532 CROSS, IDDINCS, PIRSSON, WASHINGTON 



seen, this can readily be done without transgressing the general 

 principles on which the system is based or affecting the homol- 

 ogous characters of the various divisions in different parts of 

 the scheme. 



The principles on which the divisions enumerated above are 

 made may be stated as follows : 



Class. — This, the broadest division, expresses the most gen- 

 eral chemico-mineralogical character of the magma, and is 

 therefore based on the relative proportions of the salic and femic 

 mineral groups as calculated in the standard mineral composition 

 of each magma. In Classes all the salic minerals calculated for 

 a magma are contrasted with all the femic minerals. 



Subclass. — This division is based on certain broad chemical 

 distinctions in the salic and femic groups, which make it 

 possible to divide each of them into two parts. These will be 

 explained in detail when these divisions are described at length 

 later. 



Classes and Subclasses exhibit the broadest and most general 

 characters of the magma, and are based only on the salic and 

 femic mineral groups, and the parts into which these may be 

 most broadly divided on certain chemical lines. More special 

 chemical characters of the salic and femic minerals in each class 

 are next to be distinguished, and since both cannot be indicated 

 at once, according to our principle of dealing with only two 

 factors at a time, we consider first one group of characters and 

 then the other. In accordance with the principle of giving 

 importance to constituents on the basis of their relative propor- 

 tions, the characters of the preponderant group, salic or femic, 

 will be considered first until they are fully described, and then 

 those of the subordinate group, femic or salic, will be taken up. 

 The divisions from Order to Subrang are based on the chemical 

 characters of the preponderant standard mineral group, while 

 the divisions from Grad to Subgrad will be based on the chem- 

 ical characters of the subordinate mineral group. 



Order. — The salic minerals being in the great majority of 

 cases silicates and quartz, and the silica being the most abundant 



