80 Dr. G. A. Mantell on the AnimalcuUtes of the 



into which the stream was injected. And there are innumerable 

 nodules of flint which exhibit no trace of spongeous structure ; 

 as well as veins, dikes, and sheets of tabular flint, that may be 

 regarded as pure, and free from organic remains, excepting such 

 as must necessarily have become entangled and imbedded in a 

 stream of mineral matter flowing over a sea-bottom. 



The shells of moUusks, and the crustaceous cases of echino- 

 derms, do not occur silicified in the white chalk, but their cavities 

 are very commonly filled with flint, and these casts are well 

 known as among the most common fossils of the ploughed lands 

 of chalk districts. The phosphate of lime, like the carbonate, 

 seems to have been unfavourable for the phsenomenon of silicifica- 

 tion. I have seen but two examples of bone imbedded in flint, 

 and in one of these the silex has merely incrusted the bone ; in the 

 other, a caudal vertebra of the Mososaurus from Brighton, the 

 mineral has partially invested the bone and permeated the cells, 

 but the calcareous tissue remains unchanged. A coprolite of 

 Macropoma, partially surrounded by flint, retains its calcareous 

 character ; and the teeth of fishes, although sometimes enveloped 

 in flint, are not silicified. I had teeth of the Hypsodon, and 

 Mr. Charlesworth has a portion of a jaw with teeth of the Moso- 

 saurus from the chalk, in which the pulp-cavities are filled with 

 flint, which must have permeated the parietes of the teeth, and 

 yet the calcigerous tubes remain unchanged, and are not filled 

 with silex; here probably the contents of the pulp-cavity in- 

 fluenced the pseudomorphism, as in the case of the oyster. 



But in other fossils the mineralization pervades the entire or- 

 ganism, and has been eff*ected by replacement. The original sub- 

 stance has been removed, and the silex substituted in its place ; 

 such is the common petrifaction of wood, and of most examples 

 of the softer zoophytes. The Choanites, which, from their per- 

 fect silicification, are in such request at Brighton for brooches and 

 other ornamental purposes, afibrd a good illustration of this 

 process. 



This complete transmutation of organic structures into flint, 

 quartz, or chalcedony, is very common in other divisions of the 

 chalk formation. In the well-known fossils of the Devonshire 

 whetstone, the shells are almost invariably converted into flint 

 or jasper. 



An able American mineralogist, Mr. Dana, suggests* that the 

 reason why silica is so common a material in the constitution of 

 fossilized wood and shells, as well as in pseudomorphic crystals, 

 consists in the ready solution of silex in water at high tempera- 

 tures under pressure whenever an alkali is present, (as is seen at 



* See American Journal of Science for January 1845. 



