As already stated, the mineral may be quarried in two ways: 1. By pits open to the sky; 2. By underground mining. The shape assumed by the actual excavation must depend greatly upon the nature of the mineral and its solidity. Hard solid rock, like the limestone or granite, may be allowed to stand up in high and almost vertical faces, whereas less stable materials, such as clay and sand, would slip or run in if one tried to treat them in like manner.
A very simple process of working is that adopted in Chili for obtaining nitrate of soda. The impure nitrate (caliche) is found in beds from six inches to twelve feet thick beneath a covering of hard conglomerate (costra) from one to ten feet thick. A small shaft is sunk a little below the bottom of the caliche, and enlarged in order to receive a charge of slow-burning powder made on the works. The explosion loosens and breaks up the ground over an area about 20 years in diameter. The hard overlying stratum of costra is then easily removed, and the caliche is broken up into lumps, which are taken to the lixivating and crystallizing works.
Working in steps, stopes, terraces, benches, lifts, or galleries, for all the terms are used, may be looked upon as the typical method of quarrying, and we may find examples in the case of many kinds of ores and stones.
Fig. 2 shows part of the great opencast at Rio Tinto, a huge open pit from which cupriferous iron pyrites is quarried in steps or terraces thirty-three feet to fifty feet high. Only the middle of the pit is worked for ore; the upper terraces are cut out in barren surrounding rocks, which have to be removed in like manner in order to prevent disastrous and dangerous falls of the sides.
The great Penrhyn slate quarry near Bangor is well known to all tourists in North Wales (Plate III). The valuable slate and the valueless overburden are both taken away by a series of terraces on an average sixty feet high by thirty feet wide.
The lead-bearing sandstone of Mechernich was at one time worked opencast on a very extensive scale. At the present moment the underground workings are of more importance.
The famous iron ore of Styria is mainly worked opencast, or to use what is now the legal expression in this country, is "quarried." There are in all nearly fifty terraces, in fact the workings may be looked upon as a gigantic flight of stairs 1,500 feet high, the tread of each step being on an average thirty-six feet. These workings in Styria produced in 1891, about three-quarters of a million tons of iron ore.
If the rock is firm enough to stand alone for a great height, it is sometimes taken down in one steep slice, without making a series of steps. The general appearance of Mulberry tin mine near Bodmin in Cornwall is shown in Fig. 3. Men standing at a bore and blast holes which throw the rock to B, under which a tunnel has been driven with an opening C, usually covered with timber. A wagon is then run in and is quickly and cheaply filled.
Much chalk is quarried, in Kent and Surrey for instance, in a somewhat similar fashion.
A last method of quarrying is by gigantic blasts, which bring down thousands of tons of stone at a time. It is practiced in some limestone quarries in this country, both in North Wales and in Derbyshire. A tunnel is driven into the face of the quarry, and a chamber is excavated for the reception of explosives. The tunnel is now filled up with earth or masonry, or both, and the charge is fired by long fuse, or by electricity. The result is the displacement of thousands of tons of rock, the clearing away of which may occupy the quarrymen many months.
Fig. 4 explains how this process was applied in quarrying limestone near Messina; a charge of 31 cwt. of powder in bags was placed in a special chamber at the end of the tunnel, driven into the face of the quarry, and the tunnel having been duly tamped, the powder was fired, bringing down no less than 100,000 cubic yards of rock. A B C shows the original outline of the face of the quarry, the dotted line A B' C' gives the outline after the explosion. Granite has been quarried at Baveno on the Lago Maggiore in like manner. In 1886, 17 ½ tons of gunpowder and half a ton of Nobel's dynamite were exploded simultaneously in one monster blast, displacing 500,000 cubic yards of granite.* Twenty or thirty blocks from 1,000 to 6,500 yards would be represented by a cube of 55 feet on the side.
As will be naturally understood, the task of removing the waste rock or earth, which often lies above the useful mineral, increases with the thickness, until at last a point is reached when it will no longer pay to work the quarry open. The quarry owner then endeavors to devise some method of excavating the valuable material by itself, leaving the waste rock as a roof. This leads him to true underground mining, which, from a British legal point of view, is beyond the scope of my lecture. However, when stone is obtained from such excavations, they are popularly known as quarries, and on this account they deserve a word of mention.
As the mineral quarried is usually of small intrinsic value, the cheapest method of supporting the roof is to leave portions of the deposit unworked. This method is employed in working slate at Festiniog in Merionethshire. The beds of slate are sometimes more than 120 feet thick, and are worked by chambers separated one from the other by thick partitions of rock; in other words, a huge wall of slate, 30 to 40 feet thick, is left standing between every two chambers of like dimensions. Two or three beds may be worked one above the other, care being taken that the position of the pillars shall coincide. The chambers themselves are huge caverns, the roof of which may be more than 100 feet above the heads of the workmen.
Wales is not the only part of the world where slate is obtained in underground quarries. France and Germany afford examples of similar, or somewhat similar, workings. At the slate quarries near Fumay on the Meuse, the Rimogne, near Méziéres'Charleville, the men fill up the chambers with rubbish, and stand upon it while at work.
Part of the underground workings at Ste. Anne Slate Mine, Fumay (Ardennes), France. The men are standing upon the rubbish, which is piled up so that they are in touch with the roof of the chamber. The man on the right-hand side is cutting a deep groove with a pick; the man in the center is making a nick with a saw in a thick slab of slate, so as to enable him to break it across with a blow from a sledge. A third man is in the act of carrying a slab upon his back to the most convenient tram road.
* Chamber's Journal, March, 1894, p. 80.
(To be continued.) (NOTE: The continuation is not available.)