The reef-flats and outer reefs flanking ele vated islands are partly barrier and partly fringing reefs. We may, says Agassiz, trace the passage of elevated plateaus like Guam, Tonga and some of the Fiji islands, which are partly volcanic and partly limestone, to atolls where only a small islet or a larger island of either limestone or volcanic rock is left to indicate its origin. Atolls also may be formed upon the denuded rim of a submerged volcanic crater, as in certain of the Fijis (Totoya and Trombia), and as in some of the volcanoes east of Tonga.
As to the origin of atolls by subsidence, we really have few data to support the Darwinian theory. Agassiz claims that throughout the Pacific, Inoiian Ocean and West Indies °the most positive evidence exists of a moderate, re cent elevation of the coral reefs.° This is shown by the ridges, pinnacles and undermined masses of modern or Tertiary limestones formed recently. beneath the sea, but now above sea-leveI, proving very recent uplift. The existence of honeycombed pinnacles of lime stone within the lagoons of atolls, as shoals, islands or islets, shows the extent of the solvent action of the sea upon land areas having for merly a greater extension than at the present day. The Maldivian plateau with its thousand.s of small atolls, rings or lagoon reefs rising from a depth varying from 20 to 30 fatlloms, is overwhelming testimony, says 'Agassiz, that atolls may rise from a plateau of suitable depth, wherever and however it may have been formed, and whatever may be its geological structure. In the regions examined by Agassiz the modern reef-rock is of every moderate thickness, being within the limit of depth at which reef-builders begin to grow and within which the land rims of atolls or of barrier reefs are affected by mechanical causes.
Why no coral reefs exist on the western coasts of the two Americas, and their absence at other points, is explained by Agassiz as due to the steepness of their shores and to the absence or to the crumbling nature of their sub marine foundations or platforms. Coral reefs
also cannot grow off the steep cliff-surfaces of elevated, coraliferous limestone islands.
The proof of the supposed great thickness of coral reefs, to account for which Darwin in voked subsidence, will be ascertained by boring. Thus far the evidence tends to show that the coral beds are not continuous. At Honolulu they contained several beds of volcanic ash, etc. In the Fiji Islands, shell limestones were in terstratified with coral rock. The great areas of subsidences postulated by Darwin and by Dana have been shown to be areas of elevation. The deepest depressions or °deeps" in the Pacific, judging by the Challenger maps, are in regions where there are no coral reefs or atolls.
Bibliography.-- The literature of the sub ject is extensive. The classic works are J. D. Dana's magnificent folio in the series of reports of the Wilkes Exploring Expedition, and his more popular volume, (Corals and Coral Is lands' (revised edition, New York 1890) ; and Darwin's (StruCture and Distribution of Coral Islands' (London, 3d ed., with notes by Bonney, New York 1899). Murray's papers of corals and reef-structures in the (Proceedings) of the Royal Society of Edinburgh, Vols. X (1880) and XVII (1891) are highly important. The Agassizs, father and son, directed much atten tion to this subject, and Alexander Agassiz in particular has added greatly to information in his (Visit to the Great Barrier Reef of Aus tralia' (Cambridge, Mass., 1898). The illus trated works of Saville Kent on the Australian reefs should also be remembered, as well as Heilprin's on those of Bermuda. For structure and classification consult Bourne's account of the Anthozoa in Lankester's (Treatise on Zo ology' (London 1900) ; and for fossil corals consult Zittell-Eastman, (Text-book of Palmon tology' (New York 1900), and the great (Mon ographie des polypiers fossils des terrains paleozoiques> of Milne-Edwards and Haime, issued in Paris in 1851; also Hickson, °Ccelen terata) (in Cambridge Natural History, Vol. I, London 1906). See BARRIER REEF, THE GREAT.