§ 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.
To these inferior conditions of the compact coherence we owe also the greater part of the _pretty_ scenery of the inhabited globe. The sweet winding valleys, with peeping cliffs on either side; the light, irregular wanderings of broken streamlets; the knolls and slopes covered with rounded woods; the narrow ravines, carpeted with greensward, and haunted by traditions of fairy or gnome; the jutting crags, crowned by the castle or watch-tower; the white sea-cliff and sheep-fed down; the long succession of coteau, sunburnt, and bristling with vines,--all these owe whatever they have of simple beauty to the peculiar nature of the group of rocks of which we are speaking; a group which, though occasionally found in mountain masses of magnificent form and size, is on the whole characterized by a comparative smallness of scale, and a tendency to display itself less in true mountains than in elevated downs or plains, through which winding valleys, more or less deep, are cut by the action of the streams.
§ 4. It has been said that this group of rocks is distinguished by its incapability of being separated into sheets. This is only true of it in small portions, for it is usually deposited in beds or layers of irregular thickness, which are easily separable from each other; and when, as not unfrequently happens, some of these beds are only half an inch or a quarter of an inch thick, the rock appears to break into flat plates like a slaty coherent. But this appearance is deceptive. However thin the bed may be, it will be found that it is in its own substance compact, and not separable into two other beds; but the true slaty coherents possess a delicate slatiness of structure, carried into their most minute portions, so that however thin a piece of them may be, it is usually possible, if we have instruments fine enough, to separate it into two still thinner flakes. As, however, the slaty and compact crystallines, so also the slaty and compact coherents pass into each other by subtle gradations, and present many intermediate conditions, very obscure and indefinable.
