nifera_, sponge-spicules, &c.--embedded in a siliceous
basis. In many instances the flint contains larger organisms--such
as a Sponge or a Sea-urchin. As the flint has completely surrounded
and infiltrated the fossils which it contains, it is obvious
that it must have been deposited from sea-water in a gelatinous
condition, and subsequently have hardened. That silica is capable
of assuming this viscous and soluble condition is known; and
the formation of flint may therefore be regarded as due to the
separation of silica from the sea-water and its deposition round
some organic body in a state of chemical change or decay, just as
nodules of phosphate of lime or carbonate of iron are produced.
The existence of numerous organic bodies in flint has long been
known; but it should be added that a recent observer (Mr Hawkins
Johnson) asserts that the existence of an organic structure can
be demonstrated by suitable methods of treatment, even in the
actual matrix or basis of the flint.[6]

[Footnote 6: It has been asserted that the flints of the chalk
are merely fossil sponges. No explanation of the origin of flint,
however, can be satisfactory, unless it embraces the origin of
chert in almost all great limestones from the Silurian upwards,
as well as the common phenomenon of the silicification of organic
bodies (such as corals and shells) which are known with certainty
to have been originally calcareous.]

In addition to deposits formed of flint itself, there are other
siliceous deposits formed by certain _silicates_, and also of
organic origin. It has been shown, namely--by observations carried
out in our present seas--that the shells of _Foraminifera_ are
liable to become completely infiltrated by silicates (such as
"glauconite," or silicate of iron and potash). Should the actual
calcareous shell become dissolved away subsequent to this
infiltration--as is also liable to occur--then, in place of the
shells of the _Foraminifera_, we get a corresponding number of
green sandy grains of glauconite, each grain being the _cast_
of a single shell. It has thus been shown that the green sand
found covering the sea-bottom in certain localities (as found by
the Challenger expedition along the line of the Agulhas current)
is really organic, and is composed of casts of the shells of
_Foraminifera_. Long before these observations had been made,
it had been shown by Professor Ehrenberg that the green sands of
various geological formatio