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



Each of these classifications has distinct aims of its 

 own, which can only be properly expressed by separate 

 treatment. The aims of each are legitimate and entitled 

 to full recognition ; the difficnlty has been that in mixing 

 them together and insisting on one compound classifica- 

 tion there has naturally resulted wide divergence of views 

 as to what the aims of the classification really are. "We 

 may find an analogy for these classifications in the strati- 

 fied rocks; in one system they are arranged according 

 to their geologic age ; and we speak of them as Cambrian, 

 Jurassic, etc. ; in another, according to the nature of the 

 materials composing them, and we have limestones, sand- 

 stones, etc. ; again we divide them into series, stages, 

 formations, etc. 



The geologic classification, it is presumed, is already 

 familiar to the student, and need not be further consid- 

 ered here. Petrologic classifications, with the meaning 

 mentioned above, have not yet been definitely and clearly 

 stated. The nearest approach to one is given by Rosen- 

 busch, but mingled, as has been shown, with the petro- 

 graphic system. Brogger has stated principles that 

 must be essential in their formation and has offered an 

 example with the rocks of the Christiania region, some- 

 what complicated with their geologic occurrence. Such 

 a classification really belongs in the field of theoretic 

 petrology, and should be given in a work treating of that 

 subject. 



This book is devoted to descriptive petrography, and 

 obviously the rocks should be treated in it according to 

 a petrographic system; until the student has mastered 

 this he is not in a position to comprehend fully a petro- 

 logic one. For his benefit a preliminary attempt at a 

 petrologic system is appended to that part of the book 

 dealing with the igneous rocks. 



Petrographic System of this Book. The petrographic 

 system to be employed should be based on the inherent 

 and fundamental properties that express the rock. The 

 most important one is the chemical composition, but the 

 ones that have generally been employed are the minerals 

 and the texture, since these are the more obvious and 

 readily determined. Moreover the chemical composi- 

 tion can to some extent be recognized, provided attention 

 is given to the quantitative relations of the minerals. In 



