L. V. Pirsson — Classification of Igneous Rocks. 267 



In the other we consider the kinds of material that 

 make up such occurrences and the endeavor here is to 

 classify the rocks according to those characters that would 

 be inherent in representative masses selected from the 

 rock-bodies. These characters would refer to the chem- 

 ical composition of the mass, the kinds and relative pro- 

 portions of the minerals composing it, if it is crystalline, 

 and the sizes and arrangement of the mineral grains 

 (texture), the specific gravity, etc. This we may term 

 the petrographic way of viewing the rock. The geologic 

 way considers its outward relations to other rocks, 

 whereas the petrographic one regards its inward charac- 

 ters. The student will do well to bear these two view- 

 points clearly in mind.^ 



"We do not need to describe here the various systems of 

 classification that have been suggested and more or less 

 used in the past, but which are not now in vogue. Their 

 interest is now historical and they have been clearly 

 described and their merits and defects pointed out by 

 Cross. ^ We shall consider briefly only those which are 

 more or less in force to-day, and which consequently may 

 be held to be of influence on the science of petrography. 



Qualitative Classification — One of these is what may 

 be called a purely petrographical classification and its 

 chief exponent has, perhaps, been Zirkel.^ 



According to this the rocks are divided into classes 

 dependent upon the kinds of minerals, the absence or 

 presence and varieties of feldspars being given pre- 

 dominance, and then subdivided following the texture, 

 that is whether granular, dense, glassy, etc. One other 

 feature has been incorporated with this, especially in 

 Germany, which demands notice, and this is a further 

 subdivision of the fine, dense, and glassy rocks, which 

 are often porphyritic, into two groups depending on their 

 geologic age. Those that were older than- the Tertiary 

 received one set of names, whereas those that were of 

 Tertiary age or younger were named differently, though 



^ See Whitman Cross, Geological versus Petrographical Classification of 

 Igneous Kocks; Jour, of GeoL, Vol. 6, p. 79; 1898. 



^Whitman Cross, Development of systematic Petrography in the Nine- 

 teenth Century; Jour, of GeoL, Vol. 10, p. 331, 451; 1902. Also in Quanti- 

 tative Classification of Igneous Eoeks, by Cross, Iddings, Pirsson, and Wash- 

 ington; Chicago; 1903. 



^F. Zirkel, Lehrbuch der Petrographie; Vol. I, p. 829, 1893. . 



