608 



HISTORICAL GEOLOGY. 



from the deep-seated rock that has yielded also the oil, and some higher tem- 

 perature than that of the surface was needed for its production. At a depth 



of 1415 feet in the drilling a very heavy 

 925. "gas vein" was struck, and this was 



the chief source of the gas. Ashburner 

 remarks further that several other wells 

 in the oil-regions have had similar gey- 

 sers ; and as early as 1838, in the valley 

 of the Ohio, a salt well threw jets of 

 water and gas, at intervals of 10 to 12 

 hours, to heights of 50 to 100 feet. 



The original source of the oil is sup- 

 posed, by most writers on the subject, 

 to have been a Devonian shale, like the 

 Genesee or Marcellus, below the level 

 of the Chemung beds, from which it 

 was evolved by a slow process of distil- 

 lation. The conditions necessary for 

 oil, on this view, are (1) a primary 

 source of the oil ; (2) strata to receive 

 and hold it ; and (3) overlying deposits 

 to prevent its escape to the surface and 

 consequent dissipation. These three 

 conditions are fulfilled by (1) a deep- 

 seated carbonaceous rock containing 

 abundant organic remains ; (2) an over- 

 lying porous stratum ; and (3) super- 

 incumbent shales, slates, etc. These 

 statements also apply to gas production. Slight subterranean movements 

 attending the making of the Appalachian Mountains to the east and south- 

 east would have produced some heat, and so have caused oil to escape from 

 the shales; and the vaporized oils would have risen until they were some- 

 where condensed — either in confined places in or among the rocks, or still 

 higher in the open air (Peckham, 1884). I. C. White regards the source as 

 organic materials within the sand-beds. 



Water-and-gas gej'ser. 



The oil wells of western Pennsylvania yielded, in 1891, 31,793,477 barrels of the crude 

 oil, or petroleum. Of this, 5,452,418 barrels were from the Bradford district, McKean 

 County, and 10,317,258 from Alleghany County, the county of which Pittsburg is the 

 capital. In the same year, the yield of Alleghany County, N.Y., adjoining the northern 

 end of the Pennsylvania belt, was 1,121,574 barrels; and that of West Virginia, adjoining 

 the southern end, 2,406,218 barrels. The total yield of the United States in 1891 was 

 54,291,980 barrels. Ohio produced 17,740,307 barrels, making the yield for Pennsylvania 

 and Ohio together 49,533,784 barrels. But the oil of Ohio was nearly all from the Lower 

 Silurian Trenton limestone — this formation affording 17,316,000 barrels; the Berea grit, 

 which is referred to the Subcarboniferous, supplied only a few hundred thousand barrels. 



