Fig. 9.—Section of Trenton limestone, magnified, showing that it is composed of fragments of corals, crinoids, and shells. Montreal.
Fig. 10.—Diagram showing different state of fossilisation of a cell of a tabulate coral (Dawson’s Dawn of Life).
a Natural condition, wall calcite cell empty. b Wall calcite, cells filled with the same. c Walls calcite, cells filled with silica or a silicate. d Wall silicified, cells filled with calcite. e Wall silicified, cell filled with silica.
It thus happens that many fossils are infiltrated with mineral matter. Wood, for example, may have the cavities of its cells and vessels filled with silica or silicates, with sulphide or carbonate of iron, or with limestone, while the woody walls of the cells may remain either as coaly matter or charcoal. I have often seen the microscopic cells of fossil wood not only filled in this way, but presenting under a high power successive coats of deposit, like the banded structure of an agate.
In some cases not only are the pores filled with mineral matter, but the solid parts themselves have been replaced, and the whole mass has actually become stone, while still retaining its original structure. Thus silicified wood is often as hard and solid as agate, and under the microscope we see that the wood has entirely perished, and is represented by silica or flint, differing merely in colour from that which fills the cavities. In this case we may imagine the wood to have been acted on by water holding in solution silica, combined with soda or potash, in the manner of what is termed soluble glass. The wood, in decay, would be converted into carbon dioxide, and this as formed would seize on the potash or soda, leaving the silica in an insoluble state, to be deposited instead of the carbon. Thus each particle of the carbon of the wood, as removed by decay, would be replaced by a particle of silica, till the whole became stone. By similar chemical changes corals and shells are often represented by silica, or by pyrite, which has taken the place of the original calcareous matter; and still more remarkable changes sometimes occur, as when the siliceous spicules of sponges have been replaced by carbonate of lime. The organic matter present in the fossils greatly promotes these changes, by the substances produced in its decay, and thus it often happens that the shells, corals, etc., contained in limestone have been replaced by flint, while the inclosing limestone is unchanged. [Fig. 10] shows the various conditions which a coral may assume under these different modes of treatment.