40 



LYELL'S ELEMENTS OF GEOLOGY. 



Infusoria in Tripoli Fossil Infusoria. 



The shells or shields of these infusoria are of pure silex, and 

 their forms are various, but very marked and constant in partic- 

 ular genera and species. Thus, in the family Bacillaria, (see 

 Fig. 14.,) the fossil species preserved in tripoli are seen to exhi- 

 bit the same divisions and transverse lines which characterize the 

 living shells of kindred form. With these, also, the siliceous 

 spiculse or internal supports of the freshwater sponge, or Span- 

 gilla of Lamarck, are sometimes intermingled (see the needle- 

 shaped bodies in Fig. 18.) These flinty cases and spiculae, 

 although hard, are very fragile, breaking like glass, and are 

 therefore admirably adapted, when rubbed, for wearing down 

 into a fine powder fit for polishing the surface of metals. 



Fig. 18. 



Fig. 17. 



Besides the tripoli, 

 which is formed ex- 

 clusively of infusoria, 

 there occurs in the 

 upper part of the 

 great stratum at Bi- 

 lin another heavier 

 and more compact 

 stone, a kind of se- 

 mi-opal, in which in- 

 numerable parts of 

 infusoria and spicu- 

 lse of the Spongilla 

 are filled with, and 

 cemented together by 

 siliceous matter. It 

 is supposed that the 

 shells of the more 

 delicate animalcules 

 have been dissolved 

 by water, and have 

 thus given rise to this 



Fragment of semi-opal from the great bed of tripoli, Bilin. Opal, in which the 



Fig. 17. Natural size. more durable fossils 



Fig. 18. The same magnified, showing circular artic- are preserved like in- 



ulations of a species of Gaillonella, and spi-sects in amber. This 



cu^ of Spongilla. opinion is confirmed 



by the fact that the small shells decrease in number and shafp- 

 ness of outline in proportion as the opaline cement increases in 

 quantity. 



In the Bohemian tripoli above described, as in that of Planitz 



