Account of the Skerryvore lighthouse

by Alan Stevenson

The quarry itself, as already stated, was opened in the face of the cliff, at a point where the successive beds of solid rock seemed to promise the fairest prospect of success. The preliminary operation was to remove a very thin alluvial cover which scarcely concealed from view a large mass of most beautiful granite, whose reddish colour is said to have given the name of Ross[19] to that part of Mull, the shores of which everywhere exhibit massive slopes of that fine rock. The granite is separated very abruptly from the basalts of the surrounding district, so as to leave the Ross purely granitic; but in no part of the whole coast, which abounds with creeks and bays, does the rock appear to be of equal quality, or so conveniently situated for shipping, and so easily accessible to quarriers, as at the spot we had chosen. I know of no instance of a quarry so fully answering the most sanguine expectations as that of the Ross of Mull; and I have never seen a granite quarry of equally great resources, as regards both the quantity and the quality of the material produced. The rock in general yielded easily before well-directed shots, and was separated into large masses, capable of being advantageously cut, with little loss of material, into shapely blocks, by means of wedges, which work remarkably well in that rock. A few weeks after the quarrying operations had been commenced, a single shot detached 150 tons of excellent stone, in the cutting of which into blocks for the Lighthouse Tower little loss of material occurred; and in the course of the season of 1839, although the summer was chiefly spent in the preliminary works above noticed, the Mull quarries produced nearly as much workable material for the Lighthouse Tower as the Hynish quarries had done in three years. In the course of the future working of the quarry, when it came to be more fully opened, its resources were so great, that on one occasion a single shot shook about 570 tons, while another shot detached a mass of 460 tons. In that way, between April 1839 and June 1840, material had been quarried in that single spot, sufficient to supply 4300 selected blocks, varying in weight from ³⁄₄ ton to 2¹⁄₂ tons each. The average monthly produce of the quarries was about 400 tons, and there were generally employed in them 26 quarriers, 3 labourers, and 2 smiths. The quantity of gunpowder consumed in the quarry was small, as it was almost exclusively employed in great bores about 11 feet deep, for the purpose of detaching large masses which had no open side and could not be removed by means of the pinch or crow-bar. When a mass of rock had been thus removed, it was cut up into various portions by means of wedges, and finally subdivided into blocks, hammer (or as it is called quarry) dressed, according to rough moulds, whose dimensions exceeded those of the stones of the various courses of the building by a quantity which was considered sufficient to cover any casualty in the final dressing of the block by the masons at Hynish, and which allowance was generally equal to a film of rock about 1¹⁄₂ inch in depth. The blocks thus roughly formed were shipped for Hynish, distant about 26 miles, through a tempestuous sea, open to the full reach of the Atlantic towards the south-west, sometimes in a small vessel called the “Queen,” belonging to the Commissioners, and sometimes in undecked boats of 16 tons, belonging to the adventurous men of Tyree. The freight usually paid was 5s. a ton, and yet the whole of the blocks for the Lighthouse Tower, and many of those used in the pier at Hynish, were laid down to the number of about 5000, at the rate of 2s. 1¹⁄₂d. per cubic foot, including all the expense of building barracks, opening quarries, freight of stones, and the expense of building and maintaining the small vessel called the “Queen,” above noticed. The stone of the Mull quarries is a reddish or flesh-coloured granite, in which felspar predominates. About 13·66 cubic feet weigh a ton, and it is not quite so hard as the granite of Aberdeen.

[19] Whether from any inflection of the Celtic Rhua, or Roy, or directly from the Italian Rosso, it would, perhaps, be impossible to determine.

As I am not aware that any professional work contains a detailed description of the quarrying of granite, some observations on that subject may not be unacceptable in this place; Observations on the quarrying of granite. and I therefore propose, at the risk of appearing somewhat prolix, to give a pretty minute account of the mode of opening and working a granite quarry, more especially as practised by us at North Bay.

Having laid bare the rock of earth, gravel, or other loose matter (which operation, as I have elsewhere mentioned, is in Scotland technically called terring), and having swept or washed clean the surface of the rock so as to have a good view of the natural seams or joints which traverse it, the next step is to fix the best place for putting in a bore or mine.

