280 WALKS AND TALKS. 



drained. In all cases, the filling of the lakelets has gone for- 

 ward in the manner described in Talk VIII. Nature is finish- 

 ing the world before our eyes. 



It was during the spring-time empire of water, that the 

 Great Lakes stood at the high levels described in Talk VIE. 

 To this inundation of Illinois, the prairies of the Mississippi 

 owe their origin. The prairie formation is a stratified deposit 

 of fine clay, sand, and alluvial matter. It is a fresh-water 

 deposit. It was laid down on the top of the Drift. The topo- 

 graphical and geological facts point to the great lacustrine 

 flood as the occasion. When, in the course of time, the high 

 waters subsided, the lake bottom was left exposed. It lay a 

 barren waste until the seeds of vegetation were distributed 

 over it by natural means. Birds and winds were the principal 

 agents ; but these agents transport only the lighter seeds the 

 seeds of grasses and herbs. The forest was standing thrifty 

 and green around the border of the ancient lake, but its 

 seeds found little opportunity to gain foothold on the old lake- 

 bottom. The Indian was here. He had paddled his canoe in 

 the waters above the soil which was now a prairie. When 

 the grasses and herbs had been browned by the first frosts of 

 Autumn, the Indian's torch set them ablaze. The air was 

 filled with smoke during the dry and sunny days which fol- 

 low the killing frosts. The west wind wafted the smoke to 

 New England, and our ancestors said, "The Indian Summer 

 is here." But the burning killed the shoots of the young 

 trees without injuiring the roots of the grasses and sedges. 

 So when May covered the surface again with green, the 

 grasses were there, but the woody shoot was dead. Thus the 

 prairies remained treeless. When the emigrant discovered the 

 Indian at his annual burning, he said, " That is the explana- 

 tion of the treelessness." But he never explained why the 

 region was treeless enough in the beginning to allow the sur- 

 face to come into possession of the grasses, and furnish the 

 Indian occasion for the burning. 



