ALMANDITE AND ITS SIGNIFICANCE 663 



The change involves a gain in silica, lime, and magnesia and 

 a loss of ferrous iron and soda. In this case the change in each 

 constituent, with the exception of silica, is small. If complete 

 replacement had occurred, the difference ought to have been 

 infinity and, by analogy, rocks involving smaller differences in 

 composition ought to be regarded as due to recrystallization of the 

 minerals of the preceding zone with smaller changes brought about 

 by replacement and addition. 



Now since there has been an increase in lime and magnesia in 

 the biotite gneiss, it must have been supplied either by the next 

 zone nearer to the batholith or allowed to pass through that zone 

 in solution because it did not require so much magnesia and lime 

 to maintain a state of equilibrium. Consequently the granite 

 gneiss ought to show an impoverishment of these constituents and 

 the silica might also show a decrease, although that would not 

 necessarily follow because of the intense pegmatitization. Suppose 

 the composition changes of granite gneiss are examined. 



Biotite Gneiss = Granite Gneiss X — = Difference 

 173 



SiOj 929 993 = 1033 104 



AI2O3 180 173 180 o 



FczOj IS 12 12.5 — 2.5 



FeO 103 38 40 —65 



CaO 62 45 47 —IS 



MgO 90 39 41 -49 



NazO 34 62 6s 31 



K2O 12 92 96 84 



The granite gneiss is highly impoverished in lime and magnesia, 

 while the additions of silica have decreased somewhat over the 

 amount being introduced into the preceding zone. 



Throughout the calculations the alumina has been assumed to 

 remain constant in the original rock and the changes to occur by 

 replacement of the other components. Such condition may be 

 seriously questioned since the volume of the biotite gneisses has 

 been increased somewhat over 15 per cent by the direct injection of 

 pegmatite dykes. A corresponding increase might also be expected 

 to occur through simple soaking of the rocks by pegmatitic juices. 

 When the analyses, however, are recalculated on the assumption 

 of an increase of from 15 per cent to 20 per cent in alumina, the only 

 change is the establishment of equihbrium of the iron oxides; 



