80 Dr. G. A. Mantell on the Animalculites 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 mollusks, and the crustaceous cases of echino- 
derms, do not occur silicified m 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 phenomenon 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 ; m the 
other, a caudal vertebra of the Mososaurus from Brighton, the 
mineral has partially mvested the bone and permeated the cells, 
but the calcareous tissue remains unchanged. A coprolite of 
Macropoma, partially surrounded by flint, retains its caleareous 
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 effected 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 im such request at Brighton for brooches and 
other ornamental purposes, afford 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 im water at high tempera- 
tures under pressure whenever an alkali is present, (as is seen at 
* See American Journal of Science for January 1845. 
