L. V. Pirsson — Classification of Igneous Rocks. 275 



we may cite the fact that a granite stock is frequently cut by dikes 

 of the lamprophyre called, minette. Then the granite is called a 

 plutonic rock, whereas the minette is referred to the hypabyssal 

 group ! The truth is that most petrographers who do this are 

 using, unconsciously perhaps, two classifications ; they first deter- 

 mine in their mind by petrographic characters, that is by texture 

 and mineral composition, which is one sort of classification, what 

 the kind of rock is with which they are dealing, and then place it 

 in a geologic-genetic scheme. The bearing of this will be treated 

 in a succeeding section. 



Quantitative Classification. Several American petrog- 

 raphers, including- the author, proposed in 1902 an exact 

 classification of igneous rocks based on chemical com- 

 position, expressed, however, in terms of minerals of 

 definite composition, called standard minerals.^ ^ For this 

 purpose a chemical analysis of the rock is necessary, but 

 where this cannot be obtained an approximately correct 

 result may be achieved by measurement of the minerals 

 under the microscope, computing from this their relative 

 bulk and weight, and, their composition being known, 

 reckoning from this the chemical composition of the rock 

 as a whole. 



The chemical composition is then computed, according 

 to a set plan, into the relative amounts of standard 

 minerals. These standard minerals are divided into two 

 main groups : one characterized by the presence of 

 alumina and silica, such as the feldspars, nephelite, corun- 

 dum, and quartz, but without iron or magnesia ; the second 

 characterized by iron and magnesia, but without alumina, 

 such as olivine, diopside, hypersthene, aegirite, and iron 

 ores. The complex ferromagnesian minerals which con- 

 tain alumina, such as hornblendes, biotite, augite, etc., are 

 not treated as standard minerals because it is better to 

 consider them as compounds of simpler molecules of two 

 preceding groups. The first of these is called the salic 

 (Si and Al) the second the femic (Fe and Mg) groups of 

 standard minerals, and the composition of the rock 

 computed in quantities of them is called its norm, which 

 may thus, when hornblende or biotite are really present 

 in it in notable quantities, differ considerably from its 

 actual mineral composition or mode. 



"Cross, Iddings, Pirsson, and Washington; Quantitative Classification of 

 Igneous Eocks; Chicago Univ. Press, 1903, pp. 286. See also, Jour, of Geol. 

 Vol. 10, pp. 555-690, 1902. 



