‘.286 block of nearly 8 tons had been driven before the waves at the level of :20 feet above the sea, over very rough ground, to a distance of '73 feet (figs. 7 and 8). He likewise records the moving of a 50-ton block by the waves at Barrahead, in the Hebrides.‘ At Plymouth also, blocks of several tons in weight have been known to be washed about the breakwater like pebbles.‘-3 (2.) But, besides their mechanical force, waves acquire a singular and most effective aid from the air. It is a fact familiar to engineers that, even from a vertical and appar- ently perfectly solid wall of well-built masonry exposed to heavy seas, stones will sometimes be started out of their places, and that when this happens a rapid enlargement of the cavity may be effected, as if the walls were breached by a severe bombardment. At the Eddystone lighthouse, during a storm in 1840, a door which had been securely fastened against the force of the surf from without, was actually driven outward by a pressure acting from within the tower, in spite of the strong bolts and hinges, which were broken. vacuum was formed, and that the air inside forced out the door in_its efforts to restore the equilibrium.3 This explana- tion may partly account for the way in which the sto11es are started from their places in a solidly built sea-wall. But besides this cause we must also consider a perhaps still 111ore effective one in the condensation of the air driven before the wave between the joints and crevices of the stones, and its subsequent instantaneous expansion when the wave drops. During gales when large waves are driven to shore, many tons of water are poured suddenly into each cleft and cavern within reach. These volumes of water, as ' they rush in, compress the air into every joint and pore of the rock at the further end, a11d then quickly retiring, exert such a suction as from time to time to bring down part of the walls or roof. The sea may thus gradually form an inland passage for itself to the surface above, in a “ blow—hole ” or “ puffing-hole,” through which spouts of foam and spray are in storms shot high into the air. On the more exposed portions of the west coast of Ireland numerous examples of such blow-holes occur. In Scotland, likewise, they may often be observed, as in the Bullers (boilers) of Buchan on the coast of Aberdeenshire, and the Geary Pot near Arbroath. Magnificent instances occur among the Orkney and Shetland Islands, some of the more shattered rocks of these northern coasts being, as it were, honeycombed by sea- tunnels, many of which open up into the middle of fields or moors. (3.) The sea-water which, as part of an inrushing wave, fills the gullies and chinks of the sl1ore-rocks exerts the same pressure upon the walls between which it is confined as the rest of the wave is doing upon the face of the cliff. Each cleft so circumstanced becomes a kind of hydraulic press, the potency of which is to be measured by the force with which the waves fall upon the rocks outside—a force which often amounts to three tons on the square foot. There can be little doubt that by this means considerable pieces of a cliff are from time to time dislodged. 1 Stevenson, op, cit., pp. 21-37. . ‘ The reader will bear in mind that the specific gravity of bodies 13 greatly reduced when in water, and still more in sea-water. fo1lp3r7i1)1g examples will illustrate this fact (Stevenson on Ilarbours, p. :— 5 rec. .'o. of cub. fc N" ' f feet to " lo" Gl.,“._ a ton in Rift to in sen-water of sp. I ' grav. 1-029. Basalt 2 99 I 11-9 18-26 Red granite. . ‘."71 13-? 21.30 Snndsfone..... ..u 2 41 I we 26-00 Canm.-l coal...... . .. 1'54 ' 233 70-00 3 walker: P7‘0C- Inst. C'ic. Engz'n.. i. 15; Stevenson'sIlzzrbours, p. 10. GEOLOGY We may infer that, by the sudden sinking I of a mass of water hurled against the building, a partial - ~ cliff overhanging, because the sea acts only at its base.
- the fact that in the vast majority of cases sea-cliffs-, instead
[111. DYNAMICAL. (4.) But probably by far the largest amount of erosion accomplished by the sea is due not to its own direct mechanical impetus, but to the blows dealt by the boulders, gravel, or sand which it hurls against the shores. This action was aptly compared by Playfair to a kind of artil- lery.‘ I)uring a storm upon a shingly coast we may hear, at a distance of several miles, the grind of the stones against each other, as they are dragged back by the recoil of the waves which had launched them forward. In this tear and wear the loose stones are ground smaller, and acquire the smooth round form so characteristic of a surf- beaten beach. At the same time they b1‘uise and wear down the solid cliffs against which they may be driven. Wherever the rock is much jointed, or from any cause presents less resistance to attack, it is excavated into gullies, creeks, and caves ; its harder parts standing out as promontories are pierced ; gradually a series of detached buttresses and sea-stacks appears as the cliff recedes, and these in turn are wasted until they become mere skerries and sunken surf—beaten reefs. Of this progress of destruc- tion the more exposed parts of the British coast-line f nrnish many admirable examples. The west coast of Irela.nd, ex- posed to the full swell of the Atlantic, is in innumerable localities completely undermined by caverns, into which the sea enters from both sides. In many places the cliffs are as vertical as walls, this feature depending upon their julllts‘, which enable slice after slice to be undermined and removed. The precipitous coasts of Skye, Sutherland, Caithness, Forfar, Kincardine, and Aberdeenshire abound in the most impressive lessons of the waste of a rocky sea-margin ; while the same picturesque features are prolonged into the Orkney and Shetland Islands, the magnificent cliffs of Hoy towering as a vast wall some 1200 feet above the Atlantic breakers, which are tunnelling and fretting their base. If such is the progress of waste where the materials con- sist of the 111ost solid rocks, we may expect to meet with at least equally impressive proofs of decay where the coast—line can oppose only soft sand or clay to the march of the breakers. Again, the geological student in Britain can examine for himself many illustrations of this kind of de- struction around the shores of these islands. Within the last few hundred years entire parishes with their towns and villages have been washed away, and the tide now ebbs and flows over districts which in old times were cultivated fields and cheerful hamlets. The coast of Yorkshire between Flamborough Head and the mouth of the Humber, and also that between the Wash and the mouth of the Thames, suffer at a specially rapid rate, for the cliffs in these parts consist in great measure of soft clay. In some places this loss is said to amount to 3 feet per annum. While investigating the proofs of decay along the share, the geologist endeavours to ascertain to what extent the action of the waves is assisted by that of rain, springs, frosts, and general atmospheric disintegration. He often finds that the progress of the waves depends not so much upon then own labours as upon those of the terrestrial agencies already described. A crumbling cliff, battered and wasted by the breakers, will yield to him abundant evidence of the manner in which the other agents of destruction prepare the way for its final demolition and removal by the sea ,- and he will The learn that the very blocks of stone which give the waves so much of their efficacy are in great measure furnished to them by these co-operating agents. If the cutting back of a cliff were mainly the work of the sea, we ought to find the But of overhanging, slope backward, at a greater or less angle, from the sea, shows that the waste from subacrial action is
4 Illustrations nf the Iluttomlzn Theory, sec. 97.