320 CORALS AND CORAL ISLANDS. 



Tahiti could not subside even a few scores of feet without 

 changing the even outline into one of deep coves or bays, the 

 ridges projecting out to sea on every, side, like the spread legs 

 of a spider. On the contrary, the absence of such coves, or 

 deep-valley bays, may be evidence that no subsidence has taken 

 place, or only one of comparatively small amount. 



C. Probable evidence of subsidence now in progress. 



1. A.n atoll reef without green islets, or with hut few small 

 spots of verdure. — The accumulation requisite to keep the reef 

 at the surface-level, during a slow subsidence, renders it im- 

 possible for the reef to rise above the waves and supply itself 

 with soil, unless the subsidence is extremely slow, or has whol- 

 ly ceased. 



From the above review of evidences of change of level, it 

 appears that where there are no harrier reefs, and only fring- 

 ing reefs, the corals may afford no evidence of subsidence. Bat 

 it does not follow that the existence of only fringing reefs, or 

 of no reefs at all, is proof against a subsidence having taken 

 place. For we have elsewhere shown that through volcanic 

 action, and at times other causes, corals may not have begun 

 to grow till a recent period, and, therefore, we learn nothing 

 from them as to what may have previously taken place. 

 While, therefore, a distant barrier is evidence of change of 

 level, we can draw no conclusion either one way or the other, 

 as is done by Darwin, from the fact that the reefs are small 

 or wholly wanting, until the possible operation of the several 

 causes limiting their distribution has been duly considered. 



The influence of volcanoes in preventing the growth of 

 zoophytes extends only so far as the submarine action may 

 heat the water, and it may, therefore, be confined within a few 

 miles of a volcanic island, or to certain parts only of its shores. 



