THE rHOTEROZOIC ERA. 201 



the metamorphism of the sedimentary portions of the Huronian and 

 Animikean systems. (8) After erosion of the emerged Animikean 

 system had gone on for a long period of time, flows of lava of unprec- 

 edented magnitude were poured out upon the surface of the land 

 over considerable areas and intruded into the existing terranes. (9) 

 Before the outflows ceased, sedimentation began again in the region 

 and soon predominated. Even this long category of physical events 

 is somewhat inadequate, since there are local unconformities at various 

 points, representing minor disturbances of undetermined extent, and 

 many minor changes are probably unrecorded, or their records unrecog- 

 nized. 



It will be seen that oscillations of level were one of the marked 

 characteristics of the Lake Superior region during the Proterozoic 

 era. It will be seen in the sequel that this is a characteristic confined 

 neither to this period nor to this region. All geological history indi- 

 cates that changes of the relations of sea and land are among the nota- 

 ble events of the earth's history, from the beginning even to the present 

 time. Since many other changes are dependent on them, they are 

 thought to furnish the best basis for the subdivisions of earth history. 



It is not now possible to determine, even approximately, either the 

 areal or the vertical extent of the crustal oscillations which took place 

 during the Proterozoic era, but the extent of the area in North America 

 which became land at the close of the Keweenawan — that is, at the 

 close of the Proterozoic — has been estimated with sufficient accuracy 

 to make its cartographic representation instructive (Fig. 90). 



Metamorphism. — It has been pointed out that the Huronian rocks 

 are, on the whole, more highly metamorphosed than the Animikean, 

 while the Keweenawan formations are metamorphosed scarcely at all. 

 On the other hand, the Animikean beds are locally as highly metamor- 

 phic as the Huronian, indicating intense dynamic action, at least locally, 

 after the deposition of the second 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. Independent of 

 differences due to variations in the character of the rock, metamorphism 

 has usually been intense about massive intrusions of igneous rock. 



There is scarcely a phase of metamorphism which the Proterozoic 

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

 the product of dynamic metamorphism; the quartzites are the products 



