ORE DEPOSITION. 225 
basite from the Mollie Gibson, by 8. H. Pearce and 8. L. Penfield, gave 
the following results when corrected : 
Analyses of polybasite from Mollie Gibson mine. 
| | Percent. | Per cent. 
| FS eU AG) clyy Fein Sete Se SR a RUA eg OY Set iiss 18.13 
PAT SOT Caner seers eee oe oes Deena 6. 29 7.01 
PAIN OM Ye Aen ee cere eee eo .18 30 
Silversea ss oe ae pe 59. 73 56. 90 
Coppers serene eens e eee pee een 12.91 14. 85 
ZAIN GP ee Na Na eS LO i tiie es 3.16 2.81 
TR ONS eee a a oer ate ee 100. 00 100. 00 
This polybasite ore in the Smuggler and Mollie Gibson is reduced 
along watercourses to native silver, so that the ore consists of pink or gray 
barite bound together by irregular wires and masses of silver. As this 
process is attended by some loss of bulk, the ore also becomes much jointed 
and loses cohesion. The process of change is not completely understood, but 
it seems probable that the organic material in the Weber shales which lie 
around and against this ore has been active in the reduction. The gradual 
change of polybasite to silver around the edges and along crevices can be 
seen under the microscope.. 
Near the surface these sulphides are altered, there being a gradual 
transition between pure sulphides in the deep mines and pure oxidized 
minerals in the highest mines, such as those in Tourtelotte Park. The 
principal ore in these upper zones consists of earthy carbonates and sul- 
phates, chiefly of lead (cerusite and anglesite) ; among the oxides hematite 
and limonite are very common, and the red oxides of copper and lead 
(cuprite and minium) occur in blotches in the oxidized ores, usually 
indicating the presence of silver. The black oxide of copper, melaconite, 
has been observed in thin coatings. 
GANGUE MINERALS. 
The gangues which accompany these sulphides consist chiefly of 
quartz in small crystals and crystalline aggregates ; of crystallized dolomite, 
usually ferriferous; and of barite or heavy spar. In the Mollie Gibson 
and Smuggler mines the barite connected with the polybasite ore has often 
a flesh or pink color, which is due to a small amount of iron oxide. 
MON XXXI 15 
