hence the disintegrated rubbish of the caves yields nitrate of potash
after being treated with the ley of ashes and subsequent evaporation of
the saline lixivium. The wonderfully cavernous character of the
subcarboniferous limestones of the Green River valley, and, indeed, of
these particular members of the subcarboniferous group throughout a
great part of its range in Kentucky and Indiana, is due in a great
measure to this cause, together with the solvent and eroding effects of
water charged with carbonic acid. The 'rock-houses' frequently
encountered both in this formation and in the limestones of Silurian
date, are produced by similar causes; the more easily disintegrated beds
gradually crumbling away, while the more durable remain in overhanging
ledges. By the oxidation of other elements, sulphates of oxide of iron
and alkalies result, which, by double decomposition, with carbonate of
lime, give rise to the formation of gypsums which appears in the form of
rosettes, festoons and various other imitative forms on the walls and
ceilings of the caves. Crystallizations of sulphate of soda and sulphate
of magnesia are not uncommon, both in some of the caves and in sheltered
situations under shelving rocks."

The explanations thus given of the excavation and subsequent refilling
and decoration of the limestone caves of Kentucky and Indiana apply
equally well to those of other states; but it is to be remembered that
at the time of Dr. Owen's report, onyx, the most beautiful and valuable
of dripstones, had not yet been discovered in the United States; while
now especially fine deposits are known in California, Utah, Missouri,
South Dakota and Arkansas; the Missouri supply being exceptionally
valuable on account of the marvelous delicacy and beauty of its
coloring; nor can it soon be exhausted, as deposits have been found in
eight counties and further exploration will no doubt discover more.