4 INTRODUCTION. 



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 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 exferior 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 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 sometimes the in- 

 ternal 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 ex- 

 ample of this is afforded by fossil wood which has been " silici- 

 fied " 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 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 substance is by no means necessarily 

 flint, but may be iron-pyrites, oxide of iron, sulphur, &c. ; and it 

 is not uncommon to find many other fossils besides wood pre- 

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



DEFINITION OF ROCK. 



The crust of the earth consists of various different materials, 

 produced at different successive periods, occupying certain 

 definite spaces, and not confusedly mixed together, but, on the 

 contrary, exhibiting a definite and discoverable order of arrange- 



