1876.] Plain, Prairie, and Forest. 665 
organic matter, seems to us clear. A large quantity of such mate- 
tial does collect, it is true, in the swampy places and low swales 
between the ridges or swells of the prairies; but it must be 
remembered that the higher grounds— the divides between the 
streams — are par excellence the regions of prairie. And it would 
be extremely difficult to prove that these higher grounds have 
ever been occupied by an aquatic vegetation. The extensive dis- 
trict in Wisconsin and Minnesota which has never since almost 
the earliest period of geological time been covered by water,! and 
which is as far as possible from being of a swampy nature, is 
thoroughly a prairie region, as has already been described. 
The series of events in the course of which the detrital mate- 
tials covering the greater part of the States bordering on the 
Mississippi have been distributed and arranged in their present 
position must have been a long and complicated one. We know 
that the ocean has had nothing to do with it, for not a trace of 
anything marine has ever been found in these deposits, while 
bones of land animals and fresh-water shells and plants are not 
unfrequently met with. When we consider that in going west 
m the Mississippi we rise to an elevation of more than a thou- 
sand feet above the river, while all the time the prairie soil main- 
tains its character, it becomes evident that we cannot admit that 
deposition of this detrital matter took place in the same manner 
and at the same time from one vast area of fresh water. No 
Possible barrier for this water could be found in any direction 
except to the west, for to have covered the whole prairie region 
its surface must have been nearly 2000 feet above the sea-level. 
| Everything in the prairie region indicates the slow and, as a 
_ Seneral rule, tranquil accumulation of detrital materials during 
à vast period of time, and as the result of agencies rather local 
n general, having more of a fluviatile than of a lacustrine 
character, and which must have been in operation for a long time 
before the glacial epoch commenced. The discussion, however, 
of the phenomena here alluded to would extend this paper far 
beyond any reasonable limits. ere E 
Those who are familiar with the geology of the Mississipp! 
Valley will not need to be told that the prairie region 18 one un- 
Berkain by undisturbed and nearly horizontally stratified rocka. 
_ “ey will remember that these rocks are chiefly limestones, dol- 
mites, and shales, easily acted on by water, the bulk of the ma- 
terial being bodily removed in solution, and not left as a disin- 
1 See Wisconsin Report, page 118, et seq- 
