A cliff of shale seldom shows much of the picturesque, though often a good deal of the ruinous. The rock is easily undermined by streams, and a shale ravine usually exhibits in consequence either heaps of crumbling rubbish, or, where the stream comes past with a more rapid current, perpendicular walls, jointed and laminated, but without much variety of outline. Such cliffs, however, merit the careful attention of the observer, for from their friability they are most easily decomposed and washed down by streams, to form new accumulations of similar soft argillaceous matter. A shale coast-line sometimes shows cliffs of considerable altitude, as in some parts of Skye and Pabba, where the Lias shales may be seen piled over each other often to a height of seventy or eighty feet, and spreading out along the shore as low flat reefs and skerries, brown with algæ at their seaward ends, and showing on the higher slopes of the beach the characteristic fossils of the Lias—ammonites, belemnites, and gryphææ—crowded together by hundreds. The action of the decomposing forces has operated more effectually on the soft material of the shale than on the hard crystalline lime of the included shells, so that the latter stand out in relief from the dull-brown surface of the rock, and from their numbers and prominence form one of the most marked features of the coast-line.
Probably few have ever visited a limestone district without marking the manner in which that rock yields to the action of the elements, whether in an inland part of the country where rivers have cut deep gullies through the rock, or along some exposed shore where the stone has been wasted by a still ruder assailant. An exposed cliff of hard homogeneous limestone weathers into deep clefts and holes; the entire surface assumes a pitted appearance, somewhat like a sandy beach after a showier of rain, and the planes of stratification, or lines formed by the parallel junction of the beds are often worn away until the rock looks not unlike a piece of old masonry, in which the mortar has decayed and dropped out, leaving the angles of the stones to get wasted and rounded by the action of the weather. In many districts, too, where the rock is richly fossiliferous, the broken joints of encrinites along with corals and shells may be seen crowded together by myriads, their hard skeletons protruding from the wasted rock in such a way as to show that the stone can contain very little else. By this means we often learn that a limestone bed is nothing but an old sea-bottom, where the calcareous sediment was mainly derived from broken stone-lilies, corals, and shells, though if we break off a piece of the rock the internal fracture may show very little or no trace of any organic structure. And hence if the geologist would form an accurate conception of the origin and structure of many of the stratified rocks, he must study them not in hand specimens neatly trimmed and arranged along the shelves and drawers of a cabinet, nor even in the ponderous blocks daily exhumed by the quarryman, but along some surf-beaten cliff-line or down some precipitous ravine where the rock for centuries has been exposed to the wear and tear of the elements.
Limestones and other calcareous formations are liable to more than ordinary decay, for, as we have seen above, percolating rain-water constantly carries away mineral matter from their subterranean portions. Accordingly, in some parts of the country, as for instance in Yorkshire, the interior of such rocks has been eaten away into great caverns by this form of decomposition.
Some remarkable examples occur in the island of Raasay, one of the north-eastern Hebrides. Its eastern margin shoots up from the sea to a height of over 900 feet, the cliff-line being formed of a calcareous grit as perpendicular as a wall, and fissured by deep chasms and rents. The narrow table-land between the edges of this cliff and an abrupt ridge that rises behind, is perforated by innumerable holes and clefts, into which if a stone be thrown it may be heard for several seconds rumbling far below. The edges of these pitfalls are often fringed with ferns, rushes, and long grass, so as to be nearly hidden, and it requires no little caution to traverse this elevated region in safety. Innumerable sheep have been lost by falling into the subterranean abysses, and even the wary natives seem to have sometimes lost their footing. A story is told of a woman who had crossed to the other side of the island for the purchase of some commodities, and returning by the high grounds had got nearly within sight of her own cottage, when by some unlucky accident she took a false step and instantly disappeared. Unfortunately her errand had been performed alone, so that some time elapsed ere she was missed. The search continued unremitting for two days, but no trace of the missing traveller could be found. At last on the third day her figure was seen creeping slowly along the road not many hundred yards from her own door. It appeared that she had first slid down a sheer height of about fifty feet, when her further passage was intercepted by the sides of the fissure. During the earlier part of her confinement she strove hard to re-ascend the chasm, and it was not until, the effort seeming fruitless, she had begun to resign herself to despair, that a glimmering of light from below induced her to attempt a descent. This proved no easy matter, and occupied many weary hours of labour and suspense; but at length she succeeded in worming herself to the bottom, and crawled out more dead than alive only a little way from her home. There still stand perched on some of these precipitous cliffs the remains of a few villages, the inhabitants of which were accustomed to tether their children to the soil, whence one of the hamlets received in Gaelic the soubriquet of Tethertown. Many a valuable commodity disappeared by rolling over the cliff, and I have been assured that it was no unfrequent occurrence for a pot of potatoes capsized at the doorway to tumble down the slope and make no stop until safely esconced at the sea-bottom.
