80 Washington— Pyroxenite and Hornblendite in Brazil. 



what foliated parallel to the contact with the gneiss. Cutting 

 this outer pyroxenite are many veins of white chalcedony, and 

 here and there are "bunches" of what appears to be a somewhat 

 altered biotite, a light brownish jefferisite or vermiculite. 

 There are found near the borders some quartz veins, stained 

 green by copper. A few pits have been sunk near the contact, 

 where the pyroxenite is somewhat altered and impregnated 

 with chrysocolla and malachite. It may be mentioned that as 

 a copper mine the locality is quite worthless, the amount of 

 ore being very small, and the lack of water and fuel and 

 difficulties of transportation being prohibitive of economic 



working. 



Hornblendite. 



This rock forms the upper part of the ridge and apparently 

 the interior part of the mass. It is coal black, glistening 

 and highly granular, the grains of hornblende being from 1 to 

 3 mm. in diameter. These grains are somewhat incoherent so 

 that the rock crumbles more or less under the hammer and in 

 some specimens can even be rubbed down to a coarse granular 

 powder under the fingers. The grains are quite anhedral with 

 well-developed cleavage and are apparently perfectly fresh. 

 No other mineral but hornblende is to be seen megascopically. 



In thin section the rock is seen to be noncrystalline and 

 composed almost entirely of anhedral grains of hornblende with 

 sporadic small grains of olivine and magnetite. The texture 

 is typically granitic and there is not the slightest evidence of 

 any of the structures produced by metamorphism or crustal 

 movement. The hornblende is anhedral, the grains butting 

 against each other and not interlocking. Prismatic cleavage is 

 well developed. The color is olive green, with distinct but not 

 very strong pleochroism ; a — light greenish yellow, ft = olive 

 green and 7 = olive or slightly bluish green. The absorption 

 as usual is 7>/3>a. The extinction angle is about 11°, but 

 this as well as the refractive indices and other optical 

 characters will be investigated later. The hornblende is 

 absolutely fresh and its only inclusions are the magnetite grains 

 and very rarely small olivines. 



The olivine forms small anhedra generally in the interstices 

 between the hornblende grains and less often included in the 

 mineral. It is perfectly colorless and quite fresh except for a 

 slight incipient iddingsitization on the borders. It rarely 

 carries small inclusions of magnetite. The magnetite is in 

 small anhedral grains, usually elongated and with rounded 

 outlines. It is always present as inclusions, for the most part 

 in hornblende, but less often in olivine. It seems to be quite 

 fresh but is very rarely accompanied by a brownish staining. 



