CHANGES IN ORE SINCE DEPOSITION. Dail 
and delicate threads, which occur in crevices and vues. In one case in the 
Mollie Gibson such a vug was a foot or two in diameter and was completely 
and closely festooned with threadlike silver in so large an amount that 
several small ore bags were filled with it. 
The rich ore, consisting of barite and polybasite, with tennantite, 
undergoes a change by which the sulphides disappear, their places being 
taken by native silver. This process is accompanied by a reduction, so 
that the ore becomes crumbling and consists essentially of barite held 
together by silver. It is along fractured zones and watercourses that this 
ordinarily takes place, and usually also in the neighborhood of the carbo- 
naceous Weber shales. Along the Clark fault in the Mollie Gibson some 
of the breccia has been cemented by native silver which is apparently com- 
paratively recent in deposition. The process is not clearly understood, but 
it is probable that the organic matter in the shales has operated in the 
reduction, and it is clear also that circulating waters. have probably been 
instrumental. In the concentrating works at Aspen, where ore is crushed 
and separated by means of ordinary cold water, certain iron parts of the 
apparatus become coated with native silver precipitated from the water 
which flows over them. ‘This shows that ordinary surface waters have 
power to dissolve and carry away silver, which they deposit under favor- 
able circumstances. In the highly oxidized ores near the surface, however, 
silver is not usually found native, but is apparently altered to one of its 
salts. 
Oxidation of ores —H or a variable distance below the surface the sulphide 
ores have been altered by the effects of surface action, and assume other 
forms. In the highest mines in the district—in Tourtelotte Park—the ore 
is all oxidized, while in the workings of Aspen and Smuggler mountains 
the transitional stages of oxidation are all seen, and in the lower workings 
of the deep mines the original sulphide condition prevails. On oxidation, 
the ore loses cohesion and becomes brittle or crumbling, the altered 
minerals generally assuming a pulverulent form. All the sulphides thus 
disappear and are replaced by oxides, sulphates, and carbonates. The 
barite, however, does not change. An analysis of the typical oxidized barite 
ore from the Buckhorn No. 2 mine, in Tourtelotte Park, was made by 
Dr. H. N. Stokes, of the Survey, with the following result: 
