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II. Basaltin and Basalt, or Basalton

III. Basaltin, with Porphyry

IV. Basaltin and Wacken

V. Wacken and Clay

VI. Jasper and Keralite

VII. Slate and Chlorite Slate

VIII. Felsite and Basaltin

IX. Granite and Basalt

X. Granite, with Gneiss

XI. Granite and Granitic Porphyry

XII. Gneiss and Mica Slate

XIII. Steatite and Asbestos

XIV. Shale and Coal

Various

ib.

169

170

172

ib.

173

ib.

175

ib.

176

189

ib.

191

192

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II. Vesicular Lava

III. Indurated Mud

IV. Tufo

Tufo ......

V. Pumice

VI. Obsidian

VII. Volcanic Intrite

VIII. Volcanic Glutenite

IX. Substances ejected or changed

General Remarks, and Examples of singular

Volcanoes

Fumavols ...

Veinstones

Appendix

313

328

373

378

428

443

469

503

515

......

519

545

561

391

PETRALOGY.

A TREATISE ON ROCKS.

INTRODUCTION

TO THE SECOND VOLUME.

HAVING in the former volume comprised all the Domains Accidential

rocks. which may be called Substantial, as depending upon the predominant substance, under various modes of combination, it is now necessary to enter on another field, that of the Accidential Rocks, which must of course be arranged according to their various accidences *. These accidences being, so to speak, infinitely diversified, and independent of any Mode in the sense used in the former volume, and often even of Structures and Aspects, it was necessary to adopt new denomi- New terms. nations. Even the Domains now become what might be called Dominions in the natural kingdom, as they no longer imply the preponderant or predominant substance, but grand divisions arising from natural accidences, as the Volcanic and Decomposed Rocks.

But while the term Domain still seemed unobjectionable, it became necessary to abandon the other subdivisions, which being derived from the substances, and their qualities, could have no place here. Instead of denominations strictly arising from the very essence of the subject discussed, the subdivi. sions themselves became, so to speak, accidential and arbi

Pliny has naturæ accidentia ; Cicero accidentia for res attributæ. Accidence is here used in contradistinction to accident, which, in common English, implies a moral event or incident, not an accidental circumstance in nature. Accidence is here a natural casualty, an adventitious attribute.

VOL. 11.

trary. The only idea that arose was to select terms that might indicate subdivisions of the Domains, and still, if possible, preserve some relation with chemistry, upon which the whole science of mineralogy ultimately depends. In Egypt, universally known to have been the parent country of che

mistry, the small provinces or districts were distinguished by Nome.

an appellation which the Greeks have translated Nomes, from a word simply implying divisions. But the word may be said to have remained sacred to Egypt, not having been transferred to the provinces of any other country. This

word had also the advantage of subdivisions easy to the meHyponome mory, in Hyponome and Micronome, implying greater and

.
lesser subdivisions of the Nome.

Such were the reasons for the preference of this arbitrary term to any other arbitrary term; and as it cannot be too often repeated that the chief use of any system of natural history is to assist the memory, it will perhaps be difficult to find a term less objectionable ; at least, though the plan has been deeply reconsidered for many years, none such has arisen to the author: but perhaps candid disquisition, and literary collision, may produce some more appropriate appellation, which he would be the first to adopt, having no view but the advancement of the science. Even in lithology and metallogy, Nomes will be found preferable to the Groups or Families of the Wernerians, denominations chiefly belonging to animated nature; and the clear metallic divisions of Thomson, Alloys, Sulphurets, Oxyds, and Salts, may well be styled Nomes; for the term being arbitrary there can be no objection to its occasional introduction even under Domains

which are substantial. Terms some- Above all it must not be forgotten, that in no science, times lax.

except those that are mathematical, can the terms admit mathematical precision. In the other kingdoms of natural history it is well known that disputes frequently arise whether a new object form a genus, a species, or a variety. How much more vague, therefore, must be the language of mineralogy, which depends on the infinite modifications of

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