42 A NATURALIST IN THE GREAT LAKES REGION 



islands, therefore, arose from a shallow sea that covered the 

 continental elevations. In this sea on the flanks of these early 

 nuclei were deposited the materials eroded from them that made 

 up the succeeding formations of the early Paleozoic, somewhat 

 as nowadays in the Gulf of Mexico or the Mediterranean are 

 accumulating the deposits of mud that in all probability will in 

 time transform to rock which may be upheaved to form new 

 land masses. 



In general it may be said that the rocks of the later eras — 

 Paleozoic, Mesozoic, Cenozoic — were laid down in the North 

 American region in these shallow epicontinental seas, seas that 

 constantly changed their shape as the surrounding lands were 

 raised or depressed by crustal movements, or as the ocean 

 changed its level when its floor sank or rose, in which latter 

 process accumulating sediments were a major cause. Some- 

 times the land above the ocean was a group of islands, sometimes 

 it was a continent with more or less of its central area covered 

 by a sea and this latter was connected by wide straits with the 

 Atlantic, Pacific, the warm tropical ocean, the cold Arctic, one 

 at a time or two or more simultaneously. The characters and 

 affinities of the animals and plants whose remains are found in 

 the successive rock formations give us the best evidence regard- 

 ing the oceanic connections of the seas in which these formations 

 were deposited. 



The large island made up of the Proterozoic rock complex 

 nearest to the Chicago region was extensive, covering what is 

 now northern Wisconsin, a part of the upper peninsula of 

 Michigan, much of Minnesota, and a large area extending north- 

 ward into Canada. This region now consists of quartzites, 

 shales, schists, and dolomites with the intrusive granites, 

 diorites, and other lavas. If one were to start, say, at Stevens 

 Point or at Grand Rapids, which places in Wisconsin mark about 

 the southern limit of the exposed Proterozoic rocks in our vicinity, 

 and travel southeastward to central Illinois, he would pass over 

 he edges of the succession of rock formations laid down one on 



