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_, but
differing in the fact that they secrete a shell or skeleton composed
of flint instead of lime. The _Polycystina_ occur abundantly in
our present seas; and their shells are present in some numbers
in the ooze which is found at great depths in the Atlantic and
Pacific oceans, being easily recognised by their exquisite shape,
their glassy transparency, the general presence of longer or
shorter spines, and the sieve-like perforations in the walls.
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