STRATIFICATION-MINERAL VEINS. 327 ments of civilized society in the metallic state. Most of the ores of the highest importance and utility constitute but a comparatively small portion of the components of the earth's crust; but this deficiency in their relative proportion is more than compensated by the mode of their distribution, for they are not dispersed at random, or diffused in minute quantity uniformly throughout the mass of the earth, but are collected into thin seams or beds, which form mineral veins. Man has hitherto been able to penetrate but to a very small depth into the body of the earth, the deepest excavation which he has been enabled to make being not greater, in proportion to the diameter of the earth, than the thickness of an ordinary sheet of writing paper to a globe of two feet in diameter. Geological observations have shown, and any person who has traversed a railway cutting has had a partial opportunity of convincing himself of the fact, that a great part of the superficial portions of the globe consists of a succession of beds or layers,-strata, as they are commonly termed, which rest one above another: these beds in some places retain nearly their original horizontal direction; but usually they have assumed a position more or less inclined, so as to form a considerable angle with the surface. The same stratum is liable to great variations in thickness in different parts, but each bed is found to occur in a uniform position in the series, the successive strata following each other in regular order, the uppermost being those of most recent formation. In this way the London clay rests upon the chalk, the chalk upon what is termed the green-sand, the green-sand upon the gault, and so on. The stratified or sedimentary rocks rest upon others, which, like granite, porphyry, and basalt, show no appearance of stratification, but bear marks, more or less evident, of having undergone igneous fusion. Occasionally it happens that a thin bed of metallic ore forms a part of the regular succession of the strata; in Staffordshire, for instance, over many square miles of country, thin bands or seams of the ore termed clay iron-stone, varying in thickness from 2 to 8 inches, are found lying between the beds of coal. Usually, however, the metalliferous masses occur in still older formations; such as in the mountain limestone of Cumberland and Derbyshire, or in the granite and clay-slate, as in Cornwall: they are then found in fissures which traverse the ordinary strata of the district, and assume a direction which, though it never becomes quite vertical, still approaches more or less towards this position. These fissures vary in thickness from a few inches to as many feet; they are 328 DIRECTION OF MINERAL VEINS. often filled with masses of basalt, granite, or trachyte (which have been injected from below, whilst the materials were in the molten state under the effects of subterranean heat), and then constitute what the miner terms dykes; but in other cases they are filled with metallic ores, and form mineral veins or lodes. The ore sometimes occurs nearly pure; at others mingled with quartz, fluor-spar, and various crystallized minerals, or else with earthy impurities of different descriptions. These veins extend from the surface downwards, often to a depth greater than can be followed even in the deepest mines. The veins which occur in the same district usually run in two directions, nearly at right angles to each other, the principal or original veins being traversed by the others. In Cornwall, for example, the metalliferous veins run nearly east and west, but they are occasionally intersected more or less obliquely by other lodes, to which the term of caunter (contrary) lodes, or cross courses, has been given. These cross courses, however, are by no means always metalliferous; they often appear to have been occasioned by the action of a force emanating from below, which, after bending and splitting the original strata, produced the fissures which were subsequently filled with quartz, clay, and various minerals. Such cross courses as these not seldom occasion the miner much trouble and perplexity, since the subterranean force necessary to produce them is often attended with great displacement of the original strata. A valuable vein of ore is from this cause frequently interrupted, and is sometimes lost altogether for want of knowing in what direction to seek for it. This sudden break in a vein and its displacement is, in mining language, termed a fault. It is very rarely that a single mineral vein occurs alone; usually several are found together. The thickness of the same vein, as might be expected, is subject to great variations; at one time it dwindles to a mere thread, at others it attains considerable expansion. The most productive veins usually occur near the junction of two dissimilar kinds of rock― the metallic ores having probably accumulated there in consequence of slow voltaic actions which have been going on through uncounted ages, and which have been occasioned by differences in chemical composition of the two contiguous rocks: in Cornwall, for example, where so large a proportion of the mineral wealth of Great Britain is accumulated, the most important mines occur upon the junction of the granite with the clay-slate or killas. (528) Mining Operations.