CLA SSIFICA TION OF IGNE OUS RO CKS 8 I 



Its constitution — chemical, mineralogical, structural, and phys- 

 ical — must be studied and described. The differences or simi- 

 larities exhibited by rocks in these respects lead to classes, 

 groups, and lesser divisions, and the expression of these rela- 

 tions to a system of classification and a specific nomenclature. 

 (2) The genesis of rocks is a subject of many phases. The 

 source of materials, the agencies of transportation, the condi- 

 tions of rock formation, each of these problems must be investi- 

 gated in detail. (3) The geological occurrence embraces the 

 formal relationships of rock masses to the earth and to each 

 other. (4) The genetic interrelationship of rock types is one 

 of the most difficult questions to deal with. (5) The meta- 

 morphism, and (6) the decay or destruction of rocks, each 

 embraces a wide field. To these may be added other impor- 

 tant lines of study. It is thus evident that petrology embraces 

 several lines of research, each in some degree independent, each 

 also related to the others. The results may be primarily of 

 value as applied to the general science of geology — the history of 

 the earth, or to the uses of the systematic descriptive science — 

 petrography. 



There has long been much discussion as to the objects of 

 rock classification. It has been considered, on the one hand, as a 

 mere mechanism upon which to base a nomenclature, and at the 

 other extreme of view as a means chiefly for the expression of 

 geological relationships of rocks. Mr. A. W. Jackson has said 

 that nomenclature (meaning specific nomenclature) must be 

 wholly divorced from rock classification. But that arrangement 

 of rocks, in accordance with which they are described and their 

 specific names are applied, is in itself the most important of all 

 classifications, the systematic classification. The question is as 

 to the criteria to be applied to produce this system. Here there 

 must be general agreement with Mr. Jackson r in the proposition, 

 often enunciated before, that a uniform and stable nomenclature 

 must be based on facts and laws, not on theories and hypothe- 



1 On the General Principles of the Nomenclature of the Massive Crystalline Rocks, 

 by A. Wendell Jackson, Amer. Jour. Sci. (3), XXIV, 1882, p. 113. 



