THE VOYAGE OF THE BEAGLE 501 



attention of voyagers, as well as in the no less wonderful 

 barrier-reefs, whether encircling small islands or stretching 

 for hundreds of miles along the shores of a continent, are 

 simply explained. 



It may be asked, whether I can offer any direct evidence 

 of the subsidence of barrier-reefs or atolls; but it must be 

 borne in mind how difficult it must ever be to detect a move- 

 ment, the tendency of which is to hide under water the part 

 affected. Nevertheless, at Keeling atoll I observed on all 

 sides of the lagoon old cocoa-nut trees undermined and fall- 

 ing; and in one place the foundation-posts of a shed, which 

 the inhabitants asserted had stood seven years before just 

 above high-water mark, but now was daily washed by every 

 tide: on inquiry I found that three earthquakes, one of them 

 severe, had been felt here during the last ten years. At 

 Vanikoro, the lagoon-channel is remarkably deep, scarcely 

 any alluvial soil has accumulated at the foot of the lofty 

 included mountains, and remarkably few islets have been 

 formed 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 fre- 

 quent 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 showing 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 of some kind is certain; on some atolls the islets 

 appear to have increased 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 inhab- 

 ited land. It is difficult to believe in frequent changes in the 



