CORAL-REEFS. 175 



improbability of Mangaia, a fringed island in the same 

 group (but distant 170 miles), having retained its nearly 

 perfect atoll-like structure, during any immense lapse of 

 time after its upheaval. The Red Sea, therefore, is the 

 only area in which we have clear proofs of the recent 

 elevation of a district, which, by our theory (although the 

 barrier-reefs are there not well characterised), has lately 

 subsided. But we have no reason to be surprised at 

 oscillation, of level of this kind having occasionally taken 

 place. There can be scarcely any doubt that Savage, 

 Aurora, 1 and Mangaia Islands, and several of the islands in 

 the Friendly group, existed originally as atolls, and these 

 have undoubtedly since been upraised to some height 

 above the level of the sea ; so that by our theory, there has 

 here, also, been an oscillation of level, — elevation having 

 succeeded subsidence, instead of, as in the middle part of 

 the Red Sea and at the Harvey Islands, subsidence having 

 probably succeeded recent elevation. 



It is an interesting fact, that Fais, which, from its com- 

 position, form, height, and situation at the western end 

 of the Caroline Archipelago, one is strongly induced to 

 believe existed before its upheaval as an atoll, lies exactly 

 in the prolongation of the curved line of the Mariana group, 



1 Aurora Island is described by Mr. Couthouy {Remarks, p. 58) ; it 

 lies 120 miles N.E. of Tahiti ; it is not coloured in the appended map, 

 because it does not appear to be fringed by living reefs. Mr. Couthouy 

 describes its summit as <f presenting a broad table-land which declines 

 a few feet towards the centre, where we may suppose the lagoon to 

 have been placed." It is about 200 feet in height, and consists of reef- 

 rock and conglomerate, with existing species of coral embedded in it. 

 The island has been elevated at two successive periods ; the cliffs 

 being marked half-way up with a horizontal water-worn line of deep 

 excavations. Aurora Island seems closely to resemble in structure 

 Elizabeth Island, at the southern end of the Low Archipelago. 



