pate A ee ee ee TR age! 
1878. | The Origin and Formation of Prairies. 305 
the low grounds, and these may be many rods in width. The 
theory under consideration requires us to believe that in such 
cases an enormous amount of material has been removed. And 
yet it is questionable whether, under such circumstances, the soil 
would be removed as rapidly as it would accumulate through the 
decay of vegetation. For if the surface were as level as we must 
Suppose it was, and as it frequently is in these sloughs, the water 
must have moved so slowly as to carry little sediment along with . 
it; and as the water passed through the tangled grasses, rushes 
and sedges, even this little would have been filtered out. 
Indeed, the cause assigned for the uneven surface of the prairies 
is one that tends to produce the very opposite effect, that of re- 
moving any inequalities of the surface that might have at first 
existed. For the water running down the hillsides would have 
carried with it some soil. On reaching the level slough its 
velocity would have been checked and a large part of its burden 
deposited. That this has occurred, rather than the contrary 
phenomenon, is plainly shown by the fact that the alluvium is much 
deeper in the sloughs than on the tops and sides of the hills. 
5. The theory referred to requires us to ascribe to the alluvial 
deposits of the hypothetical lakes an undue thickness. For, since 
the peculiar fine soil of the prairies is found on the hilltops, as 
well as in the valleys between; it follows that, if the surface were 
once level, the lacustrine deposit of soil must have been of a 
thickness at least equal to the height of the hilltops above the 
4 _ lowest point to which the soil extends in the valleys. We must 
then believe that the deposit was perhaps a hundred feet in depth ; 
and since the valleys have been scooped out of this, we might 
expect to find the hills composed entirely of the lacustrine sedi- 
ment—rich, black, prairie soil from ten to one hundred feet in 
depth. On the contrary the soil is comparatively thin on the 
hilltops, very deep in the valleys. 
The valleys have been excavated from the Drift formation ; and 
the general contour of the prairies must have been determined 
before the prairie soil was formed, under whatever conditions it 
may have resulted. That some portions of the prairies, especially 
those bordering our great lakes and some of our rivers, have 
originally been swamps cannot be doubted; but that they are 
destitute of trees must be attributed to other causes, the absence 
_ of which in other localities permits the growth of trees alike on — 
_ the finest or the coarsest, the sweetest or the sourest soil. 
