Such deposits are formed simply by
the precipitation of carbonate of lime from water, in consequence
of the evaporation from the water of the carbonic acid gas which
formerly held the lime in solution; but, though sometimes forming
masses of considerable thickness and of geological importance,
they do not concern us here. Almost all the limestones which
occur in the series of the stratified rocks are, primarily at any
rate, of _organic_ origin, and have been, directly or indirectly,
produced by the action of certain lime-making animals or plants,
or both combined. The presumption as to all the calcareous rocks,
which cannot be clearly shown to have been otherwise produced,
is that they are thus organically formed; and in many cases this
presumption can be readily reduced to a certainty. There are
many varieties of the calcareous rocks, but the following are
those which are of the greatest importance:--
_Chalk_ is a calcareous rock of a generally soft and pulverulent
texture, and with an earthy fracture. It varies in its purity,
being sometimes almost wholly composed of carbonate of lime,
and at other times more or less intermixed with foreign matter.
Though usually soft and readily reducible to powder, chalk is
occasionally, as in the north of Ireland, tolerably hard and
compact; but it never assumes the crystalline aspect and stony
density of limestone, except it be in immediate contact with
some mass of igneous rock.
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