[Illustration: Fig. 1.--_Trigonia longa_, showing casts to of
the exterior and interior of the shell.--Cretaceous (Neocomian).]
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 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.
[Illustration: Fig. 2.--Microscopic section of the silicified
wood of a Conifer (_Sequoia_) cut in the long direction of the
fibres. Post-tertiary? Colorado. (Original.)]
[Illustration: Footnote: Fig. 3.--Microscopic section of the wood
of the common Larch (_Abies larix_), cut in the long direction of
the fibres.
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