ATOLLS 223 



tail like pieces of a jig-saw to form the picture of the atoll 

 structure. 



Doctor Gaskell has described this work in scientific papers and 

 has voiced his propositions at learned symposiums with a wealth 

 of detail which baffles the layman, but a few weeks after the visit 

 to the Ellice Islands he gave the Challengers, in an article in the 

 ship's magazine, a simple account of the results of these experi- 

 ments. At Funafuti, he said, they had found a depth of i8oo feet 

 of rock in which the velocity of sound was 6000—8000 feet per 

 second, such a velocity being much as one would expect in coral 

 limestone. Below this lay material with a velocity of sound of 



— — — Sea level 



Speed of Sound 

 eoOO - 8000 

 FcetperSecond. 



Hard Vvlcanic core 

 Structure of Funafuti Atoll as deduced from seismic experiment 



1 2,000 feet per second, which could either be hard limestone or 

 volcanic rock. To clear up the matter he had carried the explo- 

 sions in Nukufetau right up to the edge of the reef and these 

 showed that the border between these two layers fell rapidly 

 away as the perimeter was reached, which gave a picture of a 

 supposed volcanic hump within the heart of the atoll structure. 

 Deeper again an even harder material was located, probably the 

 true volcanic core of the original volcano which had formed the 

 island. 



Thus, on the seismic evidence obtained by Challenger it would 

 seem that as the coral cannot live at a depth greater than 200 feet 

 below the surface of the ocean, the atolls forming the Ellice 



