larger blocks, as, for example, a complete continent or the whole floor of an ocean, isostasy must be assumed without question. But on a small scale, in the case of individual mountains, this law loses its validity.
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Fig. 3.—Section through the lithosphere according to the doctrine of isostasy.
Such smaller portions can be supported by the elasticity of the whole block, like a stone placed on a floating block of ice. Isostasy is effected between the block and stone as a whole and the water. In a similar manner the gravity measurements very rarely show a deviation from isostasy in continents with structures the diameter of which amounts to hundreds of kilometres. If the diameter amounts only to tens of kilometres, then at the most is there only partial compensation; and if it is only of a few kilometres, then compensation is practically non-existent.
This doctrine of isostasy, the flotation of the crust of the earth, has been confirmed to such an extent by experiments, especially those of gravity, that it belongs to-day to the firmest foundations of geophysical knowledge.
According to this doctrine, it is said that an oceanic area cannot be elevated as a whole above the level of the sea or an unloaded continent sink to the level of the deep sea floor. Small alterations of level, possibly as much as several hundred metres, such as should lead to the emergence or submergence of an area of continental shelf, are obviously possible, for example, in the case of the wandering of the poles due to the lag
