66 PEOCEEDrN'GS OE THE G-EOLOGICAL SOCIETY. 



melaphyre, which, however, would be useful if admittedly left vague. 

 Porphyry is admissible as a vague term, but objectionable in any 

 system of classification, because of its diversity of applications in the 

 past. Elvanite is a term wholly bad, because it suggests inaccurate 

 ideas, by affixing a Cornish miner's trivial name, used even in that 

 county in a very inclusive sense, to a rock which is world-wide in its 

 distribution. Plutonic and volcanic also, though useful for rough 

 divisional purposes and as general expressions, cannot for obvious 

 reasons be used for exact classification. I mention these as examples, 

 the list not being intended to be exhaustive. 



As already intimated, I should not regard the conspicuous presence 

 in a rock, whether crystalline or vitreous, of one or more minerals 

 in crystals larger than others as justifying the use of anything 

 more than an adjective ; hence in dropping the term porphyry from 

 our classification, I should continue to use the epithet porphyritic 

 (notwithstanding obvious etymological objections) ; neither should I 

 signalize the presence of macroscopic cavities, whether large or small, 

 except by an epithet, or by the employment of a trivial name for 

 general purposes of description. 



If the above premises be admitted, it is evident that our system of 

 classification must, so far as regards the igneous rocks (previously 

 limited by definition), rest upon chemistry*. To a very large extent 

 it will be also mineralogical ; but we must not say wholly minera- 

 logical, because that science will not enable us to determine the exact 

 position of a hyaline rock, and, if alone regarded, may induce us to 

 pay too much regard to minerals which can be shown to be the 

 result of secondary actions after the first consolidation of the rock. 

 Igneous rocks, we must remember, like any others, are- liable to 

 metamorphism ; and we should try, as far as possible, to separate in 

 our classification the latter from those which are comparatively 

 unchanged. Now it is well known that, cceteris paribus^ the more 

 basic a rock the more readily it assumes a holocrystalline condition. 

 Natural glasses are rare and limited in extent among the basic rocks, 

 are comparatively common and well developed among the more 

 acid, and probably are largely represented among ancient rocks, 

 which, strictly speaking, cannot now be called vitreous. 



Marked chemical and marked mineralogical differences should, 

 then, be recognized primarily in any system of nomenclature. On 

 the whole I am disposed to attach more weight to the former than 

 to the latter, because sometimes a marked mineralogical difi'erence, 

 for example the substitution of hornblende for augite, may be the 

 result of subsequent change, which, however, it may be difficult to 

 substantiate. Of course, when such changes can be proved to have 

 occurred, the rock should be removed from the category of normal 

 igneous, to that of metamorphic igneous. 



* In reality, of course, otir classification deals with the aggregate history of 

 the rock, its ontology as well as its morphology, if the phrases be permitted, 

 because our limitation iu using the epithet igneous, presumes an investigation 

 into its relations with other rocks, an investigation into its 'petrology as well as 

 into its lithology. 



