Stony matters deposited either in the way of incrustation, or within the cavities of organized substances, are called petrifactions. Calcareous earth being universally diffused, and capable of solution in water, either alone, or by the medium of carbonic acid or sulphuric acid, which are likewise very abundant, is deposited whenever the water or the acid becomes dissipated. Incrustations of limestone or of selenite, in the form of stalactites, or dropstones, are formed in this way, from the roofs of caverns, and in various other situations. Some remarkable observations relating to petrifactions are thus given by Kirwan: -
1. That those of shells are found on or near the surface of the earth; those of fish deeper, and those of wood deepest. Shells in specie are found in immense quantities at considerable depths.
2. That those organic substances that resist putrefaction most are frequently found petrified, such as shells and the harder species of woods: on the contrary, those that are aptest to putrefy are rarely found petrified, as fish, and the softer parts of animals, etc.
3. That they are most commonly found in strata of marl, chalk, limestone, or clay, seldom in sandstone, still more rarely in gypsum, but never in gneiss, granite, basaltes, or shorl; but they sometimes occur among pyrites, and ores of iron, copper, and silver, and almost always consist of that species of earth, stone, or other mineral that surrounds them, sometimes of silex, agate, or car-nelion.
4. That they are found in climates where their originals could not have existed.
5. That those found in slate or clay are compressed and flattened.