Characteristics of slaty coherents. texture. structure. and they can be scratched or crushed with much greater facility. The point of a knife will trace a continuous powdery streak upon most of the coherent rocks; 1. Softness of while it will be quite powerless against a large portion of the granular knots in the crystallines. Besides this actual softness of substance, the slaty coherents are 2. Lamination of capable of very fine division into flakes, not irregularly and contortedly, like the crystallines, but straightly, so as to leave a silky lustre on the sides of the fragments, as in roofing slate; and separating with great ease, yielding to a slight pressure against the edge. Consequently, although the slaty coherents are capable of forming large and bold mountains, they are liable to all kinds of destruction and decay in a far greater degree than the crystallines; giving way in large masses under frost, and crumbling into heaps of flaky rubbish, which in its turn dissolves or is ground down into impalpable dust or mud, and carried to great distances by the mountain streams. These characters render the slaty coherents peculiarly adapted for the support of vegetation; and as, though apparently homogeneous, they usually contain as many chemical elements as the crystallines, they constitute (as far as regards the immediate nourishment of soils) the most important part of mountain ranges. § 3. I have already often had occasion to allude to the apparent connexion of brilliancy of color with vigor of life, or purity of substance. This is pre-eminently the case in the mineral 3. Darkness and kingdom. The perfection with which the partiblueness in color. cles of any substance unite in crystallization corresponds, in that kingdom, to the vital power in organic nature; and it is a universal law, that according to the purity of any substance, and according to the energy of its crystallization, is its beauty or brightness. Pure earths are without exception white when in powder; and the same earths which are the constituents of clay and sand, form, when crystallized, the emerald, ruby, sapphire, amethyst, and opal. Darkness and dulness of color are the universal signs of dissolution, or disorderly mingling of elements.* *Compare the close of § 11, Chap. III. Vol. III., and, here, Chap. III. § 23. § 4. Accordingly, these slaty coherents, being usually composed of many elements imperfectly united, are also for the most part grey, black, or dull purple; those which are purest and hardest verging most upon purple, and some of them in certain lights displaying, on their smooth sides, very beautiful zones and changeful spaces of grey, russet, and obscure blue. But even this beauty is strictly connected with their preservation of such firmness of form as properly belongs to them; it is seen chiefly on their even and silky surfaces; less, in comparison, upon their broken edges, and is lost altogether when they are reduced to powder. They then form a dull grey dust, or, with moisture, a black slime, of great value as a vegetative earth, but of intense ugliness when it occurs in extended spaces in mountain scenery. And thus the slaty coherents are often employed to form those landscapes of which the purpose appears to be to impress us with a sense of horror and pain, as a foil to neighboring scenes of extreme beauty. There are many spots among the inferior ridges of the Alps, such as the Col de Ferret, the Col d'Anterne, and the associated ranges of the Buet, which, though commanding prospects of great nobleness, are themselves very nearly types of all that is most painful to the human mind. Vast wastes of mountain ground, covered here and there with dull grey grass, or moss, but breaking continually into black banks of shattered slate, all glistening and sodden with slow tricklings of clogged, incapable streams; the snow water oozing through them in a cold sweat, and spreading itself in creeping stains among their dust; ever and anon a shaking here and there, and a handful or two of their particles or flakes trembling down, one sees not why, into more total dissolution, leaving a few jagged teeth, like the edges of knives eaten away by vinegar, projecting through the half-dislodged mass from the inner rock, keen enough to cut the hand or foot that rests on them, yet crumbling as they wound, and soon sinking again into the smooth, slippery, glutinous heap, looking like a beach of black scales of dead fish, cast ashore from a poisonous sea, and sloping away into foul ravines, branched down immeasurable slopes of barrenness, where the winds howl and wander continually, and the snow lies in wasted and sorrowful fields, covered with sooty dust, that collects in streaks and stains at the bottom of all its thawing ripples. I know no other scenes so appalling as these in storm, or so woful in sunshine. 4. Great power of tation. § 5. Where, however, these same rocks exist in more favorable positions, that is to say, in gentler banks and at lower elevations, they form a ground for the most luxuriant vegetation; and the valleys of Savoy owe to them some of their supporting vege- loveliest solitudes,-exquisitely rich pastures, interspersed with arable and orchard land, and shaded by groves of walnut and cherry. Scenes of this kind, and of that just described, so singularly opposed, and apparently brought together as foils to each other, are, however, peculiar to certain beds of the slaty coherents, which are both vast in elevation, and easy of destruction. In Wales and Scotland, the same groups of rocks possess far greater hardness, while they attain less elevation; and the result is a totally different aspect of scenery. The severity of the climate, and the comparative durableness of the rock, forbid the rich vegetation; but the exposed summits, though barren, are not subject to laws of destruction so rapid and fearful as in Switzerland; and the natural color of the rock is oftener developed in the purples and greys which, mingled with the heather, form the principal elements of the deep and beautiful distant blue of the British hills. Their gentler mountain streams also permit the beds of rock to remain in firm, though fantastic, forms along their banks, and the gradual action of the cascades and eddies upon the slaty cleavage produces many pieces of foreground scenery to which higher hills can present no parallel. Of these peculiar conditions we shall have to speak at length in another place. 6. As far as regards ministry to the purposes of man, the slaty coherents are of somewhat more value than the slaty crystallines. Most of them can be used in the same way for rough buildings, while they furnish finer plates or sheets architecture and for roofing. It would be difficult, perhaps, to estimate the exact importance of their educational influence in the form of drawing-slate. For sculpture they are, of course, altogether unfit, but I believe certain finer conditions of them are employed for a dark ground in Florentine mosaic. 5. Adaptation to the fine arts. § 7. It remains only to be noticed, that the direction of the lamination (or separation into small folio) is, in these rocks, not always, nor even often indicative of the true direction of their larger beds. It is not, however, necessary for the reader to enter into questions of such complicated nature as those which belong to the study of slaty cleavage; and only a few points, which I could not pass over, are noted in the Appendix; but it is necessary to observe here, that all rocks, however constituted, or however disposed, have certain ways of breaking in one direction rather than another, and separating themselves into blocks by means of smooth cracks or fissures, technically called joints, which often influence their forms more than either the position of their beds, or their slaty lamination; and always are conspicuous in their weathered masses. Of these, however, as it would be wearisome to enter into more detail at present, I rather choose to speak incidentally, as we meet with examples of their results in the scenery we have to study more particularly. CHAPTER XI. OF THE MATERIALS OF MOUNTAINS:-FOURTHLY, COMPACT COHERENTS. § 1. THIS group of rocks, the last we have to examine, is, as far as respects geographical extent and usefulness to the human race, more important than any of the preceding ones. It forms the greater part of all low hills and uplands throughout the world, and supplies the most valuable materials for building and sculpture, being distinguished from the group of the slaty coherents by its incapability of being separated into thin sheets. All the rocks belonging to the group break irregularly, like loaf sugar or dried clay. Some of them are composed of hardened calcareous matter, and are known as limestone; others are merely hardened sand, and are called freestone or sandstone; and others, appearing to consist of dry mud or clay, are of less general importance, and receive different names in different localities. § 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 |