Many other fossils besides wood--such as shells, corals, sponges, &c .-- are often found silicified; and this may be regarded as the commonest form of fossilisation by replacement.

In other cases, however, though the principle of the process is the same, the replacing substance may be iron pyrites, oxide of iron, sulphur, malachite, magnesite, talc, &c.; but it is rarely that the replacement with these minerals is so perfect as to preserve the more delicate details of internal structure..