594 



FOKMATION OF COExlL EEEFS. 



[Ch. XLIX. 



abruptly to the sea-sliore are generally continued witli the 

 same slope beneath tlie water. But where the reef^ as at 

 h c ffip-. 156), is distant several miles from a steen coast, a lino 



from 



mnst 



efficient 



building corals can exists for we have seen that they cease 

 to grow in water which is more than 120 feet deep. That 

 the original rock immediately beneath the points & c is ac- 



from 



met 



mediately 



only at enormous depths. In short, the ocean is as deep as 

 might have been anticipated in the neighbourhood of a bold 

 coast ; and it is obviously the presence of the coral alone 

 which has given rise to the anomalous existence of shallow 

 water on the reef and between it and the land. 



After studying in minute detail all the phenomena above 

 described, Mr. Darwin has offered in explanation a theory 

 now very generally adopted. The coral-forming polypi, he 

 states, begin to build in water of a moderate depth, and, while 

 they are yet at work, the bottom of the sea subsides gradu- 

 ally, so that the foundation of their edifice is carried down- 

 wards at the same time that they are raising the superstruc- 

 ture. If, therefore, the rate of subsidence be not too rapid, 

 the growing coral will continue to build up to the surface ; 

 the mass always gaining in height above its original base, 

 but remaining in other respects in the same position. Not 

 so with the land : each inch lost is irreclaimably gone ; as it 

 sinks the water gains foot by foot on the shore, till in many 

 cases the highest peak of the original island disappears. 

 What was before land is then occupied by the lagoon, the 

 position of the encircling coral remaining unaltered, with the 

 exception of a slight contraction of its dimensions. 



In this manner are encircling reefs and atolls produced ; 

 and in confirmation of his views Mr. Darwin has pointed out 

 examples which illustrate every intermediate state, from that 

 of lofty islands such as Otaheite, ,encircled by coral, to that 

 of Gambler's group, where a few peaks only of land rise out 





