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176 Gargano. The fourth is bituminous, and is met with in many places, particularly in those which abound with Ichthyolitic fossils. Quartz or Firestone (Piromaco) is so frequently found in limestone of this series, that it may be considered characteristic of it. Sometimes it appears in veins at the junction of the thicker calcareous strata, but more frequently it is imbedded in them, assuming various figures, among which the spherical is remarkably perfect. Astonishment at the perfection of the globular figure of these Rognoni di Piromaco has occasionally given rise to the foolish supposition that they are areolites. The manner in which this variety of quartz is found in the limestone clearly proves it to have had the same origin as the rock which contains it; and this is corroborated by the frequent cases in which it partly or entirely takes the place of the carbonate of lime, of which the fossils enclosed in the same rock were originally formed. In such cases it is worthy of note, that the other variety of crystallized quartz is occasionally united to the Piromaco. With our present chemical knowledge, we can easily understand that silex, as well as carbonate of lime, may have been held in solution in the water by which they were deposited. But we cannot, with the same facility, account for the silex being precipitated in the state of Piromaco, or as crystallized quartz, whilst we know that silex is naturally deposited by some mineral springs, or artificially by solution in concentrated waters, with the qualities proper to Gieserite, or hydrate of silex. Perhaps arguing from the frequency with which fossils are found converted into Piromaco, we may attribute its presence to organic substances, as Becquerel observed that organic substances in a state of putrefaction, deposit crystals of pyrites in a solution of sulphate of iron.

Carbonate of magnesia, in widely-varying proportions, is always united to carbonated Apennine limestone. The large quantity which is occasionally found, we think, must be attributed to the facility with which, in some places more than others, this rock undergoes great changes, caused by the continual influence of