There are various important slate beds in Great Britain. At Ballachulish, in Argyll, quarrymen have excavated great pits in the sides of the mountains below the entrance to Glencoe. At Delabole, in Cornwall, the work of several hundred years has caused an enormous pit to be blasted out in the bare, treeless plateau. The greatest slate workings in Great Britain, however, are in the ancient mountains of Wales.
Most slates originally consisted of clay belonging to the palaeozoic strata. This clay was subjected by newer strata to tremendous pressure and also to a certain amount of heat, in the course of successive physical changes taking place in the Earth's crust. The pressure has gradually transformed the tightly packed clay into the hard rock known as slate.
Slate belongs to a group of rocks which are fissile, that is to say, they split naturally into flat leaves or plates. Shale is another fissile rock and a natural flake of shale is brittle, A corresponding piece of slate, however, has strength and a remarkable amount of elasticity.
There is good reason for this. Shale splits up into the original layers in which it was compressed. The fissile quality of slate developed long after it had been bedded, and the direction in which its flakes lie is all but parallel to the direction in which subsequent pressure was imposed upon it. In the course of the physical changes that overtook the rock, flattened mica crystals formed among the flakes. These crystals of mica are tiny. In good Welsh slate there are about 2,000 to the inch in its breadth and 6,000 to the inch in its thickness.
In slate, most of the flattened mica crystals lie broadside on to the original direction of pressure. The flattened components of a sheet of slate overlap, similarly to the scales of a fish, and it is this that gives slate its strength and great elasticity.
It is not surprising that a hard rock that can be produced in smooth, flat sheets has many uses. For many years it has provided an excellent material for roofing. Slates are almost impervious to water, and therefore little likely to be damaged by frost. They cannot be burnt and are bad conductors of heat. Most slates are free from corrosion. For this reason, slate is an invaluable material for purposes which are likely to bring it into contact with acids. It is a virtual non-conductor of electricity, and for this reason is much in demand as a material for switch panels in electrical installations.
Slate has been worked in Wales since before the time of the Eomans. It was in the eighteenth century, however, that the Welsh slate industry began to reach big proportions. The Penrhyn Quarry, near Bangor, to-day forms one of the largest excavations in the world. In 1765 the Penrhyn area was more or less spotted with small workings, but in that year the mountain passed into the ownership of Richard Tennant, afterwards Lord Penrhyn. Tennant rapidly took over all the old local workings. He aimed at the establishment of one huge quarry on a scale never before attempted. In 1780 1,800 tons of slate were produced at Penrhyn. Up to that year, transport of the slates had been by pack horse, but this method was superseded by broad-wheeled carts.
Cart transport continued until 1800, when it was superseded by a horse-drawn tramway. In that year Penrhyn produced 20,000 tons of slate, transport of an efficient type having thus made production on a large scale commercially possible. In 1874 steam traction was introduced and the quarry assumed, though still in a lesser form, the shape it has to-day, a huge terraced, horseshoe-shaped cavity in the flank of the mountain. The benches or terraces rise up the side of the mountain in a series of gigantic steps. Each intermediate bench is about 70 feet above and below its lower and upper neighbours respectively, and the uppermost bench is situated some 1,800 feet up the side of the mountain. Penrhyn is situated on the north-eastern side of a mountain called Elidyr. On the other side is another enormous quarry, rivalling that of Penrhyn. This is the Dinorwic Quarry, which has been developed along the same lines as Penrhyn. Fully equipped steam railways run along the benches, and an inclined plane connecting the several distinct systems makes possible the working of rolling stock from one level to another and from all the benches to the main transfer station. On the benches the tramways are necessarily laid to a narrow gaugeno more than 1 ft. 10-1/2 in.
Perhaps the Dinorwic Quarries afford the best and most interesting example of slate transport between the working face and the quay side. In 1824 the authorities built a 4-feet gauge railway, seven miles long, from Dinorwic to quays at Port Dinorwic, on the Menai Straits.
It is not practicable to have tramways with even as wide a gauge as 4 feet (the standard gauge for full-size railways is 4 ft. 8-1/2 in.) on the benches below the working faces. Hence the engineers responsible for Dinorwic's transport arrangements designed their wider-gauge wagons to take the small slate trucks bodily in sets of two or four, according to the capacity of the larger wagon. Thus the slates, packed at the working face, travel direct to Port Dinorwic without transhipment, despite the break of gauge on the railway line.
Dinorwic Quarries are perhaps even more imposing than those of Penrhyn on the other side of the mountain. They cover an area of 700 acres. The working face is cut into twenty-two benches, each bench being served by about two miles of tramway track. The entire quarry has a height of some 2,000 feet.
