36 Coelenterata. 



70 atolls, many of great size, and of elevated islands composed of coralliferous 

 limestone. The low atolls have been cut down by the denudation and sub- 

 marine erosion of higher islands. The great uniformity of the atolls is due 

 to the cutting down of an old ledge to a general level and the subsequent 

 building up of the atolls by the materials supplied from the reef flats and from 

 the coral slopes. Certainly subsidence alone could not have acted in such a 

 uniform way. The cutting down by erosion and denudation and the planing 

 action of the sea may be still observed. The underlying ledge of the atolls 

 is the remnant of a bed of tertiary coralliferous limestone which, at one time, 

 covered the greater part of the area of the lagoon. This limestone was gradu- 

 ally denuded and eroded to the level of the sea and passages were formed 

 on its outer edge thus allowing the sea access to the inner parts of the lagoon 

 limestone flat. The evidence of elevation in the western Paumotus is very 

 definite and the author denies that these islands are situated in an area of 

 subsidence and that this has been the great factor (Darwin and Dana) in the 

 formation of the characteristic atolls of the group. Nearly all the islands have 

 undergone elevation probably to about the same height, for the old ledge is 

 found exposed nearly everywhere at about low water. In these islands there 

 is a great development of buttresses of tertiary coralliferous limestone, these 

 are the last remnants of the former elevated land once covering a large area 

 of the atolls. In the Ellice and Marshall Is. and in some of the Gilbert Is. 

 there are similar buttresses of modern reef rock and conglomerate indicating 

 an elevation of a few feet. The reef corals of the Paumotus are remarkable 

 for the small number of genera, the small species and stunted development 

 of the corals. The outer reefs in 3 to 15 fms. are composed almost entirely 

 of dense masses of Astrseans, Pavonia, Madr&pora and Pocillopora. No 

 Mseandrinse were found and no large heads of any species of coral. This is 

 in striking contrast to the colossal masses met with on the Great Barrier Reef 

 and in the West Indies. There is also, in the Paumotus, a general absence 

 of the large fleshy Alcyonaria and there are but few sponges. Millepora and 

 Allopora are common and Tubipora was brought up from 742 fms. The bathy- 

 metrical limit of the reef building corals is 20 to 22 fms. - The Society 

 Islands are volcanic islands surrounded by barrier reefs. The fringing reefs 

 are noted for their great breadth. The reefs of the N.W. and W. coast of 

 Tahiti show well how a wide fringing reef may gradually become scooped out 

 into a barrier reef edging a shallow lagoon and ultimately a wide and deep 

 barrier reef lagoon. The Cook Archipelago consists of volcanic islands 

 with encircling reefs, of elevated coralliferous limestone islands and of low 

 atolls. The elevated islands have narrow reef platforms formed by submarine 

 erosion. Aitutaki shows plainly the manner in which the lagoon and barrier 

 reef flats have been formed by the denudation and erosion of the volcanic 

 mass which once occupied the area indicated by the outer edge of the barrier 

 reef. The Tonga Archipelago consists of an extensive area of elevated 

 coralliferous limestone greatly denuded and eroded. In the Tonga group recent 

 corals have played no part in the formation of the masses of land, but here, 

 as in the Society, Cook and Fiji Islands, which are also in areas of elevation, 

 the recent corals forms a thin living crust upon platforms which are either vol- 

 canic or calcareous and the formation of which has been independent of the 

 growth of the corals. Falcon Island and Metis Shoal perhaps indicate the 

 depth to which the action of the sea may extend in shaping platforms of sub- 

 marine erosion. Falcon Island at one time attained a height of 250 feet, now 

 forms a shoal, and Metis Shoal with a depth of 15 fms. was once an island. 



