Searching or Prospecting. Whatever the nature of the vein sought after, it is essential to have a preliminary searching or "prospecting," as it is termed, of the country or ground. For this purpose the miner may search the country for indications of veins in hill sides, precipices, and ravines; he may test the quality of springs, which generally hold in solution some of the materials through which they have passed; or he may examine the stream-drifts which have been worn and washed from the higher grounds, and which usually contain rolled fragments of the denuded veins. These shoadstones, be they of hæmatite, tin-ore, or auriferous quartz, become more numerous the nearer the vein is approached; and in this way shoading, as it is termed, becomes an important means not only of discovering veins, but of indicating the nature of the ores of which they are composed. Where none of these means are available, he may adopt the more expensive but the more certain method of costeaning— that is, of driving shallow drifts at right angles to the usual trend of the veins so as to intercept their outcrops; and from these decomposed outcrops or gossan, as it is termed in Cornwall, he can judge at once of the nature, thickness, and direction of a lode. Indeed, as this gossan is merely an altered portion of the back or upper part of a vein, it is often indicative, to the experienced miner, of the richness of the ore that lies below-and hence his expectations on the discovery of what he designates a "kindly gossan." In Britain, as in other countries, the great vein-repositories are the older rocks-the metamorphic schists of the Scottish Highlands, as in the Breadalbane district; the Cambrians and Silurians of Cornwall, Devon, Wales, and the Lake country; and the thick-bedded Carboniferous limestones of Derby, York, North Lancashire, Westmoreland, and Durham. In the older schists, gold, silver, mercury, tin, copper, manganese, and iron prevail; in the mountain limestones, lead, zinc, antimony, and iron are abundant. Occasionally some of the metals occur in a free, native, or uncombined state, as gold, platinum, silver, mercury, copper, and arsenic : more frequently as alloys, as silver, antimony, cobalt, nickel, and iron, with arsenic; silver and nickel with antimony; lead, gold, silver, and bismuth, with tellurium; silver with mercury; and platinum with gold and most abundantly as ores-that is, in a chemically combined state, as oxides, sulphides, carbonates, phosphates, and other kindred combinations. The metals peculiar, or supposed to be peculiar, to the respective systems, will be noticed in next section, and when we come to treat of the me tallic ores in Chap. XVIII.; in the mean time we merely advert to the principal areas of our metalliferous vein-development. Having ascertained the vein-structure of the country, and the presence of the minerals or metals he is in search of, the miner can then proceed with some degree of certainty to their raising or extraction. Working or Winning. The working of veins depends very much on the structure of the country, whether it is abruptly hilly and irregular or disposed in broad massive table-lands. In the former case, the veins may be approached by a system of adits, and drained by water-levels; in the latter, shafts must be sunk and water got a Vein-Mining-Shaft and Adits. quit of by pumping. In the accompanying sketch, the upper part of the vein v may be worked and drained by the adit a; the middle portion may be worked and drained by a'; while the lower portion is reached by a shaft s and adit a", and drained by pumping up to the adit a'. In Cornwall, for example, both systems are followed on a gigantic scale—the shafts being frequently of great depths and the pumping-engines proverbial for their power, the adits of great size, and the water-levels often carried for many miles (in the case of the great Gwenaap, it is said, for 30 miles) across the country. In every case, however, the engineer must be guided in his methods by the structure of the ground, the frequency, the underlie, and the intersection of the veins-his main object being to accomplish his work cheaply, by avoiding unnecessary mining, by getting readily quit of water, by raising as little "deads " or waste rubbish as he can, and thus having the smallest possible amount of "attle" or non-metalliferous stuff from the dressingtables. Ventilation is generally good in vein-mines, and there is no danger or interruption caused by the presence of explosive gases. When the underlie is small, the cheeks and walls are usually firm, and give little trouble; when it is great, the hanging wall requires not unfrequently pillaring and propping to prevent collapses. These and other minutiæ, however, belong more to the mechanical than to the geological functions of the mining engineer. In following the veins, many phenomena will present themselves which cannot be anticipated; and, generally speaking, the most successful miner is he who has studied with discernment the peculiarities of his own immediate locality. Generalisations have, no doubt, been attempted by several engineers, but these refer chiefly to the field of their own experiences, and are often inappropriate and misleading when applied to other districts. For instance, it has been set down by some that veins grow poorer as they descend into the crust; but this has been disproved by ample experience in Australia, Peru, Central America, Mexico, and California. It is true, some auriferous veins are abundantly rich at the surface; but on this point it has been well observed by Mr Belt * (an experienced miner and commissioner at the gold-mines of Chontales in Nicaragua), that "the cause of these rich deposits near the surface does not appear to me to be that the lodes originally, before they were exposed to denudation, contained more gold in their upper portion than below, but to the effect of the decomposition and wearing down