Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/471

Rh EXPLOITATION.] MINING 453 cover these cross-pieces (stempels, stull-picces] with boards or poles, and throw down the rubbish upon the platform (stull, bunning) thus formed. In the midst of the rubbish chimney- like openings (mills, passes] are reserved, lined with boards or dry-walling, and Fig. 64. Fig. 65. closed at the bottom with shoots provided with doors. The ore is thrown into these passes, which are tapped when necessary ; the ore falls into the tram- waggon placed ready to receive it. Fig. 65 gives a transverse section showing the rubbish resting on the stull. This is what may be called the typical method of stop ing, when the lode affords rubbish enough for the men to stand on and to keep them close to the rock they are attacking. Very often such is not the case, and the whole of the lode has to be sent to the surface for treatment. If the walls are firm, the lode is sometimes stoped away, a stull put in, and a sufficient heap of broken ore is left upon the stull to give the men good standing ground ; the excess is thrown over the ends of the stull, or the great heap is tapped by cutting a hole in the stull-covering, and al lowing a quantity to run down into the level. Another method con sists in putting in temporary stages upon which the men stand to do their work, whilst the excavation is left as an open space (fig. 66). Fig. 66. This mode of working is incompatible with weak walls. If a lode does not afford rubbish enough for completely filling up the exca vated space, or if it is too narrow for men to do their work comfort ably, one of the walls may be cut into and blasted down (fig. 67), so that the men always stand upon a firm bed of rubbish while at work, and there is no fear of a collapse of the mine. In certain special cases rubbish is sent down from the surface to fill up the excavations. The advantages of overhand stoping are that the miner is assisted by gravity in his work, that no ore or rock has to be drawn up by hand labour, and that less timber is required. On the other hand, the miner is always menaced by falls, but as he is close by he can constantly test the solidity of the roof and sides by sounding them with his sledge ; there is the further disadvantage that particles of ore may be lost in the rubbish, but this loss is often prevented by laying down boards or sheets of iron w r hile the lode is being broken down. Working When very wide lodes wide come to be worked, recourse lodes. is often had to special methods. The great lode Van at the famous Van mine, in mine. Montgomeryshire, is some times 40 feet in width, and the hanging wall is weak. The lode is stoped away overhand, and the cavities packed with rubbish, part of which is derived from the lode itself, whilst the greater portion is supplied from a special quarry at the surface. Fig. 68 1 explains the details of the case. A is the original cross- 1 C. Le Neve Foster, &quot;Notes on the Van Mine,&quot; Trans. Roy. Geol. Soc. Corn wall, vol. x. p. 41. Fig. 67. Fig. 68. cut (not in the line of section) by which the lode was reached B is ic flucan, C the bastard lode, generally worthless, E the main lode, H permanent levels, and K ore-pass reserved amidst the rubbish (deads] D, I pass down which rubbish is shot, N crosscut connect ing the level H with P the permanent level in the country. If the lode is not firm enough to allow of the stopes being carried for its full width, the crosscut method is adopted ; the workings in this case, instead of proceeding along the strike, are carried across the deposit from one wall to another. The lode is removed in successive horizontal slices A,B,C,D,E, and for each slice a level (L, fig. 69) is driven, either in the lode| Fig 69. Fig. 70. or partly or entirely in the country ; from this level crosscuts are put out 6 or 8 feet wide, as shown in the plan (fig. 70). These are regularly timbered according to the necessities of the case, and, when No. 1 is completed, No. 2 is begun, and the rubbish from No. 2 thrown into the empty space of No. 1 crosscut. If the quantity is insufficient, deads are brought in from the surface or from exploratory workings in worthless rock in the neighbourhood. Sometimes the crosscuts are not driven side by side, but 1 and 5 would be driven first, leaving 2, 3, and 4 as a solid pillar ; then 3 would be worked away, and finally 2 and 4 between the timber and rubbish on each side. The greater part of the timber can be re covered when the next slice above is taken off, as the props are put in with the small ends downwards, and can be drawn up with levers. M (fig. 69) is a level reserved in the deads for traffic and ventilation. This method of working is applicable not only to lodes but also to irregular masses. In working away the soft &quot; bonanzas &quot; or ore-bodies of the great Conistock Comstock lode, which are from 10 to 30 or even 40 or 50 feet wide, lode, and which are enclosed in very un stable ground, a special method of timbering is employed (figs. 71 and 72). 2 &quot;It consists in framing timbers together in rectangular sets, each set being composed of a square base placed horizontally, formed of four timbers, sills, and cross-pieces, 4 to 6 feet long, framed together, sur mounted by four posts 6 to 7 feet high, at each corner, and capped by a frame-work, similar to that of the base. These cap-pieces, forming the top of any set, are at the same time the sills or base of the next set above, the posts, as the sets rise one above the other in the stope, being gene rally placed in position directly over those below.&quot; &quot;The timbers are usually of 12-inch stuff square-hewn or sawn.&quot; Each post has a tenon 9 inches long at the upper end, and a tenon of 2 inches at the lower end, Fig. 71. Fig. 72. which fit into mortices in the cap and sill respectively ; and &quot;the sills and caps have short tenons on each end and shoulders cut to receive the ends of the post and the horizontal cross-pieces.&quot; The walls of the excavation are sus tained by a lagging of 3 -inch or 4-inch plank. The whole width of the ore-body is stoped away at once, and its place supplied by timbering, and finally the vacant space is filled with waste rock derived from dead work in the mine or from special excavations, under ground quarries in fact, in barren ground. The stoping is carried on overhand, starting from an intermediate shaft or winze, and fig. 73 will explain how the different frames are built up one above the other. Another method of working a wide lode is to attack it in slices 2 James D. Hague, United States Geological Exploration of the fortieth Parallel, vol. iii. &quot; Mining Industry,&quot; p. 112.