592 



FOEMATION OF COEAL EEEFS 



[Ch. XLIX 



Holiest part of a mountainoiis island, 01% in other cases, by 

 tlie top of a shoal. 



The following* is a brief sketch of the facts and! arguments 

 in favonr of this new view : — Besides those rings of dry 

 coral which enclose lagoons^ there are others having a similar 

 form and structure which encircle lofty islands. Of the latter 



kind is Yanikoro 



Map 



586), celebrated on 



account of the shipwreck of La Peyrouse, where the coral 



om 



the channel between it and the land having a general depth 

 of between 200 and 300 feet. This channel, therefore^ is 

 analogous to a lagoon, but with an island standing in the 

 middle like a picture in its frame. In like manner in Tahiti 

 we see a mountainous land, with everywhere round its mar- 

 gin a lake or zone of smooth salt water^ separated from the 

 ocean by an encircling reef of coral, on which a line of 



foamin 



New 



narrow island east 



New Holland, com 



partly of 



granite and partly of triassic sandstone, is surrounded by a 



This reef encompasses not only the 



reef 400 miles long, 

 island itself, but a ridge of rocks which is prolonged in the 

 same direction beneath the sea. No one, therefore, will 

 contend for a moment that in this case the corals are based 

 upon the rim of a volcanic crater, in the middle of which 

 stands a mountain or island of granite and sandstone. 



The great barrier reef, already mentioned as running paral- 

 lel to the north-east coast of Australia for nearly 1,000 miles, 

 is another most remarkable exanijole of a long strip of coral 

 running parallel to a coast. Its distance from the main- 

 land varies from 20 to 70 miles, and the depth of the 

 great arm of the sea thus enclosed is usually between 10 

 and 20 fathoms, but towards one end from 40 to 60. This 



to Mr. 



mu 



great reef would extend 



Jukes, if the growth of coral were not prevented off the shores 



of New Guinea by a muddy bottom, caused by rivers charged 



with sediment which flow from 



southern coast of that 



OTeat island.-^ 



* Quart Journ. Greol. Soc. 4, xciii. 





I 



