X. 



ON PETROLEUM, ASPHALT, PYRO- 

 SCHISTS, AND COAL. 



In the following paper on the Oil-bearing Limestone of Chicago, read before the 

 American Association for the Advancement of Science, in 1870, and published in the 

 American Journal of Science for June, 1871, will be found a summary of my conclu- 

 sions on the geological history of petroleum. To it are appended extracts from an 

 earlier paper in the same Journal for March, 1863, On Bitumens and Pyroschists, and 

 some later observations by Dawson and myself on the vegetable tissues forming coal. 

 The reader is also referred in connection with petroleum to my paper on the Geology 

 of Southwestern Ontario, in the same Journal for November, 1868, and to Notes 

 on the Oil- Wells of Terre Haute, Indiana, in that for November, 1871. 



WHEN, in 1861,* I first published my views on the petro- 

 leum of the great American palaeozoic basin, I expressed the 

 opinion that the true source of it was to be looked for in cer- 

 tain limestone formations which had long been known to be 

 oleiferous. I referred to the early observations of Eaton and 

 Hall on the petroleum of the Niagara limestone, to numerous 

 instances of the occurrence of this substance in the Trenton 

 and Corniferous formations, and, in Gaspe, in limestones of 

 Lower Helderberg age. Subsequently, in this Journal for 

 March, 1863, and in the Geology of Canada, I insisted still 

 further upon the oleiferous character of the Corniferous lime- 

 stone in southwestern Ontario, which appears to be the source 

 of the petroleum found in that region. I may here be permit- 

 ted to recapitulate some of my reasons for concluding that 

 petroleum is indigenous to these limestones, and for rejecting 

 the contrary opinion, held by some geologists, that its occur- 

 rence in them is due to infiltration, and that its origin is to be 

 sought in an unexplained process of distillation from pyro- 

 schists or so-called bituminous shales. These occur at three 

 * See the Appendix to this paper. 



