MINES; (Bergwerke, Germ.) Amidst the variety of bodies apparently infinite, which compose the crust of the globe, geologists have demonstrated the prevalence of a fewgeneral systems of rocks, to which they have given the name offormationsordeposits. A large proportion of these mineral systems consists of parallel planes, whose length and breadth greatly exceed their thickness; on which account they are called stratified rocks; others occur in very thick blocks, without any parallel stratification, or horizontal seams of considerable extent.The stratiform deposits are subdivided into two great classes; the primary and the secondary. The former seem to have been called into existence before the creation of organic matter, because they contain no exuviæ of vegetable or animal beings; while the latter are more or less interspersed, and sometimes replete with organic remains. The primary strata are characterized, moreover, by the nearly vertical or highly inclined position of their planes; the secondary lie for the most part in a nearly horizontal position.Where the primitive mountains graduate down into the plains, rocks of an intermediate character appear, which, though possessing a nearly vertical position, contain a few vestiges of animal beings, especially shells. These have been calledtransition, to indicate their being the passing links between the first and second systems of ancient deposits; they are distinguished by the fractured and cemented texture of their planes, for which reason they are sometimes called conglomerate.Between these and the truly secondary rocks, another very valuable series is interposed in certain districts of the globe; namely, the coal-measures, the paramount formation of Great Britain. The coal strata are disposed in a basin-form, and alternate with parallel beds of sandstone, slate-clay, iron-stone, and occasionally limestone. Some geologists have called the coal-measures the medial formation.In every mineral plane, the inclination and direction are to be noted; the former being the angle which it forms with the horizon, the latter the point of the azimuth or horizon, towards which it dips, as west, north-east, south, &c. The direction of the bed is that of a horizontal line drawn in its plane; and which is also denoted by the point of the compass. Since the lines of direction and inclination are at right angles to each other, the first may always be inferred from the second; for when a stratum is said to dip to the east or west, this implies that its direction is north and south.The smaller sinuosities of the bed are not taken into account, just as the windings of a river are neglected in stating the line of its course.Massesare mineral deposits, not extensively spread in parallel planes, but irregular heaps, rounded or oval, enveloped in whole or in a great measure by rocks of a different kind. Lenticular masses being frequently placed between two horizontal or inclined strata, have been sometimes supposed to be stratiform themselves, and have been accordingly denominated by the Germansliegende stocke,lying heapsorblocks.The orbicular masses often occur in the interior of unstratified mountains, or in the bosom of one bed.Nests,concretions,nodules, are small masses found in the middle of strata; the first being commonly in a friable state; the second often kidney-shaped, or tuberous; the third nearly round, and encrusted, like the kernel of an almond.Lodes, or large veins, are flattened masses, with their opposite surfaces not parallel, which consequently terminate like a wedge, at a greater or less distance, and do not run parallel with the rocky strata in which they lie, but cross them in a direction not far from the perpendicular; often traversing several different mineral planes. Thelodesare sometimes deranged in their course, so as to pursue for a little way the space between two contiguous strata; at other times they divide into several branches. The matter which fills the lodes is for the most part entirely different from the rocks they pass through, or at least it possesses peculiar features.This mode of existence, exhibited by several mineral substances, but which has been long known with regard to metallic ores, suggests the idea of clefts or rents having been made in the stratum posterior to its consolidation, and of the vacuities having been filled with foreign matter, either immediately or after a certain interval. There can be no doubt as to the justness of the first part of the proposition, for there may be observed round many lodes undeniable proofs of the movement or dislocation of the rock; for example, upon each side of the rent, the same strata are no longer situated in the same plane as before, but make greater or smaller angles with it; or the stratum upon one side of the lode is raised considerably above, or depressed considerably below, its counterpart upon the other side. With regard to the manner in which the rent has been filled, different opinions may be entertained. In the lodes which are widest near the surface of the ground, and graduate into a thin wedge below, the foreign matter would seem to have been introduced as into a funnel at the top, and to have carried along with it in its fluid state portions of rounded gravel and organic remains. In other cases, other conceptions seem to be more probable; since many lodes are largest at their under part, and become progressively narrower as they approach the surface; from which circumstance, it has been inferred that the rent has been caused by anexpansive force acting from within the earth, and that the foreign matter, having been injected in a fluid state, has afterwards slowly crystallized. This hypothesis accounts much better than the other for most of the phenomena observable in mineral veins, for the alterations of the rock at their sides, for the crystallization of the different substances interspersed in them, for the cavities bestudded with little crystals, and for many minute peculiarities. Thus, the large crystals of certain substances which line the walls of hollow veins, have sometimes their under surfaces besprinkled with small crystals of sulphurets, arseniurets, &c., while their upper surfaces are quite smooth; suggesting the idea of a slow sublimation of these volatile matters from below, by the residual heat, and their condensation upon the under faces of the crystalline bodies, already cooled. This phenomenon affords a strong indication of the igneous origin of metalliferous veins.In the lodes, the principal matters which fill them are to be distinguished from the accessory substances; the latter being distributed irregularly, amidst the mass of the first, in crystals, nodules, grains, seams, &c. The non-metalliferous exterior portion, which is often the largest, is calledgangue, from the Germangang,vein. The position of a vein is denoted, like that of the strata, by the angle of inclination, and the point of the horizon towards which they dip, whence the direction is deduced.Veins, are merely small lodes, which sometimes traverse the great ones, ramifying in various directions, and in different degrees of tenuity.A metalliferous substance is said to bedisseminated, when it is dispersed in crystals, spangles, scales, globules, &c., through a large mineral mass.Certain ores which contain the metals most indispensable to human necessities, have been treasured up by the Creator in very bountiful deposits; constituting either great masses in rocks of different kinds, or distributed in lodes, veins, nests, concretions, or beds with stony and earthy admixtures; the whole of which become the objects of mineral exploration. These precious stores occur in different stages of the geological formations; but their main portion, after having existed abundantly in the several orders of the primary strata, suddenly cease to be found towards the middle of the secondary. Iron ores are the only ones which continue among the more modern deposits, even so high as the beds immediately beneath the chalk, when they also disappear, or exist merely as colouring matters of the tertiary earthy beds.The strata of gneiss and mica-slate constitute in Europe the grand metallic domain. There is hardly any kind of ore which does not occur there in sufficient abundance to become the object of mining operations, and many are found no where else. The transition rocks and the lower part of the secondary ones, are not so rich, neither do they contain the same variety of ores. But this order of things, which is presented by Great Britain, Germany, France, Sweden, and Norway, is far from forming a general law; since in equinoxial America the gneiss is but little metalliferous; while the superior strata, such as the clay-schists, the sienitic porphyries, the limestones, which complete the transition series, as also several secondary deposits, include the greater portion of the immense mineral wealth of that region of the globe.All the substances of which the ordinary metals form the basis, are not equally abundant in nature; a great proportion of the numerous mineral species which figure in our classifications, are mere varieties scattered up and down in the cavities of the great masses or lodes. The workable ores are few in number, being mostly sulphurets, some oxides, and carbonates. These occasionally form of themselves very large masses, but more frequently they are blended with lumps of quartz felspar, and carbonate of lime, which form the main body of the deposit; as happens always in proper lodes. The ores in that case are arranged in small layers parallel to the strata of the formation, or in small veins which traverse the rock in all directions, or in nests or concretions stationed irregularly, or finally disseminated in hardly visible particles. These deposits sometimes contain apparently only one species of ore, sometimes several, which must be mined together, as they seem to be of contemporaneous formation; whilst, in other cases, they are separable, having been probably formed at different epochs. In treating of the several metals in their alphabetical order, I have taken care to describe their peculiar geological positions, and the rocks which accompany or mineralize them.In mining, as in architecture, the best method of imparting instruction is to display the master-pieces of the respective arts, which speak clearly to the mind through the medium of the eye. It is not so easy, however, to represent at once the general effect of a mine, as it is of an edifice; because there is no point of sight from which the former can be sketched at once, like the latter. The subterraneous structures certainly afford some of the finest examples of the useful labours of man, continued for ages, under the guidance of science and ingenuity; but, however curious, beautiful, and grand in themselves, they cannot become objects of a panoramic view. It is only by the lights of geometry and geology that mines can be contemplated and surveyed, either as a whole or in their details; and, therefore, these marvellous subterranean regions, in which roads are cutmany hundred miles long, are altogether unknown or disregarded by men of the world. Should any of them, perchance, from curiosity or interest, descend into these dark recesses of the earth, they are prepared to discover only a few insulated objects, which they may think strange or possibly hideous; but they cannot recognize either the symmetrical disposition of mineral bodies, or the laws which govern geological phenomena, and serve as sure guides to the skilful miner in his adventurous search. It is by exact plans and sections of subterraneous workings, that a knowledge of the nature, extent, and distribution of mineral wealth, can be acquired.698.A general view of mining operations.As there is no country in the world so truly rich and powerful, by virtue of its mineral stores, as Great Britain, so there are no people who ought to take a deeper interest in their scientific illustration. I have endeavoured in the present article to collect from the most authentic sources the most interesting and instructive examples of mining operations.To the magnificent work of Ville-Fosse,Sur la Richesse Minerale, no longer on sale, I have to acknowledge weighty obligations; many of the figures being copied from his great Atlas.Lodes or mineral veins are usually distinguished by English miners into at least four species. 