The Origin and Relations of the Carbon Minerals. 277 



semination, to be specially prone to sj^ontaneous distillntion. The 

 Utiea slifile is the lowest of these gi'eat sheets of carbonaceous 

 matter, and that supplies the hydro-carbon gases awd liquids 

 which issue from the earth at CoUmgwood, Canada, and in the 

 valley of the Cumberland. 'J'he next carbonaceous sheet is 

 formed by the great bituminous shale-beds of the Upper Devo- 

 nian, which underlie and supply the oil-wells in western Penn- 

 sylvania. In some places the shale is several hundred feet in 

 thickness, and contains more carbonaceous matter than all the 

 overlying coal strata. The outcrop of this formation, from 

 central New York to Tennessee, is conspicuously markfxl by 

 gas-springs, the flow from which is apparently unfailing. 



Petroleum is scarcely less constant in its connection Avith 

 these carbonaceous rocks than carburetted hydrogen, and it only 

 escapes notice from the little space it occupies. The two sub- 

 stances are so closely allied that they must have a common origin, 

 and they are in fact generated simultaneously in thousands of 

 localities. ' s 



During the oil excitement of some years since, when the whole 

 country was hunted over for "oil-sign," in many lagoons, from 

 which bubbles of marsh-gas were constantly escaping, films of 

 genuine petroleum were often found on the surface ; and as the 

 nnderlying strata were barren of oil, this could only have been 

 derived from the decaying vegetable tissue below. In the Bay of 

 Marquette, two or three miles north of the town, where the shore 

 is a ])eat-bog underlain by Archaean rocks, I have seen bubbles 

 of carburetted hydrogen rising in great numbers, attended by 

 drops of ]oetroleum, which spread as iridescent films on the 

 surface. 



The remarks which have been made in regard to the hetero- 

 genous nature of the solid hydro-carbons, apply with scarcely 

 less force to the gaseous and liquid pi'oducts of vegetable de- 

 composition. The gases which escape from marshes contain 

 carbonic acid, a number of hydro-carbon gases {or the materials 

 out of which they may be composed in the process of analysis), 

 and finally a larger or smaller volume of nitrogenous gas. It is 

 jDossible that the elimination of these gases takes the form of 

 fractional distillation, and definite compounds may be formed 

 directly from the wood-tissue or its derivatives, and mingle as 



