668 CROSS, IDDINGS, P1RSS0N, WASHINGTON 



magnesian minerals and the salic and femic minerals, and makes 

 evident the effect of the crystallization of the alferric minerals 

 in a given magma upon the proportions of the other minerals. 



We shall consider the problem of converting portions of salic 

 and femic minerals, already calculated from the analysis of a 

 rock, into alferric minerals. The minerals in question are : alumi- 

 nous pyroxenes, aluminous amphiboles, micas and garnets. Rarer 

 minerals of this kind will not be considered. As to the chemi- 

 cal composition of these minerals, it is known that the amounts 

 of alumina and ferric oxide in monoclinic pyroxenes, amphiboles, 

 and micas vary considerably in different cases, so that no simple 

 statement can be made regarding the ratios of alumina to other 

 constituents in these minerals. Moreover, there is no fixed rela- 

 tion between the amount of aluminous ferromagnesian minerals 

 actually crystallized in a rock and the chemical composition 

 of its magma. However, a study of the chemical composi- 

 tion of these minerals, so far as they have been analyzed, shows 

 that their composition bears some relation to that of the magma 

 from which they crystallized. In order to present this relation- 

 ship as clearly as possible, Tables XII, XIII, and XIV have been 

 arranged, as already mentioned, giving the chemical analyses of 

 aluminous pyroxenes, amphiboles, and micas, and those of the 

 rocks from which they were separated. It is evident from these 

 tables that there is need for much thorough chemical investiga- 

 tion of the rock-making pyroxenes, amphiboles, and micas before 

 we shall be in a position to cope successfully with the problem 

 before us. For the present we can explain the method of deal- 

 ing with the known factors in the problem and indicate that 

 which may be pursued in doubtful cases. 



The solution of the problem involves the transfer of alumina 

 from salic to femic molecules, the necessity of introducing it in 

 proportions corresponding to the known composition of these 

 minerals in each case, the consequent readjustment of molecules 

 among the femic minerals, and the disarrangement and readjust- 

 ment of molecules among the salic minerals. 



The process appears at first sight complex, but only involves 



