NATURE OF ORGANIC REMAINS. 4] 
_ The various states in which silex occurs have depended 
on its fluidity ; in quartz crystals the solution appears to 
have been complete; in agate and chalcedony it was in a 
- gelatinous state, assuming a spheroidal or orbicular dispo- 
_ sition, according to the motion given to its particles. Its 
condition appears also to have been modified by the influence 
of organic matter. In some polished slices of siliceous 
nodules the transition from flint to agate, chalcedony, and 
erystallized quartz, is beautifully shown. ‘The curious fact, 
that the cavities of echinites in chalk are almost invariably 
filled with flint, while their crustaceous cases are changed 
into cale-spar, is probably, in many instances, to be attri- 
buted to the animal matter having undergone silicification ; 
for the soft gelatinous part sare those which appear to have 
been most susceptible of this transmutation. In some 
specimens, the oyster is changed into flint, while the shell is 
converted into crystallized carbonate of lime. In a Tri- 
gonia from Tisbury, formerly in the cabinet of the late 
Miss Bennett, of Norton House, near Warminster, the body 
of the mollusk was completely metamorphosed into a pure 
chalcedony, the branchize or gills being as clearly defined 
as when the animal was recent. In specimens of wood 
from Australia (presented to the British Museum by Sir 
Thomas Mitchell), which are thoroughly permeated by 
silex, there are on the external surface some spots of chal- 
cedony that have apparently originated from the exudation 
of the liquid silex from the interior in viscid globules filled 
- with air, which burst, and then collapsed, and became 
solidified in their present form. 
In silicified wood the permeation of the vegetable tissues 
by the mineral matter, appears to have been effected by 
solutions of silex of a high temperature. In some examples 
the mineralization is simply a replacement: the original 
substance has been removed atom by atom, and the silex 
substituted in its place. 