1. The rake vein. 2. The pipe vein. 3. The flat or dilated vein; and 4. The interlaced mass (stock-werke), indicating the union of a multitude of small veins mixed in every possible direction with each other, and with the rock.1. Therakevein is a perpendicular mineral fissure; and is the form best known among practical miners. It commonly runs in a straight line, beginning at the superficies of the strata, and cutting them downwards, generally further than can be reached. This vein sometimes stands quite perpendicular; but it more usually inclines or hangs over at a greater or smaller angle, or slope, which is called by the miners thehadeorhadingof the vein. The line of direction in which the fissure runs, is called thebearing of the vein.2. Thepipevein resembles in many respects a huge irregular cavern, pushing forward into the body of the earth in a sloping direction, under various inclinations, from an angle of a few degrees to the horizon, to a dip of 45°, or more. The pipe does not in general cut the strata across like the rake vein, but insinuates itself between them; so that if the plane of the strata be nearly horizontal, the bearing of the pipe vein will be conformable; but if the strata stand up at a high angle, the pipe shoots down nearly headlong like a shaft. Some pipes are very wide and high, others are very low and narrow, sometimes not larger than a common mine or drift.3. Theflatordilatedvein, is a space or opening between two strata or beds of stone, the one of which lies above, and the other below this vein, like a stratum of coalbetween its roof and pavement; so that the vein and the strata are placed in the same plane of inclination. These veins are subject, like coal, to be interrupted, broken, and thrown up or down by slips, dykes, or other interruptions of the regular strata. In the case of a metallic vein, a slip often increases the chance of finding more treasure. Such veins do not preserve the parallelism of their beds, characteristic of coal seams; but vary excessively in thickness within a moderate space. Flat veins occur frequently in limestone, either in a horizontal or declining direction. The flat or strata veins open and close, as the rake veins also do.4. The interlaced mass has been already defined.To these may be added theaccumulatedvein, or irregular mass (butzenwerke), a great deposit placed without any order in the bosom of the rocks, apparently filling up cavernous spaces.The interlaced masses are more frequent in primitive formations, than in the others; and tin is the ore which most commonly affects this locality. Seefigure ofTinmine.The study of the mineral substances, calledganguesor vein-stones, which usually accompany the different ores, is indispensable in the investigation and working of mines. Thesegangues, such as quartz, calcareous spar, fluor spar, heavy spar, &c., and a great number of other substances, although of little or no value in themselves, become of great consequence to the miner, either by pointing out by their presence that of certain useful minerals, or by characterising in their several associations, different deposits of ores of which it may be possible to follow the traces, and to discriminate the relations, often of a complicated kind, provided we observe assiduously the accompanyinggangues.Mineral veins are subject to derangements in their course, which are called shifts or faults. Thus, when a transverse vein throws out, or intercepts, a longitudinal one, we must commonly look for the rejected vein on the side of the obtuse angle which the direction of the latter makes with that of the former. When a bed of ore is deranged by a fault, we must observe whether the slip of the strata be upwards or downwards; for in either circumstance, it is only by pursuing the direction of the fault that we can recover the ore; in the former case by mounting, in the latter by descending beyond the dislocation.When two veins intersect each other, the direction of theoffcastis a subject of interest, both to the miner and the geologist. In Saxony it is considered as a general fact that the portion thrown out is always upon the side of the obtuse angle, a circumstance which holds also in Cornwall; and the more obtuse the angle, the out-throw is the more considerable. A vein may be thrown out on meeting another vein, in a line which approaches either towards its inclination or its direction. The Cornish miners use two different terms to denote these two modes of rejection; for the first case, they say the vein isheaved; for the second, it isstarted.Copper lodeThe great copper lode of Carharack,d,fig.699.in the parish of Gwenap, is one of the most instructive examples of intersection. The power or thickness of this vein is 8 feet; its direction is nearly due east and west, and it dips towards the north at an inclination of two feet per fathom; its upper part being in thekillas(a greenish clay-slate); its lower part in the granite. The lode has suffered two intersections; the first produced by meeting the veinh, calledSteven’s fluckan, which runs from north-east to south-west, and which throws the lode several fathoms out; the second is produced by another veini, almost at right angles with the first, and which occasions another out-throw of 20 fathoms to the right side. The fall of the vein occurs therefore in the one case to the right, and in the other to the left; but in both it is towards the side of the obtuse angle. This distribution is very singular; for one part of the vein appears to have mounted while the other has descended.N,Sdenotes North and South.dis the copper lode running east and west.h,i, are systems of clay-slate veins called fluckans; the line overS, represents the down-shift, andd′the up-shift.General observations on the localities of ores, and on the indications of metallic mines.1.Tin, exists principally in primitive rocks, appearing either in interlaced masses, in beds, or as a constituent part of the rock itself, and more rarely in distinct veins. Tin ore is found indeed sometimes in alluvial land, filling up low situations between lofty mountains.2.Gold, occurs either in beds, or in veins, frequently in primitive rocks; though in other formations, and particularly in alluvial earth, it is also found. When this metal exists in the bosom of primitive rocks, it is particularly in schists; it is not found in serpentine, but it is met with in greywacke in Transylvania. The gold of alluvial districts,called gold of washing or transport, occurs, as well as alluvial tin, among the debris of the more ancient rocks.3.Silver, is found particularly in veins and beds, in primitive and transition formations; though some veins of this metal occur in secondary strata. The rocks richest in it are, gneiss, mica-slate, clay-slate, greywacke, and old alpine limestone. Localities of silver-ore itself are not numerous, at least in Europe, among secondary formations; but it occurs in combination with the ores of copper or of lead.4.Copper, exists in the three mineral epochas; 1. in primitive rocks, principally in the state of pyritous copper, in beds, in masses, or in veins; 2. in transition districts, sometimes in masses, sometimes in veins of copper pyrites; 3. in secondary strata, especially in beds of cupreous schist.5.Lead, occurs also in each of the three mineral epochas; abounding particularly in primitive and transition grounds, where it usually constitutes veins, and occasionally beds of sulphuretted lead (galena). The same ore is found in strata or in veins among secondary rocks, associated now and then with ochreous iron-oxide and calamine (carbonate of zinc); and it is sometimes disseminated in grains through more recent strata.6.Iron, is met with in four different mineral eras, but in different ores. Among primitive rocks, magnetic iron ore and specular iron ore occur chiefly in beds, sometimes of enormous size; the ores of red or brown oxide of iron (hæmatite) are found generally in veins, or occasionally in masses with sparry iron, both in primitive and transition rocks; as also sometimes in secondary strata; but more frequently in the coal-measure strata, as beds of clay-ironstone, of globular iron oxide, and carbonate of iron. In alluvial districts we find ores of clay-ironstone, granular iron-ore, bog-ore, swamp-ore, and meadow-ore. The iron ores which belong to the primitive period have almost always the metallic aspect, with a richness amounting even to 80 per cent. of iron, while the ores in the posterior formations become in general more and more earthy, down to those in alluvial soils, some of which present the appearance of a common stone, and afford not more than 20 per cent. of metal, though its quality is often excellent.7.Mercury, occurs principally among secondary strata, in disseminated masses, along with combustible substances; though the metal is met with occasionally in primitive countries.8.Cobalt, belongs to the three mineral epochas; its most abundant deposits are veins in primitive rocks; small veins containing this metal are found, however, in secondary strata.9.Antimony, occurs in veins or beds among primitive and transition rocks.10, 11. Bismuth and nickel do not appear to constitute the predominating substance of any mineral deposits; but they often accompany cobalt.12.Zinc, occurs in the three several formations: namely, as sulphuret or blende, particularly in primitive and transition rocks; as calamine, in secondary strata, usually along with oxide of iron, and sometimes with sulphuret of lead.An acquaintance with the general results collected and classified by geology must be our first guide in the investigation of mines. This enables the observer to judge whether any particular district, should from the nature and arrangement of its rocks, be susceptible of including within its bosom, beds of workable ores; it indicates also, to a certain degree, what substances may probably be met with in a given series of rocks, and what locality these substances will preferably affect. For want of a knowledge of these facts, many persons have gone blindly into researches equally absurd and ruinous.Formerly indications of mines were taken from very unimportant circumstances; from thermal waters, the heat of which was gratuitously referred to the decomposition of pyrites; from mineral waters, whose course is however often from a far distant source; from vapours incumbent over particular mountain groups; from the snows melting faster in one mineral district than another; from the different species of forest trees, and from the greater or less vigour of vegetation, &c. In general, all such indications are equally fallacious with the divining rod, and the compass made of a lump of pyrites suspended by a thread.Geognostic observation has substituted more rational characters of metallic deposits, some of which may be callednegativeand otherspositive.Thenegativeindications are derived from that peculiar geological constitution, which from experience or general principles excludes certain metallic matters; for example, granite, and in general every primitive formation, forbids the hope of finding within them combustible fossils (pit-coal), unless it be beds of anthracite; there also it would be vain to seek for sal gem. It is very seldom that granite rocks include silver; or limestones, ores of tin. Volcanic territories never afford any metallic ores worth the working; nor do extensive veins usually run into secondary and alluvial formations. The richer ores of iron do not occur in secondary strata; and the ores of this metal peculiar to these localities, do not exist among primary rocks.Amongpositiveindications, some are proximate and others remote. The proximate are, an efflorescence, so to speak, of the subjacent metallic masses; magnetic attraction for iron ores; bituminous stone, or inflammable gas for pit-coal; the frequent occurrence of fragments of particular ores, &c. The remote indications consist in the geological epocha, and nature of the rocks. From the examples previously adduced, marks of this kind acquire new importance when in a district susceptible of including deposits of workable ores, theganguesor vein-stones are met with which usually accompany any particular metal. The general aspect of mountains whose flanks present gentle and continuous slopes, the frequency of sterile veins, the presence of metalliferous sands, the neighbourhood of some known locality of an ore, for instance that of iron-stone in reference to coal, lastly the existence of salt springs and mineral waters, may furnish some indications; but when ferruginous or cupreous waters issue from sands or clays, such characters merit in general little attention, because the waters may flow from a great distance. No greater importance can be attached to metalliferous sands and saline springs.In speaking of remote indications, we may remark that in several places, and particularly near Clausthal in the Hartz, a certain ore of red oxide of iron occurs above the most abundant deposits of the ores of lead and silver; whence it has been named by the Germans theiron-hat. It appears that the iron ore rich in silver, which is worked in America under the name ofpacos, has some analogy with this substance; but iron ore is in general so plentifully diffused on the surface of the soil, that its presence can be regarded as only a remote indication, relative to other mineral substances, except in the case of clay ironstone with coal.Of the instruments and operations of subterranean operations.