THE PROTEROZOIC ERA 469 



to this time nor* to this region. Geological history makes it clear 

 that changes in the relations of sea and land are among the notable 

 events of the earth's history, even to the present time. Since many 

 other changes are dependent on them, they are believed to furnish 

 the best basis for the subdivision of geological history. 



It is not now possible to determine the extent of the crustal 

 oscillations which took place during the Proterozoic era; but 

 enough is known of the extent of the area in North America at the 

 close of the Proterozoic to make its cartographic representation 

 instructive. The area not covered by the early Cambrian beds 

 (Fig. 357) was land at this time, and the continent was probably 

 much more extensive than this statement implies. 



Metamorphism. The Lower Huronian rocks are, on the whole, 

 more highly metamorphosed than the Middle Huronian, and these 

 are more generally altered than the Animikean, while the Kewee- 

 nawan formations are metamorphosed scarcely at all. On the 

 other hand, the Animikean beds are locally as highly metamorphic 

 as the Lower Huronian, indicating intense dynamic action, at least 

 locally, after the deposition of the third great system. Since 

 different sorts of rock behave differently under dynamic action, it 

 follows that some beds are much more highly metamorphic than 

 others associated with them, even though subjected to the same 

 forces. 



There is scarcely a phase of metamorphism which the Protero- 

 zoic rocks do not show. The schists; slates, and gneisses are 

 especially the product of dynamic metamorphism; the quartzites 

 are the products of extreme consolidation by cementation; the 

 iron ore is the product of metasomatism, effected by ground-waters, 

 while other phases of metamorphism are due to the heat of intruded 

 rock. It is not to be understood that the metamorphism of any 

 considerable body of rock is effected by any one process alone. 

 Dynamic action, which seems on the whole the most important 

 factor in metamorphism, always generates heat, and high temper- 

 ature, especially in the presence of water, facilitates chemical and 

 mineralogical change. So, too, in the case of igneous intrusions, 

 there is often great dynamic action as well as great heat, and water, 

 an agent of chemical change, is always present. 



