Mississippi Valley Flora. 233 
forest trees, so that the prairie flora, if disturbed always culminated again 
in the grass formation. Prof. LESQUEREUX ') believed that the failure of 
forest vegetation to occupy the prairies was due to the chemic nature of the 
soil, coupled with its exceeding fineness. Most of the older geologists with 
 LESQUEREUX believed that all our prairies not only the low prairies along our 
lakes and river bottoms, but also the high rolling prairies were produced by 
the slow recession of temporary sheets of water of various extent; that these 
lakes were transferred into swamps and by and by drained and dried. That 
much of this prairie soil is not stratified, as if laid down in water, that it con- 
tains no remains of leaves, insects, fresh water or marine shells seems to 
militate against the lacustrine origin of the present prairie soils, however, it might 
have been in early Tertiary times. Several other theories have been advanced 
to account for the origin of the prairies. Dr. WHITE?) says without the least 
hesitation that the real cause of the prairies is the prevalence of annual fıres, 
originally maintained by the Indians. Professor J. D. WHITNEY concurs with 
LESQUEREUX that the cause of prairies at least east of the Mississippi is mainly 
the fineness and depth of the soil, while another writer believes the absence of 
trees is mainly dependent on the rainfall?): a view disproved by the rain charts. 
We know that during the glacial period much debris, glacial silt, gravel 
and till was carried down by the Mississippi River and its tributaries. We can 
safely imagine the formation of large temporary lakes, on the bottom of which 
would be deposited the material derived from the melting ice front and streams 
which emerged from the ice beneath. Lake Agassiz, a glacial lake of this 
type, occupied an area of more than 100,000 square miles in northwestern 
Minnesota, northeastern Dakota and a considerable portion of Manitoba. On 
the bottom of this lake was deposited during the comparatively brief time of 
its existence, silt to a depth as yet undetermined, but known to be at east 
100 feet*). Undoubtedly some of the superficial prairie soil had a lacustrine 
origin when temporary or shallow lakes were present in the great central 
valley, but the larger part of it according to the most recent studies had a 
different origin, for we find, that most of the glaciated portion of this region 
is deeply covered with fine loam called loess which in some places may 
have had its origin as a water deposit, but which in all probability was de- 
posited by wind action. In the United States, the Zoess which in the north 
lies over the glacial drift, covers thousands of square miles throughout the 
drainage basin of the Mississippi River. It is found in Ohio, Indiana, Michigan, 
Iowa, Kansas, Nebraska, Illinois, Tennessee, Alabama, Mississippi, Louisiana, 
Arkansas, Missouri, Kentucky and the Indian Territory. According to Aughey, 
Hay, O.P.: An Examination of Prof. Leo LESQUEREUxX’s Theory of the Origin and For- 
mation of Prairies. American re XI (1878) 299. 
2) WHITE: Geology of Iow 3%. 
3) Topp, J.E.: Diekärikien = Timber and Origin of Prairie in Iowa. American Naturalist 
XU (1878) 9 
4) ei; GEORGE P.: A Treatise on Rocks, Rock-weathering and Soils. 1897: 290. 
