PETROLEUM. 147 



probably at but a slight elevation of temperature, produced by the 

 same agencies which have caused elevations in the temperature of the 

 interior of the earth's crust at various points. 



Dana lias further pointed out how petroleum might be formed by 

 the reactions of the organic vegetable remains alone, the abstrac- 

 tion of some carbon and oxygen, as carbonic acid, accounting for the 

 formation of the lighter oils ; while the escape of some marsh-gas from 

 less confined material would account for the heavier oils. 



Newberry attributes the disagreeable smell of some limestone-oil 

 to its animal origin, and Dufrenoy alludes to the abundance of fish 

 fossils as a proof that the oil of various European districts was derived 

 from animal remains. 



As regards the circumstances favoring the accumulation of petro- 

 leum, it ajjpears that there should be a shale or other fine-grained 

 rock forming to protect the organic matter during its deposition, a 

 porous stratum above to be penetrated by the hydrocarbons resulting 

 from the decomposition of the organic matter, and finally another 

 shale or slate above, to prevent the further escape of the volatile prod- 

 ucts. If the sand-rock which usually forms the porous stratum is 

 filled with fissures, large quantities of oil may collect in these. 



The petroleum of Enniskillen, Canada, is ascribed by Hunt to the 

 Comiferous limestone of the Lower Devonian. Many geologists as- 

 cribe the oil of Pennsylvania, West Virginia, Ohio, and the rest of 

 this grand oil area, to the black shale or Genesee slate of the Middle 

 Devonian. Dr. J. S. Newberry, in his " Report of the Geological 

 Survey of Ohio," says of the Huron (black) shale of the Middle Devo- 

 nian in Ohio, that it is bituminous, and contains sheets of asphalt or 

 asphaltic coal. Oil and gas springs are associated with its outcrop, 

 and there is reason to believe that it supplies the wells of Oil Creek, 

 Pennsylvania. Hydrocarbons are the product of spontaneous distilla- 

 tion in the outcrops of the Huron shale in Ohio. It shows traces of 

 marine vegetation, and represents the Gardeau shale of New York, 

 with whatever there is in Ohio of the underlying Genesee slate. Its 

 materials appear to have accumulated in a quiet water-basin, being 

 marine and not terrestrial vegetation. It forms a vast repository of 

 hydrocarbonaceous matter, yielding ten to twenty gallons of oil per 

 ton by distillation. 



A line of oil and gas springs marks its outcrop, from Central New 

 York to Tennessee. Emanations of oil and gas occurring from Lower 

 Silurian rocks at Collingwood, Canada, and on the Upper Cumberland 

 River, Kentucky, are associated with similar deposits of black shale 

 representing the Utica shale (Lower Silurian) of New York. The 

 wells of Oil Creek penetrate strata immediately overlying the Huron 

 shale, and the oil is obtained from fissured and porous sheets of sand 

 stone of the Portage and Chemung groups, which lie just over the 

 Huron and offer convenient reservoirs for the oil it furnishes. It is a 



