ly BIG SPRING PRAIRIE. 
(1) The summits, the exposed slopes, and the 
outcrops. 
(2) The level median plane, and the broad shallow 
drainage valleys. 
The differences to be considered are two-fold. 
(1) Presence of species in one area not found in 
the other. 
(2) Relative abundance of the same species in the 
two areas. 
The type of plant society taken asa whole would 
be designated as White-oak, Black-oak, Hickory Forest, 
a term applied by Dr. H. C. Cowles, of the University 
of Chicago, to a similar type of forest formation at 
Glencoe, Illinois. At Glencoe, Illinois, this type oc- 
curs on hills of extensive drift formation, and in this 
region the same type in a pronounced form occurs on 
an outcrop or upheaval of Niagara Limestone, both 
representing the Xero-mesophytic form of plant society. 
The soil on the summit of the ridges and along 
the steepest hillsides is quite shallow, consisting largely 
of red clay, although various other shades of clay also 
occur. The underlying rock seems to weather into a 
slightly sandy clay soil. Numerous sink holes, now 
filled up, formerly occurred on these ridges. ‘These 
enabled the surface water to disappear readily, and re- 
tarded the progress of the plant societies upon it, thus 
partially accounting for the Xero-mesophytic forest type 
instead of the mesophytic type which sometimes occurs 
upon such areas as pointed out by Dr. H.C. Cowles in 
his ‘Plant Societies of Chicago and Vicinity.” The 
history of the plant life on this area in the past most 
probably conformed in its main features to the account 
of the ‘‘Upland Series of Plant Societies” in the work 
referred to, but the type of forest had not as yet attained 
the possible mesophytic type. 
