1879.] Native Bituinens and the Pitch Lake of Trinidad. 237 
estimated at twenty-eight millions of barrels of petroleum, or less 
than would be contained in four square miles of the oil-bearing 
limestone formation of Chicago.” 
As a rule limestone is too massive and close grained to permit 
the oil to flow freely through it to supply wells sunk in this rock; 
but overlying sandstones gradually soak up the oil, and its accu- 
mulation along the crests of anticlinal arches in the latter rock is 
due to the presence of water in the strata, which, being the 
heavier liquid, forces the oil to the top. The richest wells are 
those which tap large bodies of oil contained in the great fissures 
and cavities which, as geologists well know, usually accompany 
an anticlinal fold of the strata. Very often these subterranean 
chambers are filled partly with oil and partly with gas, and the 
latter serves a useful purpose in forcing the former to the surface. 
This gas is derived from the oil itself, and if the situation of the 
fissure or the texture of the rock are such that the gas can escape, 
its formation will continue until, in some cases at least, the petro- 
leum is reduced to a thick viscid or even solid condition. * It is by 
a similar but more rapid fractional distillation that the petroleum . 
is refined for illuminating purposes, the solid residue being chiefly 
the substance paraffine. The fissures filled with solidified or 
inspissated petroleum are not wholly theoretical, for several have 
been discovered, which, through some accident of erosion or 
faulting of the strata, are now exposed on the surface. The most 
noted of these is in New Brunswick, the material occupying the 
fissure being the famous and valuable mineral, albertite. This is 
a jet-black lustrous substance intermediate in physical characters 
between bituminous coal and asphaltum, though chemically it is 
much nearer the latter than the former, affording the formula 
Ci, Ha Oze- .This deposit bears no resemblance to a true coal 
bed, but fills a large irregular crevice cutting across the strata. 
The enclosing shales are rich in the remains of fish, and so bitu- 
minous as to be visibly oily, and to “sustain a fire without the 
aid of other fuel.” The grahamite of West Virginia is a sub- 
Stance closely resembling albertite and occurring in a similar fis- 
Sure or crevice. The same phenomena, on a smaller scale, are 
many times repeated in Canada, in the vicinity of Quebec and 
elsewhere, 
Whenever petroleum is exposed to the air for any length of 
_ time, as when it slowly exudes from the rocks, forming petroleum — 
s 
