172 THE OIL-BEARING LIMESTONE OF CHICAGO. [X. 



adjacent regions is indigenous to certain sandstone strata in 

 the Devonian and Carboniferous rocks.* 



At the meeting of the American Association for the Ad- 

 vancement of Science at Chicago, in August, 1868, in a dis- 

 cussion which followed the reading of a paper by myself on 

 the Geology of Ontario, t it was contended that, although the 

 various limestones which have been mentioned are truly oleifer- 

 ous, the quantity of petroleum which they contain is too incon- 

 siderable to account for the great supplies furnished by oil-pro- 

 ducing districts, like that of Ontario, for example. This opinion 

 being contrary to that which I had always entertained, I re- 

 solved to submit to examination the well-known oil-bearing 

 limestone of Chicago. 



This limestone, the quarries of which are in the immediate 

 vicinity of the city, is filled with petroleum, so that blocks of it 

 which have been used in buildings are discolored by the exuda- 

 tion of this substance, which, mingled with dust, forms a tarry 

 coating upon the exposed surfaces. The thickness of the oil- 

 bearing beds, which are massive and horizontal, is, according 

 to Professor "Worthen, from thirty-five to forty feet, and they 

 occupy a position about midway in ' the Niagara formation, 

 which has in this region a thickness of from 200 to 250 feet. 

 As exposed in the quarry, the whole rock seems pretty uniformly 

 saturated with petroleum, which exudes from the natural joints 

 and the fractured surfaces, and covers small pools of water in 

 the depressions of the quarry. I selected numerous specimens 

 of the rocks from different points and at various levels, witli a 

 view of getting an average sample, although it was evident that 

 they had already lost a portion of their original content of 

 petroleum. After lying for more than a year in my laboratory 

 they were submitted to chemical examination. The rock, 

 though porous and discolored by petroleum, is, when freed 

 from this substance, a nearly white, granular, crystalline, and 

 very pure dolomite, yielding 54.6 per cent of carbonate of lime. 



Two separate portions, each made up of fragments obtained 



Report Geol. Canada, 1866, p. 240. 



t American Journal of Science (2), XLVI. 355. 