—It is by the aid of geometry in the first place that the miner studies the situation of the mineral deposits, on the surface and in the interior of the ground; determines the several relations of the veins and the rocks; and becomes capable of directing the perforations towards a suitable end.The instruments are, 1. the magnetic compass, which is employed to measure the direction of a metallic ore, wherever the neighbourhood of iron does not interfere with its functions; 2. the graduated semicircle which serves to measure the inclination, which is also called the clinometer.3. The chain or cord for measuring the distance of one point from another.4. When the neighbourhood of iron renders the use of the magnet uncertain, a plate or plane table is employed.The dials of the compasses generally used in the most celebrated mines, are graduated into hours; most commonly into twice 12 hours. Thus the whole limb is divided into 24 spaces, each of which contains 15° = 1 hour. Each hour is subdivided into 8 parts.Means of penetrating into the interior of the earth.—In order to penetrate into the interior of the earth, and to extract from it the objects of his toils, the miner has at his disposal several means, which may be divided into three classes: 1.manual tools, 2.gunpowder, and 3.fire.The tools used by the miners of Cornwall and Devonshire are the following:Mining toolsFig.700.Thepick. It is a light tool, and somewhat varied in shape according to circumstances. One side used as a hammer is called thepoll, and is employed to drive in thegads, or to loosen and detach prominences. Thepointis of steel, carefully tempered, and drawn under the hammer to the proper form. The French call itpointerolle.Fig.701.Thegad. It is a wedge of steel, driven into crevices of rocks, or into small openings made with the point of the pick.Fig.702.Theminer’s shovel. It has a pointed form, to enable it to penetrate among the coarse and hard fragments of the mine rubbish. Its handle being somewhat bent, a man’s power may be conveniently applied without bending his body.Theblastingorshootingtools are:—A sledge or malletfig.703.Borer—704.Claying bar—705.Needle or nail—706.Scraper—707.Tamping bar—708.Besides these tools the miner requires a powder-horn, rushes to be filled with gunpowder, tin cartridges for occasional use in wet ground, and paper rubbed over with gunpowder or grease, for thesmiftsor fuses.Theborer,fig.704., is an iron bar tipped with steel, formed like a thick chisel, and is used by one man holding it straight in the hole with constant rotation on its axis, while another strikes the head of it with the iron sledge or mallet,fig.703.The hole is cleared out from time to time by the scraper,fig.707., which is a flat iron rod turned up at one end. If the ground be very wet, and the hole gets full of mud, it is cleaned out by a stick bent at the end into a fibrous brush, called aswab-stick.Rock blastingFig.709.represents the plan of blasting the rock, and a section of a hole ready for firing. The hole must be rendered as dry as possible, which is effected very simply by filling it partly with tenacious clay, and then driving into it a tapering iron rod, which nearly fills its calibre, called theclaying bar. This being forced in with great violence, condenses the clay into all the crevices of the rock, and secures the dryness of the hole. Should this plan fail, recourse is had to tin cartridges furnished with a stem or tube (seefig.710.,) through which the powder may be inflamed. When the hole is dry, and the charge of powder introduced, thenail, a small taper rod of copper, is inserted so as to reach the bottom of the hole, which is now ready fortamping. By this difficult and dangerous process, the gunpowder is confined, and the disruptive effect produced. Different substances are employed fortamping, or cramming the hole, the most usual one being any soft species of rock free from siliceous or flinty particles. Small quantities of it only are introduced at a time, and rammed very hard by thetamping-bar, which is held steadily by one man, and struck with a sledge by another. The hole being thus filled, the nail is withdrawn by putting a bar through its eye, and striking it upwards. Thus a small perforation or vent is left for the rush which communicates the fire.Besides the improved tamping-bar faced with hard copper, other contrivances have been resorted to for diminishing the risk of those dreadful accidents that frequently occur in this operation. Dry sand is sometimes used as a tamping material, but there are many rocks for the blasting of which it is ineffective. Tough clay will answer better in several situations.For conveying the fire, the large and long green rushes which grow in marshy ground are selected. A slit is made in one side of the rush, along which the sharp end of a bit of stick is drawn, so as to extract the pith, when the skin of the rush closes again by its own elasticity. This tube is filled up with gunpowder, dropped into the vent-hole, and made steady with a bit of clay. A papersmift, adjusted to burn a proper time, is then fixed to the top of the rush-tube, and kindled, when the men of the mine retire to a safe distance.Infig.709.the portion of the rock which would be dislodged by the explosion, is that included betweenAandB. The charge of powder is represented by the white part which fills the hole up toC; from which point to the top, the hole is filled withtamping. Thesmiftis shewn atD.Iron bucketFig.711.is an iron bucket, or as it is called in Cornwall, a kibble, in which the ore is raised in the shafts, by machines calledwhims, worked by horses. The best kibblesare made of sheet-iron, and hold each about three hundred weight of ore: 120 kibbles are supposed to clear a cubic fathom of rock.WheelbarrowFig.712.represents the wheelbarrow used under ground for conveying ore and waste to the foot of the shafts. It is made of light deal, except the wheel, which has a narrow rim of iron.VentilatorFig.713.represents Mr. Taylor’s ingenious ventilator, or machine for renewing fresh air in mines. It is so simple in construction, so complete in its operation, requires so little power to work it, and is so little liable to injury from wear, that nothing further of the kind can be desired in ordinary metallic mines. The shaft of the mine is represented atA; at either the top or bottom of which the machine may be placed, as is found most convenient, but the foul air must be discharged into a floor, furnished with a valve-door to prevent its return into the mine.Bis the air-pipe from the mine, passing through the bottom of the fixed vessel or cylinderC, which is formed of timber, and bound with iron hoops. It is filled with water nearly to the top of the pipeB, on which is fixed a valve opening upwards atD.E, the air, or exhausting cylinder of cast-iron, open at bottom, and suspended over the air-pipe, but immersed some way in the water. It is furnished with a wooden top, having an aperture fitted with a valve likewise opening upwards atF. This exhausting cylinder is moved up and down by thebobG, brought into connexion with any engine by the horizontal rodH; the weight of the cylinder being balanced, if necessary, by the counterpoiseI. The action is as follows:—When the cylinder rises, the air from the mine rushes up through the pipe and valveD; and when it descends, this valve shuts, and prevents the return of the air, which is expelled through the valveF. With a cylinder two feet in diameter and six feet long, working from two to three strokes per minute, 200 gallons of air may be discharged in the same time.Gunpowder is the most valuable agent of excavation; possessing a power which has no limit, and which can act every where, even under water. Its introduction, in 1615, caused a great revolution in the mining art.It is employed in mines in different manners, and in different quantities, according to circumstances. In all cases, however, the process resolves itself into boring a hole, and enclosing a cartridge in it, which is afterwards made to explode. The hole is always cylindrical, and is usually made by means of the borer,fig.704., a stem of iron, terminated by a blunt-edged chisel. It sometimes ends in a cross, formed by two chisels set transversely. The workman holds the stem in his left hand, and strikes it with an iron mallet held in his right. He is careful to turn the punch a very little round at every stroke. Several punches are employed in succession, to bore one hole; the first shorter, the latter ones longer, and somewhat thinner. The rubbish is withdrawn as it accumulates, at the bottom of the hole, by means of a picker, which is a small spoon or disc of iron fixed at the end of a slender iron rod. When holes of a large size are to bemade, several men must be employed; one to hold the punch, and one or more to wield the iron mallet. The perforations are seldom less than an inch in diameter, and 18 inches deep; but they are sometimes 2 inches wide, with a depth of 50 inches.The gunpowder, when used, is most commonly put up in paper cartridges. Into the side of the cartridge, a small cylindrical spindle orpierceris pushed. In this state the cartridge is forced down to the bottom of the hole, which is then stuffed, by means of the tamping bar,fig.708., with bits of dry clay, or friable stones coarsely pounded.[33]The piercer is now withdrawn, which leaves in its place, a channel through which fire may be conveyed to the charge. This is executed either by pouring gunpowder into that passage, or by inserting into it, reeds, straw stems, quills, or tubes of paper filled with gunpowder. This is exploded by a long match, which the workmen kindle, and then retire to a place of safety.[33]Sir Rose Price invented a cap of bronze alloy, to tip the lower end of the iron rod; a contrivance now generally used in Cornwall. Before the Geological Society of that county introduced this invention into practice, scarcely a month elapsed without some dreadful explosion sending the miner to an untimely grave, or so injuring him by blowing out his eyes, or shattering his limbs, as to render him a miserable object of charity for the rest of his days. Scarcely has any accident happened since the employment of the new tamping-bar. When the whole bar was made of the tin and copper alloy it was expensive, and apt to bend; but the iron rod tipped with the bronze is both cheap and effectual. An ingenious instrument, called the shifting cartridge, was invented by Mr. Chinalls, and is described in the Transactions of the above society.As thepiercermust not only be slender, but stiff, so as to be easily withdrawn when the hole is tamped, iron spindles are usually employed, though they occasionally give rise to sparks, and consequently to dangerous accidents, by their friction against the sides of the hole. Brass piercers have been sometimes tried; but they twist and break too readily.Each hole bored in a mine, should be so placed in reference to the schistose structure of the rock, and to its natural fissures, as to attack and blow up the least resisting masses. Sometimes the rock is prepared beforehand for splitting in a certain direction, by means of a narrow channel excavated with the small hammer.The quantity of gunpowder should be proportional to the depth of the hole, and the resistance of the rock; and merely sufficient to split it. Anything additional would serve no other purpose than to throw the fragments about the mine, without increasing the useful effect. Into the holes of about an inch and a quarter diameter, and 18 inches deep, only two ounces of gunpowder are put.It appears that the effect of the gunpowder may be augmented by leaving an empty space above, in the middle of, or beneath the cartridge. In the mines of Silesia, the consumption of gunpowder has been eventually reduced, without diminishing the product of the blasts, by mixing sawdust with it in certain proportions. The hole has also been filled up with sand in some cases, according to Mr. Jessop’s plan, instead of being packed with stones, which has removed the danger of the tamping operation. The experiments made in this way have given results very advantageous in quarry blasts with great charges of gunpowder; but less favourable in the small charges employed in mines.Water does not oppose an insurmountable obstacle to the employment of gunpowder; but when the hole cannot be made dry, a cartridge bag impermeable to water must be had recourse to, provided with a tube also impermeable, in which thepierceris placed.After the explosion of each mining charge, wedges and levers are employed, to drag away and break down what has been shattered.Wherever the rock is tolerably hard, the use of gunpowder is more economical and more rapid than any tool-work, and is therefore always preferred. A gallery, for example, a yard and a half high, and a yard wide, the piercing of which by the hammer formerly cost from five to ten pounds sterling, the running yard, in Germany, is executed at the present day by gunpowder at from two to three pounds. When, however, a precious mass of ore is to be detached, when the rock is cavernous, which nearly nullifies the action of gunpowder, or when there is reason to apprehend that the shock caused by the explosion may produce an injurious fall of rubbish, hand-tools alone must be employed.In certain rocks and ores of extreme hardness, the use both of tools and gunpowder becomes very tedious and costly. Examples to this effect are seen, in the mass of quartz mingled with copper pyrites, worked at Rammelsberg, in the Hartz, in the masses of stanniferous granite of Geyer and Altenberg in the Erzgebirge of Saxony, &c. In these circumstances, fortunately very