CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 33 



skeletons of animals and plants are very liable to be affected by 

 secondary changes, chiefly due to the percolation of water through- 

 out their mass. As the main result of these changes, the siliceous 

 skeletons become more or less extensively dissolved, the percolating 

 water becoming thus charged with a larger or smaller amount of 

 silica in solution. This dissolved silica is ultimately redeposited in 

 the solid form, having, however, now lost its organic structure. By 

 this partial solution of the skeletons of siliceous organisms, and the 

 subsequent precipitation of the dissolved silica thus obtained, we 

 may explain the common presence of nodules or beds of " flint " or 

 " chert " in many of the great geological formations. This subject 

 will, however, be treated of in greater detail in connection with the 

 palseontological history of the Sponges. 



The Polycystina are minute organisms belonging to the Protozoa, 

 and nearly related to the Foraminifera, from which they differ, 

 among other characters, by the fact that they secrete a " test " or 

 skeleton of flint instead of one composed of lime. The Polycystina 

 have a wide distribution in our present seas, and their skeletons are 

 very generally recognisable, in greater or less numbers, in the deep- 

 sea muds of the great 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 their 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 a " Radiolarian ooze " composed almost 

 wholly of the shells of Polycystina. Similar deposits, made up of 

 the flinty skeletons of these Radiolarians, have been formed at pre- 

 vious periods of the earth's history, and now form part of the earth's 

 crust. The two most famous of these deposits occur in Barbados 

 and in the Nicobar Islands, the former being well known to workers 

 with the microscope as the " Barbados earth" (fig. 12). 



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 de- 

 posits 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 cal- 

 culated that no less than forty-one thousand millions go to make up 

 a single cubic inch of stone. Another celebrated deposit is the so- 



vol. 1. c 



