Page:A Treatise on Geology, volume 1.djvu/347

 CHAP. VI. ornamental parts are made up by a diffusion of the other forms of Linnæan madrepores, viz. fungia, pavonia, agaricia, monticularia, echinophora, pocillopora, seriatopora, and oculina, together with gorgonia, isis, corallium, melitea, coralline, spongia, alcyonium, actinia, &c., independent of the locomotive asteriæ, echini, testacea.

The coral islands are classed by Mr. Stutchbury as circular, flat, long narrow, and encircling high land.

The coral islands of the Dangerous Archipelago (lat. S. 12° to 27°, long. W. 130° to 155°) are all of the first kind, and consist of strips or belts of coral of an annulate or circular form, from 400 or 500 yards to one mile across the ring which always incloses a lagoon; seldom raised above the water more than from 4 or 5 feet; abrupt towards the ocean, which rapidly deepens to more than 120 fathoms. The islands vary from 2 or 5 to 150 miles round; the ring, being often divided across by a fissure, admits vessels to enter the lagoon. The depth at which the coralligenous zoophyta commence their labours is said not to exceed 15 or 20 fathoms (Quoy and Gaimard say 20 or 30 feet, Mr. Darwin has recently given the same estimate as Mr. Stutchbury). The bottom of the lagoons is seen in calm weather at a depth of 100 feet or more, strewed over with dead shells and broken fragments of coral, rarely showing any living specimen below sixteen or seventeen fathoms; at which depth, smaller reefs rise within the lagoon; and beyond which depth, broken masses of rock may be seen without any living portion attached.

It would appear that, during the formation of a reef, portions of it become compact, and as dense as any limestone rock; a circumstance indicative of the partial dissolution and re-precipitation of the coral masses, and apparently analogous to the process whereby coral shells, &c. have been imbedded in the compact limestone of ancient stratified rocks. Extensive beds of particular shells appear among the islands.

Islands often occur of a fiat or tabular form, generally