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. 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 lead, 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,
Loose sand (decomposed quartz and granite),
Finely laminated reddish clay,

Loose sand (decomposed quartz and granite), with
numerous crystalline pebbles and a little gold,

151⁄2

Fine reddish clay,

1/2

Loose sand (decomposed quartz and granite), with
numerous pebbles—the main gold deposit,

Granite, water-worn surface, with large granitic boulders.

One great essential in all workings of this kind is an abundant supply of water; and where this can be obtained not only are the washing and sifting facilitated, but, as in California, powerful jets can be directed against the face of the drifts at once to bring them down and to wash out the heavier and metalliferous material from the stony debris. In the earlier days of shallow placer-working, a pick, a spade, and an iron pan formed the full equipment of a gold-digger. By-and-by these were superseded by the rocker; the rocker by the Long-Tom or mercurytrough device, and the Arastra-mill or quartz-crusher of the Mexicans; and when deep - placer became a necessity, these simple contrivances gave place to the water-gun of the Californians. In that country, where auriferous gravels cover an area 50 miles in breadth and 300 in length, from 50 to 500 in thickness, nothing but the most powerful and rapid contrivances will suffice; and hence the new system of flumes or watercourses led for many miles from the higher grounds, and terminating in the nozzle of the water-gun. "From this 6-inch tube," says Professor B. Silliman, "is projected a stream of water, which is as solid as a bar of steel as it leaves the mouth of the gun and remains so for a distance of 200 feet, or until it impinges against the face of the bank. Through that the

66

Hydraulic Placer-Working-from a Photograph.

miner is enabled to throw a continuous stream without interruption, day and night, which is equal to a measurement of not less than 1500 to 1600 cubic feet of water, projected with a

force equivalent to 140 feet of water in a second of time, with an impact equal to 1650 lb. You can understand this when I say that it is about equal to one-tenth the velocity of a cannon-ball." The preceding woodcut represents the process, the auriferous debris being carried off by a tail-flume to a lower level, there to undergo the further process of sifting and separation.

On the whole, stream or placer working, whether for metals, for ores, or for gems and precious stones, is a species of quarrying rather than of mining, and may occasionally be successfully carried on with the rudest appliances. It is always attended, however, with greater or less uncertainty; and though vast amounts of gold, tin, and precious stones have been obtained through its practice, it can never be regarded as a steady and regular species of industry. In the long-run the richest drifts get exhausted, and then the miner is driven to the veins from which they were derived, there to prosecute his calling in a settled and systematic manner.* In the mean time, the most extensive and productive stream-drifts are those of the Urals, India, Further India, New Zealand, Australia, Cape of Good Hope, Brazil, Mexico, California, and British Columbia— yielding not only native metals and metallic ores, but nearly all the gems and precious stones. Of recent years, digging for phosphatic nodules-the coprolites or "cops" of the worker-has become a considerable industry in the greensand and tertiary districts of England, and especially so in the Carolinas of North America.-See Mineral Manures.

To a country like Britain, and indeed to all other countries, in an age of so many mechanical appliances as the present, the subject of mine engineering is of the utmost importance. It is not only determining where certain minerals and metals are to be found, but in what abundance they occur, and with what facilities they can be obtained. It is evident, then, that the mining engineer should have a competent knowledge of the general principles of geology; should be able to adopt the safest and most economic method, whether it be vein-mining or stratamining in which he is engaged; and at the same time be competent to decide on suitable means of lifting, pumping, and ventilation. The difficulties that beset the practice of mining are not only numerous and perplexing, but often sudden and unforeseen; and whoever from his superior knowledge is

* These remarks are scarcely applicable to California, where the area of auriferous gravels is so wide, and their depth so great as to extend downward 400 or 500 feet to river-courses of Pliocene, if not of Miocene epoch.