48 Mr Darwin on Coral Beefs. 



It had passed through my head^ but I had not considered it with nearly 

 the attention it deserved, otherwise I assuredly would have noticed it in my 

 volume. I had always intended to examine the limestone formations of Eng- 

 land for comparison, but was prevented by bad health ; I was, however, led 

 away from the subject, and baffled when I consulted published accounts, for 

 the limestones all appeared to be uniformly spread out, andmost, if not all of 

 them, to be associated with layers of earthy matter, whereas a formation 

 of the nature of a group of atolls, would consist of separate large patches 

 of calcareous rock, which would be quite pure. — I was thus led from the 

 subject, and did not reflect on their want of thickness. The want of thick- 

 ness, however, in any limestone formation, until it be first shewn to be 

 analogous in structure, form, and composition, to a barrier-reef, an atoll 

 or group of atolls, evidently cannot be brought forward as any argument 

 against the theory of the long-continued subsidence of reefs of these 

 classes. During the elevation of all reefs in open seas, I think there can 

 be no doubt (as is dwelt on at p. 117^ 3d. vol.) that a considerable thick- 

 ness of the exterior would be denuded, and the only parts preserved would 

 be those which had accumulated in lagoons or lagoon-channels ; these 

 would be chiefly sedimentary, and in some cases might contain (p. 117) 

 scarcely any coral ; within barrier-reefs such beds would often be associated 

 with much earthy sediment. Mr Lyell, in a note just received, in which he 

 alludes to your criticisms, speaks of the limestones of the Alps and Pyre- 

 nees, as being of enormous thickness, namely, about 4000 feet. I do not 

 know what their composition is, but I have no doubt that the strata now 

 accumulating within the barrier-reef of Australia and New Caledonia, are 

 chiefly formed of horizontal layers of calcareous sediment and not of coral. 

 I suspect that denudation has acted on a far grander scale than in 

 merely peeling the outsides of upraised reefs. My theory leads me to infer 

 that the areas, where groups of atolls and barrier-reefs stand, have sub- 

 sided to agreat amount and o^ er a wide space. Now it appears to me pro- 

 bable, thata subterranean change, producing a directly opposite movement, 

 namely, a great and widely extended elevation, would be extremely slow, 

 and would be interrupted by long periods of rest, and perhaps of oscil- 

 lation of level. When I think of the denudation along the fault, which goes 

 across the northern carboniferous counties of England, where 1000 feet of 

 strata have been smoothed away ; when I think how commonly volcanic 

 islands, formed of very hard rock, are eaten back in clifis from 100 or 200 to 

 800 or 1000 feet in height, I hardly see where we can stop, with respect to 

 the probable limitsof erosiononthe comparativelysoft, generally cavernous, 

 tabular, though wide, masses of coral rock, standing exposed in great oceans 

 during very slow changes of level. Most of the atolls which have been 

 raised a few hundred feet are mere wrecks, and at the Friendly Archipelago 

 where there are upraised atolls, there are large irregular reefs, also, which I 

 have always thought were probably the basal vestiges of worn down atolls. 

 Many submerged reefs, which may have had this same origin, occur out- 

 side the line of elevation of the Salomon and New Hebrides archipelagoes. 

 The great steepness of the shores of upraised reefs (p. 65. Ehrenberg 

 quoted, and p. 51.) would probably be unfavourable to the growth of new 



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