183. Blairmachyldach Marble.—In the bed of a river, at the farm of Blairmachyldach, about three miles south of Fort William, is a singular marble, consisting of a black ground, flowered with white. It is of fine close grain, but not very hard. The flowering in it is light, and beautiful, like fine needle-work, or rather resembling the frosty fret-work upon glass windows, in a winter morning.
The cutting and polishing of marble appear to have been performed by the ancients nearly in the same manner as it is with us. In polishing, the first substance employed is a sharp, coarse-grained sand. Afterwards a finer sand is used, then emery ([58]) in different degrees of fineness. These are followed by a red powder called tripoli ([119]): and the last polish is given with putty.
184. BLACK MARBLE is a species of limestone, of uniform black colour, and easily distinguishable, by an excessively disagreeable smell, which is emitted on rubbing two pieces of it together, or striking it with a hammer.
Few minerals are susceptible of a more beautiful polish than this. It is consequently much used for chimney-pieces, small columns, vases, and other ornamental work. There are two quarries of black marble near Bakewell, in Derbyshire: and it is manufactured to a considerable extent by Messrs. Brown and Co. at Derby, who have fixed up in their ware-rooms a large slab of it as a looking-glass.
By the ancients it was much prized. Marcus Scaurus is said to have ornamented his palace with columns of black marble, each thirty-eight feet high; and many of the monuments of ancient Persepolis were executed in it. M. D’Avejan, Bishop of Alais, used a kind of black marble for paving the apartments of his palace; but the friction and heat rendered it so fetid that his successors were compelled to substitute another species of stone in its place.—The pavements, however, of many churches, and of the porticos of several galleries, on the Continent, are of black marble.
185. CALCAREOUS ALABASTER is a species of limestone of somewhat whitish or yellowish colour, translucent, and internally splendent or shining.
It is nearly a pure carbonat of lime; and occurs in masses, hanging, like immense icicles, from the roofs of lime-stone caverns, and also coating the sides of such caverns.
The formation of this substance is deserving of notice. The water which oozes through the crevices of limestone rocks, becomes strongly impregnated with minute particles of lime. This water, when it has reached the roof or side of a cavern, is generally suspended, for a considerable time, before a drop of sufficient size to fall by its own weight is formed. In the interval which thus elapses, some of the particles of lime are separated from the water, owing to the escape of the carbonic acid ([26]), and adhere to the roof. In this manner successive particles are separated, and are attached to each other, until what is called a stalactite, having somewhat the appearance of an icicle, is formed. These stalactites are sometimes solid, having a lamellar structure; sometimes of a fibrous texture, radiating from the centre to the circumference, as may be observed when they are broken; and sometimes hollow. If the water collects and drops too rapidly to allow time for the formation of a stalactite, it falls upon the floor, and there forms an irregular lump of alabaster, which has the name of stalagmite. In some caverns, the separation of the calcareous matter takes place both at the roof and on the floor; and, in course of time, the substance upon each increasing, they meet, and form pillars, sometimes of great magnitude.
Caverns of this kind occur in almost every country. Those of Derbyshire are well known; but the most celebrated stalactitic cave in the world is that of Antiparos, in the Grecian Archipelago.
The kind of limestone formed in the above manner is what the ancients generally denominated alabaster. It was employed by them for the same purposes as marble, was cut into tables, columns, vases, and sometimes even into statues. They also used it in the manufacture of vases or boxes for containing unguents. It is supposed to have been a vessel formed of this stone that is mentioned in the Gospel of St. Matthew, where it is said there came unto our Saviour “a woman having an alabaster box of precious ointment.” In the National Museum at Paris there is a colossal figure of an Egyptian deity, which is cut in a kind of alabaster brought from the mountains between the Nile and the Red Sea.