Vol. 4] Knopf-Thelen. — Geology of Mineral King. 



257 



The knotted schist is a slate-colored rock, distinctly schistose, 

 and owes its knotted character to the accumulation of biotite 

 flakes on its cleavage surfaces. Perpendicular to the foliation 

 planes the knots form flat lentils less than 1 nun. thick, and 

 varying up to 1 cm. long. Under the microscope the very finely 

 granular character of the rock becomes apparent. A large 

 amount of minute biotite scales, of chocolate brown color, is uni- 

 formly distributed throughout the slide. Equally noteworthy 

 are numberless small stout tourmaline prisms of sharp idio- 

 morphic habit. In some crushed quartz aggregates white mica 

 has been developed. The bulk of the rock is quartz, but all evi- 

 dences of clastic origin have been obliterated. Some darker 

 colored areas of irregular elliptical shape are found, and seem 

 to owe their peculiar constitut ion to aggregations of overlapping 

 scales of micaceous material. 



The hornfels is an extremely tough and compact rock of 

 dense grain and dark slate blue color. The characteristic fea- 

 ture under the lens is the rather abundant development of anda- 

 lusite, usually in rude radial aggregates. Crystallographies out- 

 lines are rare, but the sections tend to assume a longitudinal 

 habit. They are colored brownish clue to included carbonaceous 

 material ; pleochroism is not appreciable. The absorption is 

 fairly strong and is a maximum in the direction of the length. 

 The main mass of the slide is finely granular quartz, containing 

 a large amount of pyrite and carbonaceous matter. This abund- 

 ance of included material masks the presence of the andalusite 

 in ordinary light, but between crossed nicols its brighter inter- 

 ference colors (yellows and blues of the 1st order) cause it to 

 stand out in conspicuous contrast. 



Some limited cases of thermal metamorphism of the volcanic 

 rocks were found, chiefly to the west of Timber Gap. At the 

 contact of the granite and the quartz porphyries a narrow zone 

 of hornfels, two to three inches wide, has been formed. It is 

 perfectly massive, extremely tough, and has a sort of vitreous 

 lustre. Under the microscope quartz is found to be the dominant 

 constituent. It is characterized by containing numerous cir- 

 cular, and even rectangular liquid inclusions. Orthoclase is 



