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SCIENCE 



[N. S. Vol. LIII. No. 1382 



not yet well enough known to warrant any 

 statement as to whether they bear submerged 

 reefs or not ; and although new sea-level fring- 

 ing reefs are not yet developed on the Mar- 

 quesas clifFs, for Mayor reports the growth 

 there of only separate corals on the cliff faces 

 below sea level. A corollary of this last-men- 

 tioned fact is that the submergence of these 

 islands must be more recent than that of 

 Tutuila. 



Had the old barrier reef of Tutuila not been 

 drowned by a too rapid submergence — pos- 

 sibly the result of subsidence at an ordinary 

 rate reenforced by the Postglacial rise of 

 ocean level — it would have to-day formed a 

 sea-level barrier reef enclosing a fine lagoon; 

 and it would have thus imitated the barrier 

 reef which surrounds Tahiti, where the island 

 spurs are strongly cut off in plunging cliffs 

 between embayed and mostly delta-filled valley 

 mouths, thus indicating that the visible barrier 

 reef of Tahiti, like the submerged barrier reef 

 of Tutuila, has grown up from an abraded, 

 cliff-backed platform. It may be parentheti- 

 cally added that the form of the larger valleys 

 of Tahiti, now embayed and delta-filled, sug- 

 gests some such measure as 600 or 800 feet 

 for the submergence of the inferred cliff-base 

 platform ; also, as the Tahiti reef now reaches 

 sea level and as most of the drowned-valley 

 embayments there are filled with deltas, that 

 island must have been submerged less rapidly 

 and less recently than Tutuila. And to this it 

 may be added that the submergence of Tutuila 

 must, as already noted, have been less recent, 

 but perhaps not more rapid than that of the 

 Marquesas, where not even fringing reefs are 

 yet formed; and finally that local subsidence 

 of these different islands, varying in rate and 

 in amount from place to place, and not a syn- 

 chronous and uniform rise of the ocean, must 

 be taken as the cause of their non-synchronous 

 and non-uniform submergences. 



But if this view concerning barrier reefs be 

 correct, it might be objected that neither the 

 submerged barrier reef of Tutuila nor the 

 sea-level barrier reef of Tahiti was formed 

 according to Darwin's theory, because ac- 

 cording to that theory — as it is usually 



quoted — barrier reefs are supposed to have 

 developed from on-shore fringing reefs, 

 while the Tutuila and Tahiti barrier reefs ap- 

 pear to have developed from off-shore, plat- 

 form-margin reefs. Yet even this contingency 

 is provided for in Darwin's wonderfully well 

 reasoned discussion, as may now be briefly 

 pointed out. 



It is true that Darwin's type figure repre- 

 sents the initial stage of reef growth as an 

 on-shore fringe around a rather steeply sloping 

 island border; and that the fringe is trans- 

 formed, as subsidence progresses, first into a 

 barrier reef and later into an atoll; and from 

 this it has been generally inferred that, when 

 Darwin described barrier reefs and atolls as 

 developed from fringing reefs, on-shore fringes 

 must have been meant. But a closer exami- 

 nation of his text leads to a different con- 

 clusion. His chapter on fringing reefs de- 

 fines them in a manner that appears to have 

 been generally overlooked. He included in 

 that chapter not only reefs closely attached to 

 the shore of their islands, but also other reefs 

 " not closely attached." Several off-shore reefs 

 on the shelving sea floor of Mauritius and off 

 the east coast of Africa are there presented as 

 examples of detached fringing reefs: 



On the western side of Mauritius . . . the reef 

 generally lies at the distance of about half a mile 

 from the shore; but in some parts it is distant 

 from one to two, and even three miles (52). 



Again, op the eastern coast of Africa, 



For a space of nearly forty miles, from lat. 

 1° 15' to 1° 45' S., a reef fringes the shore at an 

 average distance of rather more than a mile, and 

 therefore at a greater distance than is usual in 

 reefs of this class. ... In the plan [small chart] 

 of Mombas (lat. 4° S.) a reef extends for thirty- 

 six miles, at the distance of from half a mile to 

 one mile and a quarter from the shore (56). 



None of these off-shore reefs has " an in- 

 terior deep-water channel," but only a 

 shallow one. It is therefore by the ab- 

 sence of a deep lagoon and in spite of the de- 

 tachment of these reefs from the shore that 

 they are, in Darwin's terminology, classed as 

 fringing reefs and distinguished from barrier 