-The existence of a vein having PLAN AND SECTION OF A MINE. 329 been ascertained, and its dip and general direction having been determined, the miner commences by sinking a vertical pit, or shaft, in such a manner that he calculates upon cutting through the lode at some 30 or 40 fathoms below the surface. When he has reached the lode, he drives a gallery, or level, horizontally into it, right and left, raising the ore to the surface through the shaft. If the produce be such as to encourage him to proceed, a second shaft is sunk in the course of the lode, at the distance of about 100 yards from the first, and into this the gallery or level is driven, so as to facilitate the ventilation of the mine and the extraction of the ore. In order to be able to remove the ore from other parts of the lode above and below the point at which the first level is made, the shaft is continued downwards, and other galleries, or cross cuts, as they are termed, are made, both above and below the first level, at intervals of ten fathoms, to meet the lode at different points; these cross cuts are at right angles to the levels. Fig. 324 shows a vertical cross section of the lode at the Callington Mine. E s represents the The engine shaft, v v the vein or lode, and c, c, the cross cuts. levels cannot be shown in this view; but whenever a cross cut meets the lode, a level is driven east and west, in the direction of the lode itself. Fig. 325 shows the arrangement of the levels in the same mine; Es represents the engine shaft, w, a second smaller shaft, and L, L, L, the different levels, the depths of which in fathoms are indicated by the numbers attached to them; these levels communicate at different points by short cuts, or winzes, as the Cornish miners term them; they are shown at u, u, in various parts, and are needed to facilitate the extraction of the ore from different parts of the lode. The different levels are not immediately over, or parallel to each other, but their direction and position varies with that of the inclination and direction of the lode. This is explained by fig. 326, in which the direction of these galleries is exhibited; it represents a plan of the mine, supposing the figures to refer to the levels shown in 325: the lode, it will be seen, does not preserve the same dip at all points, being much more nearly vertical at the right than at the left extremity of the plan. The cross cuts cannot be shown in fig. 325. The shaded parts in this figure indicate the portions of the lode which have been already worked away. The galleries in the mine are supported by strong timbering, the object of which is to prevent the rubbish from falling in and overwhelming the men while engaged in their work. One of the principal difficulties which the miner has to contend with is the continual oozing of water into the mine in all directions. Where the mine, as very often happens, is situated upon the side of a hill, an adit level, or watercourse, shown at a a, fig. 325, is carried from the shaft to the lowest accessible point of the surface; and through this the waters of the upper part of the mine readily escape; but when the workings extend below this point, it becomes necessary to pump more or less constantly, and for this purpose powerful steam-engines are required. The galleries and levels are so constructed that the water shall flow from them into the principal shaft of the mine, so that by pumping from the sump, or lowest part of this shaft, the whole mine is freed from water. The greater part of the water is lifted only to the adit level, but a considerable quantity is raised to the surface for the purpose of washing the ore. Much of the excavation is done by hand, with the pickaxe and wedges; but after judicious clearing, gunpowder properly applied facilitates the progress greatly. The quantities of powder used for blasting in the mines are small, usually about two ounces. MECHANICAL TREATMENT OF THE ORES. 331 The process of blasting consists in boring a hole to the depth of 18 inches or 2 feet, somewhat obliquely, under the portion of rock which is to be raised; the powder is then introduced, and the hole is closed by ramming in clay or friable rock. A copper wire runs from the surface down to the charge, and when the ramming or tamping is finished, the wire is withdrawn and its place supplied with a hollow rush charged with powder, and the train is fired by means of a fusee. A safety fusee is now commonly substituted for the copper wire and pithed reed filled with powder. The ore that is detached is raised to the surface of the mine in large wrought-iron buckets, or kibbles, which are capable of containing about 3 cwt. of ore. : (529) Mechanical Treatment of the Ores.-The extraction of metals from their ores is effected by two classes of operations; those of the first class are mechanical; by their means the earthy parts contained in the matrix or vein-stone are to a certain extent separated the operations of the second class are chemical, by which the metal itself is procured. The mechanical treatment is influenced not only by the nature and composition of the ore, but also by its market value: an ore of tin, of copper, or of lead, from the higher price which the metal bears, will be worth a more elaborate treatment than an ore of iron or of zinc. The ores of zinc and of iron are occasionally subjected to the operation of washing; for when they are accompanied by a loose friable clay, the clay admits of being readily diffused in a finely divided state through the water, and is easily removed by its means. The specific gravity of clay is not much more than 2'0, whilst that of carbonate of iron and hydrated oxide of iron varies from 3.8 to 4'0, and that of calamine is about 42; consequently particles of these materials of equal size expose a smaller surface in proportion to their weight to the action of water than the clay, and when agitated with water they subside more rapidly; and if subjected to the action of a current of water, they are held for a shorter time in suspension, and are therefore carried by it to a smaller distance. The same principles apply to the more elaborate processes of washing adopted with the ores of lead and tin. Galena has a specific gravity of 7·6; tinstone of about 7. Sulphate of barium has a density of 4'6; fluor-spar of 3'1; and quartz of 2.65. When reduced to particles tolerably uniform in size, the earthy portions may therefore to a considerable extent be separated, by the action of water, from the ores of lead and tin. The following is an outline of the mechanical operations pur |