NARRATIVE OF THE CRUISE. 



137 



lies in a N.E and S.W. direction, and it is described generally as a coral atoll ; but any 

 one who has visited coral atolls in the China Sea, Pacific, or Indian Oceans, will be at once 

 struck with some remarkable differences between these and Bermuda. The typical atoll 

 consists of a low, more or less circular, strip of land enclosing a lagoon, into which there 

 is usually a well-defined opening on the leeward side. In Bermuda the land is 260 

 feet in height at one point, and is massed to the southeast side of the atoll, with the 

 exception of a small outlier known as the " North Rock " (see Sheet 8), which is 

 composed of the same " iEolian " rocks as the mass of land to the southeast, and this 





/Eoliau " Rocks, Bermuda. 



indicates an extension of the land surface of the atoll in this direction at a former period. 

 Although the outer reef is almost continuous, there is no well-defined lagoon as in a 

 typical atoll. The whole of the northwest portion of the banks is crowded with coral 

 flats and heads, with intervening lanes and spaces of coral sand, with a depth of usually 

 4 or 5 fathoms and nowhere more than 10 fathoms. The basins, known as 

 Great Sound, Little Sound, and Castle Harbour, are almost completely enclosed by 

 the iEolian rocks, and have evidently been formed by the solvent action of the 

 sea water on these rocks. Navigators have remarked upon the light blue colour of 

 the water when compared with the deep blue of southern atolls. This arises most 

 (narr. chall. exp. — vol. i. — 1884.) 18 



