FOUNDATIONS OF THE GREAT DEEP 23 



cannot be continuous under the Atlantic. Further, the wave- 

 velocity in the oceanic sectors of the earth is so high that one 

 may well question the existence there of a continuous layer of 

 the rock type that makes up the "intermediate" layer of the 

 continental crust. 



Nevertheless, it is clear from geological observations that 

 granite and other sorts of continental rock do constitute patches 

 on the Atlantic floor as well as on the floor of the northwestern 

 part of the Indian Ocean and on the floor of the southwest 

 Pacific. In each of these three regions small islands are seen to 

 include typical continental rocks, and each emergent mass 

 must be continued out on the ocean floor for some distance. 

 Examples are: the Azores of the Atlantic; Fiji, Tonga, and 

 many other islands of the southwest Pacific; and the Seychelles 

 Islands on the Seychelles Plateau of the Indian basin (Figure 

 10). Additional evidence is furnished by the granitic frag- 

 ments ejected from explosive volcanoes that rise from the Mid- 

 Atlantic Swell. One of these is Ascension Island. Its map, 

 copied from an Admiralty chart, is given in Figure n. It was 

 the youthful Charles Darwin who first collected the granitic 

 bombs on the huge basaltic cone of Ascension and so proved 

 the surprising fact that rock of the continental type underlies 

 the deep Atlantic, halfway between Africa and South America. 

 It is, indeed, probable that the 7000-mile Mid-Atlantic Swell 

 projects somewhat above the general floor of this ocean because 

 the Swell is a greatly elongated patch of continental rock 

 resting on a basaltic crust. 



In the next chapter, dealing with the oceanic islands, we 

 shall find another reason for thinking that the crust under the 

 greater part of the deep sea is essentially of basaltic com- 

 position. 



Testimony of the Plumb-bob and Gravimeter. — Having 

 seen how the rushing earthquake waves furnish data about the 



