418 
Among the ninety-eight [llinois species for which distribution 
maps have been prepared, thirty-four belong clearly to this group of 
fishes which seem to avoid the conditions common to the flat gray 
lands of the southern part of the state. Thirty-five species, on 
the other hand, are distributed over this glaciation in a way to indi- 
cate a tolerance of its conditions if not an indifference to them, the 
data concerning the remaining twenty-ninespecies being ambiguous 
or indecisive in this respect. 
Two facts concerning the soil and waters of the lower [llinoisan 
glaciation may be held to account, at least in part, for the failure of 
certain species of fishes to thrive in its streams. Compared with the 
other regions of the state, this oldest of our glaciation areas has de- 
veloped its drainage system to a point such that the rainfall runs off 
rapidly in a large number of small streams, leaving no marshes or 
ponds to hold back the waters during periods of dry weather. Itisa 
level country whose streams fill up quickly and run down rapidly, the 
smaller ones drying up completely during the midsummer drought, 
which is here more marked than farther north. These variable and 
temporary creeks are, of course, less favorable to the maintenance of 
a varied and permanent fish population than the waters of the earlier 
Illinoisan or the Wisconsin areas. 
As a further consequence of its geological antiquity, involving 
degenerative chemical changes and a long-continued leaching, the 
soil of this lower glaciation has become an extremely fine-grained, 
light-colored clay which, when compact, sheds water almost com- 
pletely, but which washes into the streams as a fine detritus that re- 
mains persistently in suspension and renders the waters very turbid 
for a long time after a rain. Standing pools, indeed, never become 
even approximately clear. So persistent is this turbidity, due to 
very finely divided matter in suspension, that the chemists of the 
Water Survey find it almost impossible to free the water wholly from 
suspended solids even by repeated filtration. Furthermore, this soil 
has a definitely acid reaction, to which is due a notable physical dif- 
ference between the soils of this area and those of the later glacia- 
tions west and north of it. A surplus of lime in a soil coagulates or 
granulates it, causing its ultimate particles to cohere in larger gran- 
ules, while in an acid soil this effect is entirely wanting. This lack of 
granulation in a very finely divided soil increases, of course, the per- 
