mines, consequently, large quantities of such poorer ores have been left untouched that with cheap grinding would be available.
Some of the more important lodes of the Oruro silver mines are composed of iron pyrites from 1 to 6 feet wide and frequently containing from 5 to 20 per cent of tin oxide associated with ores of silver and interspersed through the mass. In these cases, the tin forms a valuable by-product, being extracted with but little extra expense by simply washing the tailings after treatment by the amalgamation process for silver. This washing has hitherto been commonly effected by hand, the tailings being fed in at the head of slightly inclined channels a foot wide and some 6 or 8 feet long, supplied with a moderate stream of water, the material being constantly raked up and turned over by means of a small, thin board; the lighter, earthy portion is gradually separated and carried off, the tin oxide finally remaining of a lye of 65 to 70 per cent, when it is dried and sacked for export. Common hand buddles are also employed, and of late Frue vanners have been introduced with fairly satisfactory results, though the unequal feeding by hand is objectionable.
The concentrated tin ore, or barrilla, is also smelted on the spot, with charcoal, in small vertical blast furnaces, and run into 50-pound slabs. The loss, however, is not less than 20 per cent; consequently, in the Oruro district but little is reduced. From Potosi, owing to higher freights to the coast (£18 per ton), all the tin is exported in the metallic form.
Toward the northern part of the Bolivian tin zone, what promises to be an important deposit has lately been opened up in the La Paz Cordillera, on the base of the snow peak Huaina Potosi. The ore is possibly superficial, but it is of the finest quality, a rough grinding and washing raising it to a lye of 70 to 73 per cent.
Further south, in the cordillera of Quimsa Cruz, lodes of fine tin exist, especially at high altitudes near the snow line. They have been worked at intervals, but, owing to the difficulties of transport, hitherto with but little profit.
Following these, are the Collquiri silver and tin mines, which have, at times, given a large production, and which, with increased facilities, will no doubt prove important. Still further south, the richest tin district is encountered in the hills and ranges immediately to the east of Oruro and Lake Poopa, where the mines of Negro Pabellon, Villacolla, Morococala, Huanuni, Llallagua, Challa, Apacheta, and Avecaya follow each other at short intervals.
All these mines are situated at an altitude of some 14,000 feet above sea level and they contain well-defined lodes of fine ore, which occasionally attain a width of 2 feet with a lye of 60 per cent.
In Negro Pabellon, tin-bearing pyrites has made its appearance at a depth of
