Part 30

We now proceed to the more compound genera.

Salts with Earths and Inflammables.

§ 257.

This genus can hardly ever occur but in countries formerly expoſed to ſubterranean fires.

Salts with Earths and Metals.

§ 258.

To be expected amongſt volcanic productions.

Salts with Inflammables and Metals.

§259.

To be ſought for in the productions of volcanoes.

Earths with Inflammables and Metals.

§ 260.

Obvious amongſt the productions of volcanoes, otherwiſe extremely rare.

Salts with Earths, Inflammables, and Metals.

§ 261.

Hardly to be expected but in volcanic mountains.

APPENDIX THE SECOND.

§ 262.

Foſſils externally reſembling animals or vegetables, originate from foreign matters, which by ſome peculiar proceſs are changed in the boſom of the earth, or are ſo impregnated by mineral particles gradually occupying the place of thoſe which have putrified, that they no longer reſemble organic ſubſtances, except in figure.—Theſe are commonly called PETREFACTIONS.

§ 263.

The harder ſhells of animals expoſed to the weather, are not always exempt from deſtruction; for their gelatinous matter being gradually deſtroyed by putrefaction, they become brittle, and in a manner calcined. In leſs expoſed ſituations, ſome of them preſerve the nature of their materials, but acquire a ſpar-like texture.

§ 264.

We muſt carefully diſtinguiſh betwixt the foreign bodies themſelves, changed or petrified, and their impreſſions upon the ſurrounding matrices. Sometimes the body is entirely deſtroyed, forming a cavity in the ſurrounding matter, and this cavity afterwards is filled with other materials. Nuclei, or kernels, are likewiſe found, formed within the cavities of the harder ſhells, and bearing the form of their internal ſurface.

§ 265.

I am far from thinking the knowledge of petrefactions is barren and uſeleſs. We may, and ought, to conſider them as medals depoſited by the hand of nature, in memory of the more remarkable changes on the ſurface of the earth, and from which the time and order of the work may, in ſome meaſure, be judged of, whilſt other monuments are ſilent. Theſe, being properly interpreted, ſhew us their native ſituations in the former ſtate of the ſurface of the earth, and teach us the unbounded empire of the ſea, and the conſequent changes. By them we learn to diſtinguiſh the ancient and modern foundations of the mineral kingdom; for thoſe which are not formed of petrefactions, and never contain them, are doubtleſs of greater antiquity than animals or vegetables; and, laſtly, by their figure they ſhew us the inhabitants of our globe, eſpecially thoſe of the greateſt depths of the ocean.

§ 266.

Mr. CRONSTEDT has admirably arranged the petrefactions; we think it right, therefore, to retain his method. The Genera are built upon the Genera of foſſils, and arranged like the four claſſes thereof; the ſpecies upon their ſpecies, and the varieties upon the organic ſubſtances that have been changed. The following are the Genera hitherto diſcovered.

Saline Calcareous Earth with an organic Form.