Waves and ground-swell are other features of oceanic action. Sometimes these waves are disastrous, but commonly they are of little consequence. The sea is never still. There is always a great though scarcely perceptible swell; but when this swell nears the shore, the upper portion of water, traveling faster than the lower, rises into Huge foam-crested billows or walls of water, which break with enormous force upon the beach. In the north of Scotland such billows often throw their spray to the height of 200 ft. It is estimated that a single roller of the ground-swell, 20 ft. high, falls with a pressure of about a ton on every square foot. The diminution. of atmospheric pres sure during a cyclone tends to raise the level of the sea within the cyclone's limits and give rise to enormous waves, On Oct. 5, 1864, during a great cyclone which passed ver Calcutta, the sea rose 4 ft., sweeping everything before it, and drowning 48,000 people.
Three chief types of sea-ice have been observed. In the arctic sounds and bays the littorahwaters freeze along the shores and form a cake of ice which, upborne by the tide, and adhering to the land, is thickened by successive additions below, as well as by snow above, until it forms a shelf of ice 120 to 130 ft. broad, and 20 or 30 ft. thick. This shelf, known as the ice-foot, serves as a platform on which the abundant debris loosened by the severe frosts of an arctic winter gathers at the foot of the cliffs. It is more or less completely broken up in summer, but forms again with the early frosts of the ensuing autumn. The surface of the open sea likewise freezes over into a contin uous sheet, which in summer breaks up into separate masses, sometimes of large extent. This is what navigators term "floe-ice," and the separate floating cakes are known as "floes." Ships fixed among these floes have been drifted with the ice for hundreds of miles, until at last liberated by its disruption. In the Baltic sea, off the coast of Labra dor and elsewhere, ice has been observed to form on the sea-bottom. It is known as ground-ice, or anchor-ice. In the Labrador fishing-grounds it forms even at consider able depths. Seals caught in the lines of these depths are brought up sometimes solidly frozen. In the Arctic regions the vast glaciers which drain the snow-fields and descend to the sea, extend for some distance from the land, until large fragments break off and float away seawards. These detached masses are icebergs. .Their shape and size vary greatly, but lofty peaked forms are common, and they sometimes rise from 200 to 300 ft. above the level of the sea. As only about a ninth part of the mass appears above water, those larger bergs may sometimes be from 2,000 to 3,000 ft. thick, from base to top. They consequently require water of some depth to float them, but they are often seen aground. In the antarctic regions, where one vast sheet of ice envelopes the land and extends as a high rampart into the sea, the detached icebergs often reach immense size, and are characterized by the frequency of a flat tabular form.
The Influence of Climate is one of the most important geological agencies. Ocean currents from warm regions raise the temperature of the places into which they flow; currents from cold regions lower it. The ocean is the great distributor of temperature over the earth. Note the opposite sides of the Atlantic. Along the North American
coast runs the cold arctic current, greatly depressing what would be the normal tempera ture. On the coast of Europe the gulf stream pours the warm water of the tropics, and correspondingly heightens the temperature. Dublin and the s.e. point of Labrador are in same latitude, yet the mean temperature of Labrador is 18° lower than that of the Irish capital.
Another great geological force is found in the works of erosion by the sea, which is accomplished in four ways: 1. the enormous force of the breakers, which suffices to tear off fragments of solid rock; 2. the alternate compression and expansion of air in the crevices of the rocks exposed to heavy breakers, which dislocate rocks even above the limits of wave action; 3. the hydraulic pressure of those portions of large waves which enter fissures and cavities, forcing asunder masses of rock; 4. the waves using loose materials to batter down the cliffs exposed -to their attacks. The dislodgement of Immense masses of loose materials especially from rocky cliffs, is too well known to need elucidatimi. All along the coasts of England, Scotland, and Norway the waves are gnawing down the rocky shores. Blocks of granite weighing 50 tons or more have been torn out and tossed about as though they were of wood. These assaults of the waves are remarkably aided by a curious action of the air. At the Eddystone light house, a door which bad been securely fastened against the surf without, was actually driven outward by a pressure from within, the strong bolts and hinges being broken. It must be inferred that the sudden sinking of a great mass of water created a pnrtiai vacuum and that the air inside the lighthouse forced itself out to restore the equilibrium. But the greatest amount of erosion accomplished by the sea is due, not to its own direct mechanical impetus, but to the blows dealt by the bowlders, gravel, or sand which it drives against the shores. This is a kind of perpetual artillery playing against the ;land, here and there makingbreaches. This incessant attack from the sea hasworn from the rocks the wonderful caves of Staffa and others of similar character along the w.,coasts of Ireland, Scotland, and the Shetland and Orkney islands. The general result of the erosive action of the sea on the land is the production of a submarine plain. As.the sea advances by cutting slice after slice away from the coast, successive lines of beach pass under low-water mark. The whole of the littoral belt, as far down as wave action has influence, is continually being ground down by the moving detritus. If no change of level between sea and land should take place, the sea might conceivably eat its way slowly far into the land, and produce a gently sloping yet almost horizontal selvage of plain covered permanently by the waves. In such a submarine plain the influence of geological structure, and notably of the relative powers of resistance of different rocks would make itself conspicuous. The present promontories caused by the superior'hardness of their component rocks would no doubt be represented by ridges on the sub-aqueous plateau, while the existing bays and creeks worn out of softer rocks would be marked by lines of valleys or :mllows.