Prairies, Flats, and Barrens in Southern Iilinois. 391 
badly permeable to water; and in depressions where rain water 
collects, the fine particles which have been held in suspension 
by the accumulating water soon close up the pores of the bottom, 
and thus shallow pools are formed, from which the water disap- 
pears slowly by evaporation. At such points observe the 
pin oak (Quercus palustris) together with the scaly-bark hickory 
(Carya alba), and sometimes the laurel oak (Quercus imbricaria). 
The flats extend either round the prairies, between them and 
the breaks or hill land, and at the same level with the edge of 
the prairies, or else they occupy the wider ridges without a cen- 
tral prairie. e most obvious difference between prairies and 
flats, apart from their different vegetation, consists in the differ- 
ent quality of their soils, which in the prairies is deeper and 
much more charged with vegetable matter; but as the soil is 
only formed in the course of time, we must search for a deeper 
seated first cause of the difference. The situation of the flats 
pe 
of which I have spoken in relation to the prairies. At last the 
soil remains slink acelin and dry. The access of air to it is 
sorb most powerfully the moisture and other gaseous elements of 
the air which are conducive to a vigorous development of vegeta- 
