208 UNIVERSITY OF VIRGINIA PUBLICATIONS 



ite substance and is formed along the cleavage and fracture directions of 

 the mineral. Alteration of cyanite to muscovite is apparent in the hand 

 specimens. The quartz frequently shows optical disturbance and contains 

 inclusions of rutile and other substances of an indeterminate character. 

 Small granules of black oxide of iron, probably magnetite, are usually pres- 

 ent, and more or less red oxide of iron is present in all the thin sections. 

 Rutile, as euhedral and anhedral crystals and nonpleochroic, is formed along 

 the boundaries of the quartz grains and as inclusions in the quartz and 

 cyanite. 



The massive quartzite which occurs on the crest and higher slopes of the 

 mountain, has essentially the same composition as the quartzite schist. 

 It is compact in texture and light nearly white in color when fresh, but near 

 the surface it is friable easily crumbling into a loose sand under pressure 

 of the hand, and is a buff color, due to the presence of some iron oxide. It 

 is fine-grained in texture, composed largely of fine sugary quartz, some 

 muscovite, and in places contains abundant large and small tables of cyanite 

 of pale green color when fresh, crystals of blue lazulite, and small grains of 

 red rutile. Pyrophyllite is rather common. Cyanite, lazulite, and rutile 

 are usually intimately associated, the first two (cyanite and lazuhte) are 

 frequently embedded one in the other, and the rutile occurs as separate 

 grains and as inclusions in the other minerals. The weathered surface of 

 the rock is quite rough and, when examined in detail, the more resistant 

 minerals, lazulite and cyanite, stand out in moderate rehef. Alteration 

 of the columnar crystals of cyanite to muscovite is often pronounced. 

 Weathering has partially discolored the cyanite a brownish yellow from 

 free hydrous oxide of iron, also to some extent the lazulite, but the most 

 noticeable effects in the latter mineral are the dulling of luster and lighten- 

 ing of color. 



Professor Shepard* remarks that the rock becomes schistose near the 

 southern extremity of the formation, and contains in addition to the min- 

 erals mentioned minute crystals of pyrite, occasional drusy cavities nearly 

 filled with massive crystaUine barite enclosing perfectly formed minute 

 crystals of quartz, and traces of gold. He adds further that the formation 

 at this point has been worked to some extent for gold, and from its resem- 

 blance to the diamond gangue (itacolumite) of Brazil it is worthy of careful 

 examination for the occurrence of diamonds. A partial chemical analysis 

 of the rock collected on top of the mountain gave Si02, 69.74 per cent; 

 AI2O3, 24.86 per cent; and Fe203, 0.53 per cent. 



* Op. cit, 1859. 



