HO 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 folio ws: 



