322 DESCRIPTIONS OF MINERALS. 



inflammable for use in lighting, to thick viscid fluids ; and 

 thence they pass by insensible gradations into asphaltum or 

 solid bitumen. The Marsh-gas series contains also gases, 

 of the composition C^Hg and C 3 H g and these, in addition to 

 Marsh-gas, often exist in connection with petroleum. 



Petroleum occurs in rocks of all ages, from the Lower 

 Silurian to the most recent ; in limestones, the more com- 

 pact sandstones, and shales ; but it is mostly obtained from 

 large cavities or caverns existing among the earth's strata. 

 Black shales and much bituminous coal afford it abundantly 

 when they are heated. But the oil obtained is not present 

 in these rocks, for when the rocks are treated with benzine, 

 the benzine takes up little or none ; instead, the rocks con- 

 tain an insoluble hydrocarbon, which yields the oil when 

 heat is applied. 



In the United States the oil, or the hydrocarbon which 

 yields it, has been observed in beds of the Lower and Upper 

 Silurian, Devonian, Carboniferous, Triassic, Cretaceous, and 

 Tertiary eras. Surface oil springs also occur in many places, 

 as at Cuba, Alleghany County, X. Y., called Seneca Oil 

 Spring ; and on a large scale in Santa Barbara, Southern 

 California ; at Rangoon in Burmah, where there are about 

 100 wells ; on the peninsula of Apcheron, on the Caspian, 

 and elsewhere. Pliny mentions the oil spring of Agrigen- 

 tum, Sicily, and says that the liquid was collected and used 

 for burning in lamps, as a substitute for oil. Moreover he 

 distinguishes the oil from the lighter and more combustible 

 naphtha, a locality of which about the sources of the Indus, 

 " in Parthia," he mentions. 



Petroleum is obtained chiefly at the present time from 

 more or less deeply-seated subterranean chambers or cavities 

 among the rock strata, reached by boring. Being under 

 pressure of gas associated with it, and also, in many cases, 

 that also of water, it rises to the surface in the boring, and 

 sometimes makes a ''spouting" well. As early as 1833, 

 Hildreth mentioned the discharge of oil with the waters 

 of the salt wells of the Little Kanawha valley ; and speaks 

 also of a well near Marietta, Ohio, which threw out at one 

 time, he says, 50 to 60 gallons of oil at ••'each eruption." 



The mineral oil of the rocks has been formed through 

 the decomposition of animal and vegetable substances. 

 From the nature of the rocks which most abound in the 

 species of hydrocarbons that yield oil, it is evident that 



