200 feet, while in Morococala, the same depth has been reached without as yet any change.
The Huanuni district has for many years been the most important tin-mining center. A splendid conical mountain, Pozoconi, rising some 1,800 feet above the level of the adjoining ravine, is traversed by a multitude of lodes and veins, which have been largely worked. While many of these, at depths of from 100 to 200 feet, run into tin pyrites, in one of the principal lodes crossing the summit of the mountain the pure ore has been followed down for 600 feet without alteration.
Most of the lodes in Negro Pabellon, Morococala, and Huanuni present continuous veins of solid tin stone, of variable width, up to some 2 feet, and, under these circumstances, as might be expected, some difficulty is experienced, owing to the extreme hardness both of the vein itself and of the adjoining rock; but seams of clay, containing interspersed nodules and grains, are also met with and are worked to advantage, while from the tin-bearing pyrites the ore is extracted by calcination in ovens and subsequent grinding and washing.
The Challa-Apacheta mines are situated about 10 miles south of Huanuni, the lodes traversing low hills, rising some 250 feet above the neighboring ravine. One of these lodes is remarkable as possessing the unusual width of from 25 to 30 feet, the tin ore, in the form of grains and sand, being so thickly interspersed through a slighty argillaceous matrix as to give a mean lye for the whole mass of some 20 per cent. This lode has been explored horizontally over an extent of 250 feet, and to about the same distance vertically below the surface. When pyrites is encountered, as might be supposed from its composition, the lode is extremely soft, the contents being worked out with pick and shovel at a low cost. It requires, however, a certain amount of grinding to reduce the tin particles to a uniform size and admit of satisfactory washing on the vanner. What appears to be a continuation of the same lode is met with in an adjoining hill, but under very different conditions. It is here a comparatively narrow hard vein, from 12 to 18 inches wide, in parts very solid, but with more or less admixture of pyrites, requiring calcination before grinding. This calcination is effected in heaps in the open, a very small amount of fuel at the base being sufficient to start combustion, which is then maintained by the sulphur present.
One of the most important and productive tin mines at the present time is that of Avecaya, situated in a lofty mountain at the southern extremity of the Oruro tin district. The lodes are from 1 to 3 feet wide, giving considerable masses of solid ore, averaging some 40 per cent of the metal.
The mining district of Berenguela is situated about 45 miles east of Oruro, on the heights immediately south of the Argue quebrada. It was extensively worked by the Spaniards, but chiefly for silver, in which many of the lodes
