[45] See again Appendix 2. Slaty Cleavage.
[46] This is a piece of the gneiss of the Montanvert, near the Châlets of Blaitière dessous.
[47] "Some idea may be formed of the nature of these incurvations by supposing the gneiss beds to have been in a plastic state, either from the action of heat or of some other unknown cause, and, while in this state, to have been subjected to pressure at the two extremities, or in some other parts, according to the nature of the curvatures. But even this hypothesis (though the best that has been thought of) will scarcely enable us to explain all the contortions which not merely the beds of gneiss, but likewise of mica slate and clay slate, and even greywacke slate, exhibit. There is a bed of clay slate near the ferry to Kerrera, a few miles south of Oban, in Argyleshire. This bed has been partly wasted away by the sea, and its structure exposed to view. It contains a central cylindrical nucleus of unknown length (but certainly considerable), round which six beds of clay slate are wrapt, the one within the other, so as to form six concentric cylinders. Now, however plastic the clay slate may have been, there is no kind of pressure which will account for this structure; the central cylinder would have required to have been rolled six times in succession (allowing an interval for solidification between each) in the plastic clay slate."—Outlines of Mineralogy, Geology, &c., by Thomas Thomson, M.D.
CHAPTER X.
OF THE MATERIALS OF MOUNTAINS:—THIRDLY, SLATY COHERENTS.
§ 1. It will be remembered that we resolved to give generally the term "coherent" to those rocks which appeared to be composed of one compact substance, not of several materials. But, as in all the arrangements of Nature we find that her several classes pass into each other by imperceptible gradations, and that there is no ruling of red lines between one and the other, we need not suppose that we shall find any plainly distinguishable limit between the crystalline and coherent rocks. Sometimes, indeed, a very distinctly marked crystalline will be joined by a coherent rock so sharply and neatly that it is possible to break off specimens, no larger than a walnut, containing portions of each; but far more frequently the transition from one to the other is effected gradually; or, if not, there exist, at any rate, in other places intervening, a series of rocks which possess an imperfectly crystalline character, passing down into that of simple coherence. This transition is usually effected through the different kinds of slate; the slaty crystallines becoming more and more fine in texture, until at last they appear composed of nothing but very fine mica or chlorite; and this mass of micaceous substance becomes more and more compact and silky in texture, losing its magnesia, and containing more of the earth which forms the substance of clay, until at last it assumes the familiar appearance of roofing-slate, the noblest example of the coherent rocks. I call it the noblest, as being the nearest to the crystallines, and possessing much in common with them. Connected with this well-known substance are enormous masses of other rocks, more or less resembling it in character, of which the following are universal characteristics.
§ 2. First. They nearly always, as just said, contain more of the earth, which is the basis of clay, than the crystalline rocks; and they can be scratched or crushed with much greater facility. Characteristics of Slaty Coherents.
1. Softness of texture. The point of a knife will trace a continuous powdery streak upon most of the coherent rocks; 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 structure. 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 preeminently the case in the mineral 3. Darkness and blueness in color. kingdom. The perfection with which the particles 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.[48]