PETRALOGY. A TREATISE ON ROCKS. INTRODUCTION TO THE SECOND VOLUME. rocks. HAVING in the former volume comprised all the Domains Accidential 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 subdivisions themselves became, so to speak, accidential and arbi Pliny has nature accidentia; Cicero accidentia for res attribute. 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. Nome. Micronome. Terms sometimes lax. 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 chemistry, the small provinces or districts were distinguished by 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 memory, 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. Above all it must not be forgotten, that in no science, 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 |