THE CORAL-REEF PROBLEM. 65 



an increased food-supply, it may be reasonably doubted if this 

 condition could obtain in the open ocean away from a land 

 area, inasmuch as by far the greater quantity of the food-sup- 

 ply would be given to the polyps as a direct down-pouring 

 from above, and independently, or nearly so, of any currental 

 action. It is true that the outer polyps or colonies would be 

 favored by having an extra supply on their exposed bor- 

 ders, but this would tend probably in the majority of cases 

 only to lateral extension, or to lateral extension combined 

 with upward growth — in other words, to a simple turbinated 

 growth with a nearly flat top. It is true that in a few instances, 

 as has been noted by Semper and Darwin, colonies, of Porites, 

 having a turbinated form, exhibit a raised border or lip, but it 

 is equally true that in by far the greater number of cases the 

 individual larger colonies assume either a clavate or a hemi- 

 spherical form, the latter condition being also distinctive of 

 the giant brain-corals. Mr. Bourne, from his researches on 

 the Diego Garcia reef, also dismisses the notion that food-con- 

 veying currents are especially instrumental in shaping the 

 reefs, and he points out that frequently the most elevated side 

 of an atoll is turned away from such currents, and, again, that 

 a large number of coral islands are placed entirely to one side, 

 or out of the path, of the prevailing ocean current. 



But even granting that through some method of accelerated 

 growth on the exterior an elevated bounding ring sho'ild be 

 formed, the difficulty in accounting for tlie existence of the 

 deep lagoon would in no wise be lessened ; for, in the first 

 place, no such ring would be formed below the line of coral 

 growth, and we should consequently be compelled to assume 

 as antecedent to its formation the complete upward growtii or 

 elevation of the submerged bank to the true coral zone, or to 

 a greatest possible depth beneath the surface of 100 or 120 

 feet. Manifestly, under such conditions there could be no 

 deep depression corresponding to lagoons of 200 or 300 feet 

 depth, unless these were subsequently formed by means other 

 than solution. Furthermore, it appears that the true energy 



