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 Spon- 

 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. 



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

 Fig. 17. Natural size. 



Fig. 18. The same magnified, showing circular artic- 

 ulations of a species of GaUlonella, and spi- 

 cuise o[ Spongilla. 



by the fact that the small shells decrease in 

 ness of outline in proportion as the opaline 

 quantity. 



In the Bohemian tripoli above described, 



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- 

 Ige 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 

 opal, in which the 

 more durable fossils 

 are preserved like in- 

 sects in amber. This 

 opinion is confirmed 

 number and sharp- 

 cement increases in 



as in that of Planitz 



