STRUCTURE OF CORAL ISLANDS. 1838 
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 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- 
may be classed with these deposits. Darwin describes this mud 
as occurring at the Maldives, and at Keeling Island (op. cit. 
p. 26); Kotzebue mentions it as common at the Marshall 
atolls, and Lieutenant Nelson observed it at the Bermidas. 
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 Astreeas ten to twelve feet 
in diameter, which once were growing where they stand, but 
are now a part of the solid lifeless rock. 
