1 84 EDSON S. BASTIN 



hornblende crystals is the result; on the weathered faces the large 

 hornblendes, some of them 2^ inches across, stand out so as to give 

 a knotty or warty appearance. The rock is exceedingly tough 

 and resistant under the hammer. 



The megascopic appearance is very similar to that described 

 by Wilhams for the type locality. The hand-specimen shows a 

 dark-green to almost black rock, with hornblendes ranging from 

 ^ to 2 J inches in diameter; when held at the proper angle, these 

 reflect the light as units, but show numerous dull, irregular inclusions 

 from -gJg^ to ^ of an inch in diameter, which under the microscope 

 prove to be olivine or its alteration product, serpentine, with an occa- 

 sional crystal of pyroxene. Light-brown biotite is the only other 

 mineral recognizable megascopically; it occurs between the horn- 

 blendes and is quite abundant. 



Under the microscope the rock is seen to be only slightly altered. 

 The hornblende is mainly massive, but is occasionally fibrous about 

 the edges of the grains, where some alteration has taken place. 

 The color ranges from light green to light brown; the pleochroism 

 is noticeable, but not very strong, a = hght yellow, b = hght green, 

 r = light green to light brown. Inclusions of magnetite are abundant 

 in the hornblende and vary greatly in size. The majority are quite 

 irregular in outhne, but show a tendency toward elongation in the 

 direction of the hornblende cleavage and toward the possession 

 of straight boundaries in this direction. In some of the horn- 

 blende crystals inclusions of magnetite and also of pyrite assume 

 minute needlelike or platelike forms, usually elongate parallel to 

 the cleavage planes, but in some cases occurring in as many as 

 three sets, inclined at widely divergent angles. This latter type 

 of inclusions gives to the mineral, when viewed in reflected light, 

 the pecuHar metallic sheen which has been described by Judd and 

 others, and has been termed '' schillerization." 



Olivine ranks next to hornblende in abundance and occurs in 

 grains of more or less rounded outline, usually inclosed by the horn- 

 blende; they vary considerably in size. The characteristic irregular 

 fracturing is present, and a few grains show slight serpentinization 

 along the cracks. The serpentine is usually hght-colored and 

 appears gray between crossed nicols; occasionally the yellov^dsh- 



