,907.1 AND CONTRACTION OF THE EARTH. 211 



earth then was than it is now ; but the separation probably took place 

 in the earlier history of our planet, before encrustation had begun. 



The Rev. O. Fisher has calculated that a layer of matter of the 

 density of granite (2.68) no more than 31 miles thick, taken from the 

 outside of our globe, would furnish a mass equivalent to that of the 

 moon. If the earth's radius was then larger than at present, the layer 

 could be correspondingly thinner ; and it probably was sensibly larger 

 then than it is now, though certainly not by so much as 25 per cent. 

 As the rupture is supposed to have originated under the disturbing 

 action of the solar tides, the tidally detached mass would not come 

 from the entire hemispheres, but mainly from the tidal protuberances 

 of the two sides, towards and from the sun. When the matter had 

 been detached most of it was at length gathered together and formed 

 into a satellite, but some considerable masses no doubt again fell back 

 to the earth. It seems most likely that the oceanic depressions and 

 continental platforms originated largely in the genesis of the moon. 

 Part of the present distribution of these features on our globe is 

 thus the work of pure chance, but others, as the Pacific, Indian and 

 Atlantic oceans, probably represent the primitive sinks left by the 

 original disruption. The inequalities remaining after certain masses 

 had again united with the earth probably gave the foundations on 

 which the continents and oceans have since been built. It is most 

 unlikely that such a disruption could occur without leaving oceanic 

 basins, and the Pacific and Indian oceans are evidently the area from 

 which the matter of the moon was mainly derived. Instead of a uni- 

 form layer 30 or 35 miles deep, a hemispherical meniscus 50 or 100 

 miles deep at the center, and thinning out at the edges is naturally 

 suggested. 



Great modification and leveling of the inequalities of the basins 

 would naturj^lly be effected by the precipitation of some of the de- 

 tached masses, and by the enormous bodily tides then at work upon 

 the globe, which was no doubt still largely molten or but thinly en- 

 crusted. The long continuation of tidal action would largely smooth 

 out the inequalities in the earth's surface, but it seems almost impossi- 

 ble that it could entirely remove the basins left by the detachment of 

 the matter now forming the moon. 



This, then, seems to be the most probable origin of the oceanic 



