President's Address. 23 



and atolls may be formed without subsidence of the sea-floor. 

 Whether this has been the usual, or only an exceptional, 

 manner of their origin, is a question that will depend for its 

 solution upon whether or not it can be shown that there 

 are general phenomena which can only be explained by 

 subsidence. Three such phenomena may be adduced. 

 I am not aware of any others that deserve serious con- 

 sideration. 



1. One of the early difficulties which Darwin's explanation 

 appeared satisfactorily to solve, was the necessity for the 

 existence of so many peaks coming up from the depths of 

 ocean just to the zone in which reef-building corals live. No 

 cause was conceivable which could have so generally arrested 

 the upward growth or upheaval of these submarine heights 

 at the limit where coral-reefs might begin. And this 

 difficulty has always been looked upon as furnishing one 

 of the strongest arguments in favour of the theory of sub- 

 sidence, for that theory removes it, by showing how, in a 

 general submergence, peak after peak would sink and come 

 within the sphere of the operation of the reef-builders. 



The difficulty is met in a totally different way by those 

 who believe it to be more formidable in appearance than in 

 reality. They contend that, while it must not be forgotten 

 that many peaks do rise above the sea-level, and many 

 submarine banks still fall far short of reaching up to the 

 coral-zone, two powerful causes conspire to bring submarine 

 banks to a common uniformity of level at a short distance 

 below the surface of the ocean. On the one hand, those por- 

 tions of volcanic mountains that rise above the sea-level are 

 worn down by the atmosphere and the waves, and, unless 

 otherwise preserved, must eventually be reduced to the lower 

 limit of effective wave-action, which is probably nearly co- 

 incident with the lower limit of reef-builders. On the other 

 hand, submarine banks in tropical seas are built up towards 

 the surface by the accumulation of the aggregated remains of 

 plants and animals which live on the bottom, or fall down to 

 it from upper waters, and the magnitude of this upward 

 growth is hardly yet adequately realised. 



In balancing these opposite views we must, I think, admit 



