Ser 
1878. ] The Origin and Formation of Prairies. 303 
no relation to that of ancient swamps; 3d. That sandy and 
gravelly bars and dams are frequently devoid of trees. 
But even if we should grant all that Prof. Lesquereux claims 
respecting the inability of trees to thrive in soils that have accu- 
mulated in swamps, I cannot admit that his theory will explain 
the occurrence of prairies over a large part of Illinois and Iowa. 
However it may be in the case of Wisconsin and Minnesota, it is 
evident that the soil of the prairies of at least large portions of 
the former States has not been formed in marshes. We have, it 
is true, evidence that at some period since the epoch of the Drift 
the surface of the whole country has been depressed much below 
its present level. We find everywhere along the Mississippi 
River‘and at many points along the Illinois an extensive deposit 
capping the bluffs and sometimes extending out into the ancient 
-river bed. This deposit is sometimes very thick near the bluffs, 
but thins rapidly towards the highlands and soon disappears. 
This deposit, called in the Report of the Illinois Survey the Loess, 
must have been thrown down during the Champlain epoch, when 
a series of lakes occupied the broad valleys of our rivers, filling 
them, no doubt, to their brim, and even extending over portions 
of the surrounding country. But that the prairie soil or subsoil 
was then deposited, or that there has been any general sub- 
mergence since, we have, I think, no sufficient reason for believing. 
On the contrary there are many serious objections that might 
be urged against the idea that the prairie soil has been deposited 
_in lakes and stagnant swamps; some of which I shall here present. 
‘1. According to Lesquereux, timber is found growing along 
dams cutting off from the body of the lake the bog that is to be- 
come prairie. Do we find our Illinois forests on the higher lands ? 
That they sometimes so occur, especially in northern Illinois, will 
not be denied ; but in such cases, instead of being long stretches of 
| 7 timber, bordering and separating prairies, they are generally small, 
_ rounded clumps. Much the greater part of the wooded country in 
Illinois is found along the river bluffs and on the bottom lands. 
2. If the prairie soil is a lacustrine deposit, it ought to be free 
_ from such coarse materials as are found in the Drift. In swamps 
= whose soil has been produced by the decay of vegetation and 
from sediment washed in by gently flowing waters, there is 
scarcely a possibility for coarse rocks and boulders to occur. The 
iscovery of a large eae boulder in the alluvium of the Missis- 
VOL. XII—NO vV. 
