52 GKEEN MOUNTAINS IN MASSACHUSETTS. 



still the appearance of the purely quartzose varieties. The feldspar occurs 

 in irregular grains fitting in between the quartz. It is partly not twinned 

 (orthoclase ?), part plagioclase, generally microline. Little crystals of rutile, 

 prisms of zircon and apatite, flakes of biotite and muscovite, masses of iron 

 hydrate, pyrite, etc., occur in nearly all the specimens. The quartz and 

 feldspars often show evidence of mechanical crushing, and part of the quartz 

 has been thus derived from larger grains. The constituents have a nearly 

 even grain. Although garnet is very rare, it is convenient to call this rock 

 the granulitic type of the quartzite. 



From this rock the transition is easy to the white gneiss proper. Sev- 

 eral varieties of this may be recognized; a banded one is common, the color 

 varies from gray, yellow to white; sometimes the banding is very fine or 

 the rock is speckled with biotite or muscovite, or both; sometimes the feld- 

 spar forms layers separated by layers of mica, or occurs in rounded or irreg- 

 ular masses. The proportions of the elements vary in every conceivable 

 way. 



A characteristic feature in the slide is seen in the round grains of feld- 

 spar of somewhat larger size than the average of the rock, inclosed in a 

 groundmass composed of little grains of quartz and feldspar in a most 

 intimate admixture, while plates of mica give the rock its banded struc- 

 ture. The larger sized feldspars are typically of a peculiar rounded shape, 

 occurring either in single crystals or in broad simple twins. They are com- 

 monly entirely without the polysynthetic twinning of plagioclase in polar- 

 ized light and might lie taken for orthoclase, but their isolation from the 

 powdered rock by the Thoulet solution shows by the specific gravity that 

 they must be generally a soda-lime feldspar near the albite end of the series, 

 although in some rocks they must contain considerable lime, judging by their 

 specific gravity. These feldspars are commonly filled with inclusions of 

 minerals found in the rock outside them — little prisms of epidote, flakes of 

 biotite and muscovite, and little rounded grains of quartz, sometimes 

 arranged like a necklace. These inclusions often lie in planes parallel to 

 the arrangement of the minerals outside the feldspar, and entirely inde- 

 pendent of crystallographic directions in the feldspar. (See PI. vn, a, 

 and PI. vm, a.) It is very rare to find any sign of mechanical deformation 

 in these feldspars. 



