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138 



GEOLOGY OF ILLINOIS. 



MONTGOMERY '-, 



A'/INC COUNTY UN 



3. Cincinnati Group. Gray and bluish limestones, with green and blue 

 shales. Total thickness, not over two hundred feet. 



4. Galena and Trenton Limestone. Porous yellowish lime- 

 stone, with some bluish beds near the base, and beds of passage 

 into the next formation below. Total thickness estimated at 

 I about two hundred feet. 



| 5. St. Peters Sandstone. Very incoherent white sandstone, 

 brought up by anticlinals. 



The accompanying reduced section, taken along the Fox river 

 in its course through this county, shows all of these formations, 

 except the first. The only outcrops of the St. Peters sandstone, 

 are where it is brought up by anticlinals on the lower course of 

 the river in this county, as represented in the section. 



The coal measures probably underlie a small area of not more 

 than three or four square miles in extent, in the extreme south- 

 western corner of the county. The underlying rocks are 

 nowhere exposed above ground in this vicinity, but the exist- 

 ence here of deposits of this age, is inferred from the strike 

 J and dip of the exposures in the adjoining counties of LaSalle 

 fc and Grundy, and not from any evidence afforded within the 

 i limits of this county. It seems highly probable, however, from 

 ; the fragments of coal, etc., found in this Drift, that at one time 

 most of the southern portion of Kendall county was overlaid by 

 deposits of this age, which have been carried off by erosion 

 during the Drift period, and it is possible that small outliers 

 may still exist, under the heavy bed of Drift clay and gravel 

 which overlies nearly the whole surface of the county. The 

 only exposure which can in any way be referred to this period, 

 is in the northeast quarter of section 16, township 35, range 8, 

 very near the section line between sections 15 and 16, where we 

 find a thin bedded bluish sandstone, overlying the gray fossilif- 

 erous limestones of the Cincinnati group, in the bed of the Au- 

 Sable at this point. The sandstone can be traced for only a few 

 rods, and the exposure is in no place good, it being generally 

 almost buried in mud and water. In making an excavation on 

 | the bank of the creek at this point, Mr. House, the owner of the 

 land, found many fragments of coal, with fire clay, and fossil 

 plants, underlying a yellowish rotten limestone reported to be 

 four feet thick, which seemed more like a mass of loose frag- 



5 ments washed together, than like a bed of rock in place. About 



i! 



j a mile north of this point, the rocks of the Cincinnati group 



again appear, no intermediate exposures being seen. 



POSTS MILLS- 



