On Rocks and Minerals from the Karakoram Himalayas. 477 



careous (effervescing with HC1), including minute dark grains. It 

 contains numerous crystals of dark mica, as much as a quarter of an 

 inch across, their outline being clearly defined and sometimes hex- 

 agonal. These commonly traverse the foliation planes at a high 

 angle, and are unusually thick. Thus the edges, which project from a 

 weathered silvery surface, have the form of oblong prisms, and some- 

 what resemble, as the colour varies from a very dark green to almost 

 black, crystals of hornblende. Under the microscope the ground 

 mass exhibits a foliated and slightly banded structure, and apparently 

 consists in part of small grains ; some seemingly calcite, while others 

 rather irregularly formed, partly free from enclosures, of a water- 

 clear mineral, giving somewhat low polarisation tints and with a 

 kind of zoned structure are probably secondary felspars, which may 

 retain traces of an original nucleus. The ground-mass contains also 

 mica with, perhaps, a little chlorite. The larger crystals of mica 

 exhibit a curious and interesting structure. They are generally a 

 light brown in colour, but they assume a greener tint near to the 

 outside. The former part is fairly dichroic, varying from a light, 

 slightly greenish- brown to a fairly rich warm brown ; but the 

 greener parts are paler and not dichroic. The crystals are usually 

 somewhat irregular in outline, but the cleavage is fairly good. In 

 parts of the slide the mica appears in numerous small patches, mixed 

 up with the ground-mass. These in some places seem to coalesce by 



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FIG. 2. A Grain of Secondary Mica in Schist from Dasskaram Needle. 



