454 MINING [EXPLOITATION. jULJL parallel to the dip, working away each slice separately as if it were a lode of ordinary dimensions, and filling up with rubbish (fig. 74). Working We now come to beds or seams. The of beds, mode of working the most important beds that occur in the earth s crust, viz., coal seams, has already been described in the article COAL (vol. vi. p. 64 sq. ), and details hav3 been given concerning the removal of the mineral by pillar working and long-wall working. Both these methods are applicable in the case of seams of other minerals. Such for instance are the beds of fire-clay and clay-ironstone which are wrought by both the processes just mentioned, and often in connexion with coal. Next in importance to coal is ironstone, and a brief account of the workings in the Cleveland district will explain the manner in which more than one-third of the iron ore raised in the British Isles is obtained by mining. It resembles the " bord and pillar " system used for working coal in Durham. deve- The Cleveland ore occurs in the form of a bed from 6 to 16 feet land thick in the Middle Lias, lying pretty level. A mainway (fig. 75) ironstone is driven about 1 2 feet work- wide for a considerable ings. distance, and at right angles to it bords are driven 5 yards wide fora length of 30 yards, and then at right angles a wall 7 or 8 feet wide and 20 yards long. By driv- ages of this kind the bed is cut up into pillars or | blocks 30 yards long by 20 yards wide. The pillars are subsequently removed in the following way. A place, or drift, ab, 6 feet wide, is driven Fig. 75. across the pillar 10 yards from the corner, and portions (lifts) about 6 yards wide are worked away in the order 1, 2, 3. After No. 1 lift has been removed, the timber put in to support the roof temporarily is withdrawn, and the roof is allowed to fall ; No. 2 is then taken, and No. 3 in the same way. While these lifts are being taken out, another place cd is being driven across the pillar 10 yards from the first, and the pillar removed entirely by a series of fresh lifts. Gypsum Fig- 76 represents in section and plan the chambers and pillars quarries, of the underground gypsum quarries - which supply the well-known plaster I of Paris to all the world. 1 The principal bed is from 50 to 60 feet in thickness ; pillars are left 10 feet square at the base, and the stalls between them are 16 feet wide. The workings are slightly arched, and are not carried up to the roof, for the purpose of better maintaining the security of the chambers, because heavy damages would have to be paid if they "caved in" and ren dered the surface useless. A simi lar layer left for the floor prevents creep (see COAL, vol. vi. p. 64), and LLL Fig. 7(J. enables the underground roads ito be kept in good repair. Underground slate quarries afford examples of very various methods of removing thick beds of mineral of comparatively little intrinsic value. At Angers in France, where the beds dip at a high angle, the underground workings are carried on like an open quarry Fio. 79. A, A, pillars of slate ; B, B, rubbish ; C, C, slate cham bers. Fig. 77. under a trong roof of slate ; the floor is continually being worked away in steps, and an immense open chamber is left. In the 1 Gallon, Lectures on Mining, vol. ii. plate xli. Festiniog district in North Wales the principal bed, or vein as it is called, is more than 100 feet thick in places, and the method of working consists in making alternate pillars and chambers each 30 feet to 50 feet wide along the strike (cross-section and plan, figs. 77 and 78). The pillars follow lines of natural cross-rending PP , which commonly make an angle of 25 to 35 with the direction of the dip. The excavations are arranged in regular lines, and form continuous chambers extending very often from the surface to the very lowest workings. A,B, C, D are the original working levels. The slate of the supporting pillars is entirely lost, as these cannot be removed with safety. This method of working requires a strong roof. In the Ardennes, on the contrary, the pillars are carried along indefinitely along the strike (fig. 79, cross-section). The slate in each longi tudinal chamber is removed in slices parallel to the bedding, and the men stand upon the rubbish, which finally fills up the chambers completely. Rock-salt constitutes another important mineral which occurs in c . the form of stratified deposits. The principal source of the Cheshire mines, salt is a bed 84 feet thick lying horizontally ; but only the bottom part, 15 feet to 18 feet thick, is mined. Pillars 10 yards square are left promiscuously about 25 yards apart, as shown in fig. 80, which represents part of Marston Hall rock-salt mine. 2 The workings are advanced by making in the upper part an excavation 5 feet 9 inches high, called the roofing (a, iig. 81) ; and then the lower two-thirds of the part worked are removed by blasting slanting holes. Many of the old salt mines have collapsed from weakness of the roof or insufficiency of the pillars, and have become inundated; the brine is then extracted by pumping and evaporated for salt. In some countries, especially when the beds of salt are impure or much mixed with clay or shale, the formation of brine is conducted regularly by making a network of drivages within a rectangular, elliptical, or circular area in thick beds of saliferous marl, and then introducing fresh water by pipes, so as to form underground ponds which gradually dissolve the roof and sides. The brine is drawn off and either pumped up or conveyed by adits to the surface. A few words remain to be said about open workings. Some minerals are always obtained in this way ; others are worked open before regular underground mining begins ; and, thirdly, it often wor kings. happens that underground and surface work are both carried on simultaneously on the same deposit. Among deposits worked open cast are peat, numerous kinds of stone, iron ore, cupreous pyrites, lead ore, gold- and tin-bearing alluvia, and diamantiferous rock. Owing to its soft, spongy, and fibrous texture, and the fact of its often lying below the water-level, peat has to be worked in a special manner. Trenches are dug about a foot deep with a sharp spade, which cuts out sods of convenient size for drying and burning. When one layer has been removed in this way, another is taken off, and so on. If water is reached the working can still be pursued by using the long spade (grand louclict, France) with a handle of 16 or 20 feet. It cuts out a sod 3 or 4 feet long at each thrust. When a deposit is more or less solid the workings are frequently arranged in steps, the height and breadth of each depending upon the firmness of the rock. In many cases the first work consists in removing worthless rock at the surface (overburden), and where the underlying deposit is thick or very valuable it will pay to remove a very great thickness of overburden, on account of the advantages of working a deposit open. These advantages are entire removal of the deposit without loss in pillars, no expense for timbering or for packing with rubbish or for ventilating or lighting the workings, better ventilation, easier supervision, longer working hours, less danger. As an example of a large open working may be mentioned the great Penrhyn slate quarry near Ban ger, employing about 3000 hands, and worked by a succession of terraces on an average 60 feet high by 30 feet wide (fig. 82). Reference has already been made to the thick lead-bearing sandstone of Mechcrnieh, which is in part worked as an open quarry. Mokta-el- Hadid, near Bona in Algeria, and the Rio Tinto mines in Spain, afford instances of extensive combined open and under ground workings for iron ore and cupriferous pyrites respectively. Local laws regulating the size of the working areas, or claims, 2 Joseph Dickinson, " Report on the Salt Districts," Reports of the Inspectori
of Afines for the year 1881, p. 6<i.