as evidences of great
convulsions of nature, whereby the earth has been violently rent
asunder. But, while mechanical forces have had their share in the work,
as has been shown, the main agent in every case has been the
comparatively gentle, invisible gas known as carbonic acid. This is
generated by the decay of animal and vegetable substances, and is to a
considerable degree soluble in water. Under ordinary circumstances one
measure of water will absorb one measure of carbonic acid; and the eye
will detect no difference in its appearance. Under pressure the power of
absorption is rapidly increased, until the water thus surcharged has an
acid taste, and effervesces on flowing from the earth, as in Saratoga
water.

"Rain-water, falling amid leaves and grass, and sinking into the soil,
absorbs large quantities of carbonic acid. On reaching the underlying
limestone, the latter is instantly attacked by the acidulated water in
which it is dissolved and carried away.

"It is agreed among geologists, amazing as the statement may seem, that
the immense caverns of Virginia, Kentucky and Indiana, including Mammoth
Cave itself (the largest of all), were eaten out of the solid mass of
limestone by the slow, patient, but irresistible action of acidulated
water."

Professor N.S. Shaler says: "The existence of deep caverns is a sign
that the region has long been above the sea."

Through the kindness of Professor C.J. Norwood, Chief Inspector and
Curator of the Geological Department of Kentucky, it is possible to
quote the first official report made on the caves of that state and
published in 1856, in Volume I., Kentucky Geological Survey Reports.
Dr. Norwood says: "Referring to the 'Subcarboniferous Limestone' (now
known as the St. Louis group of the Mississippian series), Dr. Owen
says: 'The southern belt of this formation is wonderfully cavernous,
especially in its upper beds, which being more argillaceous, and
impregnated with earths and alkalies, are disposed to produce salts,
which oozing through the pores of the stone effloresce on its surface,
and thus tend to disintegrate and scale off, independent of the solvent
effects of the carbonated water. Beneath overhanging ledges of
limestone, quantities of fine earthy rubbish can be seen, weathered off
from such causes. In these I have detected sulphate of lime, sulphate of
magnesia, nitrate of lime, and occasionally sulphate of soda. The
tendency which some calcareous rocks hav