CLA SSIFJCA TION OF IGNEO US RO CKS 689 



or more factors at once. We have been through this phase of 

 belief most, thoroughly, and have convinced ourselves that two 

 factors are all that can be handled at once without complications 

 which would destroy the practical usefulness of any system. 

 We believe that anyone who tries it will come to the same con- 

 clusion. 



Furthermore, repeated trials have led us to think that a five- 

 fold division is the most practicable one and the one best suited 

 for the needs of petrography. We have applied this logically 

 throughout, except in certain cases where a threefold seemed 

 sufficient. 



We desire also to ask the reader to bear in mind that the 

 scheme of classification and nomenclature herein proposed is of 

 much greater importance in some features than in others. The 

 important points to bear in mind, and on which our system rests, 

 are these : 



a) The bringing together of rocks of similar chemical composi- 

 tion into the same divisions. 



£) The resolution of a rock, either from its chemical or 

 microscopical analysis, into quantitative amounts of certain 

 standard minerals. 



c) The establishment of a strictly quantitative basis of com- 

 parison of rock constituents, so that all constituents are valued 

 according to their proportions. 



d) The division of the minerals into two main groups, salic 

 and femic, thus obtaining two factors, and the application of the 

 fivefold division. 



e) The recognition of the relation of the mode to the norm. 

 Time, use, and experience, if this system gains the currency 



we hope for it, may cause modifications in the smaller divisions ; 

 and the suggestions we have made in regard to nomenclature, 

 both of rocks and of descriptive terms for their textures, may 

 only in part be followed. But these are minor features and 

 their importance is relatively small compared with the main 

 points enumerated above. It is upon them that the system rests, 

 and upon their adoption or rejection that it must either stand or 

 fall. 



