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



leum of the Silurian and Lower Devonian limestones must 

 have been derived from the Utica slate beneath. This rock, 

 however, is unaltered, and moreover, the intermediate sand- 

 stones and shales of the Loraine, Medina, and Clinton forma- 

 tions are destitute of petroleum, which must, on this hypothe- 

 sis, have passed through all these strata to condense in the 

 Niagara and Corniferous limestones. More than this, the 

 Trenton limestone, which, on Lake Huron and elsewhere, has 

 yielded considerable quantities of petroleum, has no pyroschists 

 beneath it, but on Lake Huron rests on ancient crystalline 

 rocks, with the intervention only of a sandstone devoid of 

 organic or carbonaceous matter. The rock-formations holding 

 petroleum are not only separated from each other by great 

 thicknesses of porous strata destitute of it, but the distribution 

 of this substance is still further localized, as I many years since 

 pointed out. The petroleum is, in fact, in many cases, confined 

 to certain bands or layers in the limestone, in which it fills the 

 pores and the cavities of fossil shells and corals, while other 

 portions of the limestone, above, below, and in the prolon- 

 gation of the same stratum, although equally porous, contain 

 no petroleum. From all these facts the only reasonable con- 

 clusion seems to me to be that the petroleum, or rather the 

 materials from which it has been formed, existed in these lime- 

 stone rocks from the time of their first deposition. The view 

 which I put forward in 1861, that petroleum and similar bitu- 

 mens have resulted from a peculiar " transformation of vegeta- 

 ble matters, or in some cases of animal tissues analogous to 

 these in composition," has received additional support from the 

 observations of Lesley* in West Virginia and Kentucky, and 

 from the more recent ones of Peckham.t 



The objections to this view of the origin and geological rela- 

 tions of petroleum have been for the most part founded on 

 incorrect notions of the geological structure of southwestern 

 Ontario, which has afforded me peculiar facilities for studying 



* Rep. Geol. Canada, 1866, 240 ; and Proc. Amer. Philos. Soc., X. 33, 

 187. 

 J Ibid., X. 445. 