rare, the action of fire is used with advantage to diminish the cohesion of the rocks and the ores. The employment of this agent is not necessarily restricted to these difficult cases. It was formerly applied very often to the working of hard substances; but the introduction of gunpowder into the mining art, and the increase in the price of wood, occasion fire to be little used as an ordinary means of excavation, except in places where the scantiness of the population hasleft a great extent of forest timber, as happens at Kongsberg in Norway, at Dannemora in Sweden, at Felsobanya in Transylvania, &c.The action of fire may be applied to the piercing of a gallery, or to the advancement of a horizontal cut, or to the crumbling down of a mass of ore, by the successive upraising of the roof of a gallery already pierced. In any of these cases, the process consists in forming bonfires, the flame of which is made to play upon the parts to be attacked. All the workmen must be removed from the mine during, and even for some time after, the combustion. When the excavations have become sufficiently cool to allow them to enter, they break down with levers and wedges, or even by means of gunpowder, the masses which have been rent and altered by the fire.To complete our account of the manner in which man may penetrate into the interior of the earth, we must point out the form of the excavations that he should make in it.In mines, three principal species of excavations may be distinguished; viz.shafts,galleries, and thecavitiesof greater or less magnitude which remain in the room of the old workings.Ashaftorpitis a prismatic or cylindrical hollow space, the axis of which is either vertical or much inclined to the horizon. The dimension of the pit, which is never less than 32 inches in its narrowest diameter, amounts sometimes to several yards. Its depth may extend to 1000 feet, and more. Whenever a shaft is opened, means must be provided to extract the rubbish which continually tends to accumulate at its bottom, as well as the waters which may percolate down into it; as also to facilitate the descent and ascent of the workmen. For some time a wheel and axle erected over the mouth of the opening, which serve to elevate one or two buckets of proper dimensions, may be sufficient for most of these purposes. But such a machine becomes ere long inadequate. Horse-whims, or powerful steam-engines, must then be had recourse to; and effectual methods of support must be employed to prevent the sides of the shaft from crumbling and falling down.AGalleryis a prismatic space, the straight or winding axis of which does not usually deviate much from the horizontal line. Two principal species are distinguished; the galleries ofelongation, which follow the direction of a bed or a vein; and thetransversegalleries, which intersect this direction under an angle not much different from 90°. The most ordinary dimensions of galleries are a yard wide, and two yards high; but many still larger may be seen traversing thick deposits of ore. There are few whose width is less than 24 inches, and height less than 40; such small drifts serve merely as temporary expedients in workings. Some galleries are several leagues in length. We shall describe in the sequel the means which are for the most part necessary to support the roof and the walls. The rubbish is removed by waggons or wheelbarrows of various kinds. Seefig.712.It is impossible to advance the boring of a shaft or gallery beyond a certain rate, because only a limited set of workmen can be made to bear upon it. There are some galleries which have taken more than 30 years to perforate. The only expedient for accelerating the advance of a gallery, is to commence, at several points of the line to be pursued, portions of galleries which may be joined together on their completion.Whether tools or gunpowder be used in making the excavations, they should be so applied as to render the labour as easy and quick as possible, by disengaging the mass out of the rock at two or three of its faces. The effect of gunpowder, wedges, or picks, is then much more powerful. The greater the excavation, the more important is it to observe this rule. With this intent, the working is disposed in the form ofsteps(gradins), placed like those of a stair; each step being removed in successive portions, the whole of which, except the last, are disengaged on three sides, at the instant of their being attacked.The substances to be mined occur in the bosom of the earth, under the form of alluvial deposits, beds, pipe-veins, or masses, threads or small veins, and rake-veins.When the existence of a deposit of ore is merely suspected, without positive proofs, recourse must be had to labours of research, in order to ascertain the richness, nature, and disposition of a supposed mine. These are divided into three kinds;open workings,subterranean workings, andboring operations.1. Theworking by an open trench, has for its object to discover the outcropping or basset edges of strata or veins. It consists in opening a fosse of greater or less width, which, after removing the vegetable mould, the alluvial deposits, and the matters disintegrated by the atmosphere, discloses the native rocks, and enables us to distinguish the beds which are interposed, as well as the veins that traverse them. The trench ought always to be opened in a direction perpendicular to the line of the supposed deposit. This mode of investigation costs little, but it seldom gives much insight. It is chiefly employed for verifying the existence of a supposed bed or vein.Thesubterranean workingsafford much more satisfactory knowledge. They are executed by different kinds of perforations; viz. bylongitudinal gallerieshollowed outof the mass of the beds or veins themselves, in following their course; bytransverse galleries, pushed at right angles to the direction of the veins; byinclined shafts, which pursue the slope of the deposits, and are excavated in their mass; or, lastly, byperpendicular pits.If a vein or bed unveils itself on the flank of a mountain, it may be explored, according to the greater or less slope of its inclination, either by a longitudinal gallery opened in its mass, from the outcropping surface, or by a transverse gallery falling upon it in a certain point, from which either an oblong gallery or a sloping shaft may be opened.If our object be to reconnoitre a highly inclined stratum, or a vein in a level country, we shall obtain it with sufficient precision, by means of shafts, 8 or 10 yards deep, dug at 30 yards distance from one another; excavated in the mass of ore, in the direction of its deposit. If the bed is not very much inclined, only 45°, for example, vertical shafts must be opened in the direction of its roof, or of the superjacent rocky stratum, and galleries must be driven from the points in which they meet the ore, in the line of its direction.When the rocks which cover valuable minerals are not of very great hardness, as happens generally with the coal formation, with pyritous and aluminous slates, sal gem, and some other minerals of the secondary strata, theboreris employed with advantage to ascertain their nature. This mode of investigation is economical, and gives, in such cases, a tolerably exact insight into the riches of the interior. The method of using the borer, has been described underArtesian Wells.OF MINING IN PARTICULAR.The mode of working mines is two-fold; byopen excavations, andsubterranean.Workings in the open air present few difficulties, and occasion little expense, unless when pushed to a great depth. They are always preferred for working deposits little distant from the surface; where, in fact, other methods cannot be resorted to, if the substance to be raised be covered with incoherent matters. The only rules to be observed are, to arrange the workings in terraces, so as to facilitate the cutting down of the earth; to transport the ores and the rubbish to their destination at the least possible expense; and to guard against the crumbling down of the sides. With the latter view, they ought to have a suitable slope, or to be propped by timbers whenever they are not quite solid.Open workings, are employed for valuable clays, sands, as also for the alluvial soils of diamonds, gold, and oxide of tin, bog iron ores, &c., limestones, gypsums, building stones, roofing slates, masses of rock salt in some situations, and certain deposits of ores, particularly the specular iron of the island of Elba; the masses of stanniferous granite ofGeyer,Altenberg, andSeyffen, in the Erzgebirge, a chain of mountains between Saxony and Bohemia; the thick veins or masses of black oxide of iron of Nordmarch, Dannemora, &c., in Sweden; the mass of cupreous pyrites of Ræraas, near Drontheim, in Norway; several mines of iron, copper, and gold in the Ural mountains, &c.Subterranean workingsmay be conveniently divided into five classes, viz.:—1. Veins, or beds, much inclined to the horizon, having a thickness of at least two yards.2. Beds of slight inclination, or nearly horizontal, the power or thickness of which does not exceed two yards.3. Beds of great thickness, but slightly inclined.4. Veins, or beds highly inclined, of great thickness.5. Masses of considerable magnitude in all their dimensions.Subterranean miningrequires two very distinct classes of workings; thepreparatory, and those forextraction.Thepreparatoryconsist in galleries, or in pits and galleries destined to conduct the miner to the point most proper for attacking the deposit of ore, for tracing it all round this point, for preparing chambers of excavation, and for concerting measures with a view to the circulation of air, the discharge of waters, and the transport of the extracted minerals.If the vein or bed in question be placed in a mountain, and if its direction forms a very obtuse angle with the line of the slope, the miner begins by opening in its side, at the lowest possible level, a gallery of elongation, which serves at once to give issue to the waters, to explore the deposit through a considerable extent, and then to follow it in another direction; but to commence the real mining operations, he pierces either shafts or galleries, according to the slope of the deposit, across the first gallery.For a stratum little inclined to the horizon, placed beneath a plain, the first thing is to pierce two vertical shafts, which are usually made to arrive at two points in the same line of slope, and a gallery is driven to unite them. It is, in the first place, for the sake of circulation of air that these two pits are sunk; one of them, which is also destined for the drainage of the waters, should reach the lowest point of the intended workings.If a vein is intersected by transverse ones, the shafts are placed so as to follow, or, at least, to cut through the intersections. When the mineral ores lie in nearly vertical masses, it is right to avoid, as far as possible, sinking pits into their interior. These should rather be perforated at one side of their floor, even at some considerable distance, to avoid all risk of crumbling the ores into a heap of rubbish, and overwhelming the workmen.With a vein of less than two yards thick, as soon as the preparatory labours have brought the miners to the point of the vein from which the ulterior workings are to ramify, whenever a circulation of air has been secured, and an outlet to the water and the matters mined, the first object is to divide the mass of ore into large parallelopipeds, by means of oblong galleries, pierced 20 or 25 yards below one another, with pits of communication opened up, 30, 40, or 50 yards asunder, which follow the slope of the vein. These galleries and shafts are usually of the same breadth as the vein, unless when it is very narrow, in which case it is requisite to cut out a portion of the roof or the floor. Such workings serve at once the purposes of mining, by affording a portion of ore, and the complete investigation of the nature and riches of the vein, a certain extent of which is thus prepared before removing the cubical masses. It is proper to advance first of all, in this manner, to the greatest distance from the central point which can be mined with economy, and afterwards to remove the parallelopiped blocks, in working back to that point.This latter operation may be carried on in two different ways; of which one consists in attacking the ore from above; and another from below. In either case, the excavations are disposed in steps similar to a stair upon their upper or under side. The first is styled aworkingin direct or descending steps; and the second aworkinginreverse, or ascending steps.
MINES; (Bergwerke, Germ.) Amidst the variety of bodies apparently infinite, which compose the crust of the globe, geologists have demonstrated the prevalence of a fewgeneral systems of rocks, to which they have given the name offormationsordeposits. A large proportion of these mineral systems consists of parallel planes, whose length and breadth greatly exceed their thickness; on which account they are called stratified rocks; others occur in very thick blocks, without any parallel stratification, or horizontal seams of considerable extent.
