STRUCTURE OF CORAL ISLANDS. 183 



sea, with deep waters and a sandy or muddy bottom. There 

 are instances, as at the southern Maldives, of a depth of 50 

 and 60 fathoms. From 20 to 35 fathoms is the usual depth 

 in the Paumotus. This was the result of Captain Beechey's 

 investigations ; and those of the Expedition, though few, cor- 

 respond. It is however probable that deeper soundings would 

 be found in the large island of Nairsa (Dean's). In Gilbert's 

 Group, southeast of the Carolines, the depth, where examined 

 by the Expedition, varied from 2 to 35 fathoms. Mr. Darwin 

 found the latter depth at Keeling's Island. Chamisso found 

 25 to 35 fathoms at the Marshall Islands. 



The bottom of these large lagoons is very nearly uniform, 

 varying but little except from the occasional abrupt shallow- 

 ings produced by growing patches of reef. Soundings bring 

 up sand, pebbles, shells, and coral mud ; and the last men- 

 tioned material appears to be quite common, even in lagoons 

 of considerable size. It has the same character as above de- 

 scribed. The bluish clay-like mud of the harbor of Tongatabu 

 may be classed with these deposits. Darwin describes this mud 

 as occurring at the Maldives, and at Keeling's Island (op. cit. 

 p. 26) ; Kotzebue mentions it as common at the Marshall 

 atolls, and Lieutenant Nelson observed it at the Bermudas. 

 It appears, therefore, that the finer coral material of the 

 shores prevails throughout the depths of the lagoon. The 

 growing reefs within the lagoons are in the condition of the 

 inner reefs about high islands. The corals grow but little 

 disturbed by the waves, and the reef-rock often contains them 

 in the position of growth. At Taputeouea (Kingsmill's or 

 Gilbert's Group), reefs very similar to those of the Feejees 

 occur ; they contain similar large Astrseas ten to twelve feet 

 in diameter, which once were growing where they stand, but 

 are now a part of the solid lifeless rock. 



