84 CORAL-REEFS. 



beyond the line of the tropics, but Houtmans Abrolhos, 

 on the western shores of Australia in lat. 29 S., are of 

 coral formation. 



The proximity of volcanic land, owing to the lime 

 generally evolved from it, has been thought to be favourable 

 to the increase of coral-reefs. There is, however, not much 

 foundation for this view ; for nowhere are coral-reefs more 

 extensive than on the shores of New Caledonia, and of 

 north-eastern Australia, which consist of primary formations; 

 and in the largest groups of atolls, namely the Maldiva, 

 Chagos, Marshall, Gilbert, and Low Archipelagoes, there 

 is no volcanic or other kind of rock, excepting that formed 

 of coral. 



The entire absence of coral-reefs in certain large areas 

 within the tropical seas, is a remarkable fact. Thus no 

 coral-reefs were observed, during the surveying voyages of 

 the Beagle and her Tender on the west coast of South 

 America south of the equator, or round the Galapagos 

 Islands. It appears, also, that there are none 1 north of the 

 equator ; Air. Lloyd, who surveyed the Isthmus of Panama, 

 remarked to me, that although he had seen corals living in 

 the Bay of Panama, yet he had never observed any reefs 

 formed by them. I at first attributed this absence of reefs 

 on the coasts of Peru and of the Galapagos Islands, 2 to the 



1 I have been informed that this is the case, by Lieut. Ryder, R.N., 

 and others who have had ample opportunities for observation. 



2 The mean temperature of the surface sea from observations made 

 by the direction of Capt. Fitzroy on the shores of the Galapagos 

 Islands, between the 16th of September and the 20th of October 

 1835, was 68° Fahr. The lowest temperature observed was 58-5° at 

 the south-west end of Albemarle Island ; and on the west coast of this 

 island it was several times 62 and 63 . The mean temperature of the 

 sea in the Low Archipelago of atolls, and near Tahiti, from similar 

 observations made on board the Beagle, was (although further from the 



