In the valley of the Cumberland, about Burkesville, one of the oil
regions of the country, the gases escaping from the equivalent of the
Utica shale accumulate under the plates of impervious limestone above
until masses of rock and earth, hundreds of tons in weight, are
sometimes thrown out with great violence. Unless these gases had been
produced by comparatively recent distillation, such explosions could not
occur.

In opening a coal mine on a hillside, the first traces of the coal seam
are found in a dark stain in the superficial clay; then a substance like
rotten wood is reached, from which all the volatile constituents have
escaped. These appear, however, later, and continue to increase as the
mine is deepened, until under water or a heavy covering of rock the coal
attains its normal physical and chemical characters. Here it is evident
that the coal has undergone a long-continued distillation, which must
have resulted in the constant production of carbonic acid and carbureted
hydrogen.

A line of perennial oil and gas springs marks the outcrop of every
great stratum of carbonaceous matter in the country. Of these, the most
considerable and remarkable are the bituminous shales of the Silurian
(Utica shale), of the Devonian (Hamilton and Huron shales), the
Carboniferous, etc. Here the carbonaceous constituent (10 to 20 per
cent.) is disseminated through a great proportion of inorganic material,
clay and sand, and seems, both from the nature of the materials which
furnished it--cellular plants and minute animal organisms--and its
dissemination, to be specially prone to spontaneous distillation. The
Utica shale is the lowest of these great sheets of carbonaceous matter,
and that supplies the hydro-carbon gases and liquids which issue from