In selecting the position of the bore, the direction of the seams and veins of the rock must be duly considered, with a view to employ the force of the explosion to the greatest advantage in separating the natural joints or beds of the rock, instead of shattering the solid masses or posts (as they are called in the language of the quarry) into shivers or fragments.

One thing to be strictly borne in mind is, that the bore should never be in the centre of a fine or large block, but should be placed within a few inches of its back so as not to break the finer rock into small and useless fragments; and care must, at the same time, be taken to keep the bore clear of cracked or unsound rock, as the jumper, in passing through such material, is liable to be jammed by the cracks and fissures before it can be driven to the proposed depth. It is not possible to lay down precise rules for guiding the quarrier in choosing the place of the bore, as his plan must be regulated chiefly by the circumstances of each case; but it may be observed, that having first determined the depth of the hole, it will seldom be found advantageous to keep the bore farther back from the face of the rock than about four-fifths of that depth. The depth itself, also, to which the mine should be carried, is a point for deliberation with the skilful and experienced quarrier, who will take great care not to go so deep as to pass through the solid rock in which he is boring and thus to touch a bed, unless indeed he shall think it advisable to attempt to raise more than one post of rock by a single mine, in which case he will carry the bore through the first post into the second or third as the case may be. But in all cases the boring must be stopt at two inches before coming to the bed or seam of the post in which the mine is to terminate, lest the exploding powder should escape by the bottom of the bore, and thus leave the top of the rock altogether undisturbed. In endeavouring to procure large materials, the bores should in general be as deep as possible. It is only experience, however, which teaches the quarrier to form a sound prognosis as to the direction and level of the beds of the rock at any particular spot, and enables him to choose the most advantageous position and depth for the mines.

In the blasting of granite there are a few general rules which (although it may not be necessary to follow them in every case) may be considered as constituting the best practice. If the bore be a vertical one of the depth of 6 feet, 2¹⁄₂ inches diameter at the top, diminishing to 2 inches at the bottom, may be considered a proportionate caliber. If the bore be a deep one (perhaps of 14 feet), its diameter will require to be 3¹⁄₂ inches at the top, and should diminish to 2³⁄₄ inches at the bottom; and the quantity of powder required for the charge will in most cases be about as much as is required to fill ²⁄₅ths of the hole.[20]

[20] Miners and quarriers, who always work by empirical rules, disregard entirely the line of least resistance as a measure for the charge, and invariably refer to the depth of the bore.

The patent fusee having been inserted among the powder, with its end about the centre of the charge, and the upper part of the bore having been filled up with dried clay, well forced down with a wooden rammer faced with copper, a length of 3 or 4 inches of the fusee should be left outside the bore, to which the match is to be attached. Having cleared away every thing near the blast which can receive any injury and covered up the machinery of the cranes with strong planks, the mine may be said to be ready for being fired; and, on a signal given, by blowing a horn, all hands retreat to a safe distance, with the exception of the fireman, who then lights the match, and follows the others as fast as possible.

If, as already stated, the object in quarrying be to obtain large materials, the bores should, if possible, be deep; and, in that case, the rock will seldom be thrown down in fragments by the blast, but will merely be cracked, and intersected by rents about one inch in width. Recourse must therefore be had to what, in quarry language, is called a Bull, which consists in running a quantity of loose powder into the crack which has been made by the blast, at that part where its explosion seems most likely to throw out the cracked or broken mass in various fragments and disengage them from their place in the rock. In bulling, perhaps twice as much powder as was used in the bore is loosely poured into the crack, care being at the same time taken to get as much of it to go under the bottom of the rock as possible. After enough of powder has been poured into the crack, a quantity of dried smithy ashes, or dry sand, is run in over the powder, so as completely to cover it, except so much as is required to fire it by; and that coating, which is merely superficial, is employed partly to keep down the powder, and partly for security against its being accidentally fired before all things are ready. The fireman having seen every thing cleared away, gives notice to sound the alarm, when all hands escape to a distance in the direction which is supposed to be the safest. The match is then applied, and the fireman retires, as fast and as far as he can, yet so as if possible to keep in view, during his retreat, the progress of the match. The operation of bulling is far more dangerous than the firing of a bore, as the charge is much greater, and not so well confined, so that many splinters are thrown off, and the direction in which they fly varies continually with the direction of the cracks which the original bore may have produced. As might be expected, by far the greater part of the accidents which occur to firemen in granite quarries, arise from that practice.