CLA SSIFICA TION OF IGNEO US RO CKS 595 



tive amounts of soda and potash are needed. For the present 

 at . least when femic minerals are said to be alkalic, it maybe 

 assumed that they are presodic. The divisions in Sections I and 

 2 of Rangs I and II are : 



MgO 7 

 Subranar i : _ „ > - , permaarnesic. 



FeO i 



n, , M ^° 7 5, 



Subrang 2 : ^ _ < - > - , domagnesic. 

 FeO 13 ° 



« , M R° 5 3 



Subrang 3 : „ ^ < - > - , magnesiferrous. 

 FeO 3 5 



M°"0 3 1 

 Subrang 4 : „ ,^ < - > - , doferrous. 

 FeO 5 7 



Subrang 5 : ° < - perferrous. 



FeO 7 



In Sections 3 of Rangs I and II and in other Rangs of 

 Classes IV and V only three divisions are used, because of the 

 subordinate amount of miric component. These are : 



_ , MgO 5 



Subranar 1 : °^ > - , premaarnesic. 



& FeO 3 ' v & 



Ma:0 s 3 

 Subrana; 2 : .^' ^ < - > - , magnesi ferrous. 

 to FeO 3 5 



MgO x 

 Subrang 3 : < - , preferrous. 



teU 5 



Sectiom of Subra?igs. — In certain Subrangs a division into 

 Sections is necessary on account of the presence of various acid 

 radicals or for other reasons. 



In Classes I, II and III the salic mineral groups, quartz, 

 feldspar and lenads, may be considered as : (a) silica and 

 simple silicates, and (&) silicates combined with some other 

 salt, or silicates containing some other acid, as CI and S0 3 . 

 The latter are confined wholly to the lenads. This distinc- 

 tion is advisable here, rather than at Suborders, where it might 

 otherwise fall, because of the great change in the mineralogical 

 character of the lenad group induced by the presence of 

 very small amounts of CI and S0 3 (often less than I per cent. 



