498 FORMATION OF CORAL ISLANDS AND REEFS. 



In support of this doctrine, it has been stated that fragments 

 of volcanic rocks have been found in the water of a lagoon. 

 That there is nothing in the essential nature of coral structures 

 which impels them to assume this form, is evident from the fact 

 mentioned by Ehrenberg, that in the Red Sea the islands are 

 oblong or square without lagoons ; as well as from the variety 

 which we meet with even in the Polynesian Archipelago. In fact, 

 they always appear to correspond with the form of the base on 

 which they are erected ; and the evidence that some lagoon islands 

 (at least) are founded upon the tops of submarine volcanoes, 

 or upon the edges of large basins, seems therefore satisfactory. 

 But it is difficult to conceive that there should exist beneath the 

 ocean so large a number of summits, all so nearly approaching 

 its surface, as these must do in order to form a basis for coral 

 islands, not to mention the other ridges on whiuh the reefs are 

 built, which will next be noticed. This difficulty appears to be 

 solved by the very ingenious hypothesis put forth by Mr. Dar- 

 win, which will be presently explained. 



1142. .Almost all the shelving shores of tropical seas are 

 fringed more or less closely by ridjes of Coral. These are not 

 built immediately upon the edge of the land, but at some little 

 distance from it. If the wall of coral were upraised close in-shore, 

 the fresh water draining down from the land, and entering the 

 basin thus formed, would render it unfit for the habitation of 

 the Polypes. They are, therefore, endowed with a power of 

 choosing a situation more advantageous to their growth ; and we 

 accordingly find these skirting reefs upraising themselves at some 

 distance from the shore, but not so far off as to have a base of 

 greater depth, than is suitable to the constitution of the Polypes. 

 Very often such reefs run from point to point of a bay, so as 

 completely to enclose it.* But beyond these skirting reefs are 



* In the little island of Cariacou (one of the chain of Grenadines, between 

 Grenada and St. Vincent) the whole coast is a succession of such bays, and each 

 of these is shut in by such a reef ; a narrow passage into one of them being the 

 only means of access, when the Author visited it a few years since. From a hill in 

 the centre of the island, nearly every part of the coast can be seen ; and the white 

 1'nes of surf, connecting the d 3 k rocky points, present a very striking appearance. 

 The Society Islands are generally skirted by similar reefs, which are generally 400 



