86 WHITMAN CROSS 



ally connected with the properties of the objects which are to 

 be used in the further elaboration of the system. The geological 

 agencies involved in the formation of the rock may be applied 

 to produce rock classes differing in important material characters. 

 This ground of classification has often been used, though not 

 always logically carried out. It produces divisions both stable 

 and natural. More or less distinctly the criterion of geological 

 agency has been applied to form the classes called respectively 

 the sedimentary, igneous and metamorphic rocks. Modern 

 petrography has scarcely modified the old geological classification 

 of sedimentary rocks, it has not yet anything which can be con- 

 sidered a system for metamorphic rocks, but it has elaborated a 

 detailed scheme for igneous rocks, and it is now desired to review 

 this system on the basis of the principles already presented. 



Geological age has been commonly used as a criterion for 

 the first subdivision of igneous rocks. It was originally applied 

 in the belief that the older rocks differed in certain inherent and 

 essential properties from younger ones. It was assumed that cer- 

 tain material characters were in some unexplained way governed 

 by this geological factor, which thus became of prime classifica- 

 tory value. But nearly all petrographers now perceive that assump- 

 tion to have been unwarranted, and few would advocate a divi- 

 sion of igneous rocks by age were it not for the double nomen- 

 clature in existence. It is difficult to agree upon the details of 

 the simplification in this respect which all realize must eventually 

 be effected. 



It is in regard to the importance and applicability of geolog- 

 ical form or place of occurrence and association of types as 

 criteria for systematic classification that the greatest differences 

 of opinion may be found among petrographers of today. The 

 former of these factors, form or place of occurrence, has been 

 and is still applied to the classification of igneous rocks on the 

 ground that it is determinative of certain characters of rocks, 

 and especially of structure, to a degree demanding recognition 

 in this way. This usage is best represented by the well-known 

 system of Prof. Rosenbusch by which massive or eruptive rocks 



