836.] SUBSIDENCE OF CORAL-EEEFS. 



47S 



therefore, to affirm, that on the theory of the upward orowth of 

 the corals during the sinking of the land,* all the leading fea- 

 tures m those wonderful structures, the lagoon-islands or trolls 

 which have so long excited the 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 1 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 movement, the 

 tendency of which is to hide under water the part affected. Ne- 

 vertheless, at Keeling atoll I observed on all sides of the lagoon 

 old cocoa-nut trees undermined and falling; and in one place 

 he 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 remark- 

 ably deep scarcely any alluvial soil has accumulated at the foot 

 oi tne lofty included mountains, and remarkably few islets have 

 been formed by the heaping of fragments and sand on the wall- 

 ike 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 Ion- 

 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 



£££ $ *<£ C/StrS "5CS a r 7 i on of the •* 



expeditior differ with mo,, ~ rhe naturall sts, however, of this 



expeaitior ditter with me on some points respecting coral formations. 



