CLASSIFICATION OF IGNEOUS ROCKS 83 



is very different. Those rocks which are surface accumulations 

 are so open to observation that we know many particulars of 

 their origin — the sources of materials, the agencies by which the 

 materials have been brought together, and the processes by 

 which the rock has been made out of them. It is a curious fact 

 that modern petrographers have done little toward formulating 

 an adequate and logical classification and nomenclature for the 

 rocks whose relationships are most evident, while they are con- 

 tinually extending, to an ever increasing degree of refinement, a 

 systematic classification of igneous rocks upon foundations of 

 theory or clear hypothesis. The fact that many criteria now 

 used for the classification of igneous rocks are highly theoretical 

 will hardly be questioned by anyone. From a philosophical 

 standpoint it seems to me evident that such criteria cannot pro- 

 duce a stable system and must in consequence be rejected. A 

 more detailed discussion of this question will follow. 



Much has been said in recent years about the legitimate 

 demands of geology upon systematic petrography. What are 

 these demands ? Clearly, both geology and petrography have 

 certain reasonable demands to make, the one upon the other, 

 but neither science has recognized its full natural rights, and 

 hence has failed to state them properly. It is the unquestion- 

 able right of the geologist to demand of the petrographer a 

 systematic classification of rocks, and a nomenclature expressing 

 it, which shall be as natural and as stable as the controlling fac- 

 tors will allow. The petrographer must claim equal interest in 

 such a system, and his logical counter demand is that he be 

 allowed to construct that system through the application of the 

 criteria best suited to produce the result desired. That is to say, 

 he must reserve the right to reject hypotheses, theories, and 

 even facts, if they are not adaptable. 



It has often been said, from the time of the earliest classifi- 

 cations to the present, that rocks must be classified to express 

 geological relationships. I believe that the geologist who today 

 advances such a general proposition as a demand of geology 

 upon systematic petrography is not in fact claiming his just 



