Wharton's and Darwin's theories of coral reefs.

There is a passage in Gardiner's review (1) of my book on coral reefs (2) which puzzles me. He there wrote: "It may be that Wharton's views ... are 'against the consensus of geological opinion,' but I have failed to find any evidence cited against Wharton's idea of an island being cut down by the waves and currents to a flat bank 30-40 fathoms deep." Taken with the context, this can only mean that Gardiner regards Wharton's theory of atolls as still tenable; for if it be agreed that the foundations of atolls have actually been prepared by cutting down volcanic islands to flat submarine banks, as Wharton believed, the later upgrowth of the reefs around the bank margins to form atolls is acceptable enough. The puzzling part of the above quotation is therefore that Gardiner appears to "have failed to find any evidence" cogently cited against Wharton's theory; for, as I see the case, the counterevidence is not only manifest and abundant but also unquestionable and convincing. In order to make this clear, let us look somewhat closely at the theory which Wharton proposed. That experienced hydrographer was impressed, some thirty-five years ago, with the flatness of atoll-lagoon floors, in which he could not find the "deeply concave surface" of Darwin's text (1842, 93); but Wharton there misunderstood Darwin's meaning, for, although the originator of the subsidence theory of coral reefs also used other similar phrases, such as "saucer-shaped hollow" (28) and "bason-like form" (92) in connection with atoll lagoons, he elsewhere stated explicitly that "the greater part of the bottom in most lagoons is formed of sediment; large spaces have exactly the same depth" (20), thus showing that concavity is subordinate to flatness, and he very properly adduced the processes of aqueous deposition to account for the flatness with which lagoon-floor sediment are spread out. Wharton was also impressed with the moderate depth, 30 or 40 fathoms, of many lagoons, and asked whether that restricted depth "may not be the general limit of the power of oceanic waves to cut down the mass acted upon when it is fairly friable" (Brit. Assoc. Rept., 1894, 709); but he nowhere showed that "fairly friable" rocks prevail in atoll foundations. Three years later he made a fuller statement of his views in an article entitled "The Foundations of Coral Atolls" (Nature, 1v, 1897, 390-93); and here he wrote: "Toward the sides of a lagoon the depth gradually lessens"; hence the difference of opinion between Wharton and Darwin on this point would seem to have been rather verbal than factual. Darwin was considering chiefly the reef, and emphasised the slope by which descent is made from it to the flat lagoon floor; Wharton was concerned chiefly with the lagoon floor, and emphasised the general flatness of its surface, from which a gradual ascent is made to the encircling reef. It is in Wharton's article of 1897 that some account is given of certain members of the group of drowned atolls north of Fiji, for which I have proposed the name, Darwin Hermatopelago, or Sea of Banks. Wharton there wrote: "The remarkable thing about these banks is the absolute uniformity of the depth of water over their areas inside the low [submerged] rim of growing coral which encircles their edges in various degrees." The Alexis bank is represented in a full-page figure with many soundings; its outer rim has diameters of 15 and 8 miles, with depths of from 13 to 20 fathoms; its floor measures 12 by 7 miles with depths of from 24 to 28 fathoms. Other banks are said to measure 22 by 10 and 18 by 9 miles with floors of similarly equable depths. But not enough emphasis is given to the fact that, by reason of the bank rims being submerged, the banks are swept over by the swell coming, not greatly impeded, from the open ocean; or to the inference that, by the action of the swell, the exceptional smoothness of the bank floors may be accounted for. It is then asked: "What causes this remarkable similarity of depth and this extraordinarily even surface over these large banks? …