Old Red Sandstone coast-cliffs of Caithness and the Orkney Islands (Fig. 7). Where the joints are inclined away from the sea the cliffs actually overhang; but this is obviously not due to any greater demolition of their base by the waves. The true explanation will be at once understood from Fig. 8, which represents the overhanging cliffs in one of the Orkney Islands (compare also the view of the coast-line near Wick (Fig. 11), and of the Noss Head, Shetland, in Fig. 14).

No one can watch the progress of a storm on an exposed rocky coast without being strongly impressed with the powerful effects of breakers in wearing away the margin of the land. A wave which can deal a blow equal to a pressure of 6083 pounds, or nearly three tons, on the square foot (and this is the ascertained impetus of storm waves among the outer Hebrides), is no feeble instrument of abra

sion. Yet such a wave appears to have of itself little or no power to grind down the surface of the rocks on which it beats, for that surface, even after a storm, is found to be just as plentifully coated with living barnacles as before. the friction of the water could rub down the stone, these cirripedes would be removed first. It is only where its enormous weight and impetus can break off a loosened mass of rock, that a wave may be said to act by its own sheer force.

In the great majority of cases, however, breaker-action eats into a coast-line either by battering down the rocks with their own debris, or by enlisting the co-operation of the air. wave that lifts up and sweeps forward gravel, boulders, and even large blocks of stone, is a far more formidable instrument of destruction than even a large wave which is not armed with the same weapons. The stones that are thus swung on by the tempest fall with prodigious force against

the rocks of the shore; brought back by the recoil of the wave, they are caught up again by its successor and once more hurled forwards upon the rocks. And thus, by what has been aptly termed a kind of sea-artillery, even the hardest rocks of an iron-bound shore are worn away. Moreover, a huge billow, falling on a cliff that is penetrated by many cracks and passages, drives the air into these with prodigious force. The consequent contraction and expansion of the air must needs act with great effect in widening clefts and helping to disrupt rocks. The hydraulic pressure of large waves which fall upon cliffs with a force equivalent to a pressure of three tons on the square foot, cannot but tend also to dislodge masses of solid rock.

To see the character and effects of sea-action, the observer should betake himself to some rocky shore on which falls the full roll of the Atlantic. He will there find, if the coast be a precipitous one, that the rocks above the reach of the waves are rough and ragged, showing everywhere traces of that sub-aërial waste which, acting along their natural joints, has slowly shattered the crags and sent down huge blocks to the beach below. There, the fallen ruin, coming within reach of the waves, is turned into a further means of augmenting the destruction of the cliffs. Ground down by the waves into well-worn boulders, it is driven up against the cliffs, which along their base are smoothed and polished like the shingle. The line between the rough surface overhead, marking the progress of the atmospheric waste, and the well-worn zone of the beach, pointing to the work of the sea, is often singularly sharp. But the base of the cliff is not merely polished by the friction of the boulders; it is in many places hollowed out into overhanging recesses, clefts, and eaves. At the farther end of one of these excavations may be seen the rounded boulders that are carrying

on the work of demolition, likewise the passage which, worked by the compressed air through the roof, has been subsequently enlarged by many a storm, until now, when a gale sets in strongly from the sea and sweeps the breakers at high water against the cliffs, the yeasty sea-water is shot up through the opening high into the air, with a noise like the firing of a cannon. Many such spouting holes may be seen

along the Scottish coast-line. One of the best known is the Buller of Buchan, on the coast of Aberdeenshire (Fig. 9). In this case, as frequently happens, the upper part of the opening or roof of the sea-cave has fallen in; so that, from the level moor or field above, one advances to the top of a huge cauldron, at the bottom of which the chafing tides may be seen. But no one who knows such places in calm

weather only can realise their tumult and grandeur at the height of a great storm.

Between the upper limit of the tide and low-water mark on a rocky coast, crags and skerries may be noted in every stage of decay. Here rises an outjutting part of the cliff which has been separated, with the breakers meeting and bursting into foam in the narrow passage. Yonder a mass, once evidently connected with the main cliff in the same way, has been sundered by the roof of the tunnel falling in, and it now stands up as a tall massive outwork of the line of rampart behind (see Figs. 11, 13). Lower on the narrow beach, worn, tangle-covered bosses of rock rise out of the shingle and boulders, and run out to sea in low reefs, usually fringed with foam even in the calmest summer day, but rising in places into islets haunted by seal and wild-fowl. Everywhere the eye rests upon proofs of unceasing destruction. We see that the cliffs must once have stretched seaward, at least as far as yonder sea-stack, fully a furlong from their present limit, and how much farther no man can tell. It is impressively taught that the selvage of land which has been cut off has been carried away by the sea. The whole process in all its stages is before our eyes. We note the weather wasting the cliffs above, and the sea battering them below. And it is impossible to doubt, that if in a comparatively short geological period a strip of land, say a furlong broad, has been in this way planed down, there is here revealed to us a power of waste, the effects of which, if unchecked by any other natural force, can have no limit short of the total demolition of the dry land.

In looking more narrowly at the progress of this abrasion, we find it dependent not merely on the prevalent winds and the consequent fetch of the breakers, but in large measure upon the varying geological structure of the coast-line.