To Charles Lyell [September–December 1842]1
– Considering the probability of subsidence in the middle of the great oceans being very slow;—considering in how many spaces, both large ones (& small ones within areas favourable to the growth of corals) reefs are absent, which shows that their presence is determined by peculiar conditions;—considering the possible chance of subsidence being more rapid than the upward growth of the reefs;—considering that reefs not very rarely perish (as I cannot doubt) on part or round the whole of some encircled islands & atolls;—considering these things, I admit as very improbable that the polypifers should continue living on & above the same reef, during a subsidence of very many thousand feet; & therefore that they should form masses of enormous thickness, say at most above 5000 feet.2 This admission, I believe, is no ways fatal to the theory, though it is so to certain few passages in my book. In the areas, where the large groups of atolls stand, & where likewise a few scattered atolls stand between such groups, I always imagined that there must have been great tracks of land, and that on such large tracks there must have been mountains of immense altitudes.3 But now it appears to me, that one is only justified in supposing that groups of islands stood there.4 There are (as I believe) many considerable islands & groups of islands (Galapagos Id great Britain, Falkland Isd Mariana & I believe Viti groups) & likewise the majority of single scattered islands, all of which a subsidence between 4000 & 5000 ft would entirely submerge or wd leave only one or two summits above water; & hence that they would produce either groups of nothing but atolls, or of atolls with one or two encircled islands.— I am far from wishing to say that the islands of the great Oceans have not subsided, or may not continue to subside any number of feet, but if the average duration (from all causes of destruction) of reefs on the same spot is limited, then after this limit has elapsed, the reefs wd perish & if the subsidence continued they wd be carried down; & if the group consisted only of atolls only open ocean wd be left; if it consisted partly or wholly of encircled islands, these would be left naked & reefless; but should the area again become favourable for growth of reefs, new barrier-reefs might be formed round them. As an illustration, of this notion of a certain average duration of reefs on the same spot compared with the average rate of subsidence— we may take case of Tahiti, an island of 7000 ft high; now here the present barrier-reefs would never be continued upwards into an atoll, although shd the subsidence continue, at a period long after the death of the present reefs, new ones might be formed high up round its sides & ultimately over it.—5
The case resolves itself, into,—what is the ordinary height of groups of islands of the size of existing groups of atolls (excepting as many of the highest islands as there now ordinarily occur encircling barrier-reefs in the existing groups of atolls), and likewise what is the height of the single scattered islands standing between such groups of islands;—subsidence sufficient to bury all these islands (with the exception of as many of the highest, as there are encircled islands in the present groups of atolls) my theory absolutely requires, but no more.— To say what amount of subsidence would be required for this end, one ought to know the height of all existing islands, both single ones & those in groups on the face of the globe & indeed of half a dozen other worlds like ours.— The reefs may be of much greater thickness than that just sufficient on an average to bury groups of islands; and the probability of the thickness being greater, seems to resolve itself into the average rate of subsidence allowing upward growth, & average duration of reefs on the same spot— Who will say what this rate & what this duration is; but till both are known, we cannot, I think, tell whether we ought to look out for upraised coral-formations (putting on one side denudation) above the unknown limit, say between 3000 & 5000 feet, necessary to submerge groups of common islands.6 How wretchedly involved do these speculations become!
Discusses relationship of subsidence to the formation of coral reefs.
Please cite as
Darwin Correspondence Project, “Letter no. 605,” accessed on 25 August 2016, http://www.darwinproject.ac.uk/DCP-LETT-605