[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 formations are composed mainly of the internal
casts of the shells of _Foraminifera_, and we have thus another
and a very interesting example how rock-deposits of considerable
extent and of geological importance can be built up by the operation
of the minutest living beings.
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