Changes of Level in the Pacific Ocean. 241 



2. Lagoon islands or atolls. 



3. Submerged atolls. 



4. Deep bay indentations in coasts, as the terminations of 

 valleys. — In the remarks upon the valleys of the Pacific 

 Islands, it has been shewn that they were in general formed 

 by the waters of the land, unaided by the sea ; that the sea 

 tends only to level off the coast, or give it an even outline. 

 When, therefore, we find the several valleys continued on 

 beneath the sea, and inclosing ridges standing out in long 

 narrow points, there is reason 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 smaller in amount. 



5. Sea-shore alluvial flats or deposits. 



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

 interruption beneath the water, 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 impossible 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 appears that when 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 hav- 

 ing taken place. For we have elsewhere shewn 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 distant barrier is evidence 

 of change of level, we can draw no conclusion either one way 



VOL. LV. NO. CX. — OCTOBER 1853. Q 



