ANNIYEESARY ADDRESS OP THE PRESIDENT. 65 



phenomena akin to those of tension or compression appear to have 

 been permanently impressed upon the mass, only in a vast number 

 of directions. This question also can best be considered in a separate 

 excursus. Suffice it to say that, in the present state of our know- 

 ledge, it appears to me convenient to admit the existence of an inter- 

 mediate division, to contain the rocks often called " cryptocrystalline" 

 with some of the " microcrystalline,"' whatever may be the ultimate 

 fate of the division. We will therefore assume that we may expect 

 to find an igneous rock in either a holocrystalline, a hemicrystalline, 

 or a hyaline condition, admitting that the middle term is perhaps 

 rather incorrect as a symbol of facts and temporary in its existence. 

 The relative size of the constituents in rocks, and their arrange- 

 ments — such structures in short as are designated by terms such as 

 porphyritic. spherulitic, fluidal — 1 shall at present regard as only 

 varietal. 



There is, however, one question of principle in nomenclature to 

 which I must briefly advert. It is whether we should indicate 

 distinctions of minor weight by conferring separate names, or by 

 the use of qualifying adjectives — that it is to say, whether in our 

 nomenclature we should direct the mind chiefly to specific or generic 

 differences. The former is the habit in mineralogy, and it is in 

 favour with some petrologists. While fully sensible of its advantage 

 in conciseness, I believe it open to grave objection. Some questions 

 of nomenclature, those, for instance, of priority, are of little value. 

 It is comparatively unimportant, subject only to considerations of 

 euphony and orthoepy, what we call a rock, so long as the name 

 expresses as definitive an idea as the circumstances of the case 

 permit ; but it is a matter of great moment whether our nomenclature 

 suggests or obscures relationships which exist in nature. Distinction 

 is, indeed, necessaryfor clearness of thought, and is requisite as a first 

 step in scientific comparison ; but it is by comparison and recognition 

 of relationships that all great advances in morphology and in philo- 

 sophic reasoning are likely to be made ; and it is, methinks, a matter 

 of no small importance to kee]? clearly in view, in any system of 

 nomenclature, the underlying affinities rather than the superficial 

 dissimilarities. 



Our nomenclature in petrology suffers in some parts, as I shall 

 presently indicate, from a plethora, in others from a defect of names. 

 A further • difficulty exists in that there are not a few names which 

 have been so vaguely and variously used, that it seems hopeless 

 ever to render their meaning fixed or precise ; these it would be 

 better, in my opinion, to leave as admittedly vague terms, indica- 

 tive of imperfect knowledge. So applied, they become valuable. 

 Petrology needs what I may call •' travellers' terms," to be employed 

 as a naturalist would use trivial or generic names, when he either 

 had failed in getting a good view of a creature or had not yet worked 

 out the characteristics of a specimen. In this sense such terms as 

 felstone, greenstone, even trap, are valuable. Aphanite, however, is 

 not so good as compact greenstone, unless we can agree upon a 

 perfectly definite meaning for it; and the same objection applies to 



