\ aM Ne

358 V. DOLMAGE.

disseminated with increasing distance from the centers until no ore minerals can be seen without the aid of a microscope. In the leaner parts of the ore shoots, chalcopyrite becomes evident and with the bornite shows a tendency to occupy small rounded cavi- ties about the size of the phenocrysts of the unmineralized por- tions of the bed. The boundaries between the sulphides and epi-

Fic. 48. Section of ore bed of Excelsior Claim.

dote are in all cases extremely ragged. No sharply bounded veins are found in the ore.

The study of the polished sections reveals chalcopyrite, bornite, chalcocite, zinc blende, galena, covellite, hematite, magnetite, limonite and ilmenite.

Bornite constitutes about 75 per cent. of the sulphides present. It forms in places solid streaks up to 2 inches in width extending along the shoots for many feet, and is also plentifully distributed in the disseminated ore.

Considerable chalcocite is intimately associated with the bor- nite. It is of a dull grayish-blue variety, often showing streaks of a lighter color, arranged in a reticulate pattern. It forms mutual contacts with the bornite, and the graphic intergrowth is exceedingly common. Many places were found in the polished sections where the bornite after forming a narrow, regular rim between the chalcocite and the gangue would branch into the chalcocite forming a true graphic structure. The reverse of this was also found where the chalcocite formed rims and branched