CLASSIFICATION OF IGNEOUS ROCKS 



591 



Order. 1+2 <^ 



Order 5 



P+0 1 



M 7 



femic non-silicate dominant. domitic, 



femic non-silicate extreme. permitic. 



Sections of Orders. — The division of Orders into Sections is 

 needed in Classes IV and V, to express the relative proportions 

 of the subgroups of the preponderant mineral groups, that is, 

 the proportions between the pyroxene and olivine subgroups. 



In Orders 1, 2, and 3 of Classes IV and V, where the polic 

 minerals are extreme, or dominate over, or are equal to the 

 mitic, this division is carried out on a fivefold basis and results 

 as follows : 



Section 1 : 

 Section 2 : 

 Section 3 : 

 Section 4 : 

 Section 5 : 



O 1 ' 



P 7 S 



O 1 3 



P 5 3 



O 3 5 



P 3 1 



O 5 7 



P 1 



pyroxene extreme. perpyric, 



pyroxene dominant. dopyric. 



pyroxene and olivine equal, pyrolic. 



olivine dominant. domolic. 



olivine extreme. 



perolic. 



Suborders. — In Orders 4 and 5 of Classes IV and V the 

 non-silicate, mitic, minerals preponderate. The subgroups of 

 these minerals, in greatest abundance and most characteristic of 

 rocks belonging to these Orders, contain Fe 2 O s and TiO g . The 

 first subgroup includes magnetite and hematite, and is indicated 

 by the symbol H. The second subgroup includes ilmenite, 

 titanite, perofskite and rutile, and is indicated by T. For these 

 subgroups we suggest the syllables hem and til, mnemonic of the 

 minerals composing then. The relative proportions of these 

 minerals are recognized in Suborders on a fivefold basis, as 

 follows : 



Suborder 1 



H 



T 



> 



perhemic. 



