AND WATER ON THE EARTH. 319 



its center of figure. This would infer the permanence of the Pacific 

 Ocean, still a moot question among geologists ; and we must also 

 remember that Hayford and Bowie have shown that under the con- 

 tinent of North America, and, in a less convincing degree, under 

 the adjoining oceans, isostatic adjustment is complete at a very 

 small depth ; and it is only in this surface skin therefore that the 

 density of the earth is different in the two hemispheres. In many 

 parts of the known continental areas the rock has undergone changes 

 of density, wnth correcponding changes of level ; whether such 

 changes have extended over very large areas so as materially to 

 change the distribution of land and water on the globe is the still 

 unanswered problem of the permanence of the ocean basins. 

 Imagine an earth, spherically symmetrical in density ; now imagine 

 that the crust in one hemisphere to a depth of lOO miles contracts 

 so as to shorten the central radius by 3 miles and that this shortening 

 gradually diminishes to zero along the edge of the hemisphere. A 

 simple calculation shows that the crust to a depth of 100 miles would 

 be increased in density about 3 per cent. ; that the center of mass of 

 the earth would be displaced only about 70 feet, so that the level 

 surfaces would remain practically unchanged ; and therefore the 

 ocean in the center of the contracted hemisphere would be about 3 

 miles deeper than in the antipodal region. This apparently is what 

 has occurred, but why the contraction should be especially marked 

 and so general over one hemisphere is still unknown. 



The only attempt to explain the hemispherical distribution of 

 density is that of Osmond Fisher." He suggests that the material 

 that formed the moon, according to George H. Darwin's theory, 

 was collected from the superficial part of the region which is now 

 the Pacific Ocean, and was therefore of comparatively small density. 

 The scar was healed, to a large extent, by denser material from 

 below, and the two Americas were, at the time of separation of the 

 moon, cracked ofif from Europe and Africa, and floated to the west, 

 leaving the Atlantic basin underlaid by the denser material below. 

 This hypothesis is purely speculative. It runs counter to other geo- 



^ Nature, 1882, XXV., 243; also "Physics of the Earth's Crust," 2d ed., 

 1889, XXV. W. H. Pickering offered the same explanation. " The Place of 

 Origin of the Moon," Jour. Geo]., 1907, XV., 23-38. 



