L. V. Pirsson — Classification of Igneous Rocks. 271 



into three groups ; the first is identical with the abyssal 

 plutonic, in chemical and mineral composition, but differs 

 in texture, being distinguished as pronouncedly porphy- 

 ritic; it is called the granite-porphyritic group. The 

 second and third groups of dike-rocks are termed the 

 aplitic and lamprophyric groups respectively, and tliey 

 are formed on grounds of their chemical-genesis, a matter 

 discussed more fully in a succeeding section; it is suf- 

 ficient to say here that the aplitic group is felsic, the 

 lamprophyres mafic in character, and they are held to be 

 derived from the magmas which form the abyssal groups 

 by the splitting process called differentiation. The two 

 genetic groups are subdivided on mineralogical lines. 

 The system may be seen summarized in Table No. 2. 



Remarks. — In this table, No. 2, only a part of the many rocks 

 named by Rosenbusch are given; they are, however, the most 

 important ones defining the groups, and those omitted are mostly 

 of the value of subordinate varieties. Anorthosite for example 

 is treated as a variety under gabbro, though he is inclined to give 

 it a group distinction on magmatic grounds. The table, however^ 

 will serve to illustrate the chief features of the system, though it 

 is impossible in a set scheme of this kind to show the genetic rela- 

 tions of the aplitic and lamprophyric dike rocks to the plutonic 

 groups that they accompany, and also some similar relations 

 advanced for a small group of lavas that are held to be the 

 effusive representatives of the lamprophyres. 



Probably the most valuable feature of Rosenbusch's system is 

 that in it was first clearly expressed the recognition of the 

 genetic dependence of the so-called dike rocks upon the plutonic 

 types, and the mutual derivation of aplites and lamprophyres 

 from a single magma, though whether such relations furnish the 

 best basis for a systematic classification which shall define the dif- 

 ferent varieties of igneous rocks is open to question. Aside from 

 this point the chief defect of the system for petrographic pur- 

 poses is the use of geologic relations to other masses in making 

 definitions. The precise mode of occurrence of a rock-mass must 

 be known before its component substance can be determined. 

 This is of course a necessity in classifying occurrences geolog- 

 ically, but, as every one with any experience knows, it is often 

 impossible in the field to determine the mode of occurrence of a 

 rock-mass, and yet for description, mapping and other purposes 

 it is necessary to state the kind of rock composing it. In a prac- 

 tical way the petrographer is constantly confronted with the 

 problem of determining rocks of different kinds, for scientific and 

 economic purposes, of whose mode of occurrence he knows little 



Am. Jour. Sci. — Fifth Series, Vol. II, No. 11. — November, 1921. 

 19 



