The chief element of uncertainty as to the largest features of the relief of the earth’s crust is due to the unexplored area in the Arctic region and the larger regions of the Antarctic, of which we know nothing. We know that the earth’s surface if Crustal relief. unveiled of water would exhibit a great region of elevation arranged with a certain rough radiate symmetry round the north pole, and extending southwards in three unequal arms which taper to points in the south. A depression surrounds the little-known south polar region in a continuous ring and extends northwards in three vast hollows lying between the arms of the elevated area. So far only is it possible to speak with certainty, but it is permissible to take a few steps into the twilight of dawning knowledge and indicate the chief subdivisions which are likely to be established in the great crust-hollow and the great crust-heap. The boundary between these should obviously be the mean surface of the sphere.
Sir John Murray deduced the mean height of the land of the globe as about 2250 ft. above sea-level, and the mean depth of the oceans as 2080 fathoms or 12,480 ft. below sea-level.[14] Calculating the area of the land at 55,000,000 sq. m. (or 28.6% of the surface), and that of the oceans as 137,200,000 sq. m. (or 71.4% of the surface), he found that the volume of the land above sea-level was 23,450,000 cub. m., the volume of water below sea-level 323,800,000, and the total volume of the water equal to about 1⁄666th of the volume of the whole globe. From these data, as revised by A. Supan,[15] H.R. Mill calculated the position of mean sphere-level at about 10,000 ft. or 1700 fathoms below sea-level. He showed that an imaginary spheroidal shell, concentric with the earth and cutting the slope between the elevated and depressed areas at the contour-line of 1700 fathoms, would not only leave above it a volume of the crust equal to the volume of the hollow left below it, but would also divide the surface of the earth so that the area of the elevated region was equal to that of the depressed region.[16]
A similar observation was made almost simultaneously by Romieux,[17] who further speculated on the equilibrium between the weight of the elevated land mass and that of the total waters of the ocean, and deduced some interesting relations Areas of the crust according to Murray. between them. Murray, as the result of his study, divided the earth’s surface into three zones—the continental area containing all dry land, the transitional area including the submarine slopes down to 1000 fathoms, and the abysmal area consisting of the floor of the ocean beyond that depth; and Mill proposed to take the line of mean-sphere level, instead of the empirical depth of 1000 fathoms, as the boundary between the transitional and abysmal areas.
An elaborate criticism of all the existing data regarding the volume relations of the vertical relief of the globe was made in 1894 by Professor Hermann Wagner, whose recalculations of volumes and mean heights—the best results which have yet been obtained—led to the following conclusions.[18]
The area of the dry land was taken as 28.3% of the surface of the globe, and that of the oceans as 71.7%. The mean height deduced for the land was 2300 ft. above sea-level, the mean depth of the sea 11,500 ft. below, while the position of mean-sphere Areas of the crust according to Wagner. level comes out as 7500 ft. (1250 fathoms) below sea-level. From this it would appear that 43% of the earth’s surface was above and 57% below the mean level. It must be noted, however, that since 1895 the soundings of Nansen in the north polar area, of the “Valdivia,” “Belgica,” “Gauss” and “Scotia” in the Southern Ocean, and of various surveying ships in the North and South Pacific, have proved that the mean depth of the ocean is considerably greater than had been supposed, and mean-sphere level must therefore lie deeper than the calculations of 1895 show; possibly not far from the position deduced from the freer estimate of 1888. The whole of the available data were utilized by the prince of Monaco in 1905 in the preparation of a complete bathymetrical map of the oceans on a uniform scale, which must long remain the standard work for reference on ocean depths.
By the device of a hypsographic curve co-ordinating the vertical relief and the areas of the earth’s surface occupied by each zone of elevation, according to the system introduced by Supan,[19] Wagner showed his results graphically.
This curve with the values reduced from metres to feet is reproduced below.
Wagner subdivides the earth’s surface, according to elevation, into the following five regions:
Wagner’s Divisions of the Earth’s Crust:
| Name. | Per cent of Surface. | From | To |
| Depressed area | 3 | Deepest. | −16,400 feet. |
| Oceanic plateau | 54 | −16,400 feet. | − 7,400 feet. |
| Continental slope | 9 | − 7,400 feet. | − 660 feet. |
| Continental plateau | 28 | − 660 feet. | + 3,000 feet. |
| Culminating area | 6 | + 3,300 feet. | Highest. |