FORM OF SEA BOTTOM Reasoning by analogy alone, we would infer that the visible and invisible portions of the earth's outer crust must, generally speaking, present the same characteristic features of conformation ; that the irregular elevations and depressions, mountains and valleys, plateaux and plains of the land, will have their counterpart in the relief of the fax larger area covered by the waters of the ocean. In the absence of direct proof to the contrary, we would be inclined to believe that, were the vast mass of water removed, and the bed of the ocean laid bare, the scene that would meet our eyes would be but a repetition of the familiar sights on land : the same characteristic inequalities, probably intensified in proportion to the vastly larger area of the ocean—submarine plains and plateaux of vast extent, long mountain chains of enormous heights, and vast chasms of incalculable depths. And although our knowledge of the "depths of the sea" cannot as yet be said to be very extensive or absolutely accurate, yet sufficient information has been gathered, chiefly by the Challenger and other deep-sea exploration expeditions, as to enable us to form a tolerably correct idea of the general conformation of the floor of the • ocean; the position, extent, height, and depth of all the larger elevations and depressions. The general results of recent researches prove (1) that there is some—though perhaps not very exact—relation between the heights of the land and the depths of the sea ; the sea-level being, as it were, the relief equator of the earth, equidistant from the highest elevation of the land and the greatest depth of the sea ; and (2) that the sea-bottom does not, except in a few isolated instances, present abrupt descents and extreme inequalities like the land, but is, on the whole, gently undulating.
Generally speaking, the depth of water along the coast always corresponds, more or less, to the height of the adjacent land. If the land be low, the water will be shallow, but if it be high, the water will be proportionately deep. A gradual or
gentle slope of the land towards the sea is always followed by a gradual increase in the depth of the water ; the degree of inclination being, as it were, continued under the water for some distance. But if the coast be lofty and precipitous, sinking abruptly into the sea, the water deepens as suddenly and abruptly.
This "law of depth," or the correspondence between the height of the land and the depth of the water, applies only to the belt of water nearest the land, for it gives no clue whatever to the actual depth of the more distant and open areas ; but though the areas to which it may be applied are limited, it is nevertheless remarkably true for those limits. Striking in stances are found in every part of the world. Thus the level lands on the eastern coasts of England are bordered by the shallow waters of the North Sea. The lofty coasts of Norway, on the other hand, descend abruptly into corre spondingly deep water. The vast plains of northern Asia shelve gradually into the shallow Arctic Ocean, while the lofty southern extremities of Africa and America dip into the deep waters of the great Southern Ocean.
As we have already said, the general character of the bottom of the sea is "gently undulating;" the descent, even in the deepest parts, being by a gentle gradient. Occasionally the slope is very abrupt, but the extreme irregularity, so common on land, is confined to certain areas where there are vast coral reefs rising almost perpendicularly out of the ocean, or where the normal regularity of the ocean floor has been broken by violent volcanic disturbances, resulting in pre cipitous upheavals or subsidences. With these exceptions the floor of the ocean is far more regular than the surface of the land; there being, generally speaking, no extreme irregu larities as on land, although that must have been the case at one time, when the present sea-bed was dry land, and con sequently subject to active denudation.