THE CORAL-REEF PROBLEM. 57 



it of a theory of simple coral upgrowth, with structural modi- 

 fications as depending principally upon currental action and 

 food-supply. 



This theory, like its alternative, presupposes as a first neces- 

 sary condition of coral growth the existence of a submarine 

 basement within the zone of coral life (1-20 fathonis). Upon 

 this, which may be the buried slope or the summit of a volcano, 

 or merely a bank, the coral animal develops and builds to the 

 surface. Where such a sub-structure does not immediately 

 exist, or rather does not extend to the zone within which reef 

 corals are hmited, it is claimed that suitable foundations may 

 be obtained through the building up of submarine volcanoes by 

 the deposition on their summits of organic and other sediments. 

 This would explain the apparent anomaly of coral structures 

 rising from depths vastly exceeding the lower boundaries of 

 coral growth, a condition which to Mr. Darwin necessitated 

 the assumption of subsidence. It is well known that through- 

 out the greater mass of the ocean there is a constant rajn or 

 down-pouring of organic particles in the form of the calcareous 

 and siliceous tests of Foraminifera, pteropods, diatoms, etc., 

 much of which goes to form the vast accumulation of white 

 mud (Atlantic or Globigerina ooze) which covers the greater 

 part of the oceanic floor. Manifestly, such an accumulation 

 must eventually acquire great thickness. It is more than 

 doubtful, however, if any very considerable thickness- of such 

 deposit has been built up during the existing period of coral 

 growth, or that an accumulation of this kind has materially 

 aided in building up the sub-coral buttresses of the deeper 

 seas. The investigations of Mr. Murray, deduced from data 

 obtained by the "Challenger," indicate that a column of 

 oceanic water of 600 feet depth, with a transverse area of one 

 square mile, contains some 16 tons of suspended organic par- 

 ticles; these, if precipitated to the floor of the sea, would make 

 a deposit rrsUji inch in thickness. It has thus far been im- 

 possible to determine the duration of life of the organisms 

 furnishing the organic particles, mainly Foraminifera, and 



