RED SEA. 251 



to their width. Captain Moresby informs me that any one who 

 had not made actual plans of them, would be apt to think that 

 they were much more elongated than they really are. Many 

 of them rise to the surface, but the greater number lie from 

 5 to 30 fathoms beneath it, with irregular soundings on them. 

 They consist of sand and living coral ; the latter in most cases, 

 according to Captain Moresby, covering the greater part of their 

 surface. They extend parallel to the shore, and are not un- 

 frequently connected in their middle parts by short transverse 

 banks with the main land. The sea is generally profoundly 

 deep quite close to them, as it is near most parts of the coast 

 of the main land ; but this is not universally the case, for be- 

 tween lat. 15° and 17° the water deepens quite gradually from 

 the banks, both on the eastern and western shores. In many 

 parts islands rise from the banks ; they are low, flat-topped, 

 and consist of the same horizontally stratified formation with 

 that forming the plain-like margin of the main land. Some of 

 the smaller and lower islands consist of mere sand. Captain 

 Moresby informs me that small masses of rock, the remnants 

 of islands, are left on many of the banks where there is now 

 no dry land. Ehrenberg also asserts that most of the islets, 

 even the lowest, have a flat abraded basis, composed of the 

 same tertiary formation as elsewhere : he believes that as soon 

 as the surf wears down the protuberant parts of the banks to 

 just beneath the level of the sea, the surface becomes protected 

 from further abrasion by the growth of coral, and he thus 

 accounts for the existence of so many banks standing on a level 

 with the surface of this sea. It appears that most of the islands 

 are certainly decreasing in size. 



The banks and islands are curiously shaped in the parts 

 just referred to, namely, from lat. 15° to 17°, where the sea 

 deepens quite gradually : the Dhalac group, on the western 

 coast, is surrounded by an intricate archipelago of islets and 

 shoals ; the main island is irregular in outline, and includes a 

 bay seven miles long, by four across, in which no bottom was 



