CLASSIFICATION OF IGNEOUS ROCKS 599 



In Class III it is also necessary to discriminate further 

 between pyroxene and olivine, corresponding to the Suborders of 

 Orders 1, 2 and 3 of Classes IV and V. This will be necessary 

 only in Grads I, 2 and 3 of Class III. They are : 



pyroxene predominant, prepyric. 



pyroxene and olivine equal, pyrolic 



olivine predominant preolic. 



In Class IV the subordinate minerals belong to the salic 

 group. Their division into Grads is of the same kind as that 

 forming Orders in Classes I, II and III, but since their amount 

 is considerably smaller than in rocks of these Classes, the divi- 

 sion is on a threefold basis, resulting in five Grads, as follows : 



Grad 1 : — > — , quartz predominant, prequaric. 



O 5 3 

 Grad 2 : -ij| < - > — , quartz and feldspar equal, quarfelic. 



_, . O or L 3 . , , , p ,. 



Grad 3 : r= — = — < - , feldspar predominant, prefelic. 



* 5 



L S 3 

 Grad 4: — < - > — , feldspar and lenads equal, lenfelic. 



F 3 5 



Grad 5 : — > - , lenads predominant, prelenic. 



* 3 



Subgrads are formed on the general chemical character of 

 the bases of the minerals employed to form Grads, and bear the 

 same relation to the latter that Rangs bear to Orders. On 

 account of the smaller amount of the minerals involved in 

 Classes II and IV it is not thought desirable to give the distinc- 

 tions as great a taxonomic value as when the preponderant 

 minerals are concerned, and so they are made the basis of Sub- 

 grads rather than of a new taxonomic division. 



In Class II the subordinate minerals being femic the chemi- 

 cal divisions are similar to those forming Ran^s in Classes IV 



