atives, encase themselves in body walls of lime. They
absorb the lime from the sea water, and deposit it as unconsciously as
we build the bony framework of our bodies.

All the bone and shell-bearing creatures that die on the earth and in
the sea restore to the land and to the water the lime taken by the
creatures while they lived. Carbonic acid gas in the water greatly
hastens the dissolving of dead shells. Carbonic acid gas, whether free
in the air, or absorbed by percolating water, hastens the dissolving of
skeletons of creatures that die upon land. Then the raw materials are
built again into lime rocks underground.

The lime rocks are the most important group in the list of rocks that
form the crust of the earth. They are made of the elements calcium,
carbon, and oxygen, yet the different members of this calcite group
differ widely in composition and appearance. So do oyster shells and
beef bones, though both contain quantities of carbonate of lime.

Calcite is a soft mineral, light in weight, sometimes white, but oftener
of some other colour. It may be found crystallized or not. Whenever a
drop of acid touches it, a frothy effervescence occurs. The drop of acid
boils up and gives off the pungent odour of carbonic acid gas.

The reason that calcite is hard to find in rocks is that percolating
water, charged with acids, is constantly stealing it, and carrying it
away into the ocean. The rocks that contain it crumble because the limy
portions have been dissolved out.

Some limestones resist the destructive action of water. When they are
impregnated with silica they become transformed into marble, which takes
a high polish like granite. Acids must be strong to make any impression
on marble.

The thick beds of pure limestone that underlie the surface soil in
Kentucky and in parts of Virginia sometimes measure several hundred feet
in thickness, a single stratum often being twenty feet thick. They are
all horizontal, for they were formed on sea bottom, and have not been
crumpled in later time. The dead bodies of sea creatures contributed
their shells and skeletons to the lime deposit on the sea bottom. Who
can estimate the time it took to form those thick, solid layers of lime
rock? The animals were corals, crinoids, and molluscs. Little sand and
clay show in the lime rock of this period, before the marshes of the
Carboniferous Age took the place of the ancient inland sea of the
Subcarboniferous Period, the sediment