390 H. Engelmann on vegetation known as | 
have began to sprout up; at the same time their surface has — 
become drier, of which more will be said below. 
At some points in the prairies no water is obtained at moder- 
ate depths, but rocks are struck and have to be penetrated toa 
considerable depth in order to get water. At such pointswe — 
might expect to find trees, but such is usually not the case. In _ 
the vegetable kingdom also the universal rule prevails that 
the stronger gain on the weaker ones. The tenacious grasses 
growing all around such spots will then encroach upon the land 
which is fit to bear timber, and will not suffer trees to spring up, 
unless they be assisted by favorable circumstances. Such en- 
croachment.of one species upon the territory else occupied by 
another, even to the extinction of the latter, may be frequently 
oO 
The upper soil is quite shallow, and seems to be distinguished 
