Another celebrated deposit is
the so-called "Infusorial earth" of Richmond in Virginia, where
there is a stratum in places thirty feet thick, composed almost
entirely of the microscopic shells of Diatoms.
Nodules or layers of _flint_, or the impure variety of flint
known as _chert_, are found in limestones of almost all ages
from the Silurian upwards; but they are especially abundant in
the chalk. When these flints are examined in thin and transparent
slices under the microscope, or in polished sections, they are
found to contain an abundance of minute organic bodies--such as
_Foraminifera_, 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.
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