674 CROSS, IDDINGS, P IRS SON, WASHINGTON 



The liberated FeO is to be added to sufficient (Mg, FeO) from 

 hypersthene or olivine to maintain the ratio 



(Mg,Fe)0 



CaO d ± CaO a 



D . 



When the sign of CaO an is — the transfer of (Mg,Fe)0 

 and the liberated FeO is to hypersthene or olivine from diop- 

 side. 



A readjustment of Si0 3 is necessary according as (Mg,Fe)0 

 is transferred from or to hypersthene or olivine, and as Na 3 is 

 transferred from albite or nephelite. If there is extra Si0 3 it is 

 placed first with the bases having strongest affinity for it unless 

 these are already satisfied, when it appears as quartz. 



In case the transfer is to be made to the complete exhaustion 

 of one of the limiting components, the algebraic formulae which 

 can be devised for a particular case can be solved, but it fre- 

 quently happens that augite occurs in rocks together with 

 hypersthene or olivine, lime-soda-feldspars, and magnetite. The 

 algebraic problem is indeterminable, since there are more 

 unknown quantities than equations to satisfy them, unless the 

 quantity of one or more of the minerals mentioned be given. 

 More or less aluminous pyroxene within the limits of the magma 

 appears to develop in different instances. The factors controlling 

 the amount of augite crystallized in such cases are not at present 

 known. The same statement may be made with reference to 

 other aluminous ferromagnesian minerals. 



For the calculation of aluminous amphiboles : a) Hor?ible?ides 

 contain more A1 3 3 than Na 2 0, and in the great majority of 

 cases occur in rocks not having acmite molecules among the 

 femic minerals. 



To transform diopside molecules into those of hornblende 

 sufficient Na 3 must be transferred from soda-feldspars, and 

 enough CaO from anorthite with their equivalent Al 3 O s to 

 satisfy the ratios 



^ =D and A1 2 Q 3 = p 



CaO d + CaO an anU CaO d + CaO an 



