STATE OF CONSERVATION. 109 



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 is 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. 



