CLA SSIFICA TION OF IGNE OUS RO CKS 9 1 



Resume. — Rocks are too complex in their characters and 

 have too many and too varied geological relationships to per- 

 mit of one systematic classification expressing all their proper- 

 ties and relationships. A primary division on geological grounds 

 may be carried through, producing classes of different characters, 

 and such a division is universally advocated. 



Since all the geological relationships cannot possibly be 

 used in one system, it appears that a distinction must be made 

 between that classification by which rocks are grouped for pur- 

 poses of description and naming as concrete objects, and all 

 other classifications. The former may be called the systematic 

 classification, and I consider petrography to be the science pre- 

 senting and applying that system to the description and naming 

 of rocks. The broader science of petrology, using the nomen- 

 clature of petrography for specific purposes, must arrange rocks 

 in as many other ways as are desirable to express their 

 characters or relationships not introduced into the systematic 

 scheme, and for the expression of these other arrangements a 

 separate terminology is essential. It must not be appropriated 

 under redefinition by the petrographer. 



The material properties of igneous rocks afford ample cri- 

 teria for establishing a systematic classification, and the use of 

 geological relations is unnecessary. Since the geological fac- 

 tors of age, or of form or place of occurrence are not directly 

 causes of the properties used in classification, they cannot be 

 applied to produce coordinate groups. The attempts to thus 

 apply them have been unfortunate. The justification of these 

 attempts has been the belief that geology demanded that geo- 

 logical relations be recognized as petrographical relations. In 

 the view above set forth this belief is illogical, and has resulted 

 in injury both to geology and systematic petrography. 



The impossibility of setting up an all-embracing natural 

 classification of igneous rocks is not due to ignorance. It comes 

 from the nature of the rock. The more we know the less shall 

 we be able to include all relations in one classification. 



Whitman Cross. 



