Page:The American Cyclopædia (1879) Volume XV.djvu/791

 strata, veins, or masses (" tin floors ") ; in con- geries of small veins ; in large veins ; and dis- seminated in alluvial deposits. The congeries or networks of small veins (StockwerJce of the Germans) occur in granite and "elvan" (feld- spathic porphyry). The large metalliferous veins are grouped in three districts : the S. W. part of Cornwall, beyond Truro; the neigh- borhood of St. Austell ; and the neighborhood of Dartmouth, in Devonshire. The first is the richest and best explored. The tin veins be- long to different systems, having nearly the same general course, but differing in dip. It was formerly thought that tin occurred in the upper portions of the lodes only, and the ap- pearance of copper pyrites in depth was con- sidered to be a sign that the tin ore had been "cut out;" but more recently tin ore has been found at great depths and below the copper. Thus the Dolcoath mine was worked first as a tin mine for a very long period ; then as a cop- per mine for half a century ; and finally again, at still greater depth, and with considerable profit, as a tin mine. Alluvial tin ore or stream tin deposits occur on the hillsides and in the valleys, and furnished for centuries the whole of the Cornish tin. The largest works of this kind are around St. Just and St. Austell. Many of the Cornish mines have been unprofitable since 1872, on account of the great fall in prices resulting from the influx of Australian ore and metal. The mines in the East Indies might perhaps have brought about this revul- sion still earlier, since they were capable of producing tin very cheaply; but the supply from that source was limited at the will of the , governmental authorities, so as to divide the market with Cornwall, on terms which left some profit to the Cornish mines. One au- thority estimates the product in 1868 at 7,200 tons for Great Britain, and 7,500 tons for southern Asia and India. According to a re- cent writer (Berg- und Huttenmannische Zei- tung, 1875), the total product of tin in the world about 1870 was something over 11,000 tons annually, of which 6,000 tons came from Cornwall and 4,000 tons from Asia. (This es- timate for Asia is apparently too low.) But since that time, and especially since 1872, a very extraordinary development of tin mining in Australia has revolutionized the market. The Australian tin-ore deposits thus far known occur in the region of the Cordilleras, in Vic- toria, New South Wales, and Queensland. In Victoria the older outcropping rocks are pre- dominantly Silurian, and tin ore is found in small quantity in alluvial deposits, but hith- erto not in veins. In New South Wales and Queensland there is a greater quantity and va- riety of exposed rock formations, and among them granites, porphyries, and metamorphic schists, with which the tin ore is associated. In a portion of New South Wales it appears connected with more recent eruptive rocks. The alternations of rain and drought in the seasons are a serious inconvenience to alluvial TIN 761 mining, which suffers also, like the placer mi- ning of gold in Australia and the United States, from occasional excessively dry years. The existence of tin ore in this region was made known by the Rev. W. B. Clarke, colonial ge- ologist, in 1845 ; in 1869 a shepherd brought to market a considerable quantity which he had obtained by washing, without knowing its value; a population of 10,000 miners was at- tracted to the district, and a feverish specula- tion raged until near the end of 1871, followed by disastrous reaction and a gradual renewal of industry in a more reasonable way. Up to the end of 1871 the production had been about 2,000 tons of tin. The present export in metal and in ore (sent to England for reduction) is said to exceed 7,000 tons of tin ; the number of workmen is between 2,000 and 3,000. Veins are abundant, but the entire product is at pres- ent derived from alluvial mines. These occur in five principal districts, interspersed with scattered minor districts, the aggregate area being about 1,000 sq. m., the greater part of which lies south of the boundary between Queensland and New South Wales. The pla- cers usually lie along present or former water- courses, and present at the surface granitic sand and pebbles, with underlying gravel, and at the bottom, resting upon the bed rock, a layer of clay, gravel, and bowlders, in which occur tin ore, wolfram, tourmaline, quartz, and occasionally sapphire and ruby. Some- times the series is repeated, giving two layers of stanniferous gravel, of which the lowest rests upon the rock, a phenomenon familiar to placer miners for gold ; and the methods of working are similar to those of the latter. The total depth of the deposit is rarely less than 4 or more than 20 ft. The labor employed is partly Chinese; the average cost of the ore, delivered at the nearest harbor, is perhaps 40 a ton, though rich mines, favorably located, can deliver it at 30. Some furnaces have been erected near the mines to smelt the ore ; but wood, the only fuel available there, though cheap at present, is likely to be rapidly ex- hausted. Two large establishments, at Sydney and Brisbane, have successfully smelted the Australian tin ores with coal in reverberatory furnaces. The metal, however, even after re- fining, contains but 99 per cent, of tin, on ac- count of the wolfram invariably present in the ore. For this reason Australian ore is disliked by the smelters of Cornwall, and Australian tin always commands a somewhat lower price than Banca or Cornish metal. New and ex- tensive discoveries of tin ore have been re- cently reported in Tasmania. The tin ore of the island of Banca, in the Dutch East Indies, occurs as stream tin and also in veins in gran- ite. The Dutch government at present works the alluvial deposits only. These consist of 9 to 30 ft, in depth of loam, red and blue clay, coarse and fine sand, and tin ore. The tin- bearing layer is from 3 to 22 in. thick, in some cases even more. The mines are worked du-