Slate-Quarries.
Slate is the popular name for a variety of rocks which are sufficiently stratified in their structure to allow of their being cleaved into thin plates, a property which renders them valuable for a variety of purposes. Slate has superseded the use of lead for covering roofs, even of the largest buildings: from its lightness it is preferable to tile, but the latter being cheaper, in flat countries which do not contain rocks, but which yield brick-clay, slate in such localities is only used on the better class of houses. In mountainous countries, a slaty rock, which admits of being split thin, though not so much as clay slate, is used under the name of shingle.
Besides being employed for roofing, slate is used in large slabs to form cisterns, for shelves in dairies, for pavement, and similar purposes, for which its great strength and durability, coolness, and the ease with which it can be cleaned, owing to its non-absorbing property, adapt it. The latter quality renders it also of great value as a cheap substitute for paper, in the business of education; the system of teaching in large classes in National and Sunday-schools would be greatly fettered but for the use of slates.
The principal slate-quarries in Britain are in Wales, Cumberland, and various parts of Scotland; the mode of working them is generally the same. The rock is got out in tabular masses by means of large wedges, and is then subdivided by smaller to the requisite thinness; the pieces are roughly squared by a pick, or axe, and sorted, according to their sizes, for roofing. The largest called imperial, are about three and a half feet long, and two and a half wide; the smallest average half those dimensions. When wanted for paving, &c., the large blocks are sawn into thinner slabs, in the same manner as stone or marble is.
A few words respecting the position and working of some of the slate-quarries may be appropriate, as illustrating the nature of this remarkable geological formation.
A Slate-Quarry.
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.