CLASSIFICATION OF IGNEOUS ROCKS 669 



simple algebra, and when applied to a concrete example is not 

 very intricate. Its value consists in familiarizing the student or 

 investigator with the interdependence of the various mineral 

 molecules in an igneous rock, and with the ranges of variation 

 possible within rocks of the same chemical composition. 



The minerals in question being characterized by a variable 

 content of alumina, it is of first importance to note the amount 

 of alumina present in different cases and to consider what trans- 

 fer of chemical elements from salic to femic molecules would be 

 necessary in order to produce such aluminous ferromagnesian 

 minerals without destroying the stoichiometric proportions in 

 the remaining salic and femic minerals. 



In the case of a rock in which the calculation of the standard 

 mineral composition showed the presence of an excess of A1 2 3 

 over that required to form salic minerals it is evident that A1 3 3 

 may be introduced into the ferromagnesian minerals by trans- 

 ferring it from this extra A1 2 3 . 



But in the great majority of rocks there is no excess of A1 3 3 

 in the sense here employed, and the production of aluminous 

 ferromagnesian minerals affects the standard feldspathic mole- 

 cules, so that the transfer of A1 3 3 necessitates the transfer of 

 those chemical bases united with it in equal proportions, namely, 

 calcium, sodium, and potassium. 



Each of these elements may enter into the composition of the 

 minerals to be developed, whose molecules are more complex 

 than those of femic minerals. A study of the analyses of the 

 minerals in question shows that sodium enters into aluminous 

 amphiboles independently of the acmite-riebeckite molecule 

 (Na 3 0. Fe 3 3 . 4Si0 3 ), and that potassium also does, but to so 

 small an extent in most instances that the error of estimating all 

 of the alkali molecules as Na 3 is negligible. Conversely, 

 potassium enters largely into the micas, and the sodium present is 

 so small that the alkali molecules in mica may be calculated as 

 though wholly K s O without notable error. 



Several chemico-mineralogical relations appear to control the 

 amount of alumina, lime, and alkalies that may be transferred 



