Probably the most effective part of the destructive action of the sea is to be found in the battery of gravel, shingle, and loose blocks of stone which the waves discharge against cliffs exposed to their fury. These loose materials, caught up by the advancing breakers and thrown with great force upon the rocks of a coast-line, are dragged back in the recoil of the water, but only to be again lifted and swung forward. In this loud turmoil, the loose stones are reduced in size and are ground smooth by friction against each

FIG. 26.-Buller of Buchan-a caldron-shaped cavity or blow-hole worn out of granite by the sea on the coast of Aberdeenshire.

other and upon the solid cliff. The well-rounded and polished aspect of the gravel on such storm-beaten shores is an eloquent testimony to the work of the waves. But still more striking, because more measurable, is the proof that the very cliffs themselves cannot resist the blows dealt upon them by the wave-borne stones. Above the ordinary limit reached by the tides, the rocks rise with a rough ragged face, bearing the scars inflicted on it by the ceaseless attacks of the air, rain, frost, and the other agencies that waste the surface of the land. But all along the base of the cliff, within reach of the waves, the rocks have been smoothed and

polished by the ceaseless grinding of the shingle upon them, while arches, tunnels, solitary pillars, half-tide skerries, creeks, and caves attest the steady advance of the sea and the gradual demolition of the shore.

Every rocky coast-line exposed to a tempestuous sea affords illustrations of these features of the work of waves. Even where the rocks are of the most durable kind, they cannot resist the ceaseless artillery of the ocean. They are slowly battered down, and every stage in their demolition may be witnessed, from the sunken reef, which at some distance from the shore marks where the coast-line once ran, up to the tunnelled cliff from which a huge mass was detached during the storms of last winter. But where the materials composing the cliffs are more easily removed, the progress of the waves may be comparatively rapid. Thus on the east coast of Yorkshire between Spurn Point and Flamborough Head, the cliffs consist of boulder-clay, and vary up to more than 100 feet in height. At high water, the tide rises against the base of these cliffs, and easily scours away the loose debris which would otherwise gather there and protect them. Hence, within historic times, a large tract of land, with its parishes, farms, villages, and seaports, has been washed away, the rate of loss being estimated at not less than 21 yards in a year. Since the Roman occupation a strip of land between 2 and 3 miles broad is believed to have disappeared.

It is evident that to carry on effectively this mechanical erosion, the sea-water must be in rapid motion. But in the deeper recesses of the ocean, where there is probably no appreciable movement of the water, there can hardly be any sensible erosion. In truth, it is only in the upper parts of the sea, which are liable to be affected by wind, that the conditions for marine erosion can be said to exist. The space within which these conditions are to be looked for is that comprised between the lowest depth to which the influence of waves and marine currents extends, and the greatest height to which breakers are thrown upon the land. These limits, no doubt, vary considerably in different regions. In some parts of the open sea, as off the coast of Florida, the disturbing action of the waves has been supposed to reach to a depth of 600 feet, though the average limit is probably greatly less. On exposed promontories in stormy seas, such as those of the north of Scotland, breakers have been known to hurl up stones to a height of 300 feet above sea-level. But probably the zone, within which the erosive work of the sea is mainly carried on, does not as a rule exceed 300 feet in vertical range.

Within some such limits as these, the sea is engaged in gnawing away the edges of the land. A little reflection will show us that, if no counteracting operation should come into play, the prolonged erosive action of the waves would reduce the land below the sea-level. If we suppose the average rate of demolition to be 10 feet in a century, then it would take not less than 52,800 years to cut away a strip one mile broad from the edge of the land. But while the sea is slowly eating away the coast-line, the whole surface of the land is at the same time crumbling down, and the wasted materials are being carried away by rivers into the sea at such a rate that, long before the sea could pare away more than a mere narrow selvage, the whole land might be worn down to the sealevel by air, rain, and rivers (p. 28).

But there are counteracting influences in nature that would probably prevent the complete demolition of the land. What these influences are will be more fully considered in a later chapter. In the meantime, it will be enough to bear in mind that while the land is constantly worn down by the forces that are acting upon its surface, it is liable from time to time to be uplifted by other forces acting from below. And the existing relation between the amount and height of land, and the extent of sea, on the face of the globe, must be looked upon as the balance between the working of both these antagonistic classes of agencies.

FIG. 28.-Section of submarine plain. 7, Land cut into caves, tunnels, sea-stacks, reefs, and skerries by the waves, and reduced to a platform below the level of the sea (ss) on which the gravel, sand, and mud (d) produced by the waste of the coast may accumulate.

But without considering for the present whether the results of the erosion performed by the sea will be interrupted or arrested, we can readily perceive that their tendency is toward the reduction of the level of the land to a submarine plain (Fig. 28). As the waves cut away slice after slice from a coast-line, the portion of land which they thus overflow, and over which they drive the shingle to and fro, is worn down until it comes below the lower limit of breaker-action, where it may be covered up with sand or

mud. When the abraded land has been reduced to this level, it reaches a limit where erosion ceases, and where the sea, no longer able to wear it down further, protects it from injury by other agents of demolition. This lower limit of destruction on the surface of the earth has been termed "the base-level of erosion."

We see, then, that the goal toward which all the wear and tear of a coast-line tends, is the formation of a more or less level platform cut out of the land. Yet an attentive study of the process will convince us that in the production of such a platform the sea has really had less to do than the atmospheric agents of destruction. An ordinary sea-cliff is not a vertical wall. In the great majority of cases it slopes seaward at a steep angle; but if it had been formed, and were now being cut away, mainly by the sea, it ought obviously to have receded fastest where the waves attack it—that is, at its base. In other words, if sea-cliffs retired chiefly because they are demolished by the sea, they ought to be most eroded at the bottom, and should therefore be usually overhanging precipices. That this is not the case shows that some other agency is concerned which causes the higher parts of a cliff to recede faster than those below. This agency can be no other than that of the atmospheric forces—air, frost, rain, and springs. These cause the face of the cliff to crumble down, detaching mass after mass, which, piled up below, serve as a breakwater, and must be broken up and removed by the waves before the solid cliff behind them can be attacked.

ii. Accumulations formed by the Sea. It is not its erosive action that constitutes the most important claim of the sea to the careful study of the geologist. After all, the mere marginal belt or fringe within which this action is confined forms such a small fraction of the whole terrestrial area of the globe, that its importance dwindles down when we compare it with the enormously vaster surface over which the operations of the air, rain, rivers, springs, and glaciers are displayed. But when we regard the sea as the receptacle into which all the materials worn off the land ultimately find their way, we see what a large part it must play in geological history.

During the last fifteen years great additions have been made to our knowledge of the sea-bottom all over the world. Portions of the deposits accumulating there have been dredged up even from the deepest abysses, so that it is now possible to construct charts, showing the general distribution of materials over the floor of the ocean.