Origin of Coral Islands

Abstract

MR. MURRAY's concise explanation of the formation of coral reefs and islands presents advantages in more than one respect. It demands no a priori assumptions, but begins and ends with that which can be observed, while Darwin's theory requires the preliminary concession of subsidence, which never has been and never perhaps can be observed. It must appear ungracious to question a theory that accords so completely with the natural history of coral islands, but even this theory requires a geological concession, and that is stability. Coral islands, it may be supposed, after all only differ from other oceanic islands in being crusted over with coral, so that we cannot see their original state, and the question is whether we can grant such long periods of stability to them, from our experience of other oceanic islands, which are free from coral and can therefore be observed. Nearly all oceanic islands are volcanic, and it is probable that their elevation coincides more or less with the period of volcanic activity somewhere along their line. It is obvious that coral islands are not formed during this phase, because no theory would then hold good; the peaks would grow through and carry up the coral, which might leave only such small traces of its existence as we find in a single spot in Madeira. It would not be unreasonable to suppose that if the expansive and elevating force were withdrawn the peaks would slowly subside, and that if there are some lines of elevation, there must be others of subsidence, unless the earth is as a whole growing in bulk. Darwin claims the existence of areas of subsidence, and that these are eminently favourable to coral growth, and it is quite apparent that if the Island of Madeira were to sink, as it has undoubtedly risen, its last appearance in a coral sea would be as an atoll. We shall never see the interior structure of a stationary or subsiding coral island, and can only look for a re-elevated example with a crust that has been protected from solution whilst dead and submerged, and yet not sufficiently so to mask the core.