X.] THE OIL-BEARING LIMESTONE OF CHICAGO. 171 



the question. In this region, it has been maintained by Win- 

 chell that the source of the petroleum is to be sought in the 

 Devonian pyroschists. I however showed in 1866, as the re- 

 sult of careful studies of the various borings : first, that none 

 of the oil-wells were sunk in the Genesee slates, but along 

 denuded aiiticlinals, where these rocks have disappeared, and 

 ^where, except the thin layer of Marcellus slate sometimes met 

 with at the base of the Hamilton shales, no pyroschists are 

 found above the Trenton limestone. Second, that the reser- 

 voirs of petroleum in the wells sunk into the Hamilton shales 

 are sometimes met with in this formation, and sometimes, in 

 adjacent borings, only in the underlying Corniferous. Exam- 

 ples of this have been cited by me in wells in Enniskillen, 

 Bothwell, Chatham, and Thamesville, where petroleum was 

 only found at depths of from thirty to one hundred and twenty 

 feet in the Corniferous limestone, in all of these places over- 

 laid by the Hamilton shales. It was also shown, that in two 

 localities in this region, namely, at Tilsonburg and in Maid- 

 stone, where the Corniferous is covered only by post-pliocene 

 clays, petroleum in considerable quantities has been obtained 

 by sinking into the limestone.* That the supplies of petro- 

 leum in such localities are less abundant than in parts where a 

 mass of shales and sandstones overlies the oil-bearing limestone, 

 is explained by the fact that both the pores and the fissures in 

 the superior strata serve to retain the oil, in a manner analo- 

 gous to the post-pliocene 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 impreg- 

 nated with oil should sometimes occur, but the evidence of the 

 existence of indigenous petroleum, which is so clear in the 

 various limestones, is wanting in the case of the pyroschists ; 

 although concretions holding 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 petroleum of Pennsylvania, Ohio, and the 



* American Journal of Science (2), XL VI. 360 ; and Report Geol. Canada, 

 1866, pp. 241 - 250. 



