10 BIG SPRING PRAIRIE. 
Along the summits and edges of the ridges, the soil, 
varying from dark red clay to lighter shades, is exceed- 
ingly thin, the fields being covered with angular stones. 
In the shallow natural drainage courses, the soil is 
arich loam, as it is also on the flat median plain of 
West Ridge. Along the base the soil is frequently 
quite sandy. 
Whereas the greatest heights of both North and 
West Ridge are about one hundred and fifty feet each, 
and the median plains of same about thirty to fifty feet 
higher than prairie, the country to the north and west 
of the prairie is as arule, only about three to six feet 
higher than the level along edge of prarie. Even this 
elevation is a gradual slope reaching this height at a 
distance of forty to eighty rods from the edge of prairie. 
Although this slope is so very gradual, only in a few lo- 
calities is there a gradual transition from forest to prai- 
tie. For the greater portion of its extent, the line of 
demarkation between forest and prairie is quite dis- 
tinct. Bordering upon the southwestern portion of the 
prairie, there is a low wet woods in which there is no 
perceptible difference in level between it and the ad- 
joining prairie. 
CHARACTERISTIC VEGETATION OF THE RIDGES. 
For the sake of more clearly indicating the nature 
of the various plant societies in a more or less limited 
area, it will be found expedient to employ five classes 
of plant societies instead of the three given by Warm- 
ing. ‘The classes under this scheme would be: 
(1) Xerophytic area. 
(2) Xero-mesophytic area. 
(5) Mesophytic area. 
(4) Hydromesophytic area. 
(5) Hydrophytic area. 
The first, third, and fifth conform to Warming’s 
classes. 
