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502 



GEOLOGY OF THE BLACK HH^LS. 



sei)arated from the .siirrounfling mass When the nicols are crossed it 

 j)ohirizes faintly, so that tlie feldspatliic portions of the rock appear toler- 

 ably bright and white, while the groundmass remains dark; the distinction, 

 however, being made with some difficulty. On the other hand, the quartz 

 strikes the eye at once with its brilliantly colored and sharply defined grains. 

 It is scattered quite abundantly through the rock, but rather in groups and 

 irregular streaks or veins, the larger particles seldom being seen isolated. 

 It contains as inclusions some microlites and small cavities. The biotite 

 occurs quite abundantly in very ragged and irregular crystals of a green- 

 ish color; the prismatic sections are dichroitic, while the basal ones are 

 browner in color. It is too much decomposed to show any laminations 

 and it is difficult to distinguish from hornblende, except by reason of the 

 different position of the optic axes of the two minerals. Magnetite in 

 small granules is quite uniformly distributed through the rock, and by its 

 appearance on the light groundmass aids in giving the rock the character- 

 istic rhyolitic appearance. There are also some brown films of hydrated 

 oxide of iron. The groundmass is crypto-crystalline, but does not possess 

 any microlitic structure. 



The rhyolite [135] from the same locality has a similar appearance 

 to the preceding, being of a compact, uniform texture, very fine-grained, 

 and of a light-gray color with a few macroscopical black crystals of biotite 

 and quartz masses. The slide under the microscope also resembles that of 

 [134], but is a little finer grained in structure and has a few large and 

 unmistakable biotite sections. 



As before, the sanidin crystals appear as faintly outlined light spots 

 on a dark ground, but there is one crystal quite transparent and sharply 

 defined. The quartz grains are abundant and give bright colors — blue, red, 

 and yellow — with polarized light, containing also microlites and black parti- 

 cles. Most of the biotite is present in the ragged, decomposed crystals as 

 in [134]; one crystal, however, of a light-brown color is still quite trans- 

 parent and fresh. It is very strongly dichroitic, changing with one niool 

 to a very dark-brown, almost black. It has also a border of magnetite 

 grains of good size completeh^ around it, which makes it a very conspicu- 

 ous object in the almost colorless groundmass, A similar one is shown in 



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