Geologists often ask too much of the principle of isostasy. When they find that isostasy will not explain all earth movements, they think it is not a true principle. Many of the most important earth movements are not due to an attempt to reestablish a disturbed isostatic equilibrium. James Hall made a splendid contribution to our knowledge of the geologic history of mountain ranges in 1859.3 He showed that the sediments of the Appalachian Mountains were laid down in a sinking syncline, and he inferred that the sinking was due to the weight of the sediments. While this weight must have contributed to the sinking, it is clear that great depressions have taken place without the addition of material; for many of the great deeps of the ocean, and as a particular example, the Tonga Deep, are so situated that they could never have received any large amount of . . .