148 CORALS AND CORAL LSLANDS. 



and was not extricated without some difficulty. It looked like 

 a dirty brownish clay. This mud is nothing but comminuted 

 coral, so fine as to be almost impalpable. 



The lagoojis of the smaller islands are usually very shallow ; 

 and in some, merely a dry bed remains, indicating the former 

 existence of water. Instances of the latter kind are met with 

 only in islands less than three miles in diameter; and those 

 with shallow lagoons are seldom much larger. These shallow 

 waters, when direct communication with the sea is cut off, 

 become, in some instances, very salt by evaporation, and con- 

 tain no growing coral, with few signs of life of any kind ; and 

 in other cases, they are made too fresh for marine life through 

 the rains. At Enderbury's Island the water was not only ex- 

 tremely saline, but the shores of the lagoon were in some places 

 incrusted with salt. But when there is an open channel, 

 or the tides gain access over a bare reef, corals continue to 

 grow, and a considerable portion of the lagoon may be 

 obstructed by them. At Henuake, the sea is shut out ex- 

 cept at high water, and there were consequently but few 

 species of corals, and those of small size. At Ahii (Pea- 

 cock's Island), there was a small entrance to the lagoon, 

 and though com.paratively shallow, corals were growing 

 over a large part of it. 



In the larger islands, the lagoons contain but small reefs 

 compared with their whole extent ; the greater part is an open 

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

 are instances, as at the sourthern Maldives, of a depth of fifty 

 and sixty fathoms. From twenty to thirty-five fathoms is the 

 usual depth in the Paumotus. This was the result of Captain 

 Beechey's investigations ; and those of the Expedition, though 

 few, correspond. It is however probable that deeper soundings 

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

 Gilbert's Group, south-east of the Carolines, the depth, where 

 examined by the Expedition, varied from two to thirty-five 

 fathoms. Mr. Darwin found the latter depth at Keeling's 

 Island. Chamisso found twenty-five to thirty-five fathoms at 

 the Marshall Islands. 



