274 CORALS AND CORAL LSLANDS. 



C. Probable evidence of subsidence now in progress. 



I. An atoll reef without green islets, or loith but 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 wholly 

 ceased. 



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

 appears that where there are 7io barrier reefs, and only fringing 

 reefs, the corals may afford 7io evidence of subsidence. But 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 dis- 

 tribution 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. 



There are two epochs of changes in elevation which may be 

 here distinguished and separately considered, i. The subsi- 

 dence indicated by atolls and barrier reefs. 2. Elevations 

 during more recent periods. 



II. SUBSIDENCE INDICATED BY ATOLLS AND BARRIER REEFS. 



Looking at atolls as covering buried islands, we observe, 

 that through the equatorial latitudes such marks of subsidence 

 abound, from the Eastern Paumotu to the Western Carolines, 

 a distance of about six thousand geographical miles. In the 



