322 DESCRIPTIONS OF MINERALS. 
inflammable for use in lighting, to thick viseid fluids ; and 
thence they pass by insensible gradations into asphaltum or 
solid bitumen. ‘he Marsh-gas series contains also gases, 
of the composition C, H, and C, H, 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 tnsoluble 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, N. 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, 
‘fin 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 $0 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 
