1836.] CHANGES Iff COPAT.-REE^. 34) 



by the heaping of fragments and sand on the wall-like barrier-reef; 

 these facts, and some analogous ones, led me to believe that this island 

 must lately have subsided and the reef grown upwards: here again 

 earthquakes are frequent and very severe. In the Society Archipelago, 

 on the other hand, where the lagoon-channels are almost choked up, 

 where much low alluvial land has accumulated, and where in some 

 cases long islets have been formed on the barrier-reefs facts all show- 

 ing that the islands have not very lately subsided only feeble shocks 

 are most rarely felt. In these coral formations, where the land and 

 water seem struggling for mastery, it must be ever difficult to decide 

 between the effects of a change in the set of the tides and of a slight 

 subsidence : that many of these reefs and atolls are subject to changes 

 ot some kind is certain ; on some atolls the islets appear to have in- 

 creased greatly within a late period ; on others they have been partially 

 or wholly washed away. The inhabitants of parts of the Maldiva 

 Archipelago know the date of the first formation of some islets ; in other 

 parts, the corals are now flourishing on water-washed reefs, where 

 holes made for graves attest the former existence of inhabited land. It 

 is difficult to believe in frequent changes in the tidal currents of an 

 open ocean ; whereas, we have in the earthquakes recorded by the 

 natives on some atolls, and in the great fissures observed on other 

 atolls, plain evidence of changes and disturbances in progress in the 

 subterranean regions. 



It is evident, on our theory, that coasts merely fringed by reefs 

 cannot have subsided to any perceptible amount ; and therefore they 

 must, since the growth of their corals, either have remained stationary 

 or have been upheaved. Now it is remarkable how generally it can be 

 shown, by the presence of upraised organic remains, that the fringed 

 islands have been elevated; and so far, this is indirect evidence in 

 favour of our theory. I was particularly struck with this fact, when I 

 found, to my surprise, that the descriptions given by MM. Quoy and 

 Gaimard were applicable, not to reefs in general as implied by them, 

 but only to those of the fringing-class ; my surprise, however, ceased 

 when I afterwards found that, by a strange chance, all the several 

 islands visited by these eminent naturalists, could be shown by their 

 own statements to have been elevated within a recent geological era. 



Not only the grand features in the structure of barrier-reefs and of 

 atolls, and of their likeness to each other in form, size, and other 

 characters, are explained on the theory of subsidence which theory 

 we are independently forced to admit in the very areas in question, 

 from the necessity of finding bases for the corals within the requisite 

 depth but many details in structure and exceptional cases can thus 

 also be simply explained. I will give only a few instances. In barrier- 

 reefs it has long been remarked with surprise, that the passages through 

 the reef exactly face valleys in the included land, even in cases where 

 the reef is separated from the land by a lagoon-channel so wide and so 

 much deeper than the actual passage itself, that it seems hardly possible 

 that the very small quantity of water or sediment brought down could 

 Injure the corals on the reef, Now, every reef of the fri rising-class is 



