52 A NATURALIST IN THE GREAT LAKES REGION 



warm air heavy with abundant carbon dioxide, tree ferns, 

 cycads, grasses, rushes an almost impenetrable jungle a loose, 

 swampy soil, a vegetable slush into which the fallen trunks sink, 

 insects somewhat like roaches crawl over the vegetation some 

 like immense dragon flies are darting about in the air. 



In the adjacent sea are sponges, corals, graptolites, sea 

 urchins, starfish, crinoids, brachiopods, clams, snails, trilobites, 

 shrimps (Fig. 360), cephalopods, fish of the lower types, amphibi- 

 ans, and boring in the mud banks are sea worms of various sorts. 



Centuries passed by in the Chicago region, hundreds of them. 

 In the distant seas were forming the thousands of feet of rock 

 strata that make up the remainder of the Paleozoic, the Mesozoic, 

 and the Cenozoic rocks. Undoubtedly the rocks of the Chicago 

 area, disintegrated slowly under all the processes of weathering, 

 formed deep soil, and upon this there grew a rank vegetation. 

 Here, in time, occasionally perhaps roamed those huge reptiles 

 whose fossils are to be seen in the collection of the Field Museum 

 and still later various types of mammals that were precursors of 

 present-day forms. The map (Fig. 28) gives the positions and 

 relations of the main formations occurring as surface rocks over 

 a wide area about Chicago. 



A great fold of the strata occurs in Illinois, a broad up-arching, 

 its axis running southeast-northwest and continuing into Wis- 

 consin on the one hand and into Indiana on the other. The rock 

 originally lying on the top of this arch has been removed by 

 erosion as a result of which, it will be observed, there is in the 

 northern part of the state a broad central band of Galena- 

 Platteville limestone bordered on the east and west by the Rich- 

 mond (Maquoketa) shale, while farther out still lie bands of the 

 Niagara limestone on the eastern one of which Chicago is situated 

 (see Fig. 37). 



The nearest point to Chicago at which the Richmond shale 

 is found is in the neighborhood of Joliet, at the mouth of Rock 

 Run, near the old canal some four miles southwest of the city 

 and in the same neighborhood in an abandoned quarry near 



