the great dimensions of the earth and in the long periods of time which are at disposal for geological changes. This is a point which has been quite insufficiently appreciated in the previous literature, but which is of the greatest importance in geophysics. In the laboratory a small steel model of a sphere behaves in every way as a rigid body. But a steel sphere of the magnitude of the earth flows under the influence of its own attraction, at least if the necessary thousands of years are allowed for it to do so. It is the transition from the prevalence of molecular forces (degree of rigidity) to that of the molar forces (gravity), which is the factor here.[1] Isostasy signifies the predominance of the molar forces, absence of isostasy that of the molecular forces. For this reason, very small heavenly bodies, as many planetary moons, some of the small planets, and, more naturally still, the meteorites, do not possess the spherical shape; for that betokens isostasy. Isostasy prevails on the moon, taken as a whole; the great inequalities of its surface show, however, that the molar forces are there considerably less than on the earth, so that the molecular forces are more prominent. In fact, even the altitude of the mountains is no accidental magnitude, but is essentially determined by the relation of these two forces, a fact indicated by the similar heights of the Alpine peaks, to which A. Penck[2] has drawn attention. The mountain systems thus indicate the extent to which molecular forces could maintain themselves against gravity.
The question, in what manner the mere dimensions
