Prairies, Flats, and Barrens in Southern Illinois. 385 
20, feet of the level of the principal water courses, the hig 
prairies are above the heads of swiftly running tributaries of 
several miles in length. The surface of the prairies is generall 
flat or gently undulating, and some of them exhibit considerable 
difference of altitude in their different parts. They are mostly 
surrounded by timbered flats of more or less extent, and farther 
on towards the creeks by broken hills. Where there are exten- 
observe in his laboratory in filtering certain substances. ese 
pools of water remain standing on the ridges, and dry slowly 
