THE ORIGIN AND DESCENT OF ROCKS. 455 



alumina together with iron and magnesia. The second group is called femic. 



Aluminous ferromagnesian minerals, such as hornblende, augite, mica, etc.j 

 are called al ferric, and are not classed as standard minerals, because their com- 

 plexity of composition makes it better to treat them as though made up of the 

 simpler minerals of the standard list. 



The composition of all igneous rocks can be expressed in terms of the relative 

 proportions of the two groups of the standard minerals, salic and femic. By 

 subdividing these groups successively on a mineral and chemical basis, a series of 

 classificatory divisions of greater and greater precision has been formed. In 

 each stage of the series, two factors only are compared, and a simple set of ratios 

 has been selected to limit the divisions. Assuming the possiblity of a continuous 

 range of variable mixtures of the two factors (A and B) from an extreme com- 

 posed wholly of one (A), and an extreme composed wholly of the other (B), five 

 ideal cases have been chosen as types or centerpoints about which variation in 

 mixture may take place. These are: 



4=1 4 = ?- 4 = 1 4 = 1 4 = 2. 



BO' B 1' B~l' B~S' B~l' 

 Division lines half-way between these points occur as the following ratios: 



,,^ A 7 ^^^ A 7 5 ^^A 5 S ,,, A 3 1 ,,, A 1 



(1) ;g>p (2) ^<^>^, (3) -<->-, (4) -<->-, (5) -g<-. 



These ratios are used throughout the system. In (1) A is extreme; in (5) B is 

 extreme; in (2) A dominates over B ; in (4) B dominates over A ; in (3) A and B 

 are equal or nearly equal. 



All igneous rocks are grouped in five (5) primary divisions called Classes on 

 a basis of the proportions of the salic and femic minerals, thus: 



Class I. „ " >—, extremely rich in salic minerals, called persalane. 



II. <T->7r, with dominant salic minerals, called dosalane. 



Fem 13 ' 



III. :^ ^^^^' ^^^^^ ^^^ femic minerals, equal or nearly equal, called 



salfemane. 



Sal 3 1 . 



IV. :f^ — < — >—, with dominant femic minerals, called dofemane. 

 Fem 5 7 ' ' 



V. — <— , extremely rich in femic minerals, called perfemane. 



Each of these classes is divided into two subclasses according to the proportions 

 of two subgroups of the preponderant group of standard minerals. Of salic 

 minerals one subgroup includes quartz, feldspars, and the f eldspathoids ; the 

 other includes corundum and zircon. Of femic minerals one subgroup includes 

 the silicates with magnetite, ilmenite, hematite, and rutile; the other contains 



