PRAIRIES AND* THEIR TREELESSNESS. 269 



and vegetable constituents of a soil accumulated upon a 

 lake-bottom. We find in it, moreover, abundant fossil re 

 mains of a lacustrine character. Fresh -water shells of 

 species still existing in Lake Michigan are found in locali 

 ties many miles from the existing shore. Finally, we have 

 found all around the chain of the great lakes abundant 

 proofs that their waters once occupied a much higher level 

 than at present. We have discovered the obstacle which 

 dammed the waters to this extraordinary height. In short, 

 we have ascertained that the prairie region of Illinois must 

 have been a long time inundated, whether such inundation 

 contributed to the characteristics of the prairies or not. I 

 think it did. If I ascertain that the cause for an inunda 

 tion exists ; if I see the traces of an inundation all the way 

 from Niagara River to Illinois ; if the barrier which shuts 

 out Illinois from the lake is not one third the height of the 

 ancient lake-flood ; if I find throughout the region exposed 

 to inundation the peculiar soil deposited by fresh waters, 

 together with traces of lacustrine animals which never 

 wander over land, do I not discover a chain of facts which 

 necessitates my conclusion? During the flood-tide of the 

 lakes, Lake Michigan must have found an outlet toward 

 the south. We find corroboration of this. The broad, 

 and deep, and bluff-lined valley of the Illinois River was 

 never excavated by the present inconsiderable stream. 

 The deserted river valley discoverable at intervals far 

 ther north, indicates the former southward flow of a large 

 volume of water. At Lemont this valley is distinct, with 

 its bounding blufls, and its &quot; pot-holes&quot; worn in the solid 

 rock of the ancient river-bed. This was the work of the 

 lake in its declining stages. At the earlier period, when 

 the waters of Lake Michigan stood one or two hundred 

 feet above their present level, how much of the region 

 south and west of Chicago must have been submerged ? 