The stratiform deposits are subdivided into two great classes; the primary and the secondary. The former seem to have been called into existence before the creation of organic matter, because they contain no exuviæ of vegetable or animal beings; while the latter are more or less interspersed, and sometimes replete with organic remains. The primary strata are characterized, moreover, by the nearly vertical or highly inclined position of their planes; the secondary lie for the most part in a nearly horizontal position.
Where the primitive mountains graduate down into the plains, rocks of an intermediate character appear, which, though possessing a nearly vertical position, contain a few vestiges of animal beings, especially shells. These have been calledtransition, to indicate their being the passing links between the first and second systems of ancient deposits; they are distinguished by the fractured and cemented texture of their planes, for which reason they are sometimes called conglomerate.
Between these and the truly secondary rocks, another very valuable series is interposed in certain districts of the globe; namely, the coal-measures, the paramount formation of Great Britain. The coal strata are disposed in a basin-form, and alternate with parallel beds of sandstone, slate-clay, iron-stone, and occasionally limestone. Some geologists have called the coal-measures the medial formation.
In every mineral plane, the inclination and direction are to be noted; the former being the angle which it forms with the horizon, the latter the point of the azimuth or horizon, towards which it dips, as west, north-east, south, &c. The direction of the bed is that of a horizontal line drawn in its plane; and which is also denoted by the point of the compass. Since the lines of direction and inclination are at right angles to each other, the first may always be inferred from the second; for when a stratum is said to dip to the east or west, this implies that its direction is north and south.
The smaller sinuosities of the bed are not taken into account, just as the windings of a river are neglected in stating the line of its course.
Massesare mineral deposits, not extensively spread in parallel planes, but irregular heaps, rounded or oval, enveloped in whole or in a great measure by rocks of a different kind. Lenticular masses being frequently placed between two horizontal or inclined strata, have been sometimes supposed to be stratiform themselves, and have been accordingly denominated by the Germansliegende stocke,lying heapsorblocks.
The orbicular masses often occur in the interior of unstratified mountains, or in the bosom of one bed.
Nests,concretions,nodules, are small masses found in the middle of strata; the first being commonly in a friable state; the second often kidney-shaped, or tuberous; the third nearly round, and encrusted, like the kernel of an almond.
Lodes, or large veins, are flattened masses, with their opposite surfaces not parallel, which consequently terminate like a wedge, at a greater or less distance, and do not run parallel with the rocky strata in which they lie, but cross them in a direction not far from the perpendicular; often traversing several different mineral planes. Thelodesare sometimes deranged in their course, so as to pursue for a little way the space between two contiguous strata; at other times they divide into several branches. The matter which fills the lodes is for the most part entirely different from the rocks they pass through, or at least it possesses peculiar features.
This mode of existence, exhibited by several mineral substances, but which has been long known with regard to metallic ores, suggests the idea of clefts or rents having been made in the stratum posterior to its consolidation, and of the vacuities having been filled with foreign matter, either immediately or after a certain interval. There can be no doubt as to the justness of the first part of the proposition, for there may be observed round many lodes undeniable proofs of the movement or dislocation of the rock; for example, upon each side of the rent, the same strata are no longer situated in the same plane as before, but make greater or smaller angles with it; or the stratum upon one side of the lode is raised considerably above, or depressed considerably below, its counterpart upon the other side. With regard to the manner in which the rent has been filled, different opinions may be entertained. In the lodes which are widest near the surface of the ground, and graduate into a thin wedge below, the foreign matter would seem to have been introduced as into a funnel at the top, and to have carried along with it in its fluid state portions of rounded gravel and organic remains. In other cases, other conceptions seem to be more probable; since many lodes are largest at their under part, and become progressively narrower as they approach the surface; from which circumstance, it has been inferred that the rent has been caused by anexpansive force acting from within the earth, and that the foreign matter, having been injected in a fluid state, has afterwards slowly crystallized. This hypothesis accounts much better than the other for most of the phenomena observable in mineral veins, for the alterations of the rock at their sides, for the crystallization of the different substances interspersed in them, for the cavities bestudded with little crystals, and for many minute peculiarities. Thus, the large crystals of certain substances which line the walls of hollow veins, have sometimes their under surfaces besprinkled with small crystals of sulphurets, arseniurets, &c., while their upper surfaces are quite smooth; suggesting the idea of a slow sublimation of these volatile matters from below, by the residual heat, and their condensation upon the under faces of the crystalline bodies, already cooled. This phenomenon affords a strong indication of the igneous origin of metalliferous veins.
In the lodes, the principal matters which fill them are to be distinguished from the accessory substances; the latter being distributed irregularly, amidst the mass of the first, in crystals, nodules, grains, seams, &c. The non-metalliferous exterior portion, which is often the largest, is calledgangue, from the Germangang,vein. The position of a vein is denoted, like that of the strata, by the angle of inclination, and the point of the horizon towards which they dip, whence the direction is deduced.
Veins, are merely small lodes, which sometimes traverse the great ones, ramifying in various directions, and in different degrees of tenuity.
A metalliferous substance is said to bedisseminated, when it is dispersed in crystals, spangles, scales, globules, &c., through a large mineral mass.
Certain ores which contain the metals most indispensable to human necessities, have been treasured up by the Creator in very bountiful deposits; constituting either great masses in rocks of different kinds, or distributed in lodes, veins, nests, concretions, or beds with stony and earthy admixtures; the whole of which become the objects of mineral exploration. These precious stores occur in different stages of the geological formations; but their main portion, after having existed abundantly in the several orders of the primary strata, suddenly cease to be found towards the middle of the secondary. Iron ores are the only ones which continue among the more modern deposits, even so high as the beds immediately beneath the chalk, when they also disappear, or exist merely as colouring matters of the tertiary earthy beds.
The strata of gneiss and mica-slate constitute in Europe the grand metallic domain. There is hardly any kind of ore which does not occur there in sufficient abundance to become the object of mining operations, and many are found no where else. The transition rocks and the lower part of the secondary ones, are not so rich, neither do they contain the same variety of ores. But this order of things, which is presented by Great Britain, Germany, France, Sweden, and Norway, is far from forming a general law; since in equinoxial America the gneiss is but little metalliferous; while the superior strata, such as the clay-schists, the sienitic porphyries, the limestones, which complete the transition series, as also several secondary deposits, include the greater portion of the immense mineral wealth of that region of the globe.
All the substances of which the ordinary metals form the basis, are not equally abundant in nature; a great proportion of the numerous mineral species which figure in our classifications, are mere varieties scattered up and down in the cavities of the great masses or lodes. The workable ores are few in number, being mostly sulphurets, some oxides, and carbonates. These occasionally form of themselves very large masses, but more frequently they are blended with lumps of quartz felspar, and carbonate of lime, which form the main body of the deposit; as happens always in proper lodes. The ores in that case are arranged in small layers parallel to the strata of the formation, or in small veins which traverse the rock in all directions, or in nests or concretions stationed irregularly, or finally disseminated in hardly visible particles. These deposits sometimes contain apparently only one species of ore, sometimes several, which must be mined together, as they seem to be of contemporaneous formation; whilst, in other cases, they are separable, having been probably formed at different epochs. In treating of the several metals in their alphabetical order, I have taken care to describe their peculiar geological positions, and the rocks which accompany or mineralize them.
In mining, as in architecture, the best method of imparting instruction is to display the master-pieces of the respective arts, which speak clearly to the mind through the medium of the eye. It is not so easy, however, to represent at once the general effect of a mine, as it is of an edifice; because there is no point of sight from which the former can be sketched at once, like the latter. The subterraneous structures certainly afford some of the finest examples of the useful labours of man, continued for ages, under the guidance of science and ingenuity; but, however curious, beautiful, and grand in themselves, they cannot become objects of a panoramic view. It is only by the lights of geometry and geology that mines can be contemplated and surveyed, either as a whole or in their details; and, therefore, these marvellous subterranean regions, in which roads are cutmany hundred miles long, are altogether unknown or disregarded by men of the world. Should any of them, perchance, from curiosity or interest, descend into these dark recesses of the earth, they are prepared to discover only a few insulated objects, which they may think strange or possibly hideous; but they cannot recognize either the symmetrical disposition of mineral bodies, or the laws which govern geological phenomena, and serve as sure guides to the skilful miner in his adventurous search. It is by exact plans and sections of subterraneous workings, that a knowledge of the nature, extent, and distribution of mineral wealth, can be acquired.
698.A general view of mining operations.
698.A general view of mining operations.
As there is no country in the world so truly rich and powerful, by virtue of its mineral stores, as Great Britain, so there are no people who ought to take a deeper interest in their scientific illustration. I have endeavoured in the present article to collect from the most authentic sources the most interesting and instructive examples of mining operations.
To the magnificent work of Ville-Fosse,Sur la Richesse Minerale, no longer on sale, I have to acknowledge weighty obligations; many of the figures being copied from his great Atlas.
Lodes or mineral veins are usually distinguished by English miners into at least four species. 1. The rake vein. 2. The pipe vein. 3. The flat or dilated vein; and 4. The interlaced mass (stock-werke), indicating the union of a multitude of small veins mixed in every possible direction with each other, and with the rock.
1. Therakevein is a perpendicular mineral fissure; and is the form best known among practical miners. It commonly runs in a straight line, beginning at the superficies of the strata, and cutting them downwards, generally further than can be reached. This vein sometimes stands quite perpendicular; but it more usually inclines or hangs over at a greater or smaller angle, or slope, which is called by the miners thehadeorhadingof the vein. The line of direction in which the fissure runs, is called thebearing of the vein.
2. Thepipevein resembles in many respects a huge irregular cavern, pushing forward into the body of the earth in a sloping direction, under various inclinations, from an angle of a few degrees to the horizon, to a dip of 45°, or more. The pipe does not in general cut the strata across like the rake vein, but insinuates itself between them; so that if the plane of the strata be nearly horizontal, the bearing of the pipe vein will be conformable; but if the strata stand up at a high angle, the pipe shoots down nearly headlong like a shaft. Some pipes are very wide and high, others are very low and narrow, sometimes not larger than a common mine or drift.
3. Theflatordilatedvein, is a space or opening between two strata or beds of stone, the one of which lies above, and the other below this vein, like a stratum of coalbetween its roof and pavement; so that the vein and the strata are placed in the same plane of inclination. These veins are subject, like coal, to be interrupted, broken, and thrown up or down by slips, dykes, or other interruptions of the regular strata. In the case of a metallic vein, a slip often increases the chance of finding more treasure. Such veins do not preserve the parallelism of their beds, characteristic of coal seams; but vary excessively in thickness within a moderate space. Flat veins occur frequently in limestone, either in a horizontal or declining direction. The flat or strata veins open and close, as the rake veins also do.
