12 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 as a 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. 



