558 CROSS, ID DINGS, PIRSSON, WASHINGTON 



of attendant conditions upon the union of the various chemical 

 constituents of the magma, it is necessary to select a certain 

 set of salic and femic minerals as uniform standards of compari- 

 son. These standard minerals are, for the most part, species 

 commonly formed, but the aluminous pyroxenes, amphiboles, 

 and micas are necessarily excluded. In practice, the molecular 

 composition of a rock, obtained from its chemical analysis, is 

 computed by a regular method into amounts of these standard 

 minerals, and the place of the rock in the system is then easily 

 determined. 



The sta?idard mineral composition of a rock is called its norm, 

 and this may be quite different from its actual mineral composition, 

 or mode. Methods for obtaining the latter and for indicating 

 its relation to the former are set forth in detail in the Part III, 

 on Calculations. 



On the relative proportions of these two groups of standard 

 minerals, the rocks are first divided into five Classes, accordingly 

 as one or the other of these two groups alone constitutes the 

 norm, or is extremely abundant ; whether one or the other is dom- 

 inant ; or whether the two are in about equal proportions. The 

 Classes thus formed are divided into Orders on the relative pro- 

 portions of the minerals forming the predominant group in each 

 case, and in the middle group on the relative proportions of the 

 salic minerals. Thus in the preponderantly salic Classes the 

 Orders are based on the relative amounts of quartz, feldspars, 

 and feldspathoids. 



The Orders are divided into Rangs on the chemical character 

 of the bases in the minerals of the preponderant group in each 

 case ; thus, if these were feldspathic, as to whether they are alkalic, 

 alkalicalcic, or calcic. The lowest division or Grad obtains 

 only in the three intermediate Classes, and results from the con- 

 sideration of the relative amounts of the minerals composing the 

 subordinate group. 



In addition to these divisions further ones are provided for 

 where necessary by Subclasses, Suborders, Subrangs, and Sub- 

 grads. 



