548 
overflows is an important factor in soil fertility, nor to the occasional 
small forest areas that have developed from swamps in which the slowly 
decaying organic matter accumulates under water during the greater part 
of the year. The conditions described above are more prominent in the 
older glaciated regions, where we find the most extensive areas of forest 
encroachment on pre-erosion topography. ; 
The data in these soil reports also show a lower content of calcium 
in forest soils than in the adjoining prairie soils. This difference is much 
more pronounced in the subsurface soils and subsoils than in the surface 
soils, rendering them more acid than the adjoining prairie soils. Forest 
vegetation when compared with prairie vegetation therefore retards the 
accumulation of organic matter and accelerates the drain on calcium. 
Certain authors have suggested that the greater content of calcium in 
prairie soils may be one of the causes of the prairie. .The above facts 
indicate that prairie vegetation greatly retards the rate at which calcium 
leaches from the soil. 
The farmers of the state have been slow in recognizing this effect 
of the floral history upon the present value of soils. In my own com- 
munity in the Southern IlIlinoisan glaciation the clearing and cultivation 
of forested areas always seemed to have an alluring appeal entirely un- 
related to economic values. Many of the farmers continued to clear 
the forest in spite of the fact that some of the forested areas after 
ten to fifteen years of cultivation yield no better crops than adjacent 
prairie areas that have been cultivated for forty years. Many of the 
slopes formerly protected, by forest vegetation have been cleared, farmed 
for a few years, and then abandoned to the forces of erosion. An in- 
telligent constructive forest policy that can be taught and convincingly 
explained to the farmers is the only hope for remedying these regrettable 
conditions. 
The Xerarch Succession (b) on Sand 
The sand prairies of the state have been described in detail by Glea- 
son (9, 10) and Vestal (27); consequently they have received only 
minor attention in this survey. The following description deals mainly 
with the more general features which tend to show the relation of the 
floral successions on sand to those of the soil types previously discussed. 
Both of the above sources of information have been drawn upon in this 
report. Evidence collected by Gleason shows that most of the sand prai- 
ries were covered with a carpet of vegetation when settlements were 
begun in this region. Subsequent plowing and pasturing, however, has 
led to the exposure of large areas of sand to wind action and the de- 
velopment of “blowouts”. Owing to frequent movement of the sand 
by wind, the revegetation and stabilization of these xerophytic sand areas’ 
is a slow process, and progressive successions are often less prominent 
than retrogressive ones. The most xerophytic prairies of the state occur 
in these sand areas, and as Gleason has already pointed out we have 
here the greatest representation of typically western plants. In fact the 
