L. V. Pirsson — Classification of Igneous Rocks. 265 



Art. XIX. — The Classification of Igneous Rocks — A 

 Study for Students ; by L. V. Pirssoist. 



[Professor Pirsson was engaged during the last years 

 of his life in preparing a text book of petrography, based 

 on microscopical methods. This book was planned to 

 consist of 15 chapters and to cover the igneous, sedimen- 

 tary, and metamorphic rocks. Unfortunately, however, 

 but ^ve of the fifteen chapters had been completed at the 

 time of his death : the four chapters of Part I that com- 

 prise the necessary introductory matter to the volume 

 (methods of studying rocks, determination of rock min- 

 erals, rock minerals and textures, and classification of 

 rocks), and a large part of Chapter V, describing the 

 phanerocrystalline igneous rocks. 



The following paper consists of the larger part of 

 Chapter IV, with only minor verbal changes, and is pre- 

 sented here in the knowledge that Professor Pirsson 's 

 matured views on the classification of igneous rocks 

 will be of wide interest. The subtitle, ^'A Study for 

 Students," has been added in recognition of the obvious 

 purpose of the author to present the subject matter pri- 

 marily for the student. — Adolph Knopf.] 



Igneous Rocks. 



General Discussion. — There are probably few matters 

 in the domain of natural science in which there is less 

 general agreement tha-n in the classification of igneous 

 rocks. The reason for this is very simple, and it lies in 

 the fact that there are no natural absolute division lines 

 to be found among them, which are obvious to every one 

 and which compel unquestioning assent from all as a 

 basis for classification. Everywhere there are gradations 

 among igneous rocks, geologically in the form and mode 

 of occurrence of their masses, and petrographically in 

 the kinds of material composing these masses. Thus, 

 geologically, we divide them into intrusive and extrusive 

 rocks, and yet in one rock-body we may observe a dike 

 passing upward into a flow without break in the mass ; 

 indeed, every extrusive flow, unless separated by erosion, 

 must in principle pass into an intrusive mass. Again 



