peculiarly moulded outline which is quite different from the surface features produced by any other agency. If we examine a valley where the glacier once extended much further down than it does now, we shall find that the ice has left a smoothened surface of rock, rising here and there into rounded bosses or hummocks, which have a smooth, polished, and gently sloping surface on one side, with a rougher and steeper slope on the other; we shall also notice that the smoother faces of these hummocks all look up the valley towards the quarter whence the moving ice came. When viewed from a distance they have been thought to bear some resemblance to a flock of sheep lying down on the ground, and have hence been called roches moutonnées.

Passing down the valley we may also come upon the terminal moraine of the ancient glacier, still remaining as a mound of rubbish and rock débris stretching across the valley in a semicircular or horseshoe-shaped curve.

It is not only in countries where glaciers now exist that these ice-marks are found. Many valleys in the north of England, in Scotland, Ireland, and other parts of the world, present the same appearances as those of Switzerland, only the tool-marks are not quite so fresh-looking. The rounded forms of the roches moutonnées are, however, just as remarkable, and in some valleys a series of terminal moraines may be found, one above another, showing how the glacier gradually dwindled away, and shrank higher and higher up the valley.

From the above description it is clear that the passage of glacier ice down a valley will scrape out all the soils and superficial deposits that may previously have accumulated in the valley, and will sweep the materials onward to swell its terminal moraine. Any hollows or depressions which may have existed in the valley, and were, perhaps, filled up with alluvial deposits before the glacier was developed, will be cleared out by the ice, and subsequently, when the glacier has disappeared, these hollows will remain as lakes.

It has even been suggested by Sir A. Ramsay that the rock basins in which many lakes lie have actually been excavated by the erosive action of the glaciers; on this

point, however, there is much difference of opinion among geologists, but it is at any rate a noteworthy fact that lakes are most numerous in those countries which are known to have been glaciated in comparatively recent times.

CHAPTER VIII.

MARINE AGENCIES.

ACTION of Waves and Tides. The upper surface

of the sea is kept in continual agitation by the force of the winds, which raise it into waves, of all dimensions; these waves roll in upon every exposed coast-line throughout the world, and act as instruments of erosion and destruction.

Twice a day, also, the attraction of the sun and moon lifts the ocean into tides, which become currents as they approach the land. The height of the tides and the force of the currents thus produced depend upon the configuration of the coast. Their effects are greatest where shelving shores and narrow gulfs or bays confine the tidal waters into a narrower space; in such cases the rise and fall of the tide often amounts to 30 or 40 feet. In the Bristol Channel it is more than 40 feet, and the currents are proportionally powerful.

This rise and fall of the tides assists the waves by giving them a greater vertical space upon which to act, and the tidal currents serve also to transport the finer materials which are washed away from the coast. To estimate the power of sea-waves, they must be seen under the influence of a storm or gale of wind. Just as a brook in time of drought furnishes no measure of the work it does in time of flood, so the sea on a calm day gives no idea of the force with which storm waves can break on a rocky shore.

It has been ascertained that the average force of the breakers in winter time on the west coast of Scotland is equal to a pressure of 2,086 lbs. (nearly a ton) on every square foot of rock; during a storm it is greater. Mr. Stevenson found by experiments at the Bell Rock Lighthouse that the pressure was sometimes a ton and a half upon the square foot, and that at Skerryvore in the At

lantic it was doubly powerful, viz., about three tons to the square foot. At Eddystone Lighthouse the force of the waves in a storm was calculated to be equal to 3,013 lbs. (nearly 1 tons) on the square foot.

Such a force is evidently capable of dislodging and moving very large fragments of rock, especially if the blocks have been previously loosened by the action of rain and frost. Professor Geikie1 states that on the coast of the Pentland Firth blocks from 7 to 13 tons in weight had been quarried out of the cliffs at a height of 70 feet above the sea; the waves must have risen to that height, and still have been able to.bring down these immense fragments. In other places they have torn up similar masses of rock, and heaped them together at a height of 62 feet.

The damage and destruction which is often wrought upon piers and breakwaters during storms enables us to realize the enormous power of the waves, and anyone who has witnessed the making of a breach in such massive masonry will be better able to appreciate the action of the sea upon coasts and cliffs. During heavy gales at Plymouth in 1824 and 1829, blocks of limestone and granite, weighing from two to five tons, were washed about at the breakwater like pebbles. About 800 tons of such blocks were borne a distance of 200 feet up the inclined plane of the breakwater. In one place a block of limestone, seven tons in weight, was washed a distance of 150 feet. On the coast of Clare, in Ireland, blocks of rock may be seen which have been torn from rock-masses 20 feet above highwater mark, and thrown up on to the grass full 20 feet higher and 20 yards farther back."

The mere blow and force of the wave, however, is not the whole of the force exercised; when the water is dashed against a rock or cliff, some of it is injected into every crack and crevice it can find, and its pressure is exerted in forcing asunder the walls of the fissure. Moreover, the air that was previously in the fissures is suddenly compressed and impelled into the minuter cracks and planes of division. With the recess of the wave the pressure is suddenly

1 "Scenery and Geology of Scotland," p. 61.

2 De la Beche's "Geological Observer," p. 47.

3 Jukes' "Manual of Geology," second edition, p. 220.J

relieved, and the air and water rush out with a suction that is capable of loosening and dislodging large blocks of rock. No sooner are such fragments torn away from the cliff than they are immediately converted into engines of further destruction, for they are lifted by the waves and hurled against the rocks, so as to act like hammers or battering rams.

If the larger cracks and fissures run at right angles to the cliff, they form so many lines of weakness, which yield more readily to the continued action of the waves, and a long cave or else an open inlet is gradually formed. In other cases the larger cracks or master joints run parallel to the face of the cliff; shorter and wider cavities are then formed near the base of the cliff, which is thus undermined till one of the main joints is reached, when the unsupported mass above falls in ruin on the shore. The breakers then

proceed to reduce the size of the fallen fragments, dashing them one against the other, and breaking them up into pebbles and sand, till at last they are washed away, and the face of the cliff is exposed again to fresh assaults.

Instances of the action of the breakers on jointed rocks are to be seen on all coasts. The hard rocks of the western coast of Ireland afford many illustrative examples of the action as going on at present, their cliffs, caves, and rocky islets having been formed by this action. Mr. Jukes states, on the authority of Mr. W. L. Willson, late of the Geological Survey of Ireland, that in the far part of the promontory between Bantry and Dunmanus Bays there are dark holes in the fields some distance back from the edge of the cliffs, looking down into which the sea might be dimly seen washing backwards and forwards in the narrow cavern below.

"At high water, and during gales of wind, with heavy breakers rolling in upon the coast, vast volumes of water are poured suddenly into these narrow caverns, and rolling on, compress the air at their further end into every joint and pore of the rock above, and then, suddenly receding, suck both air and water back again with such force as now and then to loosen some part of the roof. Working in this way, the sea sometimes gradually forms a passage for itself to the surface above, and if that be not too lofty, forms a 'blow-hole,', or 'puffing-hole,' through which spouts of