CHAPTER IX.

OF THE MATERIALS OF MOUNTAINS:--SECONDLY, SLATY CRYSTALLINES.

-- 1. It will be remembered that we said in the last chapter (-- 4) that one of the notable characters of the whole group of the crystallines was the incomprehensibility of the processes which have brought them to their actual state. This however is more peculiarly true of the slaty crystallines. It is perfectly possible, by many processes of chemistry, to produce ma.s.ses of irregular crystals which, though not of the substance of granite, are very like it in their mode of arrangement.

But, as far as I am aware, it is impossible to produce artificially anything resembling the structure of the slaty crystallines. And the more I have examined the rocks themselves, the more I have felt at once the difficulty of explaining the method of their formation, and the growing interest of inquiries respecting that method. The facts (and I can venture to give nothing more than facts) are briefly these:--

-- 2. The mineral called mica, described in the course of the last chapter, is closely connected with another, differing from it in containing a considerable quant.i.ty of magnesia. This a.s.sociated mineral, called chlorite, is of a dull greenish color, and opaque, while the mica is, in thin plates, more or less translucent; and the chlorite is apt to occur more in the form of a green earth, or green dust, than of finely divided plates. The original quant.i.ty of magnesia in the rock determines how far the mica shall give place to chlorite; and in the intermediate conditions of rock we find a black and nearly opaque mica, containing a good deal of magnesia, together with a chlorite, which at first seems mixed with small plates of true mica, or is itself formed of minute plates or spangles, and then, as the quant.i.ty of magnesia increases, a.s.sumes its proper form of a dark green earth.

-- 3. By this appointment there is obtained a series of materials by which the appearance of the rock may be varied to almost any extent.

From plates of brilliant white mica half a foot broad, flashing in the sun like panes of gla.s.s, to a minute film of dark green dust hardly traceable by the eye, an infinite range of conditions is found in the different groups of rocks; but always under this general law, that, for the most part, the compact crystallines present the purest and boldest plates of mica; and the tendency to pa.s.s into slaty crystallines is commonly accompanied by the change of the whiteness of the mica to a dark or black color, indicating (I believe) the presence of magnesia, and by the gradual intermingling with it of chloritic earth; or else of a cognate mineral (differing from chlorite in containing a quant.i.ty of lime) called hornblende.

[Ill.u.s.tration: FIG. 5.]

Such, at least, is eminently the case in the Alps; and in the account I have to give of their slaty crystallines, it must be understood that in using the word "mica" generally, I mean the more obscure conditions of the mineral, a.s.sociated with chlorite and hornblende.

-- 4. Now it is quite easy to understand how, in the compact crystallines, the various elements of the rock, separating from each other as they congealed from their fluid state, whether of watery solution or fiery fusion, might arrange themselves in irregular grains as at _a_ in Fig. 3, p. 106. Such an arrangement constantly takes place before our eyes in volcanic rocks as they cool. But it is not at all easy to understand how the white, hard, and comparatively heavy substances should throw themselves into knots and bands in one definite direction, and the delicate films of mica should undulate about and between them, as in Fig. 5 on page 114, like rivers among islands, pursuing, however, on the whole, a straight course across the ma.s.s of rock. If it could be shown that such pieces of stone had been formed in the horizontal position in which I have drawn the one in the figure, the structure would be somewhat intelligible as the result of settlement.

But, on the contrary, the lines of such foliated rocks hardly ever are horizontal; neither can distinct evidence be found of their at any time having been so. The evidence, on the contrary, is often strongly in favor of their having been formed in the highly inclined directions in which they now occur, such as that of the piece in Fig. 7, p. 117.[45]

[Ill.u.s.tration: FIG. 6.]

-- 5. Such, however, is the simple fact, that when the compact pact crystallines are about to pa.s.s into slaty crystallines, their mica throws itself into these bands and zones, undulating around knots of the other substances which compose the rock. Gradually the knots diminish in size, the mica becomes more abundant and more definite in direction, and at last the ma.s.s, when broken across the beds, a.s.sumes the appearance of Fig. 6 on the last page.[46] Now it will be noticed that, in the lines of that figure, no less than in Fig. 5, though more delicately, there is a subdued, but continual expression of _undulation_. This character belongs, more or less, to nearly the whole ma.s.s of slaty crystalline rocks; it is one of exquisite beauty, and of the highest importance to their picturesque forms. It is also one of as great mysteriousness as beauty. For these two figures are selected from crystallines whose beds are remarkably straight; in the greater number the undulation becomes far more violent, and, in many, pa.s.ses into absolute contortion. Fig. 7 is a piece of a slaty crystalline, rich in mica, from the Valley of St.

