In both the fresh and the fossil wood (fig. 2) are
seen the discs characteristic of coniferous wood. (Original.)]
In the third class of cases we have fossils which present with
the greatest accuracy the external form, and even sometimes 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 microscope 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 composed of the original carbonaceous matter of
the wood, is now converted into flint. The only explanation that
can be given of this by no means rare phenomenon, is that the
wood must have undergone a slow process of decay in water charged
with silica or flint in solution. As each successive particle of
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.
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