No. 1.] HUNT — OIL-BEARING LIMESTONE. 51 



Corniferous limestone, in all of these places overlaid by the Ha- 

 milton shales. It was also shown, that in two localities in this 

 region, viz. in Tilsonburg and at Maidstone, where the Cornifer- 

 ous is covered only by quaternary clays, petroleum in considerable 

 quantities has been obtained by sinking into the limestone.^ That 

 the supplies are less abundant than in parts where a mass of 

 shales and sandstones overlies the oil bearing limestone is ex- 

 plained by the fact that both the pores and the fissures in the 

 superior strata serve to retain the oil, in a manner analogous to 

 the quaternary gravels in some parts of this region, which are 

 the sources of the so-called surface oil-wells. It is therefore not 

 surprising that examples of pyroschists impregnated with oil 

 should sometimes occur, but the evidence of the existence of in- 

 digenous petroleum, which is so clear in the various limestones, is 

 wanting in the case of the pyroschists ; although concretions hold- 

 ing petroleum have been observed in the Marcellus and the 

 Genesee slates of New York. There is, however, reason to 

 believe, as I have elsewhere pointed out, that much of the petro- 

 leum of Pennsylvania, Ohio and the adjacent regions, is indigen- 

 ous to certain sandbtone strata in the Devonian and Carboniferous 

 rocks. f 



At the meetinc; of the x\merican Association for the Advance- 

 ment of Science at Chicago, in August, 18G8, in a discussion 

 which followed the reading of a paper by myself on the geology 

 of Ontario, J it was contended that, although the various lime- 

 stones which have mentioned are truly oleiferous, the quantity of 

 petroleum which they contain is too inconsiderable to account for 

 the great supplies furnished by oil-producing districts, like that 

 of Ontario for example. This opinion being contrary to that 

 which I had always entertained, I resolved to submit to examin- 

 ation the well-known oil-bearinci; limestone of Chicacco. 



This limestone, the quarries of which are in the immediate 

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

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

 tions, which mingled with dust, form a tarry coating upon the 

 exposed surfaces. The thickness of the oil-bearing beds, which 



* Silliman's Journal II, xlvi, 360 ; and Report Gcol. Canada. 1866, 

 pp. 241-250. 



t Ibid, 240. 



X Silliman's Jour. II, xlvi, 355. 



