366 Revieivs — Agassiz's Coral Reefs of the Pacific, 



sponges and Algee, and by attrition on the wide reef platforms, 

 both of the sea face and lagoon side, reduced first to coarser, then 

 to fine sand and impalpable silt, which may be carried off in 

 suspension. In addition to the mechanical destruction constantly 

 going on, chemical action takes place, and water carries off in 

 solution large quantities of lime from the sea face, where we can trace 

 the extent of its action from the undercut faces of cliffs, of masses 

 of corals, and the rotten condition of smaller fragments of corals. 



" The same action takes place in the lagoon even to a greater 

 extent ; in every direction we can trace the effect of solution on 

 the beach rock beaches, the conglomerate or breccia ledges, the 

 patches of corals, the samples of the bottom, the slopes of the 

 shoals or ledges within the lagoon, all showing that solution is 

 a prominent factor in removing carbonate of lime from the interior 

 of a lagoon." (p. xxiv.) 



We may approximately classify the atolls, elevated islands, and 

 volcanic islands where reefs are found, into the following categories : 



Large volcanic islands with barrier and fringing reefs, like 

 Tahiti, Viti and Vanua Levu, and the larger Samoan Islands ; in 

 these the land mass occupies a large area as compared with that of 

 the reefs. Viti Levu and Vanua Levu, as well as Solomon, New 

 Hebrides, and Oahu in the Sandwich Islands, are partly flanked by 

 elevated coralliferous limestone. 



Next may come smaller volcanic islands with barrier and 

 fringing reefs, like the Pelew Islands, Yap, Truk, Ponapi, Kusaie ; 

 the smaller New Hebrides and Fiji Islands, like Nairai, Makongai, 

 Mbengha ; the volcanic islands of the Cook group, as Aitutaka, 

 Earatonga ; the Home Islands, Eotumah, the Marquesas, and such, 

 islands as Maupiti, Bora Bora, Eaiatea, Huaheine, Eimes, in which, 

 the area of encircling reefs is as large as, if not larger than, the 

 enclosed volcanic mass. 



Volcanoes like Totoya and Thombia in Fiji show the possibility 

 of the formation of an atoll-like island by the growth of corals on 

 the submerged or denuded rim of an extinct volcano. In Totoya 

 the extension of the volcanic reef- rim forms a barrier-reef 

 surrounding the island and enclosing a barrier-reef lagoon, with 

 ship-passes into it. 



Niuafou and Tofua in Tonga are both islands with extinct 

 craters filled with brackish water. "Were the rim cut down to 

 below the level of the sea, they would become reef-flats enclosing 

 a deep lagoon, as is the case with a part of the rim of Thombia and 

 Totoya. 



Elevated coralliferous limestone islands, probably of Tertiary age, 

 with limited areas of narrow reef-flat platforms, like Makatea, Nine, 

 Nauru, Paanopa, Naiau, Wangava, with ill-defined sinks or basins 

 occupying part of the summit. 



Elevated coralliferous limestone islands, like Mango, Kambara, 

 Tuvutha, in which the sink is better defined, and where a volcanic 

 outburst has broken through the rim or central part of the sink 

 or both. 



