phosphate
of lime (Sollas); but there are many other cases in which no actual
structure has yet been shown to exist, and as to the true origin
of which it would be hazardous to offer a positive opinion.]

The last lime-salt which need be mentioned is _gypsum_, or _sulphate
of lime_. This substance, apart from other modes of occurrence, is
not uncommonly found interstratified with the ordinary sedimentary
rocks, in the form of more or less irregular beds; and in these
cases it has a palaeontological importance, as occasionally yielding
well-preserved fossils. Whilst its exact mode of origin is uncertain,
it cannot be regarded as in itself an organic rock, though clearly
the product of chemical action. To look at, it is usually a whitish
or yellowish-white rock, as coarsely crystalline as loaf-sugar,
or more so; and the microscope shows it to be composed entirely
of crystals of sulphate of lime.

We have seen that the _calcareous_ or lime-containing rocks are
the most important of the group of organic deposits; whilst the
_siliceous_ or flint-containing rocks may be regarded as the
most important, most typical, and most generally distributed
of the mechanically-formed rocks. We have, however, now briefly
to consider certain deposits which are more or less completely
formed of flint; but which, nevertheless, are essentially organic
in their origin.

Flint or silex, hard and intractable as it is, is nevertheless
capable of solution in water to a certain extent, and even of
assuming, under certain circumstances, a gelatinous or viscous
condition. Hence, some hot-springs are impregnated with silica
to a considerable extent; it is present in small quantity in
sea-water; and there is reason to believe that a minute proportion
must very generally be present in all bodies of fresh water as
well. It is from this silica dissolved in the water that many
animals and some plants are enabled to construct for themselves
flinty skeletons; and we find that these animals and plants are and
have been sufficiently numerous to give rise to very considerable
deposits of siliceous matter by the mere accumulation of their
skeletons. Amongst the animals which require special mention in
this connection are the microscopic organisms which are known to
the naturalist as _Polycystina_. These little creatures are of the
lowest possible grade of organisation, very closely related to the
animals which we have previously spoken of as _Foraminifera_