Page:Popular Science Monthly Volume 33.djvu/836

816 under our review is, indeed, too often overshadowed and obscured by their magnitude, by the multitude of their details, and by the variety of their forms, which at first produce impressions of hopeless confusion; but, when once the idea of subordination to common laws is duly conceived, it receives confirmation at every fresh step taken.

The area of the dry land is very greatly exceeded by that which is covered with water. The whole surface of the earth being 197,000,000 square miles, about 55,000,000 are land and 142,000,000 water. The average height of the land above the sea-level is also very much less than the average depth of the sea-bottom below that level; so that a rearrangement of the surface is quite possible by which the whole of the land might be submerged with comparatively little disturbance of the present level of the sea, or reduction of its average depth. The highest measured peak of the Himalaya, known as Mount Everest, which is also the highest in the world accurately determined, just rises 29,000 feet above the sea-level, but such elevations even as 15,000 feet are, elsewhere, with the sole exception of parts of Thibet, confined to isolated peaks or very narrow bands along the crests of a few of the highest mountain-ranges. The area above 12,000 feet is about two per cent of the whole land, and that above 6,000 less than nine per cent. From a careful computation recently made, it would appear that the mean height of the surface of the land above the sea-level is about 2,250 feet; the continental areas having the following elevations: Europe, 939 feet; Asia, 3,073 feet; North America, 1,888 feet; South America, 2,078 feet; Australia, 805 feet. The greatest depths measured in the ocean exceed 27,000 feet, and it has been estimated that the mean depth is about 12,500 feet. About five per cent of the ocean area is less than 600 feet in depth, and a somewhat smaller proportion, more than 18,000 feet. About seventeen per cent is less than 3,000 feet. The ocean-bed generally appears to present very extensive, comparatively uniform plateaus, varied only by moderate undulations, possibly to be attributed to contractions of the earth's crust caused by cooling; these range in depth from 12,000 to 17,000 feet, and their general direction maintains a rough parallelism with that of the neighboring continents. Submarine deposits derived from the land do not extend beyond 300 or 400 miles from the shore; but at great depths deposits are being formed with extreme slowness, which are probably derived from decomposed organisms, or from cosmic, volcanic, or other matter, carried down through the water. Accepting these estimates, it will appear that the volume of land above the sea-level is about one fifteenth part only of the volume of the ocean.

With the latest additions made to our knowledge of the depth