110 GEOLOGICAL RECONNOISSANCE 
WASHINGTON COUNTY. 
The geology of the northern townships of this county is very analagous 
to that of Benton county. The southern extension of the cherty barren 
limestone is here, as there, the base rock of the country, the underlying 
shales only showing themselves in the deepest cuts, in the extreme north- 
east corner of the county, on White river, near Van Winkle’s mill, and 
near Blackburn’s mill, on War Eagle. At the former of these localities, 
the limestone is cavernous. Close to the Washington county line, but 
probably in the south-east corner of Benton county, a salt-petre cave is 
reported which I have not yet seen. 
One of the most interesting features of the northern part of Washing- 
ton county, consists in the noble springs, which gush forth amongst the 
ledges of limestone, in such volume and force as to afford water- powers 
for small mills, even at their very source. The Elm springs, forming the 
head of one of the branches of the Clear fork of the Illinois river, are 
one of the most remarkable of these springs. They are, no doubt, due 
to the fissured and cavernous nature of the barren limestone, reposing on 
its underlying impermeable shale. Atmospheric water, filtering with 
facility through the cavities and rents of the limestone, is arrested by the 
impervious shale beneath, and flows out along the slope of the southerly 
dip. Besides affording convenient and permanent water powers, these 
springs, no doubt, contribute greatly to the fertility of the lands in the ad- 
jacent valleys, not alone by their irrigating effects, but by reason of the 
large amount of carbonic acid and lime with which they are charged, 
which must, undoubtedly, give a remarkable impetus to vegetation; since 
they are two of the most essential fertilizers in all manures. One of 
them, carbonic acid, is, in fact, the great solvent, or vehicle, which carries 
nourishment to the plant. , 
In township 17 and the southern part of 18 north, range 29 west, the 
succession of the different beds of the subcarboniferous group is approxi- 
mately as follows: 
