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until flotation comes into play, when large masses break off from the ice-cliff, and rising up and floating, sail away seaward as icebergs. These ice-islands carry with them any soil or rock-rubbish which may have fallen upon them from inland cliffs, while they formed part of the ice-sheet of the country. The debris so borne off is, of course, thrown down upon the sea-bottom, as each berg melts away after a voyage of perhaps hundreds of miles. Year by year, whole fleets of these bergs are sent southwards in the arctic regions, so that the bed of the northern seas must be strewn with earth and boulders. As only between an eighth and a ninth part of a mass of ice appears above the sea-water on which it floats, the bulk of many bergs must be enormous. One ris

ing two hundred feet above the waves-not an uncommon height-must have its bottom more than seventeen hundred feet below them, and the thickness of the ice-cap at its outer edge must be there about two thousand feet. The antarctic ice-sheets and ice-bergs are of still more colossal dimensions.

Deeply seated in the water, bergs are acted on much more by marine currents than by winds. Hence, they are sometimes seen careering through a frozen sea in the teeth of a tempest, breaking up the thick-ribbed ice before them with a noise like the loudest thunder, yet with as much apparent ease as a ploughshare cuts the loam. Every winter, crowds of bergs are firmly fixed in the frozen sea of the arctic regions, and when summer comes, the united mass drifts southwards towards Newfoundland. Vast floes of ice, larger sometimes than the whole of Scotland, with embedded ice-hills rising two hundred feet or more above the sea-level and sinking seventeen hundred feet or more below it, are thus borne by the ocean-currents into warmer latitudes, where they break up and disappear. When such current-driven masses grate or strand on the

sea-bottom, they no doubt tear up the ooze, and bruise and scratch the rocks. In the course of long ages, a submerged hill or ridge may get its crest and sides much bruised, shorn, and striated, and the sea-bed generally may be similarly grooved and polished, the direction of the striation being more or less north and south according to the prevalent trend of the drifting ice. In some of the younger deposits of Scotland, there are indications that the lower parts of the country were submerged in an icy sea across which floes and bergs drifted to and fro. There can be no doubt, however, that the general smoothing and striation of the surface of the country has been the work of land-ice and not of icebergs.

CHAPTER V

THE HISTORY OF A LAND-SURFACE

HAVING now, as it were, watched the employment of each of the implements wielded by nature in the process of sculpturing the surface of the land, let us briefly consider the combined effect of the co-operation of the whole of these processes upon the landscapes of a country possessing such a climate and geological structure as Scotland. In the first place, as the result of the influence of the air, changes of temperature, rain and frost, there is a general disintegration of the whole surface of the land. Even the most obdurate rocks cannot permanently resist this decay, while the softer kinds yield to it with notable rapidity. The crumbled materials are ready to be blown away by wind or to be washed off by rain, leaving new surfaces exposed to a continuation of the same ceaseless attack. While the general surface of the land suffers, scope is afforded for the manifestation of differences in the degree of resistance to the progress of destruction. The harder rocks are gradually left projecting above those which, being softer, are more readily abraded.

The lines by which the drainage of the land is carried out to sea are liable to specially vigorous erosion. The

detritus produced by sub-aërial waste is washed into the rivers, and the coarser parts are employed by the running water in scouring its channels, which are thus deepened and widened, and sink inch by inch farther into the framework of the land. Under favourable conditions of climate and geological structure, the streams dig out long and deep ravines in solid rock; but more usually their work is made less obvious by the activity of the other sub-aërial agents, which attack the sides of the water-channels and lower them as fast as the streams can deepen their beds. By this combination of operations, valleys with sloping sides are hollowed

out.

In those regions where the atmospheric moisture generally falls to the ground as snow, land-ice is formed, which, when it assumes the character of an ice-cap or of distinct valley glaciers, moves downwards from the higher grounds, grinding, smoothing, polishing, and scratching the rocks over which it marches, hollowing them out into basin-shaped cavities in one place, and leaving them as projecting domes and bosses in another, but everywhere removing that angularity of feature, which they naturally assume under the ordinary influence of the weather, and replacing it with those characteristic smooth flowing contours, which are everywhere so marked a relic and evidence of ice-action.

Lastly, the sea eats into the margin of land and cuts away slice after slice. Here and there indeed, whether from the sediment carried down by rivers or from that which the tides and waves cast ashore, there is a gain of land from the sea. But such additions are merely local, and are generally insignificant. They do not seriously affect the conclusion which the evidence forces upon us that, on the whole, the sea encroaches on the land, and will continue to do so as long as any dry land is left above its level.

The

Such, then, is the order of nature as we see it now. surface of this country, and indeed of all the great terrestrial areas of the planet, is undergoing a continuous process of denudation, during which many inequalities of contour are worked out, but the ultimate result of which, if unchecked by any counterbalancing natural agency, must be to reduce. the dry land to the level, or rather a little below the level of the sea. The limit beneath which there is little effective erosion by waves and tidal currents probably does not exceed a very few hundred feet. Worn down to that limit, the degraded land would become a submarine plain, across the surface of which younger deposits might afterwards be strewn..

In speculating, however, from ascertained facts as to what may be or what has been the history of a land-surface, we are of course bound to take into consideration the effect of subterranean movements in modifying changes at the surface. It is evident that even if no disturbance were made in the drainage-lines of a country, upheaval, by increasing the declivity of the streams, would quicken their scour, and not improbably add to their volume by augmenting the rainfall. Subsidence, on the other hand, would have the contrary effect, and would carry down part of the land out of reach of atmospheric disintegration. It is certain that in the geological past there have been many uplifts, by which the solid terrestrial crust has been plicated and fractured on a colossal scale. Such a chain of mountains as the Alps, for instance, exhibits proofs of stupendous inversions and folds, whole mountainous masses of rock having there been thrown over bottom uppermost. But while the proofs of prodigious displacements are so clear, it is by no means so evident how they affected the surface of the ground at the time, or what

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