THE STORY OF OUR ROCK FOUNDATION 



changes that occurred through all these eventful ages, except the 

 tokens of the glacial period that came in late Cenozoic time, as 

 will be described shortly. 



The whole central region of Illinois is occupied by rocks of 

 the later Paleozoic era (map, Fig. 28) commonly known as the 

 coal beds. The seas (Fig. 3 5) 

 in which these rocks were 

 deposited were at times suf- 

 ficiently deep to allow mud 

 deposits to accumulate that 

 later were transformed to 

 shales and slates ; then again 

 they became so shallow they 

 were covered with swamp 

 vegetation. Here grew 

 dense jungles of cycads and 

 calami tes, rushes and sedges, 

 gigantic ferns and probably 

 conifers, a rank growth 

 whose trunks and stems 

 and leaves falling into the 

 shallow water accumulated 

 until a thick bed of vege- 



FIG. 35. North America in the Upper 

 Pennsylvanian when the Illinois coal beds 



table matter lay upon the were being deposited. Land white, seas lined, 

 floor of the sea. This Was B ^ ack areas indicate land where these strata 



are now surface formations. After Schuchert. 



covered by mud during the 



next period of depression and gradually by pressure and heat 



was converted into coal. 



The old dump piles of refuse slate, shale, and poor coal afford 

 excellent opportunities to secure fossils of this period. Fern 

 leaves, bark of the tree-fern trunks, calamite stems, fossils of 

 various animal types, including an occasional fish skeleton, and 

 that of the early amphibians are to be had by patient search 

 (Fig. 36). You can easily imagine the picture of the region 

 when the coal was forming: the dark swamp forest, the moist, 



