1876.] Plain, Prairie, and Forest. 661 
phenomena which, if any other theory be adopted, seem to be 
entirely inexplicable. 
The first question which occurs is this: Why are the prairies, 
or grassy plains in general, almost exclusively limited to areas 
which are comparatively level? No theories of climatic influ- 
ence or unequal distribution of moisture seem to have any bear- 
ing on the solution of this question. But if we consider that 
in order to carry off the finest particles produced by the disag- 
gregation of the rocks there must be currents of water having 
considerable velocity, we see that it is only in hilly regions that 
the soil will be washed out enough by the rapid flow of the 
streams to give rise to a soil sufficiently coarse to favor the 
growth of forests. Thus it happens that in the prairie region 
the growth of trees is so frequently limited to the blufis which 
border the streams; it is because the inclination is sufficiently 
rapid to cause the water, as it finds its way down to the bottom 
of the valley, to take with it the finer particles which on the up- 
lands remain undisturbed. When heavy rains fall, the water 
stands upon the surface in sheets and pools, and gradually soaks 
into the ground. For this reason the divides between the streams, 
where there is hardly any perceptible inclination of the surface, 
are occupied in preference by prairie lands. If the height of the 
bluffs be considerable and the eroding power of the stream suffi- 
cient to cut the country up into a succession of ravines with but 
little level ground between them, then the whole region will be 
more or less covered with timber, as is the case in Northeastern 
Iowa, although the conditions with regard to moisture are less 
favorable than in some other parts of the State. The groves of 
timber which stand isolated upon the prairie, in so many places, 
are found on examination to have grown upon coarser soil than 
that which surrounds them; in some cases, the deposits of coarse 
drift have escaped being covered by the prairie soil because a 
little more elevated in these spots, or the increased height pea 
vored the washing away of the finer particles. The railroa s 
Which run through Northern Illinois, where prairie soil and drift 
Soil are constantly alternating with each other, furnish excellent 
= "ections from which one can see at a glance, as he crosses the 
— Suntry, how dependent the growth of the forests is on the char- 
ter of the soil. One, even if blindfolded, could tell without 
: difficulty, in the great majority of cases, by feeling the soil, 
Whether he was in a timbered or a grassy region. Tros vA T 
* Winchell, in his description of the “ Big Woods” of Minne- 
