Graphite and pyrite. Magnetite-tremolite schist.] 

 Hornblende. Pyroxenyte. 



PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 731 



olite schist.] 

 e. 



No. 1452. GRAPHITE AND PYRITE. 



Same locality as No. 14."<1. 



Ref. Annual Report, xvi, pages 113, 126. 



Miy. A rough, apparently brecciated, rock composed largely of pyrite and a 

 black substance which is in part graphite. No section. 



Age. Archean (Keewatin). u. . o. 



No. 1453. MAGNET:TE-TREMOLITE SCHIST. 



Black River Falls, Wisconsin. Associated with magnetic ore of the York opening in the Tilden mound. 

 Ref. Annual Report, xvi, page 12<!. 



Mr;/. Magnetite mingled with short, gray fibres. 



,]//r. The amphibole is colorless, has positive elongation and ,, in the acute 

 angle of the axial plane. Its colorlessness and high double refraction with maxi- 

 mum extinction at 16 are the only known characters which necessitate the name 

 tremolite instead of actinolite. With a little calcite and much magnetite the rock is 

 rendered nearly black in hand sample. Three sections. 



Age. Taconic(?) N. H. w. 



No. 1454. HORNBLENDE. (Fibrous. ) 



Same place as the last. 



Ref. Annual Report, xvi, page 127. 



Mey. The strongly, long-fibrous mass is faintly green, and presents a fold across 

 the fibres. 



Mic. The maximum extinction angle is 22. The bisectrix (n p ) is in the acute 

 angle of the axial plane, and lies oblique to the fibration, the optic plane being 

 parallel with the fibration. These characters, with the faint pleochroism, make 

 this to be common hornblende. One section. 



Age. Taconic(?) N. H. w. 



No. 1456 (?) PYROXENYTE. 



Black River Falls, Wisconsin. Near the river, and near the granite. Just southwest from the so-called 

 Tilden mound. 



Ref. Annual Report, xvi, page 127. 



Meg. The " bedding " is about vertical, and curiously twisted and curled. 



Mic. The rock consists almost solely of pyroxene, having a few crystals of 

 x/>l/rne and a few accessory small grains of quartz. One section. 



Age. Archean (Keewatin?). 



Hemark. This rock differs so widely from the field description and from the 

 preserved hand sample (Sixteenth Annual Report, page 127), that it appears there is 

 some error in the notation. The preserved hand'sample is a fine, siliceous, jaspilitic 

 rock. N. H. w. 



