thirds times its weight
of carbonic dioxide. Lime, having a strong affinity or attraction for
this gas, has absorbed it from the air and water, forming what is known
as carbonate of lime--which is the ordinary limestone. Chalk and the
various marbles are also carbonates of lime. Limestone strata in the
crust of the earth are found in all the periods of the earth's
formation. All forms of sea shells that were once the homes of animal
life are constructed of this compound; and in the later formations of
limestone, in the Secondary and Tertiary periods, we find this rock to
be made up almost entirely of marine shells, some of them microscopic in
size. The earlier or older formations of limestone that are found deeper
down in the earth's crust are less mingled with these marine shells.
This comes from the fact that the first deposition of limestone strata
occurred before the later forms of sea life had developed. Whatever
signs of life are found in these lower stratifications are of the very
lowest order. It is not to be understood that animal life is a necessary
factor in the formation of limestone, but it has been an incidental
feature which no doubt has been the chief means of gathering up from the
water this compound and precipitating it into the great limestone
strata that are everywhere found.

Carbonate of lime is found in solution in nearly, if not quite, all of
the mineral waters, and is also found in the water of the ocean. In
earlier times it must have been held in solution in much greater
quantities than at present. The myriads of sea animals that existed, and
that still exist, gathered from the water this substance, which formed
their shells, and served as a house in which they lived. New germs were
continually forming new shells, while the older ones ceased to live as
animals, and their houses in which they lived were precipitated to the
bottom of the ocean, where they were bound together as limestone rock.
These sea animals no doubt caused a much more rapid formation of
limestone than would or could have been the case without their
existence.

One can thus readily see what an important factor animal life has been
in the process of world-building. This process is still going on, but
probably not to the same extent as in former ages, because it is not
likely that there is so much carbonate of lime held in solution as there
was before these great limestone beds were formed. Limestone, however,
is easily disint