Sec. 2. Among these rocks, the foremost position is, of course, occupied by the great group of the marbles, of which the substance appears to have been prepared expressly in order to afford to human art a perfect means of carrying out its purposes. They are of exactly the necessary hardness,--neither so soft as to be incapable of maintaining themselves in delicate forms, nor so hard as always to require a blow to give effect to the sculptor's touch; the mere pressure of his chisel produces a certain, effect upon them. The color of the white varieties is of exquisite delicacy, owing to the partial translucency of the pure rock; and it has always appeared to me a most wonderful ordinance,--one of the most _marked_ pieces of purpose in the creation,--that all the variegated kinds should be comparatively opaque, so as to set off the color on the surface, while the white, which if it had been opaque would have looked somewhat coarse (as, for instance, common chalk does), is rendered just translucent enough to give an impression of extreme purity, but not so translucent as to interfere in the least with the distinctness of any forms into which it is wrought. The colors of variegated marbles are also for the most part very beautiful, especially those composed of purple, amber, and green, with white; and there seems to be something notably attractive to the human mind in the vague and veined labyrinths of their arrangements. They are farther marked as the prepared material for human work by the dependence of their beauty on smoothness of surface; for their veins are usually seen but dimly in the native rock; and the colors they assume under the action of weather are inferior to those of the crystallines: it is not until wrought and polished by man that they show their character. Finally, they do not decompose. The exterior surface is sometimes destroyed by a sort of mechanical disruption of its outer flakes, but rarely to the extent in which such action takes place in other rocks; and the most delicate sculptures, if executed in good marble, will remain for ages undeteriorated. Sec. 3. Quarries of marble are, however, rare, and we owe the greatest part of the good architecture of this world to the more ordinary limestones and sandstones, easily obtainable in blocks of considerable size, and capable of being broken, sawn, or sculptured with ease; the color, generally grey, or warm red (the yellow and white varieties becoming grey with age), being exactly that which will distinguish buildings by an agreeable contrast from the vegetation by which they may be surrounded.