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.[6] [Footnote 6: It has been asserted that the flints of the chalk are merely fossil sponges.