Looking at the general results of the erosive work of the sea, we are led on reflection to perceive that they tend to the ultimate formation of a tolerably level surface, or what geologists call a plain of marine denudation. As it is only the uppermost layer of the sea-the part thrown into commotion by disturbance of the atmosphere-which possesses any efficient power of abrasion, the effect must be not to cut out valleys, but to eat into the land horizontally, and reduce it to a general level under the waves. This tendency is sometimes well illustrated on a small scale on a rocky beach. To the south of Girvan, for example, the Ayrshire coast exhibits between tide-marks a smooth level platform of Silurian greywacke, indenting the line of rugged crags which run along high-water mark (Fig. 17). This platform has been cut out

FIG. 17.-Inclined Silurian strata near Girvan, cut into a plane surface by the sea. of vertical strata, some of which being harder than their neighbours, rise above it into fantastic knobs and bosses. It abounds in cavities lined with sea-weeds and filled with sea-water-each a natural aquarium; and in some cases, at least, it is evident that these hollows are simply pot-holes, like those in the channel of a river, save that the boulders which lie at their bottom have been kept whirling round in the eddies of the tide, instead of a rapid brook or river. The level platform, with its hollows and outstanding crags, is a plain of marine denudation, and illustrates, on a small scale and in detail, a process of which the more gigantic results will be considered in succeeding chapters.

But, as I have already stated, we should ascribe to the

sea too great a share in the process of denudation were we to regard such a plain as mainly the work of the waves. These give it, indeed, its characteristic flatness, and put the last touches to its sculpture. But all the previous long record of waste, during which the land was worn down to the sea-level, was one in which the sub-aërial agents almost alone were concerned. These agents, sawing, filing, scraping, grooving, polishing, continue their operations as long as any land remains above the sea. It is true that the sea is also at the same time cutting away the margin of the land. But the area exposed to its attacks is a mere insignificant fraction of the whole extent of land which is subject to sub-aërial disintegration, and long before the waves could remove a strip, even a mile or two broad, from the edge of the land, the other denuding forces, working at the same rate as at present, would have reduced the dry land to the level of the sea. A plain of marine denudation, therefore, represents a base level of erosion-the limit down to which all the denuding agents, sub-aërial and marine, have reduced a mass of land, and beneath which further erosion ceases, because the ground, having got to the lowest level down to which the denuding agents can act, is thereafter protected by being covered by the sea. It is approximately a plain, because its erosion cannot be continued below a certain

average depth under the surface of the sea. But it must, doubtless, present many minor inequalities of surface, some parts remaining higher because of their superior capacity for resistance, or because of their lying less exposed to the grinding action of waves and currents which elsewhere have lowered the level of the bottom.

The sea between the British Isles and the coasts of the continent from the south of Norway to the north-western headlands of France is so shallow, that an elevation of only

600 feet would convert it all into dry land, except a long deep valley near the Norwegian coast. This wide submarine plain, if we consider the geological structure of the terrestrial areas that surround it, is undoubtedly a plain of erosion. Originally it was a portion of the general mass of the European continent. By a long succession of geological changes it was reduced to a base-level of erosion under the sea. But, after reaching that condition, it was re-elevated into land, and served as the terrestrial platform across which the present plants and animals of Britain found their way hither. Since that time it has sunk once more beneath the sea, the waves and currents are now wearing down the tops of its ridges, and strewing sand, gravel, and mud across its hollows; and by the constant gnawing away of the land on either side, the area of this submarine plain is slowly widening.

CHAPTER IV

GLACIERS AND ICEBERGS

Up to this point, the tools of earth-sculpture which we have been considering are all such as can be seen in actual operation in Scotland at the present time. In some cases, indeed, the vigour with which they are plied here is much less than in other parts of the world. Great as is the amount of wasted rock carried into the sea by an exceptionally heavy flood or 'spate' in this country, it is small, indeed, when contrasted with the rivers of mud which, during the rainy season in tropical climates, sweep down from the land, and help to heap up the long alluvial bars that, for such vast spaces, keep the sea, as it were, barred off from the land. Again, we should have but a limited conception. of the potency of frost if we took, as our type of its action, merely what we can here observe from winter to winter. Still, the action is the same in kind everywhere, and no one who has thoroughly observed it as it goes on in his own country, can have much difficulty in realising what it must. be in other parts of the world.

But all the tools which nature has used in the carving of the earth's surface are not to be seen now at work in Britain. Notably is this true of one which, at a date not by any

G

means remote in a geological sense, was in active operation here. All over these islands, except along the southern belt of England between the Bristol Channel and the mouth of the Thames, there is abundant evidence that the surface of the land, like the rest of the northern parts of Europe and America, has been modified by some kind of natural agent, very different in its effects from any of those which have been noticed in the foregoing chapters. Had the features of these northern regions been carved only by air, rain, springs, rivers, and frosts, their general outlines would have been more rugged than they are. The valleys would oftener have had scarped sides, the hills would have been sharper in form, and more deeply cleft with gullies and ravines. There would have been such an angularity of topography as one can witness on a crag or crest long exposed to sub-aërial disintegration. But on the contrary, a general smoothness of contour characterises both hill and valley, pointing to some abrading agency which has in large measure worn off the old roughnesses, and given a flowing outline to the ground. The fuller proofs of this statement will be given in a future chapter.

In the meantime, by way of illustration and explanation, let me briefly refer to the aspect of the deep inlets by which the Atlantic rolls its tides far into the heart of the great mountainous tracts of the Western Highlands. Fronting the sea, as on the coast about Arisaig, a scattered group or chain of islets rises out of the water- little bare rocky bosses, like the backs of dolphins. Inside this natural breakwater, the fjord or sea-loch, deeper, perhaps, than the sea immediately outside, winds inland between lofty mountains. On either hand, the rocks of the valley slip down into the sea, with the same smoothed outlines. It is only the loftier crests and peaks, shooting far up into the colder layers of