26 FOSSIL INFUSOKIA. [Ch. in 



upper part of the great stratum at Bilin another heavier and more compact 

 stone, a kind of semi-opal, in which innumerable parts of Diatomacea? 

 and spiculae of the Spongilla are filled with, and cemented together by. 

 siliceous matter. It is supposed that the siliceous remains of the most 

 delicate Diatomaceae 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 by the fact that the organic 

 bodies decrease in number and sharpness of outline in proportion as the 

 opaline cement increases in quantity. 



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

 ony, the species of DiatomaceaB (or Infusoria, as termed by Ehrenberg) 

 are freshwater ; but in other countries, as in the tripoli of the Isle of 

 France, they are of marine species, and they all belong to formations of 

 the tertiary period, which will be spoken of hereafter. 



A well-known substance, called bog-iron ore, often met Avith in peat- 

 mosses, has also been shown by Ehrenberg to consist of innumerable ar- 

 ticulated threads, of a yellow ochre color, composed partly of flint and 

 partly of oxide of iron. These threads are the cases of a minute micro- 

 scopic body, called Gaillonella ferruginea (fig. 18). 



It is clear that much time must have been required for the accumulation 

 of strata to which countless generations of DiatomaceaB have contributed 

 their remains ; and these discoveries lead us naturally to suspect that other 

 deposits, of which the materials have usually been supposed to be inorganic, 

 may in reality have been derived from microscopic organic bodies. That 

 this is the case with the white chalk, has often been imagined, this rock 

 having been observed to abound in a variety of marine fossils, such as 

 echini, testacea, bryozoa, corals, sponges, Crustacea, and fishes. Mr. Lons- 

 dale, on examining, Oct., 1835, in the museum of the Geological Society 

 of London, portions of white chalk from different parts of England, found, 

 on carefully pulverizing them in water, that what appear to the eye simply 

 as white grains were, in fact, well preserved fossils. He obtained above 

 a thousand of these from each pound weight of chalk, some being frag- 

 ments of minute bryozoa and corallines, others entire Foraminifera and 

 Cytheridge. The annexed drawings will give an idea of the beautiful 

 forms of many of these bodies. The figures a a represent their natural 

 size, but, minute as they seem, the smallest of them, such as a, fig. 24, 



CytheridcR and Foraminifera from the chalk. 

 Fig. 21. Fig. 22. Fig. 23. Fig. 24. 



Cythere, Mull. Portion of Cristellaria Rosalind. 



Cytheri?ia, Lam. Nodosaria. rotulata. 



are gigantic in comparison with the cases of Diatomaceae before men- 

 tioned. It has, moreover, been lately discovered that the chambers into 

 which these Foraminifera are divided are actually often filled with thou- 



