THE LAKE SUPERIOR PROTEROZOIC 339 



(9) After the deposition of this system, much of it was exposed 

 to erosion. 



This succession of events implies repeated changes of relative 

 level of land and sea in the Lake Superior region during the era. 

 We shall see that such changes are confined neither to this time nor 

 to tliis region. 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 oscilla- 

 tions which took place during this era; but enough is known of the 

 extent of land in North America at the close of the Proterozoic to 

 make its representation on maps instructive (the white areas north 

 of Mexico, Fig. 295). 



Metamorphism. The lower rocks of the Proterozoic are, on the 

 whole, more highly metamorphosed than those above, but 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 espe- 

 cially the product of dynamic metamorphism; the quartzites are 

 the products of extreme consolidation by cementation; the iron ore 

 is the product of aqueous metamorphism, 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 may be great dynamic action as well as great heat, and water, 

 an agent of chemical change, is always present. 



Events elsewhere. A series of events consonant but not neces- 

 sarily identical with those of the Lake Superior region was probably 

 in progress about every other area of Archean rock, during the 

 1'roterozoic era; but it does not follow that about every other 



