Wegener bases his theory of the drifting continents on the assumption that there are two distinct levels to be taken into account, the surface of the masses of “sal” which form the continents and the surface of the “sima” in which they float. His justification of this fundamental assumption appears to be based on the frequency curve obtained by measuring heights positively and depths negatively from sea-level, and plotting the frequencies of such measurements as in Fig. 1. Because two maxima appear, Wegener draws the conclusion that therefore two layers must be considered as having been acted upon to produce the elevations and depressions of the earth's surface, and he implies that there would be but one maximum (Fig. 1, dotted curve) if only one layer were involved.