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.)