographic conditions necessary for erosion, and on atmos- 

 pheric and thermal conditions. The sedimentary deposits of the 

 era, however, constitute the main part of its record, and are the princi- 

 pal subject of geologic investigation. 



The Proterozoic rocks include the first great series of strata which 

 imply mature weathering, and the prolonged and continuous deposition 

 in the sea of weathered material derived from the adjacent lands. It is 



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

 Surv. 



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