That nature has some process, by which the above acid is separated from the earth of bones, and probably also from the earth of shells, is evident from the state in which the bones are found in the caves of Bayreuth. Those that are the most recent, and least petrified, contain most of the phosphoric acid. Where the petrifaction has proceeded far, that acid is not found. 172. Among many of the strata, such a fluidity has prevailed, as to enable some of the substances included in them to crystallize. Calcareous spar and siliceous crystals are often found in stratified rocks, forming veins of secretion, or lining close cavities, included on all sides by the uncrystallized rock. In the instances of gneiss, and many species of marble, almost the whole matter of the stratum is crystallized. This union of a stratified and crystallized structure in the same substance, has a great affinity to that union of the crystallized with the organic structure of shells and corals which has just been mentioned; and both are doubtless to be referred to the same cause. NOTE IX. § 81. _Petroleum, &c._ 173. According to the theory of coal laid down above, its two chief materials, charcoal and bitumen, being furnished by the vegetable and animal kingdoms, both of the land and of the sea, have formed with one another a new combination, by the action of subterraneous heat; but have also, in some cases, been separated by that same action, where the degree of compression necessary for their union, happened to be wanting. The carbonic part, when thus separated from the bituminous, forms an infusible coal, which burns without flame: the bituminous part, when separated from the carbonic, is found in the various states of naphtha, petroleum, asphaltes, and jet.