PRELIMINARY CHAPTER. 11 



Xorth-west. One theory attributes them to annual fires 

 sweeping through the grass, and killing every tree, germ 

 and voung tree, almost before they could take root. In 

 some phices the fires are supposed to have encroached year 

 by year upon the forests ; in other places, as for instance 

 along the streams, in the deep hollows, or in wet places, 

 where the tires would be checked, the timber would spring- 

 up and displace the prairies. Another theory accounts for 

 the treeless character of these plains, from the lacustrine 

 origin and nature of the prairie soils and sub-soils. Trees 

 will not naturally grow in this sedimentary, finely commi- 

 nuted prairie soil, according to this theory. Others attempt 

 to explain prairie phenomena by atmospheric and climatic 

 influences, marking out certain zones of moisture and dry- 

 ness. They bound forests and prairies by certain isother- 

 mal lines. Another theory, advocated with force and plau- 

 sibility by Professor LESQUEREUX, in the first volume of 

 the Geological Reports of this State, finds all our prairies to 

 originate from causes similar to those which form peat beds, 

 and are in fact incipient peat beds, drained before com- 

 pleted. In his own clear language lie finds * k that all the 

 prairies of the .Mississippi valley have been formed by the 

 slow recession of sheets of water of various extent, first 

 transformed into swamps, and by-and-by drained and dried. 

 The high rolling prairies, the prairies around the lakes, 

 those of the bottoms along the rivers, are all the result of 

 the same causes, and form a whole in an individual system." 

 Xo one of these theories is sufficient to explain all the 

 phenomena noticed in making an examination of the prai- 

 ries. As in most such cases in theoretical geology, all of 

 them perhaps contain some truth, and may be applicable 

 to localities more or less extended. The burning of the 

 forests, in a few cases doubtless, has changed timber into 

 prairie land, and prevented the timber from invading small 

 tracts of the prairies. But the sweeping, consuming au- 

 tumnal prairie fires are not sufficient to account for the ori- 



