INTRODUCTION. 



Fig. i. — Trigonia longa^ showing casts 

 of the exterior and interior of the shell. 

 Neocomian. 



outside. In the case, in fact, of many Molluscan shells, the interior 

 cast is so unlike the exterior or unlike the shell itself, that it may be 

 difficult to determine the true origin of the former. 



It only remains to add that there is sometimes a further compli- 

 cation. If the rock be very porous and permeable by water, it may 



happen that the original shell is en- 

 tirely dissolved away, leaving the in- 

 terior cast or "mould" loose, like 

 the kernel of a nut, within the case 

 formed by the exterior cast. Or it 

 may happen that subsequent to the 

 attainment of this state of things, the 

 space thus left vacant between the 

 interior and exterior cast — the space, 

 that is, formerly occupied by the shell 

 itself — may be filled up by some 

 foreign mineral deposited there by 

 the infiltration of water. In this last case the splitting open of the 

 rock would reveal an interior cast, an exterior cast, and finally a 

 body which would have the exact form of the original shell, but 

 which would really be of much later origin and would not exhibit 

 under the microscope the minute structure of shell. 



In the third class of cases we have fossils which present with the 

 greatest accuracy the external form, and sometimes even the internal 

 minute structure, of the original organic body, but which, neverthe- 

 less, are not themselves truly organic, but have been formed by a 

 " replacement " of the particles of the primitive organism by some 

 mineral substance. The most beautiful example of this is afforded 

 by fossil wood which has been " silicified " or converted into flint. 

 In this case we have a piece of fossil wood, which presents the 

 rings of growth and fibrous structure of wood, and under the micro- 

 scope exhibits even the minutest vessels which characterise ligneous 

 tissue. The whole, however, instead of being composed of the 

 original carbonaceous matter of the wood, is now converted into 

 pure flint. The only explanation which can be given of this by no 

 means very rare phenomenon, is that the wood must have undergone 

 a slow process of decay in water holding silica or flint in solution. 

 As each particle of the wood was removed by decay, its place was 

 taken by a particle of flint deposited from the surrounding water, 

 till ultimately the entire wood was silicified. The replacing sub- 

 stance is by no means necessarily flint, but may be iron-pyrites, 

 oxide of iron, sulphur, malachite, magnesite, talc, &c. ; and it is not 

 uncommon to find many other fossils besides wood preserved in 

 this way, such as shells, corals, or sponges. 



The replacement of • the original substance of a fossil by some 



