THE VOYAGE OF THE BEAGLE 507 



archipelago, which consists of a group of atolls upheaved 

 and since partially worn down, two volcanos, and perhaps 

 more, are historically known to have been in action. On the 

 other hand, although most of the islands in the Pacific which 

 are encircled by barrier-reefs, are of volcanic origin, often 

 with the remnants of craters still distinguishable, not one of 

 them is known to have ever been in eruption. Hence in these 

 cases it would appear, that volcanos burst forth into action 

 and become extinguished on the same spots, accordingly as 

 elevatory or subsiding movements prevail there. Numberless 

 facts could be adduced to prove that upraised organic remains 

 are common wherever there are active volcanos; but until it 

 could be shown that in areas of subsidence, volcanos were 

 either absent or inactive, the inference, however probable in 

 itself, that their distribution depended on the rising or falling 

 of the earth's surface, would have been hazardous. But now, 

 I think, we may freely admit this important deduction. 



Taking a final view of the map, and bearing in mind the 

 statements made with respect to the upraised organic remains, 

 we must feel astonished at the vastness of the areas, which 

 have suffered changes in level either downwards or upwards, 

 within a period not geologically remote. It would appear 

 also, that the elevatory and subsiding movements follow 

 nearly the same laws. Throughout the spaces interspersed 

 with atolls, where not a single peak of high land has been 

 left above the level of the sea, the sinking must have been ' 

 immense in amount. The sinking, moreover, whether con- 

 tinuous, or recurrent with intervals sufficiently long for the 

 corals again to bring up their living edifices to the surface, 

 must necessarily have been extremely slow. This conclusion 

 is probably the most important one which can be deduced 

 from the study of coral formations ; and it is one which it is 

 difficult to imagine how otherwise could ever have been ar- 

 rived at. Nor can I quite pass over the probability of the 

 former existence of large archipelagoes of lofty islands, 

 where now only rings of coral-rock scarcely break the open 

 expanse of the sea, throwing some light on the distribution of 

 the inhabitants of the other high islands, now left standing 

 so immensely remote from each other in the midst of the 

 great oceans. The reef-constructing corals have indeed 



