214 ESSAYS. 
winds, what precipitation of moisture there is on our western 
plains is not from Atlantic sources, nor much from the Gulf. 
The rain-chart plainly shows that the water raised from the 
heated Gulf is mainly carried northward and eastward. It is 
this which has given us the Atlantic forest-region ; and it is the 
limitation 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, ex- 
cept far north, beyond the Great Lakes, where, in the coolness 
of high latitudes, as in the coolness of mountains, a less 
amount of rainfall suffices for forest-growth. 
We 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 Mis- 
sissippi Valley and all eastward, but not the district’ west 
of it. 
It does not altogether follow that, because rain or its equi- 
valent 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 
a stately forest along with a scanty rainfall, with rain only in 
the three winter months. All summer long, under those lofty 
trees, if you stir up the soil you may be choked with dust. 
On the other hand, the prairies of Iowa and Llinois, which 
form deep bays or great islands in our own forest region, are 
spread under skies which drop more rain than probably ever 
falls on the slopes of the Sierra Nevada, and give it at all sea- 
sons. Under the lesser and brief rains we have the loftiest 
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 I have not much to say about the other. The cause 
or origin of our prairies — of the unwooded districts this side 
of the Mississippi and Missouri— has been much discussed, 
and a whole hour would be needed to give a fair account of 
