(Original.)]
_Limestone_, like chalk, is composed of carbonate of lime, sometimes
almost pure, but more commonly with a greater or less intermixture
of some foreign material, such as alumina or silica. The varieties
of limestone are almost innumerable, but the great majority can
be clearly proved to agree with chalk in being essentially of
organic origin, and in being more or less largely composed of the
remains of living beings. In many instances the organic remains
which compose limestone are so large as to be readily visible to
the naked eye, and the rock is at once seen to be nothing more
than an agglomeration of the skeletons, generally fragmentary, of
certain marine animals, cemented together by a matrix of carbonate
of lime. This is the case, for example, with the so-called "Crinoidal
Limestones" and "Encrinital Marbles" with which the geologist
is so familiar, especially as occurring in great beds amongst
the older formations of the earth's crust. These are seen, on
weathered or broken surfaces, or still better in polished slabs
(fig. 9), to be composed more or less exclusively of the broken
stems and detached plates of sea-lilies (_Crinoids_). Similarly,
other limestones are composed almost entirely of the skeletons of
corals; and such old coralline limestones can readily be paralleled
by formations which we can find in actual course of production
at the present day.
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