The most extensive slate-quarries in Great Britain are those near Bangor, in Wales, from which slate is shipped to all parts of the world. The slate occupies the greater part of the distance from Snowdon to the Menai Straits. Upwards of two thousand men are employed in these quarries; and the proprietor is said to gain from thirty to forty thousand pounds per annum by them. Although this one is the largest, yet there is one in Cumberland in which the slate is found more remarkably situated. This is Hourston Crag, a mountain near Buttermere Lake, about two thousand feet above the level of the lake, and nearly perpendicular. On account of the difficulty of access, the workmen take their provisions for the week, and sleep in temporary huts on the summit. During the winter months they are generally involved in clouds, and not unfrequently blocked up by the snow. The slate is conveyed on sledges down a zigzag path cut in the rock, one man attending to prevent the acceleration of the descent. When the slate is emptied at the bottom the sledge is carried back on the man’s shoulders to the summit.
Notwithstanding the value of slate, few quarries are worked to a very great depth, or have subterranean galleries like mines. There is one, however, near Charleville, in France, which is an exception to this rule. The mouth of the mine is near the summit of a hill; the bed inclines forty degrees to the horizon, and is about sixty feet in thickness, but the extent and depth are unknown. It has been worked by a principal gallery to the depth of four hundred feet, and many lateral galleries have also been driven, extending about two hundred feet on the side of the main gallery. Twenty-six ladders are so placed as to give passage to the workmen and carriage for the slate. Of the sixty feet which constitutes the thickness of the bed of slate, about forty are good slate, the rest being mixed with quartz. The slate is cut into blocks of about two hundred pounds each, called faix; each workman, in his turn, carrying them on his back to the very mouth of the pit, mounting all or part of the twenty-six ladders, according to the depth of the bed where he may be working. When brought to the surface, these blocks are split into thick tables called repartons, by means of a chisel and mallet; and these repartons are divided by similar means into roofing-slates.
Another remarkable slate-quarry in France, is situated near Angers. The bed of slate extends for a space of two leagues, passing under the town of Angers, which is in great part built of slate; those blocks which are the least divisible being employed in masonry. The quarries actually explored are all in the same line, from west to east, as well as the ancient pits, the bed of the best roof-slate rising to the surface in this direction. Immediately under the vegetable earth is found a brittle kind of slate, which, to a depth of four or five feet, splits into rhomboidal fragments. A little lower is the building-stone, which is a finer but scarcely divisible slate, and is employed in the construction of houses, after it has been sufficiently hardened by exposure to the air. At fourteen or fifteen feet from the surface is found the good slate, which has been quarried to the perpendicular depth of three hundred feet, without its lower limit being attained. The interior structure of the slaty mass is divided by many veins or seams of calcareous spar and quartz, fifteen or sixteen feet in length, by two feet thick; these veins are parallel, and proceed regularly from west to east in a position rising seventy degrees to the south; they are intersected by other veins at intervals of a similar kind, but whose rise is seventy degrees north; so that when the two series meet, they form rhombs or half-rhombs. All the layers or laminæ of slate have a direction similar to those of the veins of quartz, so that the whole mass becomes divided into immense parallel rhomboids. The slate is extracted in blocks of a determinate size, which are then divided into leaves for roof-slates. When the blocks have been drawn from the quarry, if they are left exposed to the sun or the open air, they lose what is called the quarry-water, and then become hard and untractable, and can only be employed as building-stone. Frost produces a singular effect on these blocks; while frozen, they may be broken with more ease than before; but if thawed rather quickly, they become no longer divisible; yet this quality may be restored by exposing them once more to the frost.
The Process of Slating.
When the blocks of slate for roofing have been split, and the laminæ roughly squared, they are sorted, according to their size and quality, and are brought to market under the quaint names of Imperial slates, Duchesses, Countesses, &c., the first variety being the largest. The best roofing-slates come from the celebrated vale of Festiniog.
Slates are laid on battens, or thin narrow deal boards, which are nailed horizontally on the common rafters of the roof, at equal distances apart, which distance is governed by the size of the slate to be employed. An entire board is nailed along the lowest edge of the roof to receive the lead of the gutters, which are first laid, and then the lowest course of slates are nailed and pinned down to the lowermost batten; so that two-thirds the length of the slate, at least, shall lie over the lead. The next course of slates is then fixed, so that every slate shall overlap two-thirds the depth of the course below it, every slate being also laid over the joint, between two slates of that undercourse. By this construction the rain that runs through the joint between any two slates is kept from penetrating into the roof by being received on the surface of the slate beneath that joint; and the bottom course of slates is double, to continue the same principle down to the lead gutter.
The slates are fixed to the battens by two copper nails and a wooden pin when the work is well executed; holes being picked through each slate for the nails to pass through.
Paper Roofs.
Although, as intimated in a former page, in covering our imaginary dwelling with tiles or slates, we may seem to have done all that is necessary in respect to “roofing,” yet we should leave our subject only half treated if we were to omit mention of other contrivances which have been partially acted on; such as the use of paper, of asphaltum, and various other substances.