66o 



GEORGE W. BAIN 



constituent in another. Relative changes only can be determined 

 from the composition, so that in the following interpretations the 

 direction of the changes is based upon microscopic study and the 

 quantitative relationships obtained from the chemical analyses. 



In the garnet zone the structure indicates that lime and magnesia 

 went into solution and were replaced by silica, alumina, and iron 



Fig. 4. — Garnet from garnet feldspar skarn rock. (Magnification about 30 

 diameters. Ordinary light.) The photograph shows the garnets traversed by two 

 sets of fractures, the one more prominent than the other. The clouded areas are 

 feldspar and the clear colorless areas quartz. The black opaque spots are magnetite. 



in the form of garnet and feldspar. Selective solution and precipi- 

 tation have doubtless played an important part, but the exact 

 amount of each cannot be determined in as close a manner as in the 

 succeeding zones. The high differences simply indicate almost 

 complete replacement as being the dominant process in this zone 

 as compared with dominant recrystallization in those following 



(Fig. 4)- 



Consider the change from garnet rock to biotite gneiss (Fig. 

 5, A and B). In this case the change is not so great; replacement 



