236 Native Bitumens and the Pitch Lake of Trinidad. (April, 
goes on in the retorts of the gas works? Probably not. We 
have every reason to believe that the natural process is a very 
gradual one, and that the volatile products are all gaseous. In 
every bituminous coal mine in the world the two permanent gases, 
carbonic anhydride and marsh gas—the deadly choke-damp and 
fire-damp of the miners, are constantly escaping from the coal, 
but unaccompanied by any oily, petroleum-like liquid. This 
action, sufficiently long continued, must result in the production 
of anthracite, and that it has so resulted is evidenced by the fact 
that the rocks lying above the great deposits of anthracite are 
quite free from the liquid bitumens we should otherwise expect 
to find there. The fact is, that in Pennsylvania the anthracite is 
in one end of the State and the petroleum in the other; and, 
moreover, the petroleum is obtained from a formation below the 
Carboniferous, to which the coal belongs. Its origin is some- 
times referred to the carbonaceous shales or pyroschists of the 
underlying Hamilton beds; but these, like the coals, are found, 
on examination, not to contain any bitumen, and like the bitu- 
minous coals, they still retain perfectly the power of yielding 
bitumens when sufficiently heated. Beyond the limits of Penn- 
sylvania the general facts are the same, and nowhere is there any 
evidence proving a connection of the petroleum with the coals or 
pyrochists. Petroleum is generally obtained from wells sunk in 
sandstone or slate. In some cases it is probably indigenous in 
these, but usually it has been forced up by hydrostatic pressure 
or sponge-like absorption from oleiferous limestones, There are 
several extensive formations of these limestones in Eastern North 
America, and geologists are only beginning to appreciate their 
abundance and richness. The oil is found filling the pores and 
cavities of fossil shells and corals, and saturating the entire sub- 
stance of the limestone, the evidence being plain that it is indig- 
enous in this position and has not been introduced into the lime- 
` stone subsequent to the formation of the latter. Dr. Hunt has 
made a quantitative determination of the petroleum in a lime- 
stone of Niagara age occurring near Chicago, with the following 
almost incredible result: Although the formation has a thickness 
of only thirty-five feet, yet in each square mile it must’ contain 
not less than “seven and three quarter millions of barrels of 
petroleum,” He says further, “ The total produce of the great 
Pennsylvania oil region for the ten years from 1860 to 1870 is | 