4. The interlaced mass has been already defined.
To these may be added theaccumulatedvein, or irregular mass (butzenwerke), a great deposit placed without any order in the bosom of the rocks, apparently filling up cavernous spaces.
The interlaced masses are more frequent in primitive formations, than in the others; and tin is the ore which most commonly affects this locality. Seefigure ofTinmine.
The study of the mineral substances, calledganguesor vein-stones, which usually accompany the different ores, is indispensable in the investigation and working of mines. Thesegangues, such as quartz, calcareous spar, fluor spar, heavy spar, &c., and a great number of other substances, although of little or no value in themselves, become of great consequence to the miner, either by pointing out by their presence that of certain useful minerals, or by characterising in their several associations, different deposits of ores of which it may be possible to follow the traces, and to discriminate the relations, often of a complicated kind, provided we observe assiduously the accompanyinggangues.
Mineral veins are subject to derangements in their course, which are called shifts or faults. Thus, when a transverse vein throws out, or intercepts, a longitudinal one, we must commonly look for the rejected vein on the side of the obtuse angle which the direction of the latter makes with that of the former. When a bed of ore is deranged by a fault, we must observe whether the slip of the strata be upwards or downwards; for in either circumstance, it is only by pursuing the direction of the fault that we can recover the ore; in the former case by mounting, in the latter by descending beyond the dislocation.
When two veins intersect each other, the direction of theoffcastis a subject of interest, both to the miner and the geologist. In Saxony it is considered as a general fact that the portion thrown out is always upon the side of the obtuse angle, a circumstance which holds also in Cornwall; and the more obtuse the angle, the out-throw is the more considerable. A vein may be thrown out on meeting another vein, in a line which approaches either towards its inclination or its direction. The Cornish miners use two different terms to denote these two modes of rejection; for the first case, they say the vein isheaved; for the second, it isstarted.
Copper lode
The great copper lode of Carharack,d,fig.699.in the parish of Gwenap, is one of the most instructive examples of intersection. The power or thickness of this vein is 8 feet; its direction is nearly due east and west, and it dips towards the north at an inclination of two feet per fathom; its upper part being in thekillas(a greenish clay-slate); its lower part in the granite. The lode has suffered two intersections; the first produced by meeting the veinh, calledSteven’s fluckan, which runs from north-east to south-west, and which throws the lode several fathoms out; the second is produced by another veini, almost at right angles with the first, and which occasions another out-throw of 20 fathoms to the right side. The fall of the vein occurs therefore in the one case to the right, and in the other to the left; but in both it is towards the side of the obtuse angle. This distribution is very singular; for one part of the vein appears to have mounted while the other has descended.N,Sdenotes North and South.dis the copper lode running east and west.h,i, are systems of clay-slate veins called fluckans; the line overS, represents the down-shift, andd′the up-shift.
General observations on the localities of ores, and on the indications of metallic mines.
1.Tin, exists principally in primitive rocks, appearing either in interlaced masses, in beds, or as a constituent part of the rock itself, and more rarely in distinct veins. Tin ore is found indeed sometimes in alluvial land, filling up low situations between lofty mountains.
2.Gold, occurs either in beds, or in veins, frequently in primitive rocks; though in other formations, and particularly in alluvial earth, it is also found. When this metal exists in the bosom of primitive rocks, it is particularly in schists; it is not found in serpentine, but it is met with in greywacke in Transylvania. The gold of alluvial districts,called gold of washing or transport, occurs, as well as alluvial tin, among the debris of the more ancient rocks.
3.Silver, is found particularly in veins and beds, in primitive and transition formations; though some veins of this metal occur in secondary strata. The rocks richest in it are, gneiss, mica-slate, clay-slate, greywacke, and old alpine limestone. Localities of silver-ore itself are not numerous, at least in Europe, among secondary formations; but it occurs in combination with the ores of copper or of lead.
4.Copper, exists in the three mineral epochas; 1. in primitive rocks, principally in the state of pyritous copper, in beds, in masses, or in veins; 2. in transition districts, sometimes in masses, sometimes in veins of copper pyrites; 3. in secondary strata, especially in beds of cupreous schist.
5.Lead, occurs also in each of the three mineral epochas; abounding particularly in primitive and transition grounds, where it usually constitutes veins, and occasionally beds of sulphuretted lead (galena). The same ore is found in strata or in veins among secondary rocks, associated now and then with ochreous iron-oxide and calamine (carbonate of zinc); and it is sometimes disseminated in grains through more recent strata.
6.Iron, is met with in four different mineral eras, but in different ores. Among primitive rocks, magnetic iron ore and specular iron ore occur chiefly in beds, sometimes of enormous size; the ores of red or brown oxide of iron (hæmatite) are found generally in veins, or occasionally in masses with sparry iron, both in primitive and transition rocks; as also sometimes in secondary strata; but more frequently in the coal-measure strata, as beds of clay-ironstone, of globular iron oxide, and carbonate of iron. In alluvial districts we find ores of clay-ironstone, granular iron-ore, bog-ore, swamp-ore, and meadow-ore. The iron ores which belong to the primitive period have almost always the metallic aspect, with a richness amounting even to 80 per cent. of iron, while the ores in the posterior formations become in general more and more earthy, down to those in alluvial soils, some of which present the appearance of a common stone, and afford not more than 20 per cent. of metal, though its quality is often excellent.
7.Mercury, occurs principally among secondary strata, in disseminated masses, along with combustible substances; though the metal is met with occasionally in primitive countries.
8.Cobalt, belongs to the three mineral epochas; its most abundant deposits are veins in primitive rocks; small veins containing this metal are found, however, in secondary strata.
9.Antimony, occurs in veins or beds among primitive and transition rocks.
10, 11. Bismuth and nickel do not appear to constitute the predominating substance of any mineral deposits; but they often accompany cobalt.
12.Zinc, occurs in the three several formations: namely, as sulphuret or blende, particularly in primitive and transition rocks; as calamine, in secondary strata, usually along with oxide of iron, and sometimes with sulphuret of lead.
An acquaintance with the general results collected and classified by geology must be our first guide in the investigation of mines. This enables the observer to judge whether any particular district, should from the nature and arrangement of its rocks, be susceptible of including within its bosom, beds of workable ores; it indicates also, to a certain degree, what substances may probably be met with in a given series of rocks, and what locality these substances will preferably affect. For want of a knowledge of these facts, many persons have gone blindly into researches equally absurd and ruinous.
Formerly indications of mines were taken from very unimportant circumstances; from thermal waters, the heat of which was gratuitously referred to the decomposition of pyrites; from mineral waters, whose course is however often from a far distant source; from vapours incumbent over particular mountain groups; from the snows melting faster in one mineral district than another; from the different species of forest trees, and from the greater or less vigour of vegetation, &c. In general, all such indications are equally fallacious with the divining rod, and the compass made of a lump of pyrites suspended by a thread.
Geognostic observation has substituted more rational characters of metallic deposits, some of which may be callednegativeand otherspositive.
Thenegativeindications are derived from that peculiar geological constitution, which from experience or general principles excludes certain metallic matters; for example, granite, and in general every primitive formation, forbids the hope of finding within them combustible fossils (pit-coal), unless it be beds of anthracite; there also it would be vain to seek for sal gem. It is very seldom that granite rocks include silver; or limestones, ores of tin. Volcanic territories never afford any metallic ores worth the working; nor do extensive veins usually run into secondary and alluvial formations. The richer ores of iron do not occur in secondary strata; and the ores of this metal peculiar to these localities, do not exist among primary rocks.
Amongpositiveindications, some are proximate and others remote. The proximate are, an efflorescence, so to speak, of the subjacent metallic masses; magnetic attraction for iron ores; bituminous stone, or inflammable gas for pit-coal; the frequent occurrence of fragments of particular ores, &c. The remote indications consist in the geological epocha, and nature of the rocks. From the examples previously adduced, marks of this kind acquire new importance when in a district susceptible of including deposits of workable ores, theganguesor vein-stones are met with which usually accompany any particular metal. The general aspect of mountains whose flanks present gentle and continuous slopes, the frequency of sterile veins, the presence of metalliferous sands, the neighbourhood of some known locality of an ore, for instance that of iron-stone in reference to coal, lastly the existence of salt springs and mineral waters, may furnish some indications; but when ferruginous or cupreous waters issue from sands or clays, such characters merit in general little attention, because the waters may flow from a great distance. No greater importance can be attached to metalliferous sands and saline springs.
In speaking of remote indications, we may remark that in several places, and particularly near Clausthal in the Hartz, a certain ore of red oxide of iron occurs above the most abundant deposits of the ores of lead and silver; whence it has been named by the Germans theiron-hat. It appears that the iron ore rich in silver, which is worked in America under the name ofpacos, has some analogy with this substance; but iron ore is in general so plentifully diffused on the surface of the soil, that its presence can be regarded as only a remote indication, relative to other mineral substances, except in the case of clay ironstone with coal.
Of the instruments and operations of subterranean operations.—It is by the aid of geometry in the first place that the miner studies the situation of the mineral deposits, on the surface and in the interior of the ground; determines the several relations of the veins and the rocks; and becomes capable of directing the perforations towards a suitable end.
The instruments are, 1. the magnetic compass, which is employed to measure the direction of a metallic ore, wherever the neighbourhood of iron does not interfere with its functions; 2. the graduated semicircle which serves to measure the inclination, which is also called the clinometer.
3. The chain or cord for measuring the distance of one point from another.
4. When the neighbourhood of iron renders the use of the magnet uncertain, a plate or plane table is employed.
The dials of the compasses generally used in the most celebrated mines, are graduated into hours; most commonly into twice 12 hours. Thus the whole limb is divided into 24 spaces, each of which contains 15° = 1 hour. Each hour is subdivided into 8 parts.
Means of penetrating into the interior of the earth.—In order to penetrate into the interior of the earth, and to extract from it the objects of his toils, the miner has at his disposal several means, which may be divided into three classes: 1.manual tools, 2.gunpowder, and 3.fire.
