524 W. M. DAVIS SUBSIDENCE OF REEF-EXCIRCLED ISLANDS 



are still free from fringing reefs, as if their submergence were more 

 recent than that of Tutnila. The above opinion al:)out Tntiiila is, how- 

 ever, onh' provisional, until fuller information is gained about the "mod- 

 ern offshore marine platform" beneath the fringing reef. It is therefore 

 to be hoped that Mayer may be successful in his plan to bore through the 

 reef and "study the existence or non-existence of a submerged marine 

 platform'^ (1917, 526) underneath it. 



The spur-end cliffs of Tutuila and the Marquesas Islands, as well as 

 those of Tahiti, seem at first thought to give support to the Glacial-control 

 theory of coral reefs ; but in reality they contradict it, for if the cliffs of 

 these exceptional islands were cut under the conditions which that theory 

 postulates, then the central islands of all barrier reefs should be clift, and 

 they are not. The cliffs of volcanic islands are best explained as the result 

 of conditions determined by the islands themselves, as stated below. 



VNCO^'FORMABLE FRINGING REEFS IN THE 80L02ION ISLANDS 



Many other occurrences of unconformable fringing reefs of a new gen- 

 eration might be adduced, but space will be allowed only for a striking 

 example on the small island of Fauro, in the western part of the Solomon 

 group. The island is described by Guppy as "the basal wreck of some 

 huge volcanic cone" (1887, 33) ; '"^so great has been the degradation of 

 the surface that we have nothing more than the cores or basal remains of 

 the ancient volcanic cones, which have built up this island" (40) : some 

 of the higher knobs consist of massive igneous rocks, and are interpreted 

 as denuded volcanic necks (37). This painstaking observer says: "It is 

 worthy of note that neither in this . . . island . . . nor in the 

 adjacent smaller islands of volcanic formation did I find any raised coral 

 rock. Xarrow shore reefs fringe the coast" (38). Although Guppy 

 mentions the "deeply indented sea-border" of Fauro as well as the great 

 denudation that the island has suffered, he did not infer subsidence either 

 from the embayed shoreline of the island, well shown in figure 11, or 

 from the manifestly unconformable fringing reefs, or from the partly 

 reef -rimmed platform that surrounds the island, on which the northward 

 increase of depth strongly suggests a gentle tilting in that direction. On 

 the contrary, he rejected Darwin's theory, which he had previously held, 

 and regarded all the reefs of the Solomon group, whether at sealevel or 

 above it, as having been formed during "an alternation of long periods 

 of upheaval with lengthened intervals of repose" (127). 



Not only so: Guppy 's book was favorably reviewed in. Nat are and in 

 the Geological Magazine, and the reviewers completely failed to notice 

 the neglect of matters so manifest and so critically significant as embayed 



