FRINGING REEFS ON CLIFF-RIMMED ISLANDS 523 



UNCONFORMABLE FRINGING REEFS IN SAMOA 



According to the recent observations of Mayer, Tutuila, a well embayed 

 member of the Samoan group, has strongly clift spurs, "some of the sea- 

 cliffs being 500 feet high"; and the cliffs are fronted by a "fringing reef 

 which forms a mere veneer over the modern offshore marine platform, 

 and extends a short distance seaward, its precipitous outer edge being 

 from 5 to about 20 fathoms deep" (1917, 523, 522). A recent small 

 emergence is indicated, for ^^a platform about 8 feet above high tide juts 

 out to seaward from the -base of practically every promontory." The 

 composition of this platform is not stated and its genetic relations are 

 not explained, but it would seem to be a young platform cut at mid- 

 height in partly submerged sea-cliffs. An unpublished chart of Tutuila, 

 which I have lately had opportunity of seeing in the Hydrographic Office, 

 Washington, contains a great number of new soundings, and reveals the 

 existence of a submerged platform, from one to three miles in breadth 

 and from 30 to 50 or more fathoms in depth, the presence of which could 

 not have been proved by the scanty soundings of earlier charts. The outer 

 part of the platform is usually somewhat shallower than at half distance 

 offshore, as if a poorly developed barrier reef inclosed it; the most strik- 

 ing example of such a reef rises to less than 10 fathoms depth outside of 

 Pagopago harbor, the chief embayment of the island; inside of this reef 

 the platform has its maximum depth of 66 fathoms. Such a platform 

 must have been backed with high cliffs, now partly submerged. 



In view of these facts, it seems probable that the partly submerged cliffs 

 of the spur ends were, like those of Tahiti, cut back by the sea when the 

 island stood some 50 fathoms higher than now, and that the valleys, now 

 embayed, were eroded while the cliffs were cut back, under conditions that 

 prevented reef growth, as outlined on a later page. If this be the case, a 

 rock platform of marine abrasion, from one to three miles in breadth,- 

 50 fathoms or more below present sealevel, and covered by later deposits, 

 must extend seaward from the submerged base line of the great cliffs ; the 

 imperfect barrier reef on the platform margin would then be explained 

 as the first result of a moderate submergence, afterward followed by a 

 rapid submergence, when the incipient reef was drowned and young cliffs 

 were cut at mid-height in the great cliffs. The narrow platform of the 

 young cliffs is now slightly emerged and margined by a fringing reef. 



The Marquesas Islands resemble Tutuila, for the spur-ends of their 

 embayed shorelines are strongly clift, and the cliffs plunge into water 

 10 or 20 fathoms in depth near shore, except that young cliffs and plat- 

 forms are cut in the great cliff faces at present sealevel ; but these islands 



