THE FOSSILIFEROUS ROCKS. 33 
and their shells are present in some numbers in the ooze which 
is found at great depths in the Atlantic and Pacific oceans, 
being easily recognised by their exquisite shape, their glassy 
transparency, the general presence of longer or shorter spines, 
and the sieve-like perforations in the walls. 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). 
In addition to flint- producing animals, we have also the 
great group of fresh-water and marine microscopic plants 
Fig. 15.— Shells of Polycystina from Fig. 16.—Cases of Diatoms in the Rich- 
‘*Barbadoes earth;” greatly magnified. mond ‘‘Infusorial earth;” highly magni- 
(Original.) fied. (Original.) 
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 un- 
commonly 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.. 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 fmf, or the impure variety of flint 
fe 
