442 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



dark greenish gray hornblende gneiss of basic character: The essential minerals 

 are idiomorphic green hornblende and allotriomorphic 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 crystallized magnetite are accessory in large amounts. The hornblende 

 prisms are often twinned parallel to (100). 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 batholithic 

 mass. It is believed to represent a new secondary rock formed by the recrystal- 

 lization of the materials leached out of the other two metamorphic 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 granulation of the minerals 

 exposed them to wholesale solution, whether in water and other fluids 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 work their way through the rock in its mechanical readjustments.* 

 Escape for the mineral-laden fluids (perhaps chiefly water freed from combina- 

 tion in biotite or from solid solution in hornblende) was most ready in the zones 

 of maximum shear. Thither the fluids were drawn, and there some of the 

 dissolved material recrystallized so as to develop the darker coloured bands of 

 biotite-epidote gneiss, biotite schist, and hornblende gneiss. 



Where the granulation was least the granodiorite retains nearly its original 

 composition, though epidote may be formed; the specific gravity averages 2 -730. 

 Where the granulation was more pronounced, as in the first metamorphic type 

 described, much of the hornblende, titanite, magnetite, and apatite have been 

 leached out and abundant metamorphic biotite and epidote have formed ; the result 

 is a biotite-hornblende-epidote gneiss with a density less than that of the original 



* This conclusion has in this instance been deduced from the study of thin-sections. 

 In general it accords with the results of experiment. Miiller has found that in car- 

 bonated water hornblende and apatite are much more soluble than either orthoclase 

 or oligoclase. Magnetite is less soluble than any of those minerals, but the relatively 

 minute size of its crystals in granodiorite would allow of its complete solution and 

 migration before the essential minerals had lost more than a fraction of their sub- 

 stance. It is also possible that magnetite would suffer especially rapid corrosive 

 attack from fluid in which the chlorine-bearing apatite has gone into solution. Cf. R. 

 Miiller in Tschermak's Miner, und Petrog. Mittheilungen, 1877, p. 25. 



