Irregularities.—It is only in a general view, however, that there is a close approximation to a perfect spheroidal surface. In detail there are very notable variations from it. Geodetic surveys seem to have shown that the equatorial diameters are not all equal, even when the measurements are reduced to sea-level, but research along this line has not reached a sufficient stage of completeness to permit satisfactory discussion. It is, however, highly probable that the ocean surface as well as the average land surface is warped out of the perfect spheroidal form to some notable degree. This is very likely due to inequalities in the density of the earth’s interior. The fact that the larger portion of the water is gathered on one side of the globe, while the land chiefly protrudes on the opposite side, is very possibly due to unequal specific gravity in the interior of the earth.
The most obvious departure from a spheroidal form is found in the protrusion of the continents and in the sinking away of the earth surface under the oceans. As these inequalities present themselves to-day, they are known as continental platforms and ocean basins. These do not correspond accurately with the present land and water surfaces. About the continental lands there is a submerged border extending some distance out from the shore, and constituting a sea-shelf beyond which the surface descends rapidly to the great depths of the ocean. This slightly submerged portion, known as the continental shelf, belongs as properly to the continent as the adjacent low lands which are not submerged. The submergence of the edge of this shelf at present is usually about 100 fathoms, so that if the upper 600 feet of the ocean were removed, the outlines of the land would correspond quite closely with the border of the true continental platform.
BATHYMETRICAL CHART OF THE OCEANS
SHOWING THE “DEEPS” ACCORDING TO SIR JOHN MURRAY
It is customary to look upon the protrusions of the continents as the great features of the earth’s surface, but in reality the oceanic depressions are the master phenomena. In breadth, depth, and capacity they much exceed the continental protrusions, and if the earth be regarded as a shrunken body, the settling of the ocean bottoms has doubtless constituted its greatest surface movement. From the estimates of Murray, Gilbert has derived the following tables, showing the relative areas of the lithosphere above, below, and between certain levels.[1]
From these estimates it appears that if the surface were graded to a common level by cutting away the continental platforms and dumping the matter in the abysmal basins, the average plane would lie somewhere near 9000 feet below the sea-level. The continental platform may be conceived as rising from this common plane rather than from the sea-level.
| Contours. | Percent. of Surface above. | Percent. of Surface below. |
|---|---|---|
| Contour 24,000 feet above sea-level | 0.004 | 99.996 |
| “ 18,000 “ “ “ | 0.09 | 99.91 |
| “ 12,000 “ “ “ | 0.7 | 99.3 |
| “ 6,000 “ “ “ | 2.3 | 97.7 |
| Sea level | 27.7 | 72.3 |
| Contour 6,000 feet below sea-level | 42.5 | 57.5 |
| “ 12,000 “ “ “ | 57.3 | 42.7 |
| “ 18,000 “ “ “ | 96.8 | 3.2 |
| “ 24,000 “ “ “ | 99.93 | 0.07 |
| Percent. | |
|---|---|
| More than 6000 feet above sea-level | 2.3 |
| Between sea-level and 6000 feet above | 25.5 |
| Between sea-level and 6000 feet below | 14.8 |
| Between 6000 and 12,000 feet below sea-level | 14.8 |
| Between 12,000 and 18,000 feet below sea-level | 39.4 |
| Between 18,000 feet and 24,000 feet | 3.1 |
Epicontinental seas.—Those shallow portions of the sea which lie upon the continental shelf, and those portions which extend into the interior of the continent with like shallow depths, such as the Baltic Sea and the Hudson Bay, may be called epicontinental seas, for they really lie upon the continent, or at least upon the continental platform; while those other detached bodies of water which occupy deep depressions in the surface are to be regarded as true abysmal seas, as, for example, the Mediterranean and Caribbean seas and the Gulf of Mexico, whose bottoms are as profound as many parts of the true ocean basin itself.
Diversities of surface.—The bottoms of the oceanic basins are diversified by broad undulations which range through many thousands of feet, but they are not carved into the diversified forms that give variety to land surfaces. The ocean bottoms are also diversified by volcanic peaks, many of which rise to the surface and constitute isolated islands. Some of them have notable platforms at or near the surface, cut by the waves or built up by the accumulation of sediment and of coralline and other growths about them. Aside from these encircling platforms, the solid surface usually shelves rapidly down to abysmal depths, so that the islands constitute peaks whose heights and slopes would seem extraordinary if the ocean were removed.