CLASSIFICATION OF IGNEOUS ROCKS 665 



other salic and femic components, according to the method 

 given for calculating the standard minerals. 



The determinations of Rang and Subrang as well as of Grad 

 and Subgrad follow the calculation of the chemical components. 



THE CALCULATION OF THE MODE FROM THE CHEMICAL ANALYSIS 



OF A ROCK. 



This is often desired in order to compare the two, and to 

 determine the amount of the chemical composition of some of 

 the component minerals, not otherwise determinable. 



But it is evident that the knowledge of the actual minerals 

 present in the rock cannot be learned directly from the chemical 

 analysis, since various mineral combinations may be formed 

 within certain limits. It is therefore necessary to determine the 

 presence of these minerals by a study of the rock, and this 

 involves microscopical investigation in nearly all cases. This 

 has generally meant the simple enumeration of the kinds of 

 minerals present, with the crudest statement of their relative 

 proportions. 



In some instances this would furnish data enough for the 

 solution of the problem, but it is clear that such cases must be 

 those in which all the minerals present are salic and femic, with- 

 out intermediate, alferric, kinds, or those cases in which the alfer- 

 ric minerals are developed to their limit, as for example, horn- 

 blende present to the exclusion of diopside, or to the exclusion 

 of hypersthene and olivine; biotite present to the exclusion of 

 hypersthene and olivine, or to the exclusion of potash-feld- 

 spathic minerals, or to the exhaustion of available alumina. 



Under such circumstances it would be possible to calculate 

 the composition and proportions of the actual minerals. 

 But even in these cases the variable chemical character of 

 amphiboles and micas renders the solution of the problem 

 untrustworthy unless the composition of the particular amphi- 

 bole or mica be known exactly or approximately. 



This involves the separation and analysis of one or both 

 of these minerals, or the reasonable assumption that they have 