Nicolas, below Zermatt. The rock from which it was broken was thrown into coils three or four feet across: the fragment, which is drawn of the real size, was at one of the turns, and came away like a thick portion of a crumpled quire of paper from the other sheets.[47]

[Ill.u.s.tration: FIG. 7.]

Typical character of Slaty Crystallines.

-- 6. I might devote half a volume to a description of the fantastic and incomprehensible arrangement of these rocks and their veins; but all that is necessary for the general reader to know or remember, is this broad fact of the _undulation_ of their whole substance. For there is something, it seems to me, inexpressibly marvellous in this phenomenon, largely looked at. It is to be remembered that these are the rocks which, on the average, will be oftenest observed, and with the greatest interest, by the human race. The central granites are too far removed, the lower rocks too common, to be carefully studied; these slaty crystallines form the n.o.blest hills that are easily accessible, and seem to be thus calculated especially to attract observation, and reward it.

Well, we begin to examine them; and first, we find a notable hardness in them, and a thorough boldness of general character, which make us regard them as very types of perfect rocks. They have nothing of the look of dried earth about them, nothing petty or limited in the display of their bulk. Where they are, they seem to form the world; no mere bank of a river here, or of a lane there, peeping out among the hedges or forests: but from the lowest valley to the highest clouds, all is theirs--one adamantine dominion and rigid authority of rock. We yield ourselves to the impression of their eternal, unconquerable stubbornness of strength; their ma.s.s seems the least yielding, least to be softened, or in anywise dealt with by external force, of all earthly substance. And, behold, as we look farther into it, it is all touched and troubled, like waves by a summer breeze; rippled, far more delicately than seas or lakes are rippled; _they_ only undulate along their surfaces--this rock trembles through its every fibre, like the chords of an Eolian harp--like the stillest air of spring with the echoes of a child's voice. Into the heart of all those great mountains, through every tossing of their boundless crests, and deep beneath all their unfathomable defiles, flows that strange quivering of their substance. Other and weaker things seem to express their subjection to an Infinite power only by momentary terrors: as the weeds bow down before the feverish wind, and the sound of the going in the tops of the taller trees pa.s.ses on before the clouds, and the fitful opening of pale s.p.a.ces on the dark water as if some invisible hand were casting dust abroad upon it, gives warning of the anger that is to come, we may well imagine that there is indeed a fear pa.s.sing upon the gra.s.s, and leaves, and waters, at the presence of some great spirit commissioned to let the tempest loose; but the terror pa.s.ses, and their sweet rest is perpetually restored to the pastures and the waves. Not so to the mountains. They, which at first seem strengthened beyond the dread of any violence or change, are yet also ordained to bear upon them the symbol of a perpetual Fear: the tremor which fades from the soft lake and gliding river is sealed, to all eternity, upon the rock; and while things that pa.s.s visibly from birth to death may sometimes forget their feebleness, the mountains are made to possess a perpetual memorial of their infancy,--that infancy which the prophet saw in his vision: "I beheld the earth, and lo, it was without form and void, and the heavens, and they had no light. I beheld the mountains, and lo, they _trembled_; and all the hills _moved lightly_."

Serviceable characters of the Slaty Crystallines.

1. Fitness for building with.

-- 7. Thus far may we trace the apparent typical signification of the structure of those n.o.ble rocks. The material uses of this structure are not less important. These substances of the higher mountains, it is always to be remembered, were to be so hard as to enable them to be raised into, and remain in, the most magnificent forms; and this hardness renders it a matter of great difficulty for the peasant to break them into such ma.s.ses as are required for his daily purposes. He is compelled in general to gather the fragments which are to form the walls of his house or his garden from the ruins into which the mountain suffers its ridges to be naturally broken; and if these pieces were absolutely irregular in shape, it would be a matter of much labor and skill to build securely with them. But the flattened arrangement of the layers of mica always causes the rock to break into flattish fragments, requiring hardly any pains in the placing them so as to lie securely in a wall, and furnishing light, broad, and unflawed pieces to serve for slates upon the roof; for fences, when set edgeways into the ground; or for pavements, when laid flat.

