94 A, Gray—Forest Geography and Archeology. 
owing to a deficiency of rain. That, the rain-charts settle, as 
Professor Whitney well insists. 
The prairies which indent or are enclosed in our Atlantic 
forest-region, and the plains beyond this region, are different 
things. But, as the one borders—and in Iowa and Nebraska 
passes into—the other, it may be supposed that common causes 
ave influenced both together, perhaps more than Professor 
Whitney allows. 
He thinks that the extreme fineness and depth of the usual 
prairie soil will account for the absence of trees; and Mr. Les- 
quereux equally explains it by the nature of the soil, in a dif- 
ferent way. ese, and other excellent observers, scout the 
idea that immemorial burnings, in autumn and spring, have 
any effect. Professor Shaler, from his observations in the 
the reconversion of. prairie into woodland. 
I am disposed, on general considerations, to think that the 
line of demarcation between our woods and our plains is not 
where it was drawn by nature. Here, when no physical bar- 
rier is interposed between the ground that receives rain enoug 
for forest, and that which receives too little, there must be a 
debateable border, where comparatively slight causes will turn 
the scale either way. Difference in soil citend 
—pra racticed for hundreds of years by our nomade prede- 
cessors, may have had a very marked effect. Za suspect that 
the irregular border line may have in this way been rendered 
more irregular, and have been carried farther eastward wher- 
ever nature of soil or circumstances of exposure predisposed 
to 
it. 
It does not follow that trees would re-occupy the land when 
the operation that destroyed them, or kept them ery: ceased. 
The established turf or other occupation of the soil, and the 
[To be continued.] 
