36 



A NATUMAlIsrS WANDEniNOS 



sftTiie height, extending over areas of many thousand square 

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

 dpnee of the foundatioits on wlneh the atolls were lyrimariJy haml, 

 together with the upward growth of the reef-construciing eoraU.^^ * 

 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 Espodition, the Tt(scm*ora, the 

 Blahe, and our own Chtllenger voyages. These have put us in 

 possession of a large l>ody of facts scarcely guessed at when 

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

 Professor Semper, Professor Agassiz and Mr. Mntraj of the 

 Challenger staff, have also specially made C4>ral reefs a subject 

 of study. These three *ast named investigators have showTi 

 that the explanation of coral reef formation may be in other 

 causes than those of elevation and subsidence* Great submarine 

 banks have l)een 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 

 modem limestone lie in the very track of the great -oceanic 

 currents suflieiently shows that these currents hold the immense 

 quantity of carl)onate of lime needed in the growth of the 

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

 and flora extend . , as experiments seem conclnsively to 

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

 carljonate of lime for every sqnare mile 100 fathoms deep. 

 But the -greater the depth at w^hich these plateaux begin to 

 form, tl»e less rapid must be their formation. Deep water 

 itself being, as Professor Ditmar has rec<^ntly shown,t a greater 

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

 proportion of free carbonic acid, but l»ecause of its depth,) than 

 shallower water, would dissolve up all the lighter and thinner 

 calcareous shells and (Uhris; while in less deep w^ater, the dead 

 siliceous and calcareous shells of Fcjraminifera, Sponges, Hy- 

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

 these plateaux," with a calcareous conglomerate, " "Whenever 



* "Hie Btnicttirc and Disfrilnjtion of Coml Reefs,' by Charles Darwin, 

 1841!, 146-7. TIic italics are the present ftutlior's. 



f Official Beport nf the Scientific Keen Its of the Voyage of H.M.S. 

 C%nf}ej}ger : Physii^ and f MieiniRtry. YoL I. 



