T. S. Hunt — Oil-hearing Limestone of Chicago. 421 



depths of manv hundred feet, present any evidence of having 

 been submitted to the temperature required for the generation 

 of volatile hydrocarbons. On the contrary they still retain the 

 nroperty of yielding such products when exposed to a sufficient 

 heat, at the same time undergoing a charring process by which 

 their brown color is changed to black. In other words these 

 pvroschists have not yet undergone the process of destructive 

 distillation. 



Third, the conditions which the oil occurs in the limestones, 

 are inconsistent with the notion that it has been introduced into 

 these rocks by distillation. The only probable or conceivable 

 source of heat, in the circumstances, being from beneath, the 

 process of distillation would naturally be one of ascension, the 

 more so as the pores of the underlying strata would be tilled 

 with water. Such being the case, the petroleum of the Upper 

 Jilunan and Lower Devonian limestones must have been 

 denved from the Utica slate beneath. This rock, however, is 

 unaltered, and moreover, the intermediate sandstones and shales 

 ot the Loraine, Medina and Clinton formations are destitute of 

 petroleum, which must, on this hypothesis, have passed through 

 all these strata to condense in the Niagara and the Comiferous 

 limestones. More than this, the Trenton Hmestone which, on 

 ijake Huron and elsewhere, has yielded considerable quantities 

 oi petroleum, has no pyroschists beneath it, but on Lake Huron 

 rests on ancient crystalline rocks, with the intervention only of 

 a sterile sandstone. The rock-formations holding petroleum 

 arp y^r.^ r.^1^ separated from each other by great thicknesses of 

 trata destitute of it, but the distribution of this sub- 

 rr^rS, ®*^^^ ^rther localized, as I many years since pointed 

 t9 X. petroleum is in fact in many cases, confined to cer- 

 3f^^^ds or layers in the limestone, in which it fills the pores 

 ^Q the cavities of fossil shells and corals, whHe other portions 

 ^i the hmestone, both above, below, and in the prolonga- 

 tion 01 the same stratum, though equally porous, contain no 

 F^iroieum. Fj-qj^ ^^ ^-^^^^ ^^^^^ ^j^^ ^^^ reasonable conclusion 

 ZT 1°^^ *^ be that the petroleum, or rather the materials 

 ™m which it has been formed, existed in these limestone rocks 

 i^m the timp rsf +\.^;^ ^-„i. j7„_ -i^^^ rri,A „io™ ^uhioh 1 



or in peculiar *' transtonnaxion oi vegcwtuic i 



j)Q_-T.^°^e cases of animal tissues analogous to these i 



^«mon, has received additional support from the observations 

 recent ^^* '"^ ^^^* Virginia and Kentucky, and from the more 

 recent ones of Peckham.f 



t IWd" ?^°h^^°^•^^' ^866, 240, and Proc. Amer. Philos. Soc., x, 33, 187. 





