824 L. Lesquereux on the Origin and Formation of Prairies. 
it is that fine impalpable clay, evidently a residue of the decom- 
position of its plants. At the depth of three and a half to four 
feet, this vegetation suddenly ceases and the bottom of the lakes 
is pure sand and pebbles with shells. Nearer to the borders, 
on the contrary, at the depth of one foot, the carpet of mosses, 
etc., begins to be intermixed with some plants of sedges, becom- 
ing more and more abundant in proportion as the depth de 
into woody matter, under atmospheric influence, and their detri- 
tus is at first clayey mould, and then pure black mould, the 
upper soil of the prairies. Of course, near the borders of the 
rivers, or under peculiar circumstances, the formation 1s some 
what modified by the addition of transported matter, or of for- 
eign elements. The clay may thus take a different color, and 
have a somewhat different composition; but the process of form 
where the ground is naturally naked or without trees. It gives 
the reason of the formation of the prairies from the base of the 
Rocky Mountains to the borders of the Mississippl poe: 
a 
toms of our southern rivers; of the Platas of the Madeyra 
the plains on the shores of the North and of the Baltic Sea, a é 
in Asia, the vast steppes of the Caspian, etc., etc., we find every 
where the same appearances, and the same results of a ge? pene 
The glades on the slopes of some mountains of Arkansas, 
| Alleghanies, etc., have been quoted as a phenomenon 
tradicting the theory of formation of prairies by water. I Jogi- 
fully examined those glades in connection with the geo 
"= R.W. Wells, this Journal, i, 334, 1819. 
