THE SCOPE OF PALAHONTOLOGY. 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 (sz/ex). 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- 
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Fig. 2.— Microscopic section of the Fig. 3.—Microscopic section of the wood 
silicified wood of a Conifer (Seguota) cut of the common Larch (A dies darix), cut in 
in the long direction of the fibres. Post- the long direction of the fibres. In boththe 
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 
