L. V. Pirsson — Classification of Igneous Rocks. 273 



the deep-seated stocks and batholiths, and thus at a higher 

 level form an intermediate group with the surface lavas. 

 These differences in the occurrences produce correspond- 

 ing variations in the physical conditions attending the 

 solidification of a magma, and this shows itself in the 

 textures. Thus a suitable magma forms a plutonic 

 granite, a hypabyssal granite porphyry, and a rhyolite 

 lava. This group includes the dike rocks of Rosenbusch, 

 but is obviously of greater scope. The division of these 

 geologic groups into families (granite, syenite, etc.), 

 although based in theory on chemical composition, in 

 practice is mostly performed by considering the minerals, 

 thus Brogger in the effort to solve the problem presented 

 by rocks containing approximately equal quantities of 

 orthoclase and plagioclase, mentioned in the foregoing 

 section, proposed the recognition of a new family inter- 

 mediate between granite and quartz diorite, when the 

 rock contains quartz, and between syenite and diorite 

 when quartz is absent, and of equal value with them in 

 classification.^ To this family he gives the name of 

 monzonite, previously used by a few geologists to desig- 

 nate a rock of this character occurring at Monzoni in the 

 Tyrol. This proposal has been generally accepted and 

 the name widely used, especially by the geologists of the 

 U. S. Geological Survey. 



Brogger urges even more strongly than Rosenbusch 

 the principle of the common genesis of groups of rocks 

 from particular magmas distinguished by their chemical 

 properties, as a means of classification. Thus he would 

 not classify all aplites by themselves in one group and 

 all lamprophyres in another group, but would assign to 

 each plutonic mass, representing a magma (or family) 

 the aplites and lamprophyres to which it has given rise 

 by differentiative splitting. The mutual relation that 

 the aplites and lamprophyres bear to another he desig- 

 nates under the term of "complementary rocks.' '^ It 

 will be seen from what has been stated that Brogger 

 classifies the igneous rocks, partly on geological, partly 

 on genetic, and partly on petrographic characters, the 

 genetic predominating. He has applied his method very 



« Triadische Eruptionsf olge bei Predazzo, Erupt. Gest. Kristiania-gebietes ; 

 Videnskab. Skrift. M. N. KL. 1895; p. 21. 

 ^ Basic Eruptive Eocks of Gran; Quar. Jour. Geol. Soc. Vol. 50, p. 15, 1894. 



