170 



W. H. HOBBS — METAMORPIIISM OF BERKSHIRE SCHISTS. 



seen. This may be largely peripheral, as in a specimen (number 3230) * 

 from near Ore Hill, or it may include nearly the entire crystal, as in a 

 number of thin sections (number 3224) from Miles mountain. Such 

 pronounced granulation seems, however, to be most developed in the 

 vicinity of shear planes, and is accompanied by a stretching and tearing 

 of the other constituents. Figure 1, A shows the effect of this action on 

 two adjacent garnets (number 3230). 



Granophyre Structure m Feldspar. — Those structures which 1 desire here 

 more especially to emphasize, however, seem to be found farther removed 

 from shear planes, and concern phases of metamorphism where mechan- 

 ical movement has been a minimum. In such localities the feldspars 

 have frequently a mottled appearance, like D in figure 1 (numbers 3111, 

 3326, 3328, 3463). 



__s 



<o=> 



C- 



Figure 1.— Examples of Deformation and modified Growths of Minerals in Schists. 



A = stretched garnets. B = secondary growth of tourmaline ; the oval core has brown tones, 

 the enlargement blue or plum tones; the cloud} 7 material near the junction is probably graphite. 

 C= zonal structure in tourmaline. D = mottled feldspar. E = parallel growth of ilmenite and 

 chlorite. 



The included areas sometimes take the form of curving canals, at 

 others polygonal outlines, and, in short, exhibit all the peculiarities of 

 the micropegmatite or granophyre structure. Hexagonal outlines char- 

 acterize many of the areas, and there can be little doubt that they are in 

 these cases quartz (number 3463). 



The inclosed quartz extinguishes alike over considerable areas, but 

 sometimes shows several orientations within a single crystal of feldspar .f 

 The feldspars which show this structure exhibit in many cases the min- 

 imum of crushing and but little effect of stress, while in other cases the 

 granophyre structure coexists with a pronounced granulation and exhi- 

 bition of secondary twinning. In the latter ease the structure is very 

 complicated, and it is difficult to distinguish the secondary quartz from 

 the mosaic of feldspar, it was frequently observed (in those cases where 

 but little deformation could be made out) that the granophyre occupies 

 the center of a crystal, leaving a clear rim. 



*The numbers of sections are those of the Uniti'.l States Geological Survey collection. 

 •f-Cf. Iddings, Obsidian Cliff, Yellowstone National Turk . Seventh Ann. Rep. V. 8. God Sur., 1888, 

 p. 275, plate xv, fig. 5. 



