OF THE MATERIALS OF MOUNTAINS:--THIRDLY, SLATY COHERENTS.
Sec. 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.
Characteristics of Slaty Coherents.
1. Softness of texture.
2. Lamination of structure.
Sec. 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 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. Darkness and blueness in color.
Sec. 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 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]
Sec. 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 supporting vegetation.
Sec. 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
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.
5. Adaptation to architecture and the fine arts.
Sec. 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.
Sec. 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.
FOOTNOTES
[48] Compare the close of Sec. 11, Chap. III. Vol. III., and, here,
Chap. III. Sec. 23.