Prare VI.

A. Microphotograph of polished ore from No. 4
vein of the Rocher De Boule mine, magnified 200
diameters. Molybdenite replacing chalcopyrite and
hornblende. 1, hornblende; 2, chalcopyrite; 3,
pyrrhotite; 4, molybdenite. (Page 13.)

B. Microphotograph of ore from the Great Ohio
property, magnified 100 diameters. A peculiar
structure due to the replacement of marcasite by
siderite. The edges of the veins of siderite are against
fresh marcasite; away from the veins, in the interior
of the grains of marcasite, the alteration is very
marked, leaving only limonite and_ skeletons of
marcasite. The chief gangue mineral is hornblende.
1, gangue; 2, marcasite; 3, siderite. (Page 18.)