The tools used by the miners of Cornwall and Devonshire are the following:
Mining tools
Fig.700.Thepick. It is a light tool, and somewhat varied in shape according to circumstances. One side used as a hammer is called thepoll, and is employed to drive in thegads, or to loosen and detach prominences. Thepointis of steel, carefully tempered, and drawn under the hammer to the proper form. The French call itpointerolle.
Fig.701.Thegad. It is a wedge of steel, driven into crevices of rocks, or into small openings made with the point of the pick.
Fig.702.Theminer’s shovel. It has a pointed form, to enable it to penetrate among the coarse and hard fragments of the mine rubbish. Its handle being somewhat bent, a man’s power may be conveniently applied without bending his body.
Theblastingorshootingtools are:—
Besides these tools the miner requires a powder-horn, rushes to be filled with gunpowder, tin cartridges for occasional use in wet ground, and paper rubbed over with gunpowder or grease, for thesmiftsor fuses.
Theborer,fig.704., is an iron bar tipped with steel, formed like a thick chisel, and is used by one man holding it straight in the hole with constant rotation on its axis, while another strikes the head of it with the iron sledge or mallet,fig.703.The hole is cleared out from time to time by the scraper,fig.707., which is a flat iron rod turned up at one end. If the ground be very wet, and the hole gets full of mud, it is cleaned out by a stick bent at the end into a fibrous brush, called aswab-stick.
Rock blasting
Fig.709.represents the plan of blasting the rock, and a section of a hole ready for firing. The hole must be rendered as dry as possible, which is effected very simply by filling it partly with tenacious clay, and then driving into it a tapering iron rod, which nearly fills its calibre, called theclaying bar. This being forced in with great violence, condenses the clay into all the crevices of the rock, and secures the dryness of the hole. Should this plan fail, recourse is had to tin cartridges furnished with a stem or tube (seefig.710.,) through which the powder may be inflamed. When the hole is dry, and the charge of powder introduced, thenail, a small taper rod of copper, is inserted so as to reach the bottom of the hole, which is now ready fortamping. By this difficult and dangerous process, the gunpowder is confined, and the disruptive effect produced. Different substances are employed fortamping, or cramming the hole, the most usual one being any soft species of rock free from siliceous or flinty particles. Small quantities of it only are introduced at a time, and rammed very hard by thetamping-bar, which is held steadily by one man, and struck with a sledge by another. The hole being thus filled, the nail is withdrawn by putting a bar through its eye, and striking it upwards. Thus a small perforation or vent is left for the rush which communicates the fire.
Besides the improved tamping-bar faced with hard copper, other contrivances have been resorted to for diminishing the risk of those dreadful accidents that frequently occur in this operation. Dry sand is sometimes used as a tamping material, but there are many rocks for the blasting of which it is ineffective. Tough clay will answer better in several situations.
For conveying the fire, the large and long green rushes which grow in marshy ground are selected. A slit is made in one side of the rush, along which the sharp end of a bit of stick is drawn, so as to extract the pith, when the skin of the rush closes again by its own elasticity. This tube is filled up with gunpowder, dropped into the vent-hole, and made steady with a bit of clay. A papersmift, adjusted to burn a proper time, is then fixed to the top of the rush-tube, and kindled, when the men of the mine retire to a safe distance.
Infig.709.the portion of the rock which would be dislodged by the explosion, is that included betweenAandB. The charge of powder is represented by the white part which fills the hole up toC; from which point to the top, the hole is filled withtamping. Thesmiftis shewn atD.
Iron bucket
Fig.711.is an iron bucket, or as it is called in Cornwall, a kibble, in which the ore is raised in the shafts, by machines calledwhims, worked by horses. The best kibblesare made of sheet-iron, and hold each about three hundred weight of ore: 120 kibbles are supposed to clear a cubic fathom of rock.
Wheelbarrow
Fig.712.represents the wheelbarrow used under ground for conveying ore and waste to the foot of the shafts. It is made of light deal, except the wheel, which has a narrow rim of iron.
Ventilator
Fig.713.represents Mr. Taylor’s ingenious ventilator, or machine for renewing fresh air in mines. It is so simple in construction, so complete in its operation, requires so little power to work it, and is so little liable to injury from wear, that nothing further of the kind can be desired in ordinary metallic mines. The shaft of the mine is represented atA; at either the top or bottom of which the machine may be placed, as is found most convenient, but the foul air must be discharged into a floor, furnished with a valve-door to prevent its return into the mine.Bis the air-pipe from the mine, passing through the bottom of the fixed vessel or cylinderC, which is formed of timber, and bound with iron hoops. It is filled with water nearly to the top of the pipeB, on which is fixed a valve opening upwards atD.E, the air, or exhausting cylinder of cast-iron, open at bottom, and suspended over the air-pipe, but immersed some way in the water. It is furnished with a wooden top, having an aperture fitted with a valve likewise opening upwards atF. This exhausting cylinder is moved up and down by thebobG, brought into connexion with any engine by the horizontal rodH; the weight of the cylinder being balanced, if necessary, by the counterpoiseI. The action is as follows:—When the cylinder rises, the air from the mine rushes up through the pipe and valveD; and when it descends, this valve shuts, and prevents the return of the air, which is expelled through the valveF. With a cylinder two feet in diameter and six feet long, working from two to three strokes per minute, 200 gallons of air may be discharged in the same time.
Gunpowder is the most valuable agent of excavation; possessing a power which has no limit, and which can act every where, even under water. Its introduction, in 1615, caused a great revolution in the mining art.
It is employed in mines in different manners, and in different quantities, according to circumstances. In all cases, however, the process resolves itself into boring a hole, and enclosing a cartridge in it, which is afterwards made to explode. The hole is always cylindrical, and is usually made by means of the borer,fig.704., a stem of iron, terminated by a blunt-edged chisel. It sometimes ends in a cross, formed by two chisels set transversely. The workman holds the stem in his left hand, and strikes it with an iron mallet held in his right. He is careful to turn the punch a very little round at every stroke. Several punches are employed in succession, to bore one hole; the first shorter, the latter ones longer, and somewhat thinner. The rubbish is withdrawn as it accumulates, at the bottom of the hole, by means of a picker, which is a small spoon or disc of iron fixed at the end of a slender iron rod. When holes of a large size are to bemade, several men must be employed; one to hold the punch, and one or more to wield the iron mallet. The perforations are seldom less than an inch in diameter, and 18 inches deep; but they are sometimes 2 inches wide, with a depth of 50 inches.
The gunpowder, when used, is most commonly put up in paper cartridges. Into the side of the cartridge, a small cylindrical spindle orpierceris pushed. In this state the cartridge is forced down to the bottom of the hole, which is then stuffed, by means of the tamping bar,fig.708., with bits of dry clay, or friable stones coarsely pounded.[33]The piercer is now withdrawn, which leaves in its place, a channel through which fire may be conveyed to the charge. This is executed either by pouring gunpowder into that passage, or by inserting into it, reeds, straw stems, quills, or tubes of paper filled with gunpowder. This is exploded by a long match, which the workmen kindle, and then retire to a place of safety.
[33]Sir Rose Price invented a cap of bronze alloy, to tip the lower end of the iron rod; a contrivance now generally used in Cornwall. Before the Geological Society of that county introduced this invention into practice, scarcely a month elapsed without some dreadful explosion sending the miner to an untimely grave, or so injuring him by blowing out his eyes, or shattering his limbs, as to render him a miserable object of charity for the rest of his days. Scarcely has any accident happened since the employment of the new tamping-bar. When the whole bar was made of the tin and copper alloy it was expensive, and apt to bend; but the iron rod tipped with the bronze is both cheap and effectual. An ingenious instrument, called the shifting cartridge, was invented by Mr. Chinalls, and is described in the Transactions of the above society.
[33]Sir Rose Price invented a cap of bronze alloy, to tip the lower end of the iron rod; a contrivance now generally used in Cornwall. Before the Geological Society of that county introduced this invention into practice, scarcely a month elapsed without some dreadful explosion sending the miner to an untimely grave, or so injuring him by blowing out his eyes, or shattering his limbs, as to render him a miserable object of charity for the rest of his days. Scarcely has any accident happened since the employment of the new tamping-bar. When the whole bar was made of the tin and copper alloy it was expensive, and apt to bend; but the iron rod tipped with the bronze is both cheap and effectual. An ingenious instrument, called the shifting cartridge, was invented by Mr. Chinalls, and is described in the Transactions of the above society.
As thepiercermust not only be slender, but stiff, so as to be easily withdrawn when the hole is tamped, iron spindles are usually employed, though they occasionally give rise to sparks, and consequently to dangerous accidents, by their friction against the sides of the hole. Brass piercers have been sometimes tried; but they twist and break too readily.
Each hole bored in a mine, should be so placed in reference to the schistose structure of the rock, and to its natural fissures, as to attack and blow up the least resisting masses. Sometimes the rock is prepared beforehand for splitting in a certain direction, by means of a narrow channel excavated with the small hammer.
The quantity of gunpowder should be proportional to the depth of the hole, and the resistance of the rock; and merely sufficient to split it. Anything additional would serve no other purpose than to throw the fragments about the mine, without increasing the useful effect. Into the holes of about an inch and a quarter diameter, and 18 inches deep, only two ounces of gunpowder are put.
It appears that the effect of the gunpowder may be augmented by leaving an empty space above, in the middle of, or beneath the cartridge. In the mines of Silesia, the consumption of gunpowder has been eventually reduced, without diminishing the product of the blasts, by mixing sawdust with it in certain proportions. The hole has also been filled up with sand in some cases, according to Mr. Jessop’s plan, instead of being packed with stones, which has removed the danger of the tamping operation. The experiments made in this way have given results very advantageous in quarry blasts with great charges of gunpowder; but less favourable in the small charges employed in mines.
Water does not oppose an insurmountable obstacle to the employment of gunpowder; but when the hole cannot be made dry, a cartridge bag impermeable to water must be had recourse to, provided with a tube also impermeable, in which thepierceris placed.
After the explosion of each mining charge, wedges and levers are employed, to drag away and break down what has been shattered.
Wherever the rock is tolerably hard, the use of gunpowder is more economical and more rapid than any tool-work, and is therefore always preferred. A gallery, for example, a yard and a half high, and a yard wide, the piercing of which by the hammer formerly cost from five to ten pounds sterling, the running yard, in Germany, is executed at the present day by gunpowder at from two to three pounds. When, however, a precious mass of ore is to be detached, when the rock is cavernous, which nearly nullifies the action of gunpowder, or when there is reason to apprehend that the shock caused by the explosion may produce an injurious fall of rubbish, hand-tools alone must be employed.
