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 Polycystma from Fig. i6. — 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-cailed " Infusorial earth " of Richmond m 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 



