142 



CORAL ISLANDS. 



Fig. 224. Coral island in the Pacific Ocean. 



(fig. 224), with a lake in the centre, enclosed on all sides ; some- 

 limes they form broken circles, having one or more openings 

 through which the centre may be reached ; again, they are in 

 groups of islands, arranged in a circle, and frequently there are 

 several in a series. In these internal seas the water is often very 

 deep but sometimes very shallow, and an immense number of 

 polypa'ria are developed, which sooner or later fill up the space. 

 It appears evident that these circular reefs are the edges of different 

 upheaved craters, upon which the polyps have established them- 

 selves ; this is inferred from the volcanic nature of most islands in 

 the Pacific, and from the manner in which submarine eruptions 

 sometimes take place. Nevertheless, this explanation is not re- 

 ceived as satisfactory in respect to many reefs of the kind, and 

 particularly those which constitute the Maldives and Lakadives, 

 groups in the Indian Ocean. The great number of circular groups 

 found in certain localities, and the immense expanse which we 

 must suppose craters of elevation to have in other places, are facts 

 urged in objection to the explanation. 



Around coral reefs, as well as in the lakes they enclose, soft and white 

 mud of a calcareous nature, analogous to chalk, has been observed, which 

 has sometimes been referred to the disintegration of madrepores, and some- 

 times to dejections of worms which pierce the polypa'ria, or to those of 

 fishes which feed on them. In many places in the South Seas this mud 

 seems to constitute considerable deposits. 



41. When a reef has reached the level of the water, the sea 

 often covers it with debris of every kind, on which vegetation is 

 afterwards developed. Most low islands in the Pacific have been 

 produced in this way, all of which rest on masses of polypa'ria. 

 A great many other islands have sprung up on their coasts ' 



me way ; and there are many which will sooner or later 



same 



in the 

 grow 



41 How are coral islands formed ? 



