§ XXII. 2. Forms of Earth (Crystals). It may be asked why I do not say rocks or mountains? Simply, because the nobility of these depends, first, on their scale, and, secondly, on accident. Their scale cannot be represented, nor their accident systematised. No sculptor can in the least imitate the peculiar character of accidental fracture: he can obey or exhibit the laws of nature, but he cannot copy the felicity of her fancies, nor follow the steps of her fury. The very glory of a mountain is in the revolutions which raised it into power, and the forces which are striking it into ruin. But we want no cold and careful imitation of catastrophe; no calculated mockery of convulsion; no delicate recommendation of ruin. We are to follow the labor of Nature, but not her disturbance; to imitate what she has deliberately ordained,[64] not what she has violently suffered, or strangely permitted. The only uses, therefore, of rock form which are wise in the architect, are its actual introduction (by leaving untouched such blocks as are meant for rough service), and that noble use of the general examples of mountain structure of which I have often heretofore spoken. Imitations of rock form have, for the most part, been confined to periods of degraded feeling and to architectural toys or pieces of dramatic effect,--the Calvaries and holy sepulchres of Romanism, or the grottoes and fountains of English gardens. They were, however, not unfrequent in mediæval bas-reliefs; very curiously and elaborately treated by Ghiberti on the doors of Florence, and in religious sculpture necessarily introduced wherever the life of the anchorite was to be expressed. They were rarely introduced as of ornamental character, but for particular service and expression; we shall see an interesting example in the Ducal Palace at Venice. § XXIII. But against crystalline form, which is the completely systematised natural structure of the earth, none of these objections hold good, and, accordingly, it is an endless element of decoration, where higher conditions of structure cannot be represented. The four-sided pyramid, perhaps the most frequent of all natural crystals, is called in architecture a dogtooth; its use is quite limitless, and always beautiful: the cube and rhomb are almost equally frequent in chequers and dentils: and all mouldings of the middle Gothic are little more than representations of the canaliculated crystals of the beryl, and such other minerals: