CLA SSIFICA TI ON OF IGNE OUS RO CKS 5 6 1 



should control the formation of a systematic classification of 

 igneous rocks, may approve of our method of applying them. 



We wish to acknowledge our obligations to all those whose 

 ideas and writings have influenced us consciously or uncon- 

 sciously; influences which will be evident in many places in the 

 succeeding pages. It is not always possible to credit a particular 

 conception to a particular author. The science of petrography 

 has developed so rapidly, and so many workers have been 

 engaged upon kindred problems, that similar ideas have forced 

 themselves upon investigators. Especially, in our own case, fre- 

 quent interchanges of thought have so blended and modified 

 our ideas that is difficult for any one of us to identify his own. 



In particular we acknowledge our obligations to those 

 masters of petrography from whom we caught inspiration for our 

 petrographical careers. To Professor Ferdinand Zirkel and to 

 Professor Heinrich Rosenbusch, our instructors, whose work and 

 thought have opened the way to a host of students of rocks, we 

 are indebted for much that has influenced us in shaping the 

 system proposed, which is in fact a natural outgrowth of present 

 petrographical conditions. 



Defects of present systems. — From the review of rock classifi- 

 cation in an earlier part of this volume by one of us, it appears 

 that all past and existing classifications of rocks have fundamental 

 weaknesses arising either from the use of theoretical concepts, or 

 from an inconsequent or illogical application of the characters 

 of igneous rocks as bases of their systematic arrangement. 

 That a new nomenclature is required for a new system can 

 scarcely be disputed. The present confusion in petrography is 

 in no small degree due to redefinition of old terms, and to this 

 confusion we have no desire to add. Without going into an 

 extensive discussion of the vitally weak points of existing systems 

 of petrography, upon which all of us have expressed individual 

 opinions, some of the most unsatisfactory features may be 

 briefly stated as follows : 



i. There are few definite, clearly enunciated and generally 

 applicable guiding principles, the consequence being that the 



