288 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



others and less foliated. It has been both more thoroughly crushed and 

 more completely cemented by intiltrated quartz. In section it shows strik- 

 ingly the effect of this crushing and cementation. The quartz ground is 

 plainly clastic and like that of the other three specimens, but many second- 

 ary grains have grown into the interstices between the other grains, and the 

 biotite crystals have rubbed out into thin scales, twisted and raveled out 

 completely and, except in rare cases in the center of the larger fragments, 

 changed from brown to green. 



5. The quartzite east of the Connecticut, along Perchee Brook and at 

 Howe's mine, on the north line of Northfield A white, fine-grained, massive 

 quartzite, friable to compact, containing galena, barite, fluor, pyrite, and 

 dolomite in the abundant fissures. It resembles loaf sugar. 



Under the microscope this is made up of clastic grains of quartz, among 

 which the perfectly fresh and often idiomorphic feldspars, ji;st visible to 

 the eye as shining porphyritic spots, are plainly of secondary origin, being 

 limpid at center and outwardly crowded full of inclosed quartz grains like 

 those outside. These are sometimes single crystals, sometimes polysyn- 

 thetic, but usually formed of two individuals with observed extinctions 2°, 

 5°, and 16° on either side the suture in different individuals. Most of these 

 are carlsbad twins of orthoclase, and one section gave extinction of 21°, 

 being cut parallel to M, exactly as in fig. 216 of Rosenbusch.^ Many gi-ains 

 of magnetite occur. 



6. The middle outcrop of gneissoid quartzite, east of the north end of 

 the Purple blind road, Bernardston. A light-gray, fine-grained rock, almost 

 massive, but with faint parallel structure from films of biotite. It has wholly 

 the appearance of a fine-grained, gray gneiss; contains quartz, biotite, an 

 untwinned feldspar, muscovite, leucoxene. 



This shows under the microscope a completely granular, clastic mass 

 of minute, rounded quartz grains. In this the rectangular and elongate 

 sections of feldspar are plainly' of later gi'owth, inclosing often rounded 

 quartz grains in large numbers, especially toward the outer portion. These 

 feldspar sections are plainly visible with lens, are very fresh and clearly 

 idiomorphic, and dotted often with the contained quartz grains. 



The red biotite is also notched at the edges, from contact with quartz 

 grains, and incloses many of the latter. It is constantly associated with the 



'Mic. Pliys., 1892, under Orthoclase, p. 633. 



