X.] THE OIL-BEARING LIMESTONE OF CHICAGO. 169 



distinct horizons in the New York system, and are known as 

 the Utica slate, immediately above the Trenton limestone, and 

 the Marcellus and Genesee slates which lie above and below 

 the Hamilton shales ; the latter being separated from the un- 

 derlying Corniferous limestone by the Marcellus state. 



First, these various pyroschists do not, except in rare in- 

 stances, contain any petroleum or other form of bitumen. 

 Their capability of yielding volatile liguid hydrocarbons or 

 pyrogenous oils, allied in composition to petroleum, by what is 

 known to chemists as destructive distillation, at elevated tem- 

 peratures, is a property which they possess in common with 

 wood, peat, lignite, coal, and most substances of organic origin, 

 and has led to their being called bituminous, although they are 

 not in any proper sense bituminiferous. The distinction is one 

 which will at once be obvious to all those who are familiar 

 with chemistry, and who know that pyroschists are argilla- 

 ceous rocks containing in a state of admixture a brownish 

 insoluble and infusible hydrocarbonaceous matter, allied to lig- 

 nite or to coal. 



Second, the pyroschists of these different formations do not, 

 so far as known, in any part of their geological distribution, 

 whether exposed, at the surface or brought up by borings from 

 depths of many 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 

 property 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 

 pyroschists have not yet undergone the process of destructive 

 distillation. 



Third, the conditions in which the oil occurs in the lime- 

 stones are inconsistent with the notion that it has been intro- 

 duced 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 ae the pores of the underlying strata 

 would be filled with water. Such being the case, the petro- 



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