The process whereby these fissures and caverns originate is the same as that noticed already in the Spar Cave. Water containing an impregnation of carbonic acid filters down through cracks and fissures of the calcareous rock, dissolving out in its passage a portion of the lime which it eventually carries back to the surface, and either deposits there or transports into streams, and thence to the sea. Thus atom by atom is removed wherever the percolating water reaches, until in the course of ages an irregular cavern of greater or less extent is produced. The decomposition of limestone at the surface results from the same kind of action, that of carbonated water. Every shower of rain insensibly carries away a fraction of the constituent parts of the rock, so that the size and form of detached blocks as well as of exposed cliffs is constantly changing. How often do we see the same decay going on with a melancholy rapidity among the exposed marble tombstones of our churchyards. In a few years the tablet gets worn and furrowed as though it had stood there for centuries. Eventually, too, the inscription becomes effaced, and perhaps ere the bones of the deceased have mouldered away and mingled with their kindred dust, the epitaph that recorded for the admiration of posterity his many virtues and his vigorous talents, has faded from the stone—often, alas! only too fit an emblem of how speedily the memory of the dead may fade away out of the land of the living.
CHAPTER IX.
Mechanical forces at work in the disintegration of rocks—Rains Landslips—Effects of frosts—Glaciers and icebergs—Abrading power of rivers—Suggested volume on the geology of rivers—Some of its probable contents—Scene in a woody ravine—First idea of the origin of the ravine one of primeval cataclysms—Proved to be incorrect—Love of the marvellous long the bane of geology—More careful examination shows the operations of Nature to be singularly uniform and gradual—The doctrine of slow and gradual change not less poetic than that of sudden paroxysms—The origin of the ravine may be sought among some of the quieter processes of Nature—Features of the ravine—Lessons of the waterfall—Course of the stream through level ground—True history of the ravine—Waves and currents—What becomes of the waste of the land—The Rhone and the Leman Lake—Deltas on the sea-margin—Reproductive effects of currents and waves—Usual belief in the stability of the land and the mutability of the ocean—The reverse true—Continual interchange of land and sea part of the economy of Nature—The continuance of such a condition of things in future ages rendered probable by its continuance during the past.
The forms of decomposition noticed in the last chapter were chiefly of a chemical kind. Their effects were observable alike on the surface of the earth and below ground; in the latter case we saw them excavating caverns and long irregular chasms, in the former we noted the production of debris which if undisturbed went to the formation of soils. It must be borne in mind however, that in these operations other forces than simply those of a chemical kind come into play. The percolation of water and the removal of insoluble particles on the exposed parts of rocks rank as mechanical processes. So also do those by which new surfaces of mineral masses are brought within the sphere of the chemical agencies, such as the action of frosts, rains, rivers, and waves. In short, as already noticed, any subdivision of the forces at work in effecting the decomposition of rocks must ever be more or less arbitrary; but it remains nevertheless useful, if we bear in mind that the exactly defined boundary lines are of our making, not Nature's. With this caution we may proceed to examine what are termed the mechanical agencies in the disintegration of mineral masses, and in so doing, we shall find that the chemical forces are not less helpful to the mechanical than the latter to the former.
First, we may notice the effect of rains in washing away the disintegrated particles to lower levels or into river-courses whereby fresh portions of rock become exposed to the decomposing forces. Rains also act powerfully in altering the form of cliff-lines and steep declivities, especially where these consist more or less of friable earthy matter. After a long continuance of wet weather, I have seen the abrupt sides of a river-channel that were formed of a stiff blue clay completely cut up by rents of various dimensions, whereby large masses had subsided many feet, while others had rolled down altogether and lay in the bed of the stream where they were undergoing a rapid abrasion. The cause of this alteration was obvious. The rains pouring down from the sloping grounds on either side of the river had excavated deep channels on the abrupt face of the cliffs, while a considerable quantity of water finding its way through the soil, had permeated through joints and crevices in the clay some feet from the edge of the bank. By the combined operation of these causes, masses of clay several yards in extent lost their cohesion and either settled down a few feet, or found their way to the bottom. Such landslips are of frequent occurrence where large masses of rock of a hard compact nature rest upon loose shales and clays more or less inclined. Whole hills have been known to be hurled in this way into the valleys below.
But these results become perhaps still more marked where to the ordinary operations of water there are added those of intense frost. The effects of a severe winter (such, for instance, as a Canadian one), in loosening the particles of rocks and facilitating the breaking-up of large masses, must be ranked among the most powerful agencies of nature. In such a season, the percolating water with which nearly every surface-rock is charged becomes frozen, and in the act of congelation expands. The result of this dilatation is to exert great pressure on the particles of the rock, and thereby loosen their cohesion. When thaw comes the frozen liquid contracts again, but the loosened particles have no such elastic power, and so, having lost hold of each other, crumble down. If the season be a changeable one, frost and thaw quickly alternating, the amount of waste produced becomes very great. Not only is the outer surface of the stone decomposed, but the water filtering through the joints of the rock freezes there, and thus on the arrival of milder weather vast masses become detached from the cliffs, and roll down, to be worn by the grinding action either of waves or of rivers, as the case may be. Spring at last sets in with its warmth and its showers; the snow rapidly melts away; the whole country streams with water; every valley and hollow has its red turbid rivulet, that bears a burden of muddy sediment into the nearest river; and thus the loosened portions of the rocks get washed away down to sea, leaving a new surface for the action of next winter. We can easily understand, therefore, that in certain regions the combined effects of frost and thaw may work in the course of ages changes of almost inconceivable extent, and that the agency of ice must be not less varied and important on the land than, in the case of the boulder clay, we found it to be in the ocean.