Hence, in spite of the efforts of nature to heal the wounds with a covering of vegetation, fresh raw scars are always appearing. Among solid rocks similar effects are to be seen, though the rate of degradation is there much slower. Where a thick stratum of pervious stone lies upon another of more impervious kind-a massive sandstone upon shale or clay, for example-and where these rocks end off in a cliff or steep bank, the conditions are favourable for the occurrence of landslips. Water oozing out along the outcrop of the lower bed loosens the support of the overlying stratum, which, consequently, from time to time breaks off in large blocks that roll down to join the masses that have already fallen. In this way, the vertical edge of a harder bed forms an escarpment, which, by continual loss along its face, creeps backward. FROST. In close connection with the disintegrating effects of springs on cliffs and steep slopes of rock, comes the influence of frost. When the water, which is trickling between the joints of a cliff, is frozen, it expands, and in so doing exerts a vast disparting force on the rocks within which it is confined. On the thawing of the ice, the rocks which have been thus separated do not return to their former position; the severance remains until it is increased by another frost. Winter after winter, as the loosened masses are separated, the wedge of ice is driven farther in between them, and at last, losing cohesion and support, those on the outside fall with a crash from the face of the cliff, leaving a raw scar to mark whence they have come. Every mountain group in Scotland will be found to supply examples of this operation. Some rocks being more jointed or opening more easily along their joints than others, are more readily broken up by frost. Granite, for instance, is remarkable for the perfection of its jointing and also generally for its toughness. It may crumble away on the surface, but otherwise may remain coherent and durable, though there are some varieties that decay far down into their mass. Its numerous joints, however, afford admirable scope for the action of frost. On lofty mountain crests, accordingly, granite frequently presents a most impressive array of splintered crags. Pinnacles and buttresses of the most varied forms and dimensions rise along the face of the precipices. Vast rifts, descending for several hundred feet, show where the joints have most easily opened, and naked vertical walls mark where the ice-wedges, driven home by the winters of centuries, have at last detached huge slices from the face of the cliffs. The tourist who has climbed Lochnagar will well remember the grim precipice that yawns beneath him as he gains the crest of the mountain and looks northward to the valley of the Dee (Fig. 37). There, screened from the sun, the snow lingers long into the summer, and frost finds a congenial home. Inch by inch the vertical joints are there being opened farther into the face of the cliff. Along the edge one can, as it were, watch all the stages of the process from the fine rift just starting like a crack in a window pane, up to the loosened pillar which now stands gaunt and alone in front and awaits the fate that is eventually to hurl it into the gulf below. Far down, between the base of the precipice and the little tarn that lies gleaming in the shadow of the mountain, we can see the grey slopes cumbered with debris, and can hardly believe, so much does height deceive us, that these long slopes are not mere trails of sand but avalanches of blocks, many of them hundreds of tons in weight, which, in the course of ages, have been wedged off from the cliffs, and are now travelling slowly to the plains, still, however, a prey to frost and rain, sun and storm, and slowly breaking up into loose fragments as they descend. Besides splitting solid rocks into separate blocks, frost disintegrates their surface, as the water contained between their particles freezes. This result, especially observable in tracts where there is abundant moisture in summer and great cold in winter, is familiarly seen in the pulverising of soil. Every one knows that when thaw comes after a long black frost, the country roads are often in as bad condition as after long rain or snow, the reason being that the frost, having separated the particles of the earth, has allowed the thawed moisture to mix thoroughly with them. Hence the value of a hard frost upon bare ploughed land. The soil is, as it were, ground down finer, new portions are added to it from the surface of stones which it may contain, and fresh mineral matter is thus provided for the rootlets of plants. CHAPTER III THE SEA AND ITS WORK ON THE SCOTTISH COAST-LINE IN contemplating the gradual waste to which the surface of the earth is everywhere subjected, the observer is soon struck with the signal proofs of decay furnished by that outer border of the land which is washed by the sea. The abrupt cliffs that shoot up from high-water mark, the skerries that rise among the breakers a little way from the shore, and the sunken reefs that lie still farther out to sea-all tell of the removal of masses of solid rock. A little reflection leads us to perceive that the abrading power of the sea must be confined to that upper part of the water which is affected by winds and tides, and that in the deeper abysses there is probably no sensible erosion of hard rock, though the currents there may be capable of carrying along fine ooze and silt. The waste which takes place along the line where land and sea meet has a twofold character. In the first place, there is a direct abrasion by the sea itself, and it is this loss which is now to be considered. But, in the second place, cliffs and precipitous banks overlooking the waves subject to that never-ceasing atmospheric waste described in the last chapter; and the sea, in many places, does little are more than remove from tide-mark the debris which has been loosened from the cliffs by springs and frosts. Thus sea-cliffs, like the walls of river-gorges, receive no permanent protection from an accumulation of their own ruins in front of them. In looking at the results of the wear and tear of a coast-line, we are apt to assign, perhaps, too much importance to the action of the breakers, and too little to the less obtrusive but more constant influences of rains, springs, and frosts. It may be impossible to give to each agency its due share in the wasting of the shore, but it should not be forgotten that in what is usually called marine denudation the atmospheric influences play a great part. Some indication of the relative potency of the sea and sub-aërial agents is afforded by the forms of sea-cliffs. The sea only wears away the base of a precipice, but air, rain, springs, and frost attack every accessible part of it. Hence, other things being equal, where a cliff is so eaten away below as to overhang, it not improbably points to greater rapidity of breaker-action. Where, on the other hand, the cliff recedes towards the top, it proves that its upper portion has been more worn away by sub-aërial disintegration than its lower part has been by the sea; and as overhanging cliffs are quite exceptional, it follows that in the demolition of a cliff, and the constant advance of the sea landwards across a plain of erosion, sub-aërial disintegration takes a larger share than the waves and currents of the sea. It occasionally happens that a rock breaks away along lines of clean-cut joints which, when vertical, leave smooth perpendicular walls of rock. These joints are opened from above by the atmospheric agents, and slice after slice is cut away from the precipices, which are thus enabled to retain. their wall-like character. Nowhere in Britain can these features be so impressively seen as along the great ranges of |