34 ON CORAL REEFS AND ISLANDS. 



zontal for eighty to two hundred feet, and then there is a gradual 

 rise of three to four feet. Over this portion there are large slabs 

 of the beach conglomerate, along with masses from the reef-rock, 

 and some thick plates of a huge foliaceous Madrepora; and these 

 slabs, many. of which are six feet square, lie inclining quite regu- 

 larly against one another, as if they had been taken up and laid 

 there by hand. They incline in the same direction with the 

 slope of the beach. The large Madrepora alluded to has the 

 mode of growth of the Madrepora palmata; and probably the 

 entire zoophyte extended over an area twelve or fifteen feet in 

 diameter. The fragments are three to four inches thick, and 

 thirty square feet in surface. 



As a key to the explanation of the peculiarities here observed, 

 it may be remarked that the tides in the Paumotus are two to 

 three feet, and about Enderby's Island five to six feet in height. 



Maldives. — Chagos Bank. — The Maldives have been often 

 appealed to in illustration of coral structures. They are particu- 

 larly described by Mr. Darwin, from information communicated 

 to him by Captain Moresby, and from the charts of this officer 

 and Lieutenant Powell.* The point of special interest in their 

 structure is the occurrence of atolls or rings within the larger 

 atolls. The islets of the lagoon, and those of the encircling reef, 

 are in many instances annular reefs, each with its own little lake. 

 Gems within gems are here clustered together. 



The annular islets of the main encircling reef are oblong, and 

 lie with the longest diameter, which is sometimes three miles 

 long, in the line of the reef. Those of the lagoon are generally 

 less than two miles across. The lagoons they contain vary from 

 five fathoms or less to twelve fathoms in depth. 



The Maldives are among the largest atoll-reefs known ; and 

 they are intersected by many large open channels; and Mr. Dar- 

 win observes, that the interior atolls occur only near these chan- 

 nels, where the sea has free access. We may view each large 

 island in the archipelago as a sub-archipelago of itself. Although 

 thus singular in their features, they illustrate no new principles 

 with regard to reef- formations.! 



* Darwin on Cora! Reefs, p. 32. See also Journal of the Royal Geographical So- 

 ciety, on the Geography of the Maldives, by J. J. Horsburgh, ii, p. 12 ; and by Cap- 

 tain" W. F. W. Owen, ibid, p. 81 ; also vol. v, p. 39S, on the Northern Atolls of the 

 Maldives, by Captain Moresby. 



f Mr. Darwin thus remarks, (Op. cit. pp. 33, 34,) — " I cnn in fact point out no 

 essential difference between these little ring-formed reefs, (which, however, are 

 larger, and contain deeper lagoons than many true atolls that stand in the open 

 sea,) and the most perfectly characterized atolls, excepting that the ring-formed reefs 

 are based on a. shallow foundation instead of the floor of the open sea, and that 

 instead of being scattered irregularly, they are grouped closely together." — "It ap- 

 pears from the charts on a large scale, that the ring-like structure is contingent on 

 the marginal channels or branches being wide, and consequently on the whole inte- 

 rior of the atoll being freely exposed to the waters of the open sea. When the 



