236 THE OCEAN 



silica on the continental areas, and conse- 

 quently the continents would become the 

 lighter portions of the external crust and neces- 

 sarily stand at a higher level than the floor 

 of the oceans. In this way through the action 

 of forces which we can now observe in opera- 

 tion the surface features of the external crust 

 appear to have been slowly developed. 



We have seen as a result of the study of 

 deep-sea soundings that the continental blocks 

 of the lithosphere stand on the whole about 

 two and a half miles above the sub-oceanic 

 blocks, and physicists generally believe, for 

 mechanical reasons, that these must be 

 the lighter portions of the lithosphere, or they 

 would not be elevated above the depressed 

 portions ; and many observations go to con- 

 firm this view. In a paper discussing the 

 recent observations on the measurement of the 

 intensity of gravity on the ocean, G. W. Little- 

 hales says : " Concerning the dispute as to 

 whether the oceans have always had the same 

 general extent and positions since the waters 

 were gathered together, or as to whether, by 

 alternate rising and sinking of the earth's 

 crust, oceans and continents have succes- 

 sively occupied the same areas, the deciding 

 stroke appears to have been delivered in 

 favour of the permanence of the ocean basins, 

 on account of the extreme improbability that 



