q6 THE FLOOR OF THE OCEAN 



Why have peridotite and closely allied kinds of rock been 

 thrust into the mountain roots? No geologist has yet offered 

 an answer free from speculative elements. According to the 

 theory outlined in the first chapter, the layer immediately un- 

 derlying the earth's solid skin, a true crust, formerly consisted 

 of a thin basaltic shell in the glassy state. This vitreous shell in 

 its turn rested on a thicker peridotitic shell, also in the glassy 

 state. See Figure 30. Let us assume that this kind of stratifica- 

 tion really characterized the East Indian sector at the time of 

 this mountain-making, and then apply the main principle of 

 the Vening Meinesz theory. If, accordingly, the mountain root 

 was formed while the crust was bent down, it is not difficult to 

 imagine that the down-warped crust pushed aside the basaltic 

 material and made contact with the lower vitreous layer. Any 

 deeply penetrating crack in the depressed crust would be a 

 channel for the upward injection of the peridotitic melt. If, 

 simultaneously, the depressed crust was broken into blocks, 

 forced away from one another, the rising melt would come to 

 occupy large chambers within the broken crust, there to solid- 

 ify as crystalline peridotite or its derivatives. Just such fractur- 

 ing of the crust is implied by a mechanically sound version of 

 the Vening Meinesz theory — as Professor Hess also seems to 

 have assumed in his discussion. 



Conclusions 



The principal conclusions of this chapter may now be re- 

 viewed and thereby made easier of comparison with those of 

 the first chapter, which outlined a theory of the earth-shells 

 under both land and sea. Incomplete as the survey of the deep- 

 sea mountains has been, it has broadened the field of observed 

 facts on which that theory has been based. Let us run through 

 the list of these facts. 



