547 
were not abundant in either of the Andropogon associations but might 
occur abundantly on eroding soils. Some of the plates accompanying this 
report show these same features for Spartina Michauxiana, Calama- 
grostis canadensis, and Panicum virgatum. 
To sum up, both the historical data and the data’ obtained from relic 
prairie areas lead to the conclusion that Andropogon furcatus is the 
climax grass of the whole upland prairie region of the state, and that 
in the successions leading to this climax from the more xerophytic up- 
lands and exposed clay soils Andropogon scoparius is the most important 
species. Figure, 7 shows the successions of associations that may occur 
in this xerarch series. 
Andropogon furcatus 
Mixed ays Andropogon scoparius 
grass aa 
Bi herbs 
Fic. 7. Diagram showing the principal associations in the xerarch suc- 
cession on xerophytie upland soils. 
During the long postglacial period forests: working back from the 
water courses invaded these upland prairie soils. The progress of this 
invasion as would be expected is much farther advanced in the Lower 
Illinoisan glaciation than in the Wisconsin glaciation. While this en- 
croachment of the forest on the prairie is generally considered to have 
kept pace with the development of drainage and the soil erosion involved, 
there are rather extensive areas in which the encroaching forest preceded 
erosion. Since these forest soils bordering the upland prairie soils had the 
same origin, the differences in them today are largely a result of their floral 
history. In this connection the county soil reports (3) have some very in- 
teresting data showing the effect of vegetation upon soil types. The or- 
ganic matter of the forest soils ranges from about 25-50 per cent. of the 
amount present in the adjoining prairie soils. This gives the prairie 
soils a much darker color than the forest soils and is the result of a more 
complete decomposition of the plant remains of a forest than of a prairie. 
To quote from one of these reports: “The leaves and twigs of the trees 
fall upon the surface of the ground and decay completely; whereas the 
prairie grasses form a mass of roots in the soil which, when they die, 
are prevented from complete decay by the absence of sufficient oxygen. 
In this way prairie grasses and other plants cause a gradual accumula- 
tion of organic matter. If prairie land becomes forested the organic 
matter is slowly diminished to a low point.” This statement apparently 
is not intended to apply to flood-plain forests, where deposition during 
