* 
300 The Origin and Formation of Prairies. [ May, 
formed as we may now see prairies of far less extent being pro- 
duced along the shores of the lakes of the West and along the 
banks of rivers. “ Where the waves or currents strike the shores 
or the low grounds and there heap material, sand, pebbles, mud, 
etc., they build up more or less elevated dams or islands. These 
dams are not always built along the shores, but often enclose wide 
shallow basins, whose waters are thus sheltered against any move- 
ment. Here the aquatic plants, sedges, rushes, grasses, etc., soon 
appear, these basins become swamps, and, as it can be seen near 
the borders of Lake Michigan, though the waters may surround 
them, the trees never invade them, never grow upon them, even 
when the swamps become drained and dried by some natural or 
artificial cause.” Prof. Lesquereux states that such marginal 
swamps, generally fringed with trees, can be seen also along the 
shores of Lake Erie, and along the Mississippi and Minnesota 
rivers, outside the line of slack water. All gradations are to be 
found between such swamps and dry prairies. He hence con- 
cludes that all our prairies, not only the low prairies along our 
lakes and rivers bottoms, but also the high rolling prairies, have 
been produced by the slow recess of sheets of water of various 
extent; that these lakes have first been transformed into swamps 
and by and by drained and dried. The soil of these ancient 
swamps, having been produced by the slow and incomplete 
decomposition of aquatic plants, must be of impalpable fineness 
and thoroughly impregnated with ulmic acid; the former condi- 
tion proving deleterious to the germination of the seeds of trees, 
the latter condition favoring the growth of the peculiar vegetation 
of the prairies. 
But if the prairies were at one time swamps, why is their ae 
face not now everywhere level, or nearly so? Or, if the existing 
elevations have been formed as low islands or dams in lakes, why 
are they not now wooded? Mr. Lesquereux believes that the 
surface was originally horizontal; but that it has been made to 
assume its present undulating character by the slow and long 
continued erosive action of water;—in short, that the broad, 
gently sloping valleys have been worn out by running waters as 
have the beds of our rivers and creeks; the difference being that 
in the former case the waters have had a very gentle, in the latter 
a more rapid motion. 
_. Let us now consider the facts and argument presented by Prof. 
-Lesquereux, to sustain his opinion. 
