18 GEOLOGY OF ILLINOIS. 



ness. At this point, it appears to occupy the eastern portion of an ancient 

 valley, excavated by some cause in operation before the formation of the exist- 

 ing rivers, but now, in part occupied by them, and also in part, by the alluvial 

 deposits to which they have given origin. The hills around the Salt Spring, 

 and between that and the bottom lands on the Mississippi, are composed of 

 Loess. Where these marly deposits are subjected to a leaching process, they 

 contain numerous calcareous concretions, some of which assume fantastic forms 

 like the " clay stones " of the Connecticut valley, but more frequently they as- 

 sume an irregularly spherical form, and are known by the popular name of 

 " petrified potatoes." Bleached specimens of .the living species of land and fresh 

 water shells of the adjacent region are found in this deposit, and it frequently 

 affords the teeth and bones of extinct Mammalia, but we are not aware that 

 any have been found in it, in this vicinity. 



Economical Geology* 



Building Stone. No county in the State contains a greater variety, or more 

 abundant supply of excellent building stone than this. First in value and im- 

 portance, is the Niagara limestone, which outcrops along the river bluffs on the 

 west side of the county, from Hamburg to Gilead, and thence trending back 

 for a mile or two from the river bluffs, it continues southward nearly to Cap 

 au Gres, whence it bends abruptly east across the narrow divide between the 

 Illinois and the Mississippi to Monterey. The whole thickness of the forma- 

 tion is exposed here, and from this point it extends northwardly on the east 

 side of the county, appearing occasionally in outcrops at the base of the bluffs, 

 as far north as Farrowton, opposite to Columbiana. At all the outcrops seen 

 between Gilead and Monterey, this limestone is an evenly bedded buff or brown 

 dolomite, very similar to the rock at Joliet and Grafton, and fully equal in 

 quality to the building stone obtained from either of the above named locali- 

 ties. The only drawback to the immediate availability of this valuable build- 

 ing material, is its situation, a mile or more distant from the Illinois river on 

 the east, and about a half mile from the Mississippi, on the west; but this 

 difficulty could be readily overcome, by the construction of a cheap railroad 

 track i'rom the quarries to the river bank. This formation is from fifty to 

 seventy-five feet in thickness in this county, and the whole mass in townships 

 11 and 12 south, is an evenly bedded buff, or brown magnesian limestone, and 

 equal in quality to any building stone to be found in the State. 



In the northern portion of the county, the Burlington limestone outcrops 

 along the river bluffs, and on most of the small streams. It affords a very good 

 building stone, though not equal to that afforded by the Niagara limestone. 

 The upper part of the formation is a coarse, semi-crystaline limestone, that is 



