IV. 



THE CORAL-REEF PROBLEM. 



Perhaps no class of phenomena has been so frequently ap- 

 pealed to in evidence of subsidence on a grand scale as that 

 presented in the formation of coral reefs. Scattered freely over 

 a large expanse of the oceanic surface, these structures consti- 

 tute features there as distinctive and prominent as do the 

 mountain masses on the continents. Rising in most cases 

 from a deep sea, and with a limited extent, their presence, as 

 organic accumulations, immediately suggests peculiarities of 

 geological construction which are to be found in no other 

 form of relief. It is an ascertained fact, as has been variously 

 demonstrated that the conditions governing the existence and 

 distribution of reef-building corals (Porites, Diploria, Msean- 

 drina, Madrepora, Tubipora, Fungia, Astrsea, etc.) are drawn 

 within narrow limits, and that they are equally of a general 

 and of a special character. Broadly stated these conditions 

 are : A surface temperature of the water never falling below 

 70 or 68 F. ; an absence of muddy sediment ; freedom from 

 contact with fresh waters ; the necessity, in some cases, of surf 

 action. Accordingly, we find that reef-structures are practi- 

 cally confined to the warm tropical or subtropical seas, and 

 that they are largely wanting in tracts where exceptional cold 

 currents have wedged a path into the warmer waters, or where, 

 as at the mouths of outflowing streams, there is a free dis- 

 charge of both freshwater and sediment. To this must be ad- 

 ded the all-important fact that the reef-building corals are con- 

 fined to a superficial zone of the sea not exceeding 100 or 120 