of the higher parts, and the concentration of the gold they contained in the lode below that worn away. We have seen that in the decomposed parts of the lode, the gold exists in loose fine grains. During the wet season water percolates freely from the surface down through the lodes, and the gold set free by the decomposition of the ore at the surface must be carried down in it, so that in the course of ages, during the gradual degradation and wearing away of the surface, there has, I believe, been an accumulation of loose gold in the upper part of the lodes from parts that originally stood much higher, and have now been worn away by the action of the elements." Again, it has been observed when a vein passes from one rock-formation to another, as from slate to granite, it is usually richer at their contact; and farther, when veins cross each other, there is generally a fuller development of ore at their intersection. When a vein, however, passes through alternations of harder and softer rocks, as in the limestones and shales of the Carboniferous, it is often rich in the former, and is poor or altogether "nipped" in the latter. If, again, a vein splits into * The Naturalist in Nicaragua, by Thomas Belt, F.G.S. strings, either vertically or longitudinally, it is regarded as a sign of impoverishment, and not unfrequently this splitting or diffusion renders the vein unworkable. It has been already mentioned that the Paleozoic systems and their associated igneous rocks are the great repositories of metalliferous veins in Britain; and it is from these ancient rocks that almost all our metals are derived. In a few instances (according to Professor Phillips), veins of small value, producing lead and copper, pass through the magnesian limestone; but not a single example is known of a true metallic vein in the oolitic, cretaceous, or tertiary system. The secondary rocks have been subjected to comparatively slight volcanic agency, and it is chiefly in and around igneous centres that veins most abound. It has also been attempted to associate the occurrence of certain metals with certain rocks, as gold with the older schists, tin with slates and granites, copper and silver with slates and porphyries, and lɛad, zinc, and antimony with limestones; but as yet such attempts to establish relations must be received merely as tentative, and as by no means established. It has been farther attempted to establish relations between the nature of the walls and that of the vein-stuff. It is true that quartz - veins occur most abundantly in silicious rocks, and calc-spars in limestones; but there are many veins in the older rocks which are filled with baryta and strontia, and many again in limestones which contain calc-spar, fluor-spar, rock-crystal, &c., either in successive layers or confusedly interblended. Werner insists on the fact, that certain associations of minerals can be traced in veins; but the assertion must be received with reserve. He notices the concurrence of leadglance, zinc-blende, and copper-pyrites; of cobalt, copper, nickel, and native bismuth; of tin, wolfram, tungsten, molybdena, and arsenical pyrites; of topaz, fluor-spar, apatite, schorl, mica, chlorite, and lithomarge; of brown ironstone, black ironstone, manganese, and heavy spar. He says, where tin occurs, ores of silver, lead, and cobalt, and vein-stuffs of heavy spar, calcareous spar, and gypsum, are rarely found. Cinnabar and other ores of mercury scarcely ever occur with the ores of other metals, except iron-ochre and iron pyrites. IV. STREAM OR PLACER WORKING. Stream or placer working consists in turning over, washing, and searching for metals, metallic ores, and minerals, the sands, gravels, and shingly debris which occur in river-valleys, and which have been worn and transported from the higher grounds. It is evident that the veins which traverse the higher grounds will be subjected to the same tear and wear as the rocks which contain them, and that in course of time the denuded portions will be carried by rain, streams, and freshets to the valleys below. If, for example, the vein be one of auriferous quartz, the veinstone will be more and more broken up as it is carried down by the stream; the debris will accumulate more in flats, and creeks, and eddies; and, generally speaking, the metal or orestone will, from its greater specific gravity, be the first to settle or subside in this downward course. In this way the stream-drifts become richer the nearer the vein is approached; and hence the practice of shoading for veins, as already alluded to. Stream-working is usually a simple process, and has been long practised in searching for gold, tinstone, and precious stones. And yet, simple as it appears to be, a great deal of skill may be shown in selecting the site for operations. Wherever the course of a stream is interrupted by sudden bendings, creeks, and level reaches, there will the drifted matter accumulate in greater abundance. In course of time every river-valley changes less or more its conformation, and it is by a skilful calculation of what that conformation may have been in former ages that the placer-worker is frequently led to the richest accumulations. Where the deposits are shallow, little is required beyond shovelling, separating, washing, and sifting; but where they are of great thickness, and occasionally overlaid, as in Australia, with sheets of lava, not only digging, but mining in a rude way, has to be resorted to. The following, for example, is a section of an auriferous digging (Mr Cleghorn's) near Uralla in Queensland :— Red rich clay, Stiff red clay, Mottled clay-volcanic ashes, Basaltic lava, Brown laminated clay, Loose sand (decomposed quartz and granite), Black peaty clay, with numerous leaves and stems, Fine reddish clay, Loose sand (decomposed quartz and granite), with Loose sand (decomposed quartz and granite), with Granite, water-worn surface, with large granitic boulders. |