374 PEECT SLADEN TEUST EXPEDITION. 



and frail frustules so numerous in tlie surface waters are never found in tlie deeper 

 deposits. 



In one of the preceding paragraphs the high salinity of the Red Sea, the Arabian 

 Sea, and the western half of the Indian Ocean was contrasted with the low salinity of 

 the Bay of Eengal and the seas about Sumatra and the China Sea. In the latter region 

 there is the largest rainfall in the world, and the low salinity is the result of the 

 enormous run-off from the land-masses along with the slow rate of evaporation. It is 

 in areas like this that great quantities of colloidal clayey matter are carried into the sea 

 from the land-surfaces and held in suspension, and from this suspended matter the 

 silica-secreting organisms appear to obtain the silica for their frustules and skeletons ; 

 hence Radiolarian Oozes are found in the greatest dejjths towards the eastern part of 

 the Indian Ocean. Generally it may be stated that silica-secreting plankton organisms 

 abound where sea-water is mixed with the run-off from land-surfaces — as off the em- 

 bouchures of rivers, in the Antarctic, and in the western tropical Pacific. 



Coral-Beefs. — The considerations which have been urged above regarding the solution 

 of dead pelagic shells as they fall through the water to the bottom of the ocean led me 

 during the ' Challenger ' Expedition to apply them to the explanation of the formation 

 of coral-reefs and islands. These views were published in a short paper in the year 

 1880 *, and I have not seen reason to change the ideas therein expressed. This view 

 of the origin of coral-atolls is often called the solution theory, but the solution of 

 the dead calcium carbonate structures is only a part of the theory. The mechanical 

 transport of material from the lagoons, the preparation of foundations, the building- 

 out of coral-reefs on a talus of their own formation were all clearly stated in the original 

 paper. 



It is a remai'kable fact that in recent years not one naturalist has been able to point 

 to a single atoll undoubtedly formed in the manner stated by Darwin. Funafuti has, 

 of course, been held up as an example, and in a recent work by Joly it is said : — "The 

 boring, which was carried to a depth of 185 fathoms by David, met with no founda- 

 tional layer of chalk or calcareous ooze. This should appear at about 30 fathoms down 

 if the foundation of the coral rock indeed rests upon ooze, and there had been no vertical 

 movement. But as only true coral rock was encountered the entire 1100 feet of descent, 

 clearly subsidence must be admitted. Although the true foundation was not reached 

 by boring, Darwin's view that there has been a subsidence is in this case established. 

 The coral rock now at a depth below sea-level of 1100 feet must once have been at or 

 near the surface " f . Now I do not in any way accept this statement. 



At one of the early meetings of the Funafuti Committee I urged tliat, when the 

 position of the bore was settled, each member of the Committee should write down what 

 he expected to find as the boring proceeded in depth, but no single member would agree 



* Proc. Eo}'. Soc. Edin., vol. x. p. 505. 



t See Joly, ' Radioactivity and Geology,' pp. 120-121 (London, 1909). 



