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. [Illustration: =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. -------------------------------------+----------------+-------------- | Percent. of | Percent. of Contours. | Surface | Surface | above. | 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 -------------------------------------+----------------+--------------