ing, and the same volcanic action has been occurring during
many successive geological periods and millions of years; so that it
is difficult to conceive limits to the magnitude of the stores of
petroleum which may be awaiting discovery in the subterranean
depths.[2]

[Footnote 2: Professor J. Le Conte, when presiding recently at the
International Geological Congress at Washington, mentioned that in
the United States extensive lava floods have been observed,
covering areas from 10,000 to 100,000 square miles in extent and
from 2,000 to 4,000 feet deep. We have similar lava flows and
ashes in the North of England, in Scotland, and in Ireland,
varying from 3,000 to 6,000 feet in depth. In the Lake District
they are nearly 12,000 feet deep. Solfataras are active during the
intermediate, or so-called "dormant," periods which occur between
acute volcanic eruptions.]

Gypsum may also be an indication of oil-bearing strata, for the
substitution in limestone of sulphuric for carbonic acid can only be
accounted for by the action of these hot sulphurous gases. Gypsum is
found extensively in the petroleum districts of the United States, and
it underlies the rock salt beds at Middlesboro, where, on being
pierced, it has given passage to oil gas, which issues abundantly,
mixed with brine, from a great depth.

III. Besides the space occupied by "natural gas," which is very
extensive, 17,000 million gallons of petroleum have been raised in
America since 1860, and that quantity must have occupied more than
100,000,000 cubic yards, a space equal to a subterranean cavern 100
yards wide by 20 feet deep, and 82 miles in length, and it is
suggested that beds of "porous sandstone" could hardly have contained
so much; while vast receptacles may exist, carved by volcanic water
out of former beds of rock salt adjoining the limestone, which would
account for the brine that usually accompanies petroleum. It is
further suggested that when no such vacant spaces were available, the
hydrocarbon vapors would be absorbed into, and condensed in,
contiguous clays and shales, and perhaps also in beds of coal, only
partially consolidated at the time.

There is an extensive bituminous limestone formation in Persia,
containing 20 per cent. of bitumen, and the theory elaborated in the
paper would account for bitumen and oil having been found in Canada
and Tennessee embedded in limestone, which fact is cited by Mr.
Peckham as favoring