We have, first, Land, rising above the sea-level, and consisting of plains, undulations, hills, mountains. It covers altogether less than one-third of the Earth’s surface, and it is called THE CONTINENTAL AREA, though Islands as well as Continents belong to it. We have, secondly, the Ocean-floor under deeper parts of the Ocean; that which lies beyond a depth of about two miles. This division has been described as the “great submerged plain,” and it comprises about one-half of the Earth’s surface. It is known as THE ABYSMAL AREA. We have, thirdly, a middle region, which may be spoken of as a kind of borderland under the sea, connecting the dry land with the greater ocean-depths. It amounts to about one-sixth of the Earth’s surface, and it has been named THE TRANSITIONAL AREA. By “connecting” the two, I do not mean that it must always be between the two. It does very generally so lie, but there are exceptions. Some deeper portions of the sea are close to land, and some parts of the Transitional Area are found far out at sea. The meaning of the word “Continental” needs no explanation; and the very word “Abysmal” carries its own sense. More, however, will be said in future chapters about those reaches of ocean known as the “Transitional Area.” A curious law seems to have governed the grouping of land and water. Putting aside innumerable small islands, scattered about, we find that the great mass of land clusters towards and round the north pole, with a water-and-ice-filled hollow for its centre. While, on the contrary, the greater mass of water may be said to cluster towards and round the south pole, with--so far as we can conjecture--a large extent of land for its centre. The conditions of north and south thus seem to be exactly reversed. Not long ago it was believed that the ocean’s floor might be a fairly close imitation of that which we see on land. The differences, however, have been found to be greater than was expected. Perhaps it is not surprising, when one thinks of the immense levelling power of water. That must be a firm make of rock which can permanently resist the effect of sea waves breaking upon or near the shore. And even deeper down, where waves are not and currents may be slow, some movement must still exist, since the ocean is nowhere quite stagnant. Such movements, no matter how gentle, would tend to shift all loose and soft substances. The ocean-bed is held to be generally flat, though with gradual slopes here and there, leading up or down to higher or lower levels. Many submarine mountains rear their heads, sometimes near the surface, sometimes above it. In places high mountain-ridges run for a long distance below the sea, with profound depths on either side; and these again often show their peaks, forming groups of islands.