Sg ae Se ee a Re TR ee eee eR ee eo 
A. Gray—Forest Geography and Archeology. 98 
and least on the remote interior. From the lower Mississippi 
eastward and northward, including the Ohio River basin, an 
so to the coast, and up to Nova Scotia, there is an average 
of forty-seven inches of rain in the year. This diminishes 
rather steadily westward, especially northwestward, and the 
western border of the ultra-Mississippian plain gets less than 
twenty inches. 
ndeed, from the great prevalence of westerly and southerly 
winds, what precipitation of moisture there is on our western 
— is not from Atlantic sources, nor much from the Gulf. 
he rain-chart plainly shows that the water raised from the 
heated Gulf is mainly carried northward and eastward. Itis this 
which has given us the Atlantic forest region ; and it is the limi- 
tation of this which bounds that forest at the west. The line on 
the rain-chart indicating twenty-four inches of annual rain is 
not far from the line of the western limit of trees, except far 
north, beyond the Great Lakes, where, in the coolness of high 
latitudes, as in the coolness of mountains, a less amount of 
rain-fall suffices for forest growth. 
e see, then, why our great plains grow bare as we proceed 
from the Mississippi westward; though we wonder why this 
should take place so soon and so abruptly as it does. But, as 
already stated, the general course of the wind-bearing rains 
from the Gulf and beyond is such as to water well the Missis- 
sippi valley and all eastward, but not the district west of it. 
oes not altogether follow that, because rain or its equiva- 
lent is needed for forest, therefore wherever there is rain 
enough, forest must needs cover the ground. At least there 
are some curious exceptions to such a general rule,—excep- 
tions both ways. In the Sierra Nevada we are confronted with 
trees we know: under the more copious and well-dispersed rain, 
we have prairies, without forest at all. 
There is little more to say about the first part of this para- 
dox ; and T have not much to say about the other. The cause 
