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Letter 1169

Darwin, C. R. to De la Beche, H. T.

7 Apr [1848]

    Summary Add

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    CD discusses questions about his coral reef theory that were raised by HTDlaB [in his Geol. Soc. Anniversary Address (1848)].

Transcription

Down Farnborough Kent

April 7th

Dear Sir Henry

As you were so kind as to suggest my writing to you on the point referred to in your Address, I will begin seriatim with the Barrier reefs— At p. 49 of my Volume on Coral Reefs, (please read whole page) you will see the reasons, why I do not think it probable that these reefs grow on the submarine margin of the denuded coast-land.

With respect to atoll-like reefs being thus formed, I here remarked (p. 101) that if a bank either of rock or hardened sediment “lay a few fathoms submerged, the simple growth of the coral, without the aid of subsidence, would produce a structure scarcely < > true atoll; for in all cases < > Again at p. 57 (& elsewhere, as at p. 89) I have sai<d> < > origin just mentioned “sometimes < > should not be overlooked) the appea< > differ from them &c &c.”

The Red Sea is the only instance where there is evidence that worn down islands have been coated by coral-reefs; & here the coating begins as soon as ever the surface gets under water; such, I believe, would be the usual occurrence, & accordingly I have endeavoured to show at p. 93 that atolls cannot have been thus formed. If it be supposed that the islands are worn down to a depth of 20 or 30 fathoms, before the corals become attached to them; then we have the case proposed by Chamisso, against which I have urged some considerations at p. 89. That I had the special case of worn-down craters in my mind, you will see in the note to p. 94; but I really thought it superfluous to argue separately against worn-down craters, or islands of other rocks, or upraised coral-reefs, or banks of sediment, but I lumped them together under the term of submarine < > repeatedly in my volume endea= < > that there may be < > atolls & barrier reefs < > formed by subsidence; thus at p 89. I state < > Chamisso's view “is inapplicable to the greater number of atolls”. In same page I have remarked that some atolls may be formed on the crest of a crater (of which case I suspect Rose Isd is an instance). I consider my theory as only applicable with any certainty to the groups of atolls; & consequently in my final chapter, I remark (p 124) “The evidence from a single atoll or a single encircling barrier-reef, must be received with some caution, for the former may possibly be based on a submerged crater or bank, and the latter on a submerged margin of sediment or of worn-down rock.”— Whether you consider my remarks satisfactory or not, I trust that you will find that I have not proceeded without consideration of the sources of error: I assure you, I did not spare time or labour in examining thousands of charts & all voyages.— But forgive the length of this letter; a man is as tender of his theories as of his child<ren> & values one & the other at a hundred < >

Y< >
To | Sir H. Delabeche

If you are interested on this subject; pray look at the great charts of of E. Indian Company of the Maldiva islands: my reduced copy of Mahlos Madoo is quite insufficient to show their truly wonderful structure. The reefs in the extreme west part of the W. Indian archipelago, are the greatest puzzlers to me.—

    Footnotes Add

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    f1 1169.f1
    De la Beche's anniversary address to the Geological Society on 18 February 1848 cited observations made at the Great Barrier Reef off the coast of Australia by Joseph Beete Jukes (De la Beche 1848, pp. xc–xcvi). The point in question was whether, as Jukes contended, atolls might have been formed by the growth of corals on the foundations of sediment from denuded coastland without subsidence, or whether subsidence was a necessary condition as CD maintained.
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    f2 1169.f2
    Coral reefs, p. 101: ‘would produce a structure scarcely to be distinguished from a true atoll’.
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    f3 1169.f3
    Coral reefs, p. 101: ‘for in all cases the corals on the outer margin of a reef, from having space and being freely exposed to the open sea, will grow vigorously and tend to form a continuous ring, whilst the growth of the less massive kinds on the central expanse, will be checked by the sediment found there’.
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    f4 1169.f4
    Coral reefs, p. 57. CD referred to modified fringing reefs in shallow seas, such as the Persian Gulf and the East Indian Archipelago, stating that they ‘sometimes assume, (and this circumstance ought not to be overlooked,) the appearance of atolls; but they differ from atolls in their central expanse being much less deep, in their form being less defined, and in being based on a shallow foundation.’
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    f5 1169.f5
    Adelbert von Chamisso, who accompanied Otto von Kotzebue on his voyage to the Pacific, 1815–18. CD discussed Chamisso's theory of the growth of coral reefs in Coral reefs, p. 89.
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    f6 1169.f6
    In Coral reefs, p. 92, CD referred to the West Indian formations as a possible exception to his theory: ‘A conjecture will perhaps be hazarded, that the requisite bases might have been afforded by the accumulation of great banks of sediment, which … did not quite reach the surface,—as actually appears to have been the case in some parts of the West Indian Sea.’
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