STATE OF CONSERVATION. 
81 
of them, glutinous or gelatinous matter forms the base, and is more or 
less hardened by admixture with carbonate of lime. Soft corallines, 
echini, and the coverings of crustaceous animals, contain likewise some 
phosphate of lime, but generally in small quantity. Four principal 
states of preservation may be distinctly observed among these fossils. 
First, when the coralline or shell retains not only its external figure 
and appearance, but even its internal texture and almost all its original 
substance. Such specimens look as if obtained from the sea in a dead 
state, with no other loss than that of colour and brilliancy. This perfect 
state of preservation is well exemplified in the beautiful fossils which lie 
in the comparatively recent strata near London and Paris. The fossil 
shells of Speeton on the Yorkshire coast are very little altered except by 
the loss of their gelatinous matter, which causes them to be of a chalky 
or friable consistence. 
In the second condition of fossil shells and corallines, the figure and 
general appearance are little or not at all altered, but the composition is 
completely changed by the insinuation of extraneous matter : thus the 
calcareous substance of shells and corals, and the horny fibres of sponge, 
are become flint. In such cases the new substance appears to have been 
introduced gradually, so as to fill the pores of the perishing original 
body. The same explanation probably applies to the petrifaction of 
wood. 
The third condition is exemplified by those stony masses, frequently 
found in limestone quarries, which have the general figure of shells, but 
not their structure nor texture. These are casts or moulds in the cavities 
of shells, which have been dissolved and carried away from the places 
they once occupied in the rock. In consequence, the cavity left retains 
the exact impression of the outside of the shell, and encloses a stony mass 
which was moulded within it. The same explanation applies to the 
flint moulds in the cavities of echini, and to the * screw-stones’ which are 
casts in the central hollow of crinoidal columns. 
M 
