&. : 3 — 
Parr Il. § vi] FRAGMENTAL ROCKS—ORGANIC. 178 
classes the clays or shales (of all geological ages) which are 
hydrocarbonaceous, and yield by distillation volatile hydrocarbons, 
inflammable gas, &c. 
Petroleum, a general term, under which is included a series of 
natural mineral oils. These are fluid hydrocarbon compounds, varying 
from a thin, colourless, watery liquidity to a b:ack, epaque, tar-like 
viscidity, and in specific gravity from 0-8 to 1-1. The paler, more 
limpid varieties are generally called naphtha, the darker, more viscid 
kinds mineral tar, while the name petroleum, or rock-oil, has 
been more generally applied to the intermediate kinds. 
Petroleum occurs sparingly in Europe. A few localities for it 
are known in Britain. It is found in large quantity along the 
country stretching from the Carpathians, through Gallicia and 
Moldavia, also at Baku on the Caspian. The most remarkable and 
abundant display of the substance, however, is in the so-called oil- 
regions of North America, particularly in Western Canada and 
Northern Pennsylvania, where vast quantities of it have been obtained 
in recent years. In Pennsylvania it is found especially in certain 
porous beds of sandstone or “sand-rocks,” which occur as low down. 
as the Old Red Sandstone, or even as the top of the Silurian system. 
In Canada it is largely present in still lower strata. Its origin in 
these ancient formations, where it cannot be satisfactorily con- 
nected with any destructive distillation of coal, is still an unsolved 
problem.’ 
Asphalt.—A smooth, brittle, pitch-like, black or brownish-black 
mineral, having a resinous lustre and conchoidal fracture, streak 
paler than surface of fracture, and specific gravity of 1:0 to 1:68. It 
melts at about the temperature of boiling water, and can be easily 
kindled, burning with a bituminous odour and a bright but smoky 
flame. It is composed chiefly of hydrocarbons, with variable ad- 
mixture of oxygen and nitrogen. It occurs sometimes in association 
with petroleum, of which it may be considered a hardened oxidized 
form, sometimes as an impregnation filling the pores or chinks of 
rocks, sometimes in independent beds. In Britain it occurs as a 
product of the destructive distillation of coals and carbonaceous 
shales by intrusive igneous rocks, as at Binny Quarry, Linlith- 
gowshire, but also in a number of places where its origin is not 
evident, as in the Cornish and Derbyshire mining districts, and 
among the dark flagstones of Caithness and Orkney, which are laden 
with fossil fishes. At Seyssel (Département de l’Aine) it forms a 
deposit 2500 feet long and 800 feet broad, which yields 1500 tons 
annually. It exudes in a liquid form from the ground round the 
borders of the Dead Sea. In Trinidad it forms a lake 14 miles 
in circumference, which is cool and solid near the shore, but increases 
in temperature and softness towards the centre. 
Graphite.—This mineral occurs in masses of sufficient size and 
1 See Second Geol. Survey of Pennsy'vania, vol. ii, 1877. Also Ashburner, Proe. 
Amer. Phil. Soe. Deccmber, 1876, 
