FOSSILISATION. 5 



It only, remains to add that there is sometimes a further 

 complication. If the rock be very porous and permeable 

 by water, it may happen that the original shell is entirely 

 dissolved away, leaving the interior cast loose, like the ker- 

 nel 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 inte- 

 rior and exterior cast the space, that is, formerly occupied 

 by the shell itself may be filled up by some foreign min- 

 eral 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 be really a much later formation, and which 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 even some- 

 times the internal minute structure, of the original organic 

 body, but which, nevertheless, 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 elegant 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 which under the 

 microscope exhibits even the minutest vessels which char- 

 acterise 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 explana- 

 tion 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 surround- 

 ing water, till ultimately the entire wood was silicified. The 

 replacing substance 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 



