CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 



23 



the walls.. In many places, in fact, especially in the colder 

 portions of the great oceans, or at very great depths, the 

 " Globigerina ooze " disappears, and its place is taken by an 

 ooze composed almost wholly of the shells of Polycystina. 

 Similar deposits, made up of the flinty skeletons of these 

 Eadiolarians, have been formed at previous periods of the 

 earth's history, and now form part of the earth's crust. The 

 two most famous of these deposits occur in Barbadoes and 

 in the Mcobar Islands, the former being well known to 

 workers with the microscope as the " Barbadoes earth " 

 (fig. 7). 



In addition to flint-producing animals, we have also the 

 great group of fresh -water and marine microscopic plants 



Fig. 7. Shells of Polycystina from 

 " Barbadoes earth ; " greatly magnified. 

 (Original. ) 



Fig. 8. Cases of Diatoms in the Rich- 

 mond " Infusorial earth ; " highly magni- 

 fied. (Original.) 



known as Diatoms, which likewise secrete a siliceous skele- 

 ton, 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 that forty-one thousand millions go 

 to make up a single cubic inch of stone. Another celebrated 



