THE PROTEROZOIC ERA 327 



while the younger consists of slates, quartzites, and lime- 

 stones which are neither closely folded nor much altered. 



GENERAL CHARACTERISTICS OF THE PROTEROZOIC GROUP 



Sedimentary rocks but with some igneous. Having 

 learned something about the Proterozoic systems in widely 

 separated regions, we may proceed to consider the things 

 which are characteristic of the group as a whole and of the 

 long periods of time during which it was being formed. In 

 each case the rocks which make up the group were derived 

 chiefly from ordinary sediments. They were once gravel, 

 sand, clay, and ooze spread out upon the sea floor or upon the 

 low-lying lands. They have since been cemented into solid 

 rocks; they have been folded, mildly in some places and 

 intensely in others ; and some of them have been metamor- 

 phosed into slates, schists, and gneisses. Many kinds of lava 

 have been forced up through them at different times. These 

 either spread out on the surface as flows, or solidified beneath 

 in the form of dikes, sills, batholiths, and other intrusions, 

 which interrupt the stratified rocks and complicate the study 

 of the structure. As would be expected, the older Proterozoic 

 formations are often much more deformed than the younger, 

 because they have passed through more epochs of folding. 



The Archsean system, we learned, includes some beds of 

 sedimentary rock, but the vast body of that ancient mass is 

 of either igneous or doubtful origin. In the Proterozoic 

 group, on the other hand, the proportions are reversed, and 

 the sedimentary strata predominate overwhelmingly. 



Unconformity general but not universal. We have seen 

 that in each district where the rocks have been fully studied 

 the Proterozoic group is separated from the Archseozoic by a 

 great unconformity. This clearly shows that the regions had 

 been lands cut down by weathering and erosion until the very 

 roots of the Archaean mountains were laid bare and planed 

 off, and all this before the Proterozoic sediments began to 



