90 WHITMAN CROSS 



of the genetic relationships of types which has come within the 

 past few years largely as a result of a promulgation of the theo- 

 retical ideas lying back of the systematic scheme advocated by 

 Rosenbusch. One may well deny the desirability of the Dike 

 rock group of Rosenbusch and be at the same time an ardent 

 advocate of the theories upon which the group was established, 

 and which have little connection with the fact of geological occur- 

 rence expressed in the name. Classifications of rocks, express- 

 ing working hypotheses as to their genesis, are necessary and 

 may be set up at will if disconnected with the systematic classi- 

 fication. A stable nomenclature for rocks as objects will facili- 

 tate rather than hinder development of theoretical science. 



I believe, then, that geological occurrence is not a practi- 

 cable criterion for systematic classification of igneous rocks, and 

 that it has been applied to that purpose through a misunder- 

 standing of the true position of systematic petrography to the 

 broader science of rocks. The petrographer has for many years 

 failed to perceive, or, at least, to acknowledge, the right of the 

 geologist to any special nomenclature of rocks. He has taken the 

 time-honored terms of the geologist, redefined them to suit his 

 own special purposes, until the geologist who is not a petrog- 

 rapher is almost afraid to use the simplest and most plainly 

 denotive terms lest he be denounced as unscientific. But I 

 maintain that it is the petrographer who has been unscientific, 

 when he has misappropriated the natural nomenclature of the 

 geologist, and when he has defined structural terms to express a 

 genetic theory, or has applied them to certain rocks only out of 

 all those possessing the structures in question. The definition 

 of the granular structure as one having a certain mode of origin, 

 instead of stating what the structure really is ; the appropriation 

 of gramdite and granite for certain granular rocks, leaving no 

 appropriate name for all rocks of that structure ; the misuse 

 oi porphyry in analogous ways, — these are illustrations of thor- 

 oughly undesirable precision of definition, undesirable because 

 at the expense of the geologist who has a prior and logical claim 

 to these terms and needs them in their original senses. 



