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 has further pointed out how petroleum might be formed by the reactions of the organic vegetable remains alone, the abstraction 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 petroleum, it appears 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 products. 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 Coniferous limestone of the Lower Devonian. Many geologists ascribe 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 Devonian 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 distillation 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
