How, then, can we account for the present regularity of the sea bottom? The idea that it has been "planed down," as it were, by vast deep-sea currents, is absurd : for would not their action produce the very irregularities which they are supposed to have removed I Besides, it is now well known that, although there is undoubtedly a "set" of the water along the bottom of the ocean (generally from the poles towards the equator), yet the movement is very slow, and scarcely perceptible. This secular movement, the "creeping flow" of polar waters along the ocean floor, can hardly be called a cur rent, and it certainly has no denuding action—otherwise the light fleecy-like deposits of calcareous and silicious shells which cover a large part of the sea - bottom, and into which the sounding-weight sinks several feet, would have been either washed away, or so mixed and triturated that the tiny delicate shells of the foraminifera would have been ground to powder ; instead of which the specimens brought up are very frequently perfect, and scarcely mixed with any other foreign matter —thus suggesting most forcibly the idea of "perfect repose at the bottom of the deep sea." The comparative smooth ness of the floor of the ocean is most probably due to marine erosion during repeated submergences ; and, since its resumption of its present position, the deposition of matter eroded by waves, or brought down by rivers, and distributed by currents—combined with a constant shower, as it were, of the calcareous and silicious shells, and skeletons of myriads of marine forms of life with which the ocean teems from the surface to its lowest depths—which would gradually fill up and cover any extreme irregularities which still existed. And the same process is going on unceasingly—hour after hour the floor of the ocean is constantly strewn with the materials of future continents.
The position, direction, and extent of the principal submarine elevations or " ridges " may be approximately marked by connecting the lines of islands in each ocean— islands being, in fact, but the visible summits of submarine mountains. Such a ridge, frequently rising above the surface of the water, may be traced in the Aleutian Islands, extending in an unbroken chain from Alaska to Kamtchatca. A not less
striking example occurs in the West India Islands, enclosing the Caribbean Sea. A few points of the long ridge in the South Atlantic rise above the surface in the islands of Ascension, St. Helena, St. Paul, the Azores, and others. The continuity of chains or groups of islands, therefore, proves the existence of submarine mountain chains, of which the islands are the highest summits.
As we shall again describe the " bed " of each of the great oceans separately, we shall not pursue the subject further here ; but we must remind the student that, unless he bears in mind the fact that sections of the sea-bottom are scarcely ever drawn to a true scale, he will be in danger of forming exagge rated notions as to the real conformation of the floor of the ocean. Being intended chiefly to show the exact depth at any given place, the actual proportion of the vertical and horizontal measurements are disregarded, the vertical height being greatly exaggerated, so that what looks in the section almost a per pendicular declivity, is in reality a gentle slope. This is unavoidable, especially in sections of large tracts of the sea, for the horizontal distances are so much in excess of the vertical depth, that they could not be conveniently shown in their actual relation on a true scale. Thus a section of the Atlantic, from Sandy Hook to Bermuda, 700 miles distant, showing a maximum depth of 2,850 fathoms, by a vertical line 3i inches in length, would have to be five feet wide if drawn to a true scale. In the accompanying sections those of the Atlantic present apparently almost perpendicular descents, but actually, if it were a mere question of gradients, a waggon could be driven along the bottom of the sea from Ireland to Newfoundland without any difficulty.' As the best preventative against forming incorrect and exaggerated ideas of the shape of the sea-bottom, the student would do well to select the most irregular portions of the sections given, and draw them to a true scale. This can be easily done by marking off the horizontal and vertical distances from the same scale, say 1/10 or of an inch to the mile.