The system of cutting out the mountain side into benches or terraces is used at Penrhyn and at Dinorwic, but quarry engineering takes different forms elsewhere. The quarry may be excavated downwards, to form a huge terraced pit, as at Delabole, in Cornwall, or there may be underground workings, sometimes on a large scale, so that the works become a mine rather than a quarry.
The great pressure to which the slate-bearing strata have been subjected in the past has made them follow a decidedly eccentric course in places. Open bench working is resorted to where the stratum is steeply inclined. At Delabole, the great chasm has been excavated from the flat top of a plateau. Where the stratum is inclined at a low angle, however, as, for instance, when a bed of slate passes into the side of a hill at a gentle inclination, possibly with masses of other material or top of it, then mining becomes necessary. An example of underground slate working is that in the district of Blaenau Festiniog, high up in the mountains of Merioneth. There are three railway approaches to the district. The L.M.S. line runs from the north coast, and passes through a mile-long tunnel under the mountain, before it emerges into a world of slate. There is slate or slate waste everywhere. From Bala and the south runs a G.W.R. branch line and there is also a narrow-gauge mountain railway from Portmadoc.
It was this narrow-gauge railway, dating from 1836, that first provided an outlet for Festiniog slates. The tiny train twists and turns on its tortuous way up from the coast, and at intervals meets and passes immensely long trains of slate wagons descending by gravity. The railway is the oldest narrow-gauge railway in the world, with the exception of the mining tramroads of Silesia and Transylvania.
Quarry engineers at Blaenau Festiniog and other places in Merioneth first drive inclined shafts into, the face of the mountain containing the slate. From these they bore "levels," or further shafts, into the slate strata. As they remove the slate a cavity forms, and as the cavity grows they leave pillars of slate to support the roof and to prevent a disastrous collapse from taking place. Each cavity increases in size until it approaches the safety limit. The miners work in several levels, and at different points on each level. Thus one of the Merioneth slate mines consists of an extensive series of artificial caves inside the mountain, from which the newly-won slate is brought out by tramway wagons running on rails in the inclined shafts.
Caverns of Enormous Size
One of the mines at Blaenau Festiniog contains fifteen different levels, and the lowest of these is at a depth of more than 1,500 feet below the top of the tramway tunnel serving it. The cavities themselves are often of enormous size. A large cavity may be no less than 120 feet long, 40 feet wide and 100 feet high from floor to roof. The workings may be more than a mile in length, and may contain no fewer than fifty cavities. In the same way as other rocks, slate, in its natural state, contains "joints" or deep regular cracks at intervals. These joints run fairly evenly at right angles to one another. They may be visible or invisible, simple planes of weakness, but whichever form they take, they tend to divide the virgin slate up into regular, rectangular blocks. The removal of these natural blocks from the face of the quarry or underground chamber is a relatively simple matter. Small blocks may be removed by hand ; heavier and more difficult blocks may require the use of explosives. A block thus detached will weigh many tons. The quarrymen then attack and reduce it to a number of smaller blocks, the largest of which will probably not exceed two tons in weight.
These smaller blocks are then loaded up on the tramway wagons and sent along the bench to sheds, where they are cut up into sheets. The cutting of the raw material into plates or sheets is effected by splitting them along the planes of cleavage, each sheet thus treated having a thickness of about 3 in. These sheets are then sawn into sections equal in length to the future roofing slate. The mason then takes the sawn pieces and splits them up into tile slabs by hand.
He holds a chisel against the edge of a slab and gives it a smart blow with a mallet. The blow produces a crack in the slate along the line of cleavage. The mason presses gently on the chisel, this action being enough to split the slab into two pieces. He repeats the process again on each of two pieces until the original slab has been reduced to anything up to eighteen tiles.
The degree of thinness to which the mason can reduce his slabs depends on the purpose for which the slate is to be used, on its size, and above all on its quality. A good Welsh roof slate is about J in. thick. The final finishing process is the trimming of the slates. This may be carried out by hand methods, in which the mason goes about his work with an apparatus similar to a printer's guillotine, incorporating a fixed straightedge and a moving knife which shears down on the firmly held slab. Mechanical trimming is carried out by rotating knives, the apparatus somewhat resembling a giant mowing machine.
Wales contains to-day some sixty slate quarries and mines within her borders, apart from older quarries now no longer worked. As in many other spheres of activity, one great engineering enterprise depends on another. The Welsh branch railway would not live without the slate industry, and the quarry in its turn cannot live without the railway.
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