§ 15. I have never had an opportunity of studying the effects of these tones upon rocks of porphyry; but the beautiful color of that rock in its interior substance has rendered it one of the favorite materials of the architects of all ages, in their most costly work. Not that all porphyry is purple; there are green and white porphyries, as there are yellow and white roses; but the first idea of a porphyry rock is that it shall be purple,--just as the first idea of a rose is that it shall be red. The purple inclines always towards russet[44] rather than blue, and is subdued by small spots of grey or white. This speckled character, common to all the crystalline rocks, fits them, in art, for large and majestic work; it unfits them for delicate sculpture; and their second universal characteristic is altogether in harmony with this consequence of their first.
Their second characteristic. _Toughness._
§ 16. This second characteristic is a tough hardness, not a brittle hardness, like that of glass or flint, which will splinter violently at a blow in the most unexpected directions; but a grave hardness, which will bear many blows before it yields, and when it is forced to yield at last, will do so, as it were, in a serious and thoughtful way; not spitefully, nor uselessly, nor irregularly, but in the direction in which it is wanted, and where the force of the blow is directed--there, and there only. A flint which receives a shock stronger than it can bear, gives up everything at once, and flies into a quantity of pieces, each piece full of flaws. But a piece of granite seems to say to itself, very solemnly: "If these people are resolved to split me into two pieces, that is no reason why I should split myself into three. I will keep together as well as I can, and as long as I can; and if I must fall to dust at last, it shall be slowly and honorably; not in a fit of fury." The importance of this character, in fitting the rock for human uses, cannot be exaggerated: it is essential to such uses that it should be hard, for otherwise it could not bear enormous weights without being crushed; and if, in addition to this hardness, it had been brittle, like glass, it could not have been employed except in the rudest way, as flints are in Kentish walls. But now it is possible to cut a block of granite out of its quarry to exactly the size we want; and that with perfect ease, without gunpowder, or any help but that of a few small iron wedges, a chisel, and a heavy hammer. A single workman can detach a mass fifteen or twenty feet long, by merely drilling a row of holes, a couple of inches deep, and three or four inches apart, along the surface, in the direction in which he wishes to split the rock, and then inserting wedges into each of these holes, and striking them, consecutively, with small, light, repeated blows along the whole row. The granite rends, at last, along the line, quite evenly, requiring very little chiselling afterwards to give the block a smooth face.