the earth at Collingwood, Canada, and in the valley of the Cumberland.
The next carbonaceous sheet is formed by the great bituminous shale
beds of the upper Devonian, which underlie and supply the oil wells in
western Pennsylvania. In some places the shale is several hundred
feet in thickness, and contains more carbonaceous matter than all the
overlying coal strata. The outcrop of this formation, from central New
York to Tennessee, is conspicuously marked by gas springs, the flow from
which is apparently unfailing.

Petroleum is scarcely less constant in its connection with these
carbonaceous rocks than carbureted hydrogen, and it only escapes notice
from the little space it occupies. The two substances are so closely
allied that they must have a common origin, and they are, in fact,
generated simultaneously in thousands of localities.

During the oil excitement of some years since, when the whole country
was hunted over for "oil sign," in many lagoons, from which bubbles of
marsh-gas were constantly escaping, films of genuine petroleum were
found on the surface; and as the underlying strata were barren of oil,
this could only have been derived from the decaying vegetable tissue
below. In the Bay of Marquette, two or three miles north of the town,
where the shore is a peat bog underlain by Archaean rocks, I have seen
bubbles of carbureted hydrogen rising in great numbers attended by drops
of petroleum which spread as iridescent films on the surface.

The remarks which have been made in regard to the heterogeneous nature
of the solid hydrocarbons apply with scarcely less force to the gaseous
and liquid products of vegetable decomposition. The gases which escape
from marshes contain carbonic acid, a number of hydrocarbon gases (or
the materials out of which they may be composed in the process of