THE SCOPE OF PALEONTOLOGY. 13 



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 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 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 (silex). In such cases 

 we have fossil wood which presents the rings of growth and 

 fibrous structure of recent wood, and which under the micro- 

 scope exhibits the minutest vessels which characterise ligneous 

 tissue, together with the even more minute markings of the 

 vessels (fig. 2). The whole, however, instead of being com- 



Fiff 2. Microscopic section of the Fig. 3. Microscopic section of the wood 



silicified wood of a Conifer (Sequoia) cut of the common Larch (Abies larix), cut in 

 in the long direction of the fibres. Post- the long direction of the fibres. In both the 

 tertiary? Colorado. (Original.) fresh and the fossil wood (fig. 2) are seen 



the discs characteristic of coniferous wood. 



(Original.) 



posed of the original carbonaceous matter of the wood, is now 

 converted into flint. The only explanation that can be given 



