114 GEOLOGY OF ILLINOIS. 



The artesian well bored by the Chicago, Burlington and Quincy railroad 

 company, at their work-shops, in the City of Aurora, affords a means of ascer- 

 taining the thickness of the older geological formations at that point. In this 

 boring, after passing through thirty, or more, feet of the alluvial surface de- 

 posits of the river valley, the section afforded is as follows : 



FEET. 



1. Alternating beds of grayish white and gray limestone, sixty-eight feet, followed by 



forty feet of buff or brown limestone, Niagara group 108 



2. Sixty-four feet of light grayish limestone, underlaid by one hundred and one feet of 



shale and shaly beds, the middle portion dark colored and bituminous, Cincinnati 

 group 165 



3. Gray, buff, and nearly white limestone, Galena and Trenton 232 



4. Buff and reddish yellow sandstones, St. Peters 158 



A comparison between the record of this boring and the Chicago section, 

 given in the report on Cook county, will show a very considerable diminution 

 in the thickness of the different formations above the St. Peters sandstone. 

 In the Chicago section, the total vertical thickness of all the strata between the 

 base of the Niagara and the top of the St. Peters sandstone, is six hundred and 

 thirty feet, which is here decreased to three hundred and ninety-seven feet, a 

 very noticeable difference. As, however, none of the beds below the upper 

 portion of the Galena limestone are represented in .the surface exposures within 

 the limits of this district, the remainder of this section only is of general 

 interest. 



Niagara Group. This formation, underlies the whole eastern portion of the 

 district, including the whole of DuPage county, and the greater part, if not 

 all of Kane county. Its western border cannot be located with any certainty. 

 It seems quite probable, indeed, that it extends westward through the central 

 portion of DeKalb county, but, from the want of outcrops, this point cannot 

 be determined. 



The lower part of this group, which alone is exposed in this district, consists 

 of gray, buff and sometimes nearly white, limestones, in some cases dolomitic 

 in composition, and in others nearly pure, and affording a good material for the 

 manufacture of quick lime. The beds also contain much chert, unequally dis- 

 tributed throughout the strata, in thin seams and lenticular masses. A large 

 portion of the rock, however, is quite free from this material, and answers 

 excellently well as building stone. Its aggregate thickness in this district can- 

 not be easily ascertained. In Kane county, the section before given probably 

 includes all the beds exposed, but to the eastward the outcrops are not so easily 

 identified with it. At the utmost, however, it will probably not exceed one 

 hundred and fifty feet, if, indeed, it approaches that thickness. The principal 

 outcrops are as follows : 



