REVIEWS—GEOLOGICAL SURVEY OF IOWA. 199 
examination, yet we conceive that the extreme fineness of the 
particles of which the prairie soil is composed, is probably the 
principal reason why it is better adapted to the growth of its 
peculiar vegetation, than to the development of forests. It cannot 
fail to strike the careful observer that where the prairie occupies the 
surface, the soil and superficial material have been so comminuted as 
to be almost in the state of an impalpable powder. This is due 
partly to the peculiar nature of the underlying rocks and the facility 
with which they undergo decomposition, and partly to the mechani- 
cal causes which have acted during and since the accumulation of the 
sedimentary matter forming the prairie soil. If we go toa thickly 
wooded region like that of the nothern peninsula of Michigan, and 
examine those portions of the surface that have not been invaded by 
the forest, it will be observed that the beds of ancient lakes which 
have been filled up by the slowest possible accumulation of detrital 
matter, and are now perfectly dry, remain as natural prairies, and 
are not trespassed on by the surrounding woods. We can imagine 
no other reason for this than the extreme fineness of the soil which 
occupies these basins, and which is the result of the slow and quiet 
mode in which they have been filled up...... Applying these facts to 
the case of the prairies of larger dimensions farther south, we infer, 
on what seems to be reasonable grounds, that the whole region now 
occupied by the prairies of the northwest was once an immense lake, 
in whose basin sediment of almost impalpable fineness graduaily 
accumulated ; and that this basin was drained by the elevation of the 
whole district, but at first so slowly, that the finer particles of the 
Superficial deposits were not washed away, but allowed to remain 
where they were originally deposited. After the more elevated 
portions of the former basin had been laid bare, the drainage becom- 
ing concentrated in narrow channels, the current thus produced, 
aided perhaps by a more rapid rise of the region, acquired sufficient 
velocity to wear down through the finer material on the surface, wash 
away a portion of it altogether, and mix the rest so effectually with 
the underlying drift materials, or with abraded fragments of the rocks 
in place, as to give rise to a different character of soil in the valleys 
from that of the elevated land. This valley soil being much less 
homogeneous in its composition, and containing a larger proportion 
of coarse materials than that of the uplands, seems to have been 
adapted to forest vegetation; and, in consequence of this, we find 