2. Stability in debris.

-- 8. Farther: whenever rocks break into utterly irregular fragments, the ma.s.ses of debris which they form are not only excessively difficult to walk over, but the pieces touch each other in so few points, and suffer the water to run so easily and so far through their cavities, that it takes a long series of years to enable them either to settle themselves firmly, or receive the smallest covering of vegetation. Where the substance of the stone is soft, it may soon be worn down, so that the irregular form is of less consequence. But in the hard crystallines, unless they had a tendency to break into flattish fragments, their ruin would remain for centuries in impa.s.sable desolation. The flat shape of the separate pieces prevents this; it permits--almost necessitates--their fitting into and over each other in a tolerably close ma.s.s, and thus they become comparatively easy to the foot, less permeable to water, and therefore retentive both of surface moisture and of the seeds of vegetation.

3. Security on declivities.

-- 9. There is another result of nearly equal importance as far as regards the habitableness of the hills. When stones are thrown together in rounded or ma.s.sy blocks, like a heap of hazel nuts, small force will sometimes disturb their balance; and when once set in motion, a square-built and heavy fragment will thunder down even a slightly sloping declivity, with an impetus as unlikely to be arrested as fatal in its increase. But when stones lie flatly, as dead leaves lie, it is not easy to tilt any one of them upon its edge, so as to set it in motion; and when once moved, it will nearly always slide, not roll, and be stopped by the first obstacle it encounters, catching against it by the edge, or striking into the turf where first it falls, like a hatchet. Were it not for the merciful ordinance that the slaty crystallines should break into thin and flattish fragments, the frequent falls of stones from the hill sides would render many spots among the greater mountain chains utterly uninhabitable, which are now comparatively secure.

4. Tendency to form the loveliest scenery.

-- 10. Of the picturesque aspects which this mode of cleavage produces in the mountains, and in the stones of the foreground, we shall have to speak presently; with regard to the uses of the materials it is only necessary to note farther that these slaty rocks are of course, by their wilful way of breaking, rendered unfit for sculpture, and for nearly all purposes of art; the properties which render them convenient for the peasant in building his cottage, making them unavailable for the architecture of more elaborate edifices. One very great advantage is thus secured for the scenery they compose, namely, that it is rarely broken by quarries. A single quarry will often spoil a whole Alpine landscape; the effect of the lovely bay of the Lago Maggiore, for instance, in which lie the Borromean Islands, is, in great part, destroyed by the scar caused by a quarry of pink granite on its western sh.o.r.e; and the valley of Chamouni itself has lost some of its loveliest rock scenery in consequence of the unfortunate discovery that the boulders which had fallen from its higher pinnacles, and were lying in ma.s.sy heaps among its pines, were available for stone lintels and door-posts in the building of its new inns. But the slaty crystallines, though sometimes containing valuable mines, are hardly ever quarried for stone; and the scenes they compose retain, in general, little disturbed by man, their aspect of melancholy power, or simple and n.o.ble peace. The color of their own ma.s.s, when freshly broken, is nearly the same as that of the compact crystallines; but it is far more varied by veins and zones of included minerals, and contains usually more iron, which gives a rich brown or golden color to their exposed sides, so that the coloring of these rocks is the most glowing to be found in the mountain world. They form also soil for vegetation more quickly, and of a more fruitful kind than the granites, and appear, on the whole, intended to unite every character of grandeur and of beauty, and to const.i.tute the loveliest as well as the n.o.blest scenes which the earth ever unfolds to the eyes of men.

FOOTNOTES

[45] See again Appendix 2. Slaty Cleavage.

[46] This is a piece of the gneiss of the Montanvert, near the Chalets of Blaitiere 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 cla.s.ses pa.s.s 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, pa.s.sing 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 ma.s.s 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 a.s.sumes the familiar appearance of roofing-slate, the n.o.blest example of the coherent rocks.

I call it the n.o.blest, as being the nearest to the crystallines, and possessing much in common with them. Connected with this well-known substance are enormous ma.s.ses of other rocks, more or less resembling it in character, of which the following are universal characteristics.

Characteristics of Slaty Coherents.

1. Softness of texture.

2. Lamination of structure.

-- 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. 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 capable of very fine division into flakes, not irregularly and contortedly, like the crystallines, but straightly, so as to leave a silky l.u.s.tre 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 ma.s.ses 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 h.o.m.ogeneous, they usually contain as many chemical elements as the crystallines, they const.i.tute (as far as regards the immediate nourishment of soils) the most important part of mountain ranges.

3. Darkness and blueness in color.

-- 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 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 const.i.tuents 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]

-- 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 s.p.a.ces 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 s.p.a.ces 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 a.s.sociated ranges of the Buet, which, though commanding prospects of great n.o.bleness, 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 gra.s.s, 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 ma.s.s 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 ash.o.r.e 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 supporting vegetation.

-- 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 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 princ.i.p.al 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.

5. Adaptation to architecture and the fine arts.

-- 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 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.