36 A NATURALIST'S WANDERINGS 



same height, extending over areas of many thousand square 

 miles, there is but one alternative ; namely, the prolonged svhsi- 

 dence of the foundations on which the atolls were primarily based, 

 together with the upward growth of the reef -constructing corals." * 

 Since Mr. Darwin published this theory, several expeditions 

 expressly directed towards the examination of the floor of the 

 great oceans have taken place, prominent among them being 

 the United States Exploring Expedition, the Tuscarora, the 

 Blake, and our own Challenger voyages. These have put us in 

 possession of a large body of facts scarcely guessed at Avhen 

 Mr. Darwin broke deep ground on this subject. Mr. Dana, 

 Professor Semper, Professor Agassiz and Mr. Murray of the 

 Challenger staff, have also specially made coral reefs a subject 

 of study. These three last named investigators have shown 

 that the explanation of coral reef formation may be in other 

 causes than those of elevatiqn and subsidence. Great submarine 

 banks have been discovered, " covered by deposits of Pteropods 

 and Globigerina ooze serving as foundations for barrier reefs 

 and atolls, while their volcanic substratum has been completely 

 hidden." "The fact that these great submarine banks of 

 modern limestone lie in the very track of the great oceanic 

 currents sufficiently shows that these currents hold the immense 

 quantity of carbonate of lime needed in the growth of the 

 banks. . . . Mr. Murray has shown that if the pelagic fauna 

 and flora extend . . ., as experiments seem conclusively to 

 prove, to a depth of 100 fathoms, we should have 16 tons of 

 carbonate of lime for every square mile 100 fathoms deep. 

 But the greater the depth at which these plateaux begin to 

 form, the less rapid must be their formation. Deep water 

 itself being, as Professor Ditmar has recently shown,t a greater 

 solvent (not from, as has been held, its containing a much greater 

 proportion of free carbonic acid, but because of its depth,) than 

 shallower water, would dissolve up all the lighter and thinner 

 calcareous shells and debris; while in less deep water, the dead 

 siliceous and calcareous shells .of Foraminifera, Sponges, Hy- 

 droids, Corals, MoUusca, etc., would accumulate and build up 

 these plateaux," with a calcareous conglomerate. " Whenever 



» ' The Stiuctnie and Distribution of Coral Reefs,' b^- Chailes Darwin, 

 1842, pp. 146-7. The itaHcs are the present author's.' 



f Official Eeport of the Scientific Results of Iho Toj-age of H.M.S. 

 Challenger: Physics and Chemistry. Vol. I.- 



