unt of clay or hydrated
silicate of alumina in their composition. Under this head come
clays, shales, marls, marl-slate, clay-slates, and most flags
and flagstones.

B. CHEMICALLY-FORMED ROCKS.--In this section are comprised all
those Aqueous or Sedimentary Rocks which have been formed by
chemical agencies. As many of these chemical agencies, however,
are exerted through the medium of living beings, whether animals
or plants, we get into this section a number of what may be called
"_organically-formed rocks_." These are of the greatest possible
importance to the palaeontologist, as being to a greater or less
extent composed of the actual remains of animals or vegetables,
and it will therefore be necessary to consider their character
and structure in some detail.

By far the most important of the chemically-formed rocks are
the so-called _Calcareous Rocks_ (Lat. _calx_, lime), comprising
all those which contain a large proportion of carbonate of lime,
or are wholly composed of this substance. Carbonate of lime is
soluble in water holding a certain amount of carbonic acid gas
in solution; and it is, therefore, found in larger or smaller
quantity dissolved in all natural waters, both fresh and salt,
since these waters are always to some extent charged with the
above-mentioned solvent gas. A great number of aquatic animals,
however, together with some aquatic plants, are endowed with
the power of separating the lime thus held in solution in the
water, and of reducing it again to its solid condition. In this
way shell-fish, crustaceans, sea-urchins, corals, and an immense
number of other animals, are enabled to construct their skeletons;
whilst some plants form hard structures within their tissues
in a precisely similar manner. We do meet with some calcareous
deposits, such as the "stalactites" and "stalagmites" of caves,
the "calcareous tufa" and "travertine" of some hot springs, and
the spongy calcareous deposits of so-called "petrifying springs,"
which are purely chemical in their origin, and owe nothing to the
operation of living beings. 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 st