SYSTEMATIC PETROGRAPHY 493 



units and that classification consists in the grouping by more or 

 less artificial means of these fundamental units. Others have 

 sought to make the system of rocks in some degree natural by 

 applying geological factors of occurrence, or genesis, as bases of 

 classification. The view is apparently held by some that in time 

 there will be a comprehensive system expressing all important 

 relations of rocks and that until that result is achieved all 

 arrangements must be regarded as unsatisfactory and temporary. 



It appears to the writer that those who hold this attractive 

 and apparently philosophical view may not have in mind the 

 distinction between the formal unit and the rock substance of 

 that unit, or that distinction between the various cross- 

 classifications of petrology and the one system of petrography, 

 with which the nomenclature is specially connected. 1 The belief 

 expressed by Lossen that "geological relations must be recog- 

 nized as petrographical relations" and the assertion by Rosen- 

 busch that "petrography is an historical science" illustrate this 

 point. 



If the system of petrography is to be hierarchical, as the 

 writer believes it should be, the natural element in system is to 

 be provided for in the judicious selection of broad geological 

 factors so related to important characters of rocks that the com- 

 pleted system in the construction of which those characters have 

 been used, will have a logical and appropriate co-ordination and 

 sequence of parts. That this aim has not controlled in the past 

 is evident from the following partial list of designations given to 

 the rocks which are actually consolidation products of magmas : 

 "Composite, crystalline-granular, and porphyritic" (Zirkel) ; 

 " Non-clastic, composite, massive " (Zirkel) ; "Composite-simple " 

 (von Lasaulx) ; Unstratified, Anogene, Massive, Plutonic, Vol- 

 canic, Eruptive, Igneous. Here are expressed a number of nat- 

 ural relations, to be recognized in the proper place, but only 

 the last term refers directly to the relation most appropriate for 

 petrographic system. It was not the fact that eruptive force was 



'See "The geological versus the petrographical classification of rocks," by 

 Whitman Cross, Jour. Geol., Vol. VI, p. 79, 1898. 



