JAN. \\\ lit21 PARDEE, LARSEN, & STEIGER: BEMENTITE 27 



nocalcite, rhodonite, rhodochrosite, barite, and manganophyllite. At 

 the Apex mine a dense, hard, fine-textured, dark green to black ma- 

 terial composed chiefly of manganese oxides forms numerous irregu- 

 lar streaks and bunches that are inclosed by the bementite rock. 

 Under the microscope thin sections of this material show an irregular 

 frame-work or skeleton of transparent bementite imbedded in the 

 opaque oxides. The boundaries between the silicate and the oxides 

 are sharp and the two appear to be of contemporaneous origin. The 

 mineral species of the oxides has not been determined. 



A rather striking local feature of the bementite rock is its associa- 

 tion with native copper. At the Black and White mine this metal 

 is disseminated as fine specks and flakes through a considerable body 

 of the bementite rock to which it imparts a noticeable red color. 

 Similar, though less abundant, occurrences of copper were observed 

 at Ginnett's prospect and a few places elsewhere. Very small amounts 

 of chalcocite were also observed in specimens from the Black and 

 White and other deposits in that vicinity. 



Some specimens of the bementite rock are cut by thin veinlets of 

 dark brown to black amorphous neotocite, associated with calcite, 

 quartz and barite. 



The principal weathering product of the ordinary bementite rock 

 is a soft, dull black, amorphous manganese oxide. Weathered spec- 

 imens of the copper-bearing rock commonly show a little green copper 

 stain and specimens from the Black and White mine contain in addi- 

 tion bright red coatings composed of felted aggregates of fine prisms 

 of cuprite or chalcotrichite. 



The source of the manganese of the bementite rock or the form in 

 which it was first deposited is not known. Its distribution along beds 

 of limestone, however, suggests it to be of sedimentary origin, and 

 the character of the deposits and of the rocks that inclose them indi- 

 cates the bementite to be a product of regional metamorphism. 



The appearance of some thin sections makes it seem probable that 

 the bementite was derived from a granular mineral, possibly tephroite, 

 that was associated with more or less rhodonite. In some specimens 

 veinlets of fresh rhodonite cut the bementite, in others crystals of 

 rhodonite are embedded in the bementite, but these may represent 

 veinlet cuttings and crystals associated with the mineral from which 

 the bementite was derived and the rhodonite may have resisted de- 

 composition. However, none of the minute veinlets of bementite 



