J. D. Dana — Origin of Coral Beefs and Islands. 181 



ward. Mr. Crosby argues that the great thickness of the now- 

 elevated reefs could have been produced only " during a pro- 

 gressing subsidence," so that " we have apparently no recourse 

 but to accept Darwin's theory." 



C. It has been urged by Mr. Semper, Dr. J. J. Eein, Mr. 

 A. Agassiz, Mr. Murray, Dr. Geikie-and others, that since the 

 growing calcareous deposits of the sea-bottom are slowly rising 

 toward the surface by successive accumulations of the shells 

 and other debris of marine species, they may have been built 

 up locally in various regions of the deep seas (as they actually 

 are now about some islands) until they were near enough to 

 the surface to become next a plantation of corals ; and that in 

 •this way, atolls became common within the area of the tropical 

 oceans. The method is regarded as setting aside subsidence, 



a. The advocates of this hypothesis have not pointed to such 

 a mound now approaching the ocean's surface on the western 

 border of the Gulf Stream, where the depth over the remark- 

 ably luxuriant region is least ; and none over any part of the 

 tropical Pacific. It is suggested that the Chagos Bank may be 

 one example ; but it is not known to be so. Professor Semper 

 states that he found evidence of pelagic life instead of modern 

 corals in the lower part of the elevated reefs of the Pelews. 

 Dr. Geikie cites from letters by Dr. Guppy in Nature of Nov. 

 29, Dec. 6, 1883, and Jan. 12, 1884, the fact that in elevated 

 reefs on the Salomon Islands, 100 to 1200 feet high, the coral 

 rock forms a comparatively thin layer over impure earthy 

 limestone abounding in foraminifers and other pelagic organ- 

 isms, such as Pteropods. Such observations have great 

 interest, but they only prove that, in coral-reef seas, corals will 

 grow over any basis of rock that may offer where the water is 

 right in depth, and do not nullify any of the evidences of sub- 

 sidence. This point should be kept before the mind in all 

 future study of coral-reef regions. Borings in coral islands, as 

 recommended on a former page, are the true means of investi- 

 gating it. 



b. The old hypothesis that atolls may have been built upon 

 the summits of submerged mountain-peaks or volcanic cones 

 at the right distance under water for growing reef-corals, or, if 

 not at the right level, brought up to it by other organic depo- 

 sitions, or down to it by abrasion, is urged by Mr. Murray. 



This writer observes that " the soundings of the Tuscarora and 

 Challenger have made known numerous sub-marine elevations : 

 mountains rising from the general level of the ocean's bed at a 

 depth of 2500 or 3000 fathoms up to within a few hundred 

 fathoms of the surface." But " a few hundred fathoms," if we 



