PROFESSOR J. P. n.4A^A\S E]'I PENCE. 89 



sand out of the southern part, and thus leave an area particularly favourably situated for the growth of corals. Mr. 

 Murray points out that larger atolls generally have deeper lagoons than small atolls, and urges this fact in support of 

 his theory ; but here again the facts in the C'hagos group are against him. \'ictory Bank is a submerged atoll ; the 

 Solomons is an atoll with a large extent of dry land. In each the lagoon attains a depth of seventeen- — eighteen 

 fathoms, and in Diego Ciarcia the lagoon, although far larger, does not attain a greater depth. Peros B.iuhos is far 

 smaller than the Great Chagos Bank, yet in both the lagoons attain nearly the same maximum depth, viz., forty- 

 one fathoms for Peros Bauhos, forty-four fathoms for the Great Chagos Bank. Speaker's Bank is very little larger 

 tlian I'eros Bauhos : its lagoon is far shallower, having a maximum depth of twenty-four fathoms. ' " 



By way of placin.a^ on record some of the testiinony in direct support of Mr. Darwin's 

 subsidence theory, which was brought forward in response to the interpretation advocated b\' Dr. 

 Murray, a short extract may be made from a paper contributed by Professor J. D. Dana, " On 

 the Origin of Coral-Reefs and Islands," to the American Journal of Science for the year 18S5. — 



In this communication Professor Dana "calls special attention to the eastern half of tlie Fiji .Vrchipelago 

 where several of the great barrier-reefs from ten to twenty miles long have but one or two emergent peaks of land ; 

 Nanuku, for instance, has one little point near its south-eastern angle, a mile of peak within a barrier island, 200 

 square miles in area. Bacon's Isles are the last two little peaks of a still greater lagooiv ... A dozen of the 

 easternmost islands are actually atolls — the last peak is gone. But in case it should be answered that these are 

 the emergent portions of submarine volcanoes, in which case the ring-shaped barriers l>ecome difficult of explanation, 

 while they are easy on the theory of subsidence, Professor Dana adds, that movement in this direction is proved 

 by the existence of deep fiord-like indentations in rocky coasts of islands, both of those inside of barriers, and 

 those not bordered by reefs. As examples of this structure, generally admitted to be one of the strongest evidences 

 of subsidence all the world over, he quotes the Marquesas Islands with the Gambler and the Hogoleu Islands, 

 Raiata, Bolabola, and the Tahiti group, and the Exploring Isles of the Fijis. Professor Dana also calls attention to 

 the general parallelism between the average trends of coral islands and the courses, not only of the groups of which 

 they form part, but also of the groups of high islands, not far distant, and refers to the arguments drawn by Mr. 

 Darwin from the fact that the larger coral islands have the same diversity of form as is found in the barrier-reefs of high 

 islands, and exhibit grouping such as would result from the sinking of a large island of ridges and peaks with 

 encircling reefs. The depth of the lagoon, and of the channels inside of barrier-reefs — in many cases two or three 

 times greater than twenty fathoms — is very difficult to explain if there has been no subsidence, so is that of the ocean 

 near to atolls.'' 



Among the more important evidence, recently adduced, that gives direct support to 

 Mr. Darwin's subsidence interpretation of barrier and atoll-reef construction, reference 

 may be made to a second communication contributed by Captain Wharton, R.N., to Natnre, 

 Vol. XXXVI., p. 413. This communication referred to a recent survey made by Captain 

 Maclear, R.N., in H.M.S. Flying Fish, of the small island of Masamarhu, in the Red Sea, and 

 embodies two sections, true to scale, of the coral-reefs that surround it. These sections, through 

 the kind courtesy of the publishers, Messrs. Macmillan & Co., are herewith reproduced, together 

 with the explanatory description of them given in Professor Bonney's appendix to Mr. Darwin's 

 work. — 



