ON CHANGES OF LEVEL IN THE PACIFIC OCEAN. 119 



off the coast, or give it an even outline. When therefore, we 

 find the several valleys continued on beneath the sea, and their 

 enclosing ridges standing out in long narrow points, there is rea- 

 son to suspect that the island has subsided after the formation of 

 its valleys. For such an island as 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. The absence of such 

 coves, on the contrary, is evidence that any subsidence which has 

 taken place, has been comparatively small in amount. 



5. Seashore alluvial flats or deposits. 



6. The lava surface of a volcanic island, sloping without in- 

 terruption beneath the ivater, instead of terminating in a shore 

 cliff of a hundred feet or so. 



C. Probable evidence of subsidence now in progress. 



1. An atoll reef without green islets, or with but few small 

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

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

 ble for the reef to rise above the waves unless the subsidence is 

 extremely slow. 



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

 pears that where there are no barrier reefs, and only fringing 

 reefs, the corals afford no 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 previously have taken place. While, therefore, a dis- 

 tant barrier is evidence of change of level, we can draw no con- 

 clusion either one way or the other, as is done by Darwin, from 

 the fact that the reefs are small or wholly wanting, until the pos- 

 sible operation of the several causes limiting their distribution 

 has been duly considered, 



The influence of volcanoes in preventing the growth of zoo- 

 phytes, 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 three epochs of changes in elevation which may be 

 distinguished and separately considered. 1. The subsidence in- 

 dicated by atolls and barrier reefs. 2. Elevations during more 

 recent periods, and also during the same epoch of subsidence. 

 3. Changes of level anterior to the atoll subsidence and the 

 growth of recent corals. On this last point, we have few facts. 



