CORAL REEFS, AND ISLANDS. 253 



Coming now to the atolls, or lagoon islands, we 

 may repeat that they resemble in many points the 

 encircling reefs just described, but differ in enclosing 

 a lake or lagoon instead of mountainous islands. 

 The lagoon is indeed only an enclosed portion of the 

 sea, surrounded by a more or less complete wall or 

 belt of coral rock, which is but little elevated above 

 the surface, and here and there surmounted by 

 vegetation, which have the appearance of small islets. 

 " In many of the smaller atolls the lagoon has lost 

 its ocean character, and become a shallow lake, and 

 the green islets of the margin have coalesced in some 

 instances into a continuous line of foliage. Traces 

 may, perhaps, be still detected of the passage, or 

 passages, over which the sea once communicated 

 with the internal waters, though mostly concealed by 

 the trees and shrubbery which have spread around 

 and completed the belt of verdure (fig. 53). The coral 

 island is now in its most finished state ; the lake rests 

 quietly within its circle of palms, hardly ruffled by 

 the storms that madden the surrounding ocean." l 



It must not be assumed that the atolls are always 

 ring-shaped, although that may be the most perfect 

 form, for they are not only triangular, quadrangular, 

 and very irregular, but sometimes are completed only 

 on one side, with the opposite so much submerged that 



1 Dana, p. 131. 



