METAMORPHISM OF THE GRANODIORITE 345 



A second metamorphic type is a yet more highly schistose biotite-epi- 

 clote gneiss often transitional into biotite schist. The essential constit- 

 uents are biotite, epidote, orthoclase, andesine, and quartz. The accesso- 

 ries include very rarely apatite and magnetite, while titanite seems to have 

 entirely disappeared along with the hornblende. Orthoclase seems here 

 to be more abundant than plagioclase. The quartz and feldspars are in- 

 tensely granulated and, with polarized light, are full of strain shadows. 

 The rock is more richly charged with biotite than the hornblende-bearing 

 gneiss. 



The third metamorphic type occurs in immediate association with the 

 gneiss just described, being interbanded with it. It is a fine grained, 

 strongly schistose, dark greenish gray hornblende gneiss of basic character. 

 The essential minerals are idiomorphic green hornblende and allotrio- 

 morphic feldspars in mosaic with considerable interstitial quartz ; the last 

 is hardly more than accessory. The feldspar is mostly unstriated and 

 not easy of determination. Orthoclase seems to be dominant, but, as 

 shown by extinctions on (010), approaches soda-orthoclase in composition. 

 The plagioclase is possibly andesine. Titanite, apatite, and well crys- 

 tallized magnetite are accessory in large amounts. The hornblende prisms 

 are often twinned parallel to (010). That crystallographic plane now 

 lies characteristically parallel to the plane of schistosity. Except for the 

 soda content of the orthoclase, the minerals all appear to have the same 

 characters as in the granodiorite. 



This third phase occurs in zones of maximum shearing in the batho- 

 lithic mass. It is believed to represent a new secondary rock formed by 

 the recrystallization of the materials leached out of the other two meta- 

 morphic phases just noted and out of the granodiorite as it was crushed. 

 The recrystallization either accompanied or followed the very closing- 

 stage of the orogenic crushing. This fact is demonstrated by the entire 

 absence of granulation or even undulatory extinctions in the mineral 

 components. 



The probable history of the metamorphism may now be summarized. 

 After the complete solidification of the original granodiorite, very intense 

 crushing stresses affected the whole body. The straining and granula- 

 tion of the minerals exposed them to wholesale solution, whether in water 

 and other fluids mechanically inclosed in the rock or in fluids of exotic 

 origin. This process of solution was hastened by the rise of temperature 

 incident to violent crushing. All the minerals must have been affected, 

 but it appears that the hornblende, biotite, magnetite, apatite, and titanite 

 were most likely to be dissolved and so migrate with the fluids that slowly 



XXXII— Bull. Geol. Soc. Am., Vol. 17, 1905 



