§ 11. The first, that these crystalline rocks being, as we saw above, generally the oldest and highest, it is from them that other soils of various kinds must be derived; and they were therefore made a kind of storehouse, from which, wherever they were found, all kinds of treasures could be developed necessary for the service of man and other living creatures. Thus the granite of Mont Blanc is a crystalline rock composed of four substances; and in these four substances are contained the elements of nearly all kinds of sandstone and clay, together with potash, magnesia, and the metals of iron and manganese. Wherever the smallest portion of this rock occurs, a certain quantity of each of these substances may be derived from it, and the plants and animals which require them sustained in health.
The second reason appears to be that rocks composed in this manner are capable of more interesting variety in form than any others; and as they were continually to be exposed to sight in the high ranges, they were so prepared as to be always as interesting and beautiful as possible.
And divisible into two classes, Compact Crystallines and Slaty Crystallines.
§ 12. These crystalline or spotted rocks we must again separate into two great classes, according to the arrangement, in them, of the particles of a substance called mica. It is not present in all of them; but when it occurs, it is usually in large quantities, and a notable source of character. It varies in color, occurring white, brown, green, red, and black; and in aspect, from shining plates to small dark grains, even these grains being seen, under a magnifier, to be composed of little plates, like pieces of exceedingly thin glass; but with this great difference from glass, that, whether large or small, the plates will not easily break _across_, but are elastic, and capable of being bent into a considerable curve; only if pressed with a knife upon the edge, they will separate into any number of thinner plates, more and more elastic and flexible according to their thinness, and these again into others still finer; there seeming to be no limit to the possible subdivision but the coarseness of the instrument employed.
§ 13. Now, when these crystals or grains, represented by the black spots and lines in Fig. 3, lie as they do at _a_ in that figure, in all directions, cast hither and thither among the other materials of the stone,--sometimes on their faces, sometimes on their sides, sometimes on their edges,--they give the rock an irregularly granulated appearance and structure, so that it will break with equal ease in any direction; but if these crystals lie all one way, with their sides parallel, as at _b_, they give the rock a striped or slaty look, and it will most readily break in the direction in which they lie, separating itself into folia or plates, more or less distinctly according to the quantity of mica in its mass. In the example Fig. 4, a piece of rock from the top of Mont Breven, there are very few of them, and the material with which they are surrounded is so hard and compact that the whole mass breaks irregularly, like a solid flint, beneath the hammer; but the plates of mica nevertheless influence the fracture on a large scale, and occasion, as we shall see hereafter, the peculiar form of the precipice at the summit of the mountain.[43]