An island or group of islands which forms the summit of an isolated fragment of sial swimming in the sima must manifestly be surrounded by this gravity disturbance in a ring-like manner. On account of this, the gravity is greater than normal on the island itself, and especially so on the shore, whilst outside it there is a circular area on the sea where it is below normal. The observation that was early made that the pendulum measurements on islands yield gravities greater than normal is explained in this manner. The view of many authors that the Pacific islands are purely volcanic cones which are simply mounted on the floor of the ocean, and are supported by it, cannot be established by gravity measurements. These favour the ideas advocated, for example, by Gagel for the Canary Islands and by Haug for many Pacific isles, that all these islands are pieces of the sialsphere, but that they are in many cases so completely covered with lava that their sial cores are nowhere exposed.
These circumstances may be considered in yet another light, and one which is adapted to illustrate directly their effects. In a continental block, the pressure must manifestly increase with the depth, according to a law different from that which prevails in oceanic areas. If we compare the pressures[1] at similar depths, we find that all over the continental block—with the exception of its upper surface and its under surface—the pressure is greater than in oceanic areas. If we take as a basis the numerical proportion assumed in Fig. 5 (p. 30), we obtain as the values for
- ↑ Strictly speaking, the vertical pressures. According to Rudzki there are altogether six pressures acting on a cubical mass of a solid body, namely, three normal to its walls (compressive), and three tangential (shearing). Expansion (dilatation) can be conceived as being a negative compression, so that the pressure can be positive or negative. In our case shearing forces will be assumed to be non-existent.
