1903.] Barrier Reefs and of the Different Types of Atolls. 



413 



Australia, Florida, Honduras, and the Bahamas, are underlaid by 

 outliers of the adjoining land masses, which crop out as islands and 

 islets on the very outer edge of the Barrier Reefs. Some of the 

 Barrier Reefs of the Society Islands, of Fiji, and of the Carolines, 

 show that the wide and deep lagoons, separating them from the land 

 mass, have been formed by erosion, from a broad fringing reef flat. 

 Encircling reefs, such as characterise especially the Society Islands, 

 hold to their central island or islands the same relation which a 

 Barrier Reef holds to the adjoining land mass. Denudation and 

 submarine erosion fully account for the formation of platforms upon 

 which coral reef and other limestone organisms may build, either 

 barrier or encircling reefs, or even atolls, rising upon a volcanic base, 

 of which the central mass may have disappeared, as in Fiji, the 

 Society and Caroline Islands. 



We may next take the type of elevated islands of the Paumotus, 

 the Fiji, the Gilbert, and the Laclrones, many composed only of 

 tertiary limestones, others partly of limestone, and partly volcanic. 

 We can follow the changes from an elevated island, like Niue, or 

 Makatea in the Paumotus, to an island like Niau, through a stage like 

 Rangiroa to that of the great majority of the atolls in the Paumotus. 

 The reef-flats and outer reefs flanking elevated islands, hold peculiar 

 relation to them, they are partly those of Barrier Reef and partly of 

 Fringing Reef. We may also trace the passage of elevated plateaux 

 like Tonga, Guam, and islands in Fiji, partly volcanic and partly lime- 

 stone, to atolls where only a small islet or a larger island of either 

 limestone or volcanic rock is left to indicate its origin. Atolls may 

 also be formed upon the denuded rim of a volcanic crater, as at 

 Totoya and Thombia in Fiji, as well as in some of the volcanoes east 

 of Tonga. 



In the Ellice and Marshall group and the Line Islands, are a number 

 of atolls, the underlying base of which is not known, and where wc 

 can only follow the formation of the land rim of the atoll, as far as it 

 is due to the agency of the trades or of the monsoons in constantly shift- 

 ing the superficial material (prepared by boring organisms) which goes to 

 form its rim. Many of the atolls in the Pacific are merely shallow 

 sinks, formed by high sandbanks, thrown up around a central area. 



Throughout the Pacific, the Indian Ocean, and the West Indies the 

 most positive evidence exists of a moderate, recent elevation of the 

 coral reefs. This is shown by the bosses, pinnacles, and undermined 

 masses of modern or tertiary limestone left to attest it. The existence 

 of honeycombed pinnacles of limestone within the lagoons of atolls, 

 as shoals, islands, or islets, shows the extent of the solvent action of 

 the sea upon land areas, having formerly a greater extension than at 

 the present day. Signs of this solvent action are to be seen every- 

 where among coral reefs. Atmospheric denudation has played an 



VOL. LXXI. 2 H 



