Stresses set up by (h) Contraction due to change of state and cooling by conduction.(i) Expansion due to heating-up of the earth's interior.(j) Contraction (h) and expansion (i) alternating in time (Joly's hypothesis of thermal cycles).(k) Convection currents in the substratum (the hypothesis developed in this paper).(a) Tidal stresses, due mainly to the frictional drag of shallow seas against the crust, retard the earth's rotation. This involves a bodily contraction of the earth and an approach towards a spherical form (65, p. 293). Schafler has recently developed the geological consequences (91). Lee, however, adopts the opposite view that, despite this effect, the earth's rotation has been periodically accelerated as a result of gravitational condensation (79). Taylor appeals to tidal forces as the chief cause of Tertiary mountain building and continental drift. He requires, however, that the earth should have captured the moon towards the close of Cretaceous time (99, p. 158). Wegener regards a westerly drift of the continents, due to bodily tidal friction, as a possible