662 



GEORGE W. BAIN 



doubtless plays an important part, but the dominant processes are 

 granulation and recrystallization with increase in rock volume as 

 shown by the change of garnet and sillimanite to chlorite (Fig. 6). 



Fig. 6. — Biotite gneiss. (Magnification about 30 diameters. Ordinary light.) 

 The photograph shows a great abundance of biotite (dark) and a complete absence of 

 garnet due to the completion of the alteration. A single elongated prism of sillimanite 

 occurs in the center of the field. Along the cleavages it is altered to chlorite (dark). 

 The rectangular outline at the end of the prism is a basal section of sillimanite altered 

 to chlorite. 



As before, the alumina of the original rock is assumed to remain 

 behind and additions and subtractions of the other constituents 

 to occur to produce the required changes. 



Garnet Rock = Biotite Gneiss X^ = Difference 

 180 



SiOz 880 929 =1235= 355 



AI2O3 239 180 239 o 



FczOi 18 15 20 2 



FeO 156 103 137 —19 



CaO 21 62 ■ 82 61 



MgO 60 90 120 60 



Na^O 47 34 45 -2 



K2O 16 12 16 o 



