Both in Barbadoes and in the Nicobar islands occur geological
formations which are composed of the flinty skeletons of these
microscopic animals; the deposit in the former locality attaining
a great thickness, and having been long known to workers with
the microscope under the name of "Barbadoes earth" (fig. 15).
[Illustration: Fig. 15.--Shells of _Polycystina_ from "Barbadoes
earth;" greatly magnified. (Original.)]
[Illustration: Fig. 16.--Cases of Diatoms in the Richmond "Infusorial
earth;" highly magnified. (Original.)]
In addition to flint-producing animals, we have also the great
group of fresh-water and marine microscopic plants known as
_Diatoms_, which likewise secrete a siliceous skeleton, often of
great beauty. The skeletons of Diatoms are found abundantly at the
present day in lake-deposits, guano, the silt of estuaries, and in
the mud which covers many parts of the sea-bottom; they have been
detected in strata of great age; and in spite of their microscopic
dimensions, they have not uncommonly accumulated to form deposits
of great thickness, and of considerable superficial extent. Thus
the celebrated deposit of "tripoli" ("Polir-schiefer") of Bohemia,
largely worked as polishing-powder, is composed wholly, or almost
wholly, of the flinty cases of Diatoms, of which it is calculated
that no less than forty-one thousand millions go to make up a
single cubic inch of the stone.
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