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Oil Coral Reefs and Islands. 39 



zorrtal for eighty to two hundred feetj and then there is a gradual 

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

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

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

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

 larly against one aiiother, 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, — Cha^os 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 o( 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 

 longj 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 amoi ^ 

 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 



ird to reef-formations.t 



»fid_^ 



' 'T 



^ Darwin on Coral Eeefe, p. 32. See also Journal of the Rojal Geograpliical So- 

 ciety, on the Geogi-aphy of the Miiklives, by J. J. Horsburgb, ii, p. *72 ; and by Cap- 

 tain W. F. W. Owen, ibid, p. 81 ; also vol. t, p. 398, on the Northern Atolls of the 

 Maldives, by Captain Moresby. 



f Mr. Darwin thus remarks, (Op. cit, pp. 33, 34,)^" I can iu fact point oot no 

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

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

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

 are bai;ed on a shallow foundation instead of tlie floor of the open sea,^ and that 

 instead of beincr scattered irreguhirly, they are grouped closely ^^b^etJier "-^ It ap- 

 pears from tiWHiFi lit on a large ^cale, that tlie ring-like structure Ls rontil%ent on 

 the marginal cbminels or branches being wide, and coni=equci^y on the wh*)le inte- 

 rior of the at<rfl being freely expose^ to the waters of tht openj^a. When the 



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