In certain rocks and ores of extreme hardness, the use both of tools and gunpowder becomes very tedious and costly. Examples to this effect are seen, in the mass of quartz mingled with copper pyrites, worked at Rammelsberg, in the Hartz, in the masses of stanniferous granite of Geyer and Altenberg in the Erzgebirge of Saxony, &c. In these circumstances, fortunately very rare, the action of fire is used with advantage to diminish the cohesion of the rocks and the ores. The employment of this agent is not necessarily restricted to these difficult cases. It was formerly applied very often to the working of hard substances; but the introduction of gunpowder into the mining art, and the increase in the price of wood, occasion fire to be little used as an ordinary means of excavation, except in places where the scantiness of the population hasleft a great extent of forest timber, as happens at Kongsberg in Norway, at Dannemora in Sweden, at Felsobanya in Transylvania, &c.
The action of fire may be applied to the piercing of a gallery, or to the advancement of a horizontal cut, or to the crumbling down of a mass of ore, by the successive upraising of the roof of a gallery already pierced. In any of these cases, the process consists in forming bonfires, the flame of which is made to play upon the parts to be attacked. All the workmen must be removed from the mine during, and even for some time after, the combustion. When the excavations have become sufficiently cool to allow them to enter, they break down with levers and wedges, or even by means of gunpowder, the masses which have been rent and altered by the fire.
To complete our account of the manner in which man may penetrate into the interior of the earth, we must point out the form of the excavations that he should make in it.
In mines, three principal species of excavations may be distinguished; viz.shafts,galleries, and thecavitiesof greater or less magnitude which remain in the room of the old workings.
Ashaftorpitis a prismatic or cylindrical hollow space, the axis of which is either vertical or much inclined to the horizon. The dimension of the pit, which is never less than 32 inches in its narrowest diameter, amounts sometimes to several yards. Its depth may extend to 1000 feet, and more. Whenever a shaft is opened, means must be provided to extract the rubbish which continually tends to accumulate at its bottom, as well as the waters which may percolate down into it; as also to facilitate the descent and ascent of the workmen. For some time a wheel and axle erected over the mouth of the opening, which serve to elevate one or two buckets of proper dimensions, may be sufficient for most of these purposes. But such a machine becomes ere long inadequate. Horse-whims, or powerful steam-engines, must then be had recourse to; and effectual methods of support must be employed to prevent the sides of the shaft from crumbling and falling down.
AGalleryis a prismatic space, the straight or winding axis of which does not usually deviate much from the horizontal line. Two principal species are distinguished; the galleries ofelongation, which follow the direction of a bed or a vein; and thetransversegalleries, which intersect this direction under an angle not much different from 90°. The most ordinary dimensions of galleries are a yard wide, and two yards high; but many still larger may be seen traversing thick deposits of ore. There are few whose width is less than 24 inches, and height less than 40; such small drifts serve merely as temporary expedients in workings. Some galleries are several leagues in length. We shall describe in the sequel the means which are for the most part necessary to support the roof and the walls. The rubbish is removed by waggons or wheelbarrows of various kinds. Seefig.712.
It is impossible to advance the boring of a shaft or gallery beyond a certain rate, because only a limited set of workmen can be made to bear upon it. There are some galleries which have taken more than 30 years to perforate. The only expedient for accelerating the advance of a gallery, is to commence, at several points of the line to be pursued, portions of galleries which may be joined together on their completion.
Whether tools or gunpowder be used in making the excavations, they should be so applied as to render the labour as easy and quick as possible, by disengaging the mass out of the rock at two or three of its faces. The effect of gunpowder, wedges, or picks, is then much more powerful. The greater the excavation, the more important is it to observe this rule. With this intent, the working is disposed in the form ofsteps(gradins), placed like those of a stair; each step being removed in successive portions, the whole of which, except the last, are disengaged on three sides, at the instant of their being attacked.
The substances to be mined occur in the bosom of the earth, under the form of alluvial deposits, beds, pipe-veins, or masses, threads or small veins, and rake-veins.
When the existence of a deposit of ore is merely suspected, without positive proofs, recourse must be had to labours of research, in order to ascertain the richness, nature, and disposition of a supposed mine. These are divided into three kinds;open workings,subterranean workings, andboring operations.
1. Theworking by an open trench, has for its object to discover the outcropping or basset edges of strata or veins. It consists in opening a fosse of greater or less width, which, after removing the vegetable mould, the alluvial deposits, and the matters disintegrated by the atmosphere, discloses the native rocks, and enables us to distinguish the beds which are interposed, as well as the veins that traverse them. The trench ought always to be opened in a direction perpendicular to the line of the supposed deposit. This mode of investigation costs little, but it seldom gives much insight. It is chiefly employed for verifying the existence of a supposed bed or vein.
Thesubterranean workingsafford much more satisfactory knowledge. They are executed by different kinds of perforations; viz. bylongitudinal gallerieshollowed outof the mass of the beds or veins themselves, in following their course; bytransverse galleries, pushed at right angles to the direction of the veins; byinclined shafts, which pursue the slope of the deposits, and are excavated in their mass; or, lastly, byperpendicular pits.
If a vein or bed unveils itself on the flank of a mountain, it may be explored, according to the greater or less slope of its inclination, either by a longitudinal gallery opened in its mass, from the outcropping surface, or by a transverse gallery falling upon it in a certain point, from which either an oblong gallery or a sloping shaft may be opened.
If our object be to reconnoitre a highly inclined stratum, or a vein in a level country, we shall obtain it with sufficient precision, by means of shafts, 8 or 10 yards deep, dug at 30 yards distance from one another; excavated in the mass of ore, in the direction of its deposit. If the bed is not very much inclined, only 45°, for example, vertical shafts must be opened in the direction of its roof, or of the superjacent rocky stratum, and galleries must be driven from the points in which they meet the ore, in the line of its direction.
When the rocks which cover valuable minerals are not of very great hardness, as happens generally with the coal formation, with pyritous and aluminous slates, sal gem, and some other minerals of the secondary strata, theboreris employed with advantage to ascertain their nature. This mode of investigation is economical, and gives, in such cases, a tolerably exact insight into the riches of the interior. The method of using the borer, has been described underArtesian Wells.
OF MINING IN PARTICULAR.
The mode of working mines is two-fold; byopen excavations, andsubterranean.
Workings in the open air present few difficulties, and occasion little expense, unless when pushed to a great depth. They are always preferred for working deposits little distant from the surface; where, in fact, other methods cannot be resorted to, if the substance to be raised be covered with incoherent matters. The only rules to be observed are, to arrange the workings in terraces, so as to facilitate the cutting down of the earth; to transport the ores and the rubbish to their destination at the least possible expense; and to guard against the crumbling down of the sides. With the latter view, they ought to have a suitable slope, or to be propped by timbers whenever they are not quite solid.
Open workings, are employed for valuable clays, sands, as also for the alluvial soils of diamonds, gold, and oxide of tin, bog iron ores, &c., limestones, gypsums, building stones, roofing slates, masses of rock salt in some situations, and certain deposits of ores, particularly the specular iron of the island of Elba; the masses of stanniferous granite ofGeyer,Altenberg, andSeyffen, in the Erzgebirge, a chain of mountains between Saxony and Bohemia; the thick veins or masses of black oxide of iron of Nordmarch, Dannemora, &c., in Sweden; the mass of cupreous pyrites of Ræraas, near Drontheim, in Norway; several mines of iron, copper, and gold in the Ural mountains, &c.
Subterranean workingsmay be conveniently divided into five classes, viz.:—
1. Veins, or beds, much inclined to the horizon, having a thickness of at least two yards.
2. Beds of slight inclination, or nearly horizontal, the power or thickness of which does not exceed two yards.
3. Beds of great thickness, but slightly inclined.
4. Veins, or beds highly inclined, of great thickness.
5. Masses of considerable magnitude in all their dimensions.
Subterranean miningrequires two very distinct classes of workings; thepreparatory, and those forextraction.
Thepreparatoryconsist in galleries, or in pits and galleries destined to conduct the miner to the point most proper for attacking the deposit of ore, for tracing it all round this point, for preparing chambers of excavation, and for concerting measures with a view to the circulation of air, the discharge of waters, and the transport of the extracted minerals.
If the vein or bed in question be placed in a mountain, and if its direction forms a very obtuse angle with the line of the slope, the miner begins by opening in its side, at the lowest possible level, a gallery of elongation, which serves at once to give issue to the waters, to explore the deposit through a considerable extent, and then to follow it in another direction; but to commence the real mining operations, he pierces either shafts or galleries, according to the slope of the deposit, across the first gallery.
For a stratum little inclined to the horizon, placed beneath a plain, the first thing is to pierce two vertical shafts, which are usually made to arrive at two points in the same line of slope, and a gallery is driven to unite them. It is, in the first place, for the sake of circulation of air that these two pits are sunk; one of them, which is also destined for the drainage of the waters, should reach the lowest point of the intended workings.If a vein is intersected by transverse ones, the shafts are placed so as to follow, or, at least, to cut through the intersections. When the mineral ores lie in nearly vertical masses, it is right to avoid, as far as possible, sinking pits into their interior. These should rather be perforated at one side of their floor, even at some considerable distance, to avoid all risk of crumbling the ores into a heap of rubbish, and overwhelming the workmen.
With a vein of less than two yards thick, as soon as the preparatory labours have brought the miners to the point of the vein from which the ulterior workings are to ramify, whenever a circulation of air has been secured, and an outlet to the water and the matters mined, the first object is to divide the mass of ore into large parallelopipeds, by means of oblong galleries, pierced 20 or 25 yards below one another, with pits of communication opened up, 30, 40, or 50 yards asunder, which follow the slope of the vein. These galleries and shafts are usually of the same breadth as the vein, unless when it is very narrow, in which case it is requisite to cut out a portion of the roof or the floor. Such workings serve at once the purposes of mining, by affording a portion of ore, and the complete investigation of the nature and riches of the vein, a certain extent of which is thus prepared before removing the cubical masses. It is proper to advance first of all, in this manner, to the greatest distance from the central point which can be mined with economy, and afterwards to remove the parallelopiped blocks, in working back to that point.
This latter operation may be carried on in two different ways; of which one consists in attacking the ore from above; and another from below. In either case, the excavations are disposed in steps similar to a stair upon their upper or under side. The first is styled aworkingin direct or descending steps; and the second aworkinginreverse, or ascending steps.