or continuing in existence.



COMMENCEMENT OF ORGANIC LIFE--SEA PLANTS, CORALS, ETC.



We can scarcely be said to have passed out of these rocks, when we
begin to find new conditions in the earth. It is here to be observed
that the subsequent rocks are formed, in a great measure, of matters
derived from the substance of those which went before, but contain
also beds of limestone, which is to no small extent composed of an
ingredient which has not hitherto appeared. Limestone is a carbonate
of lime, a secondary compound, of which one of the ingredients,
carbonic acid gas, presents the element CARBON, a perfect novelty in
our progress. Whence this substance? The question is the more
interesting, from our knowing that carbon is the main ingredient in
organic things. There is reason to believe that its primeval
condition was that of a gas, confined in the interior of the earth,
and diffused in the atmosphere. The atmosphere still contains about
a two-thousandth part of carbonic acid gas, forming the grand store
from which the substance of each year's crop of herbage and grain is
derived, passing from herbage and grain into animal substance, and
from animals again rendered back to the atmosphere in their expired
breath, so that its amount is never impaired. Knowing this, when we
hear of carbon beginning to appear in the ascending series of rocks,
we are unavoidably led to consider it as marking a time of some
importance in the earth's history, a new era of natural conditions,
one in which organic life has probably played a part.