Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/61

Rh uniform pressure) at 114 km., which is identical with the underside of the continental blocks, from the deviations of the plumb-line at more than a hundred stations in the United States. Practically the same figure, namely 120 km., was found by Helmert from gravity measurements with the pendulum at fifty-one coastal stations. The close agreement of these figures, obtained as they are in different ways, naturally gives to them a heightened degree of accuracy, but it should not be thought that this thickness can be universally assigned to the continental blocks. This would not be consistent with isostasy.

The thickness must be estimated as much less in the case of continental shelves, and as much greater in elevated regions such as Tibet, so that about 50 to 300 km. can be assumed as limits.

We can now easily calculate what the specific gravity of the sial and sima must be in order that a block of sial of about 100 km. thickness (M) may be elevated 4.8 km. above the ocean floor, thus being submerged to a total depth of 95.2 km. (compare Fig. 7). Equilibrium of pressure exists on the lower margin of the continental block; that is, a column of unit cross section extending