L. V. Pirsson — Classification of Igneous Rocks. 269 



principles by grouping some of the more important rocks 

 into a table, No. 1, whick will show its practical appli- 

 cation. It should be noted that the coarser granular, 

 non-porphyritic rocks are not divided according to geo- 

 logic age. 



Remark s.^Aside from the question of age, which is now prac- 

 tically obsolete, it should be observed that the system is defective 

 in that little or no account is taken of the relative quantities of 

 the component minerals that may be present in a rock. Thus 

 syenite may contain some pyroxene, but a rock composed mostly 

 of pyroxene with a very little alkalic feldspar, although chemi- 

 cally it may differ widely from the former, is equally classed as a 

 syenite. The chemical composition of a rock is, however, its most 

 fundamental property. The defect runs through the whole 

 scheme and it is for this reason spoken of as the qualitative 

 system. 



It is to be noted also that the system is arbitrary; thus in 

 texture, whether a rock shall be classed as granular, porphyritic, 

 or dense is purely a matter of opinion in many cases. Also what 

 is meant by ' ' predominant" may be very often an open question. 

 It is customary to class rocks as syenites when they contain a little 

 quartz, but not if they contain a little nephelite ; they then pass 

 into the next division. Such examples might be greatly multi- 

 plied, but these are sufficient to indicate that the division lines are 

 largely artificial, or arbitrary. The valuable feature of the sys- 

 tem is, from the petrographic point of view, that it is based on 

 the inherent, inward properties of the rocks, and not upon out- 

 ward geologic relations. 



Geologic-Petro graphic Classification. — In this system 

 of classification, which has been given a wide circulation 

 and usage by the writings of Rosenbusch,^ the effort is 

 made to combine the geologic outward relations with the 

 inward petrographic characters of the rocks. They are 

 divided into abyssal (deeply-formed intrusive or plu- 

 tonic) rocks (Tiefengesteine), and the volcanic or extru- 

 sive lavas (Ergussgesteine), as the chief groups, with 

 a smaller subordinate group of dike rocks (Gangges- 

 teine), with intermediate characters. The effusive 

 rocks, or lavas, were again divided according to their 

 geologic age on the same basis as in the system previously 

 mentioned, but in the last edition of Rosenbusch's work 



^ Mikro-physiograpMe der massigen Gesteine 1st, 2d, 3d, and 4tli Eds. 

 1877-1908. Elemente der Gesteinslehre, 1st, 2d, and 3d Eds. 1898-1910. 



