A Manual of Palæontology: For the Use of Students, With a General Introduction on the Principles of Palæontology (Classic Reprint) - Softcover

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Excerpt from A Manual of Palæontology: For the Use of Students, With a General Introduction on the Principles of Palæontology

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 ofshefl.

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.

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