68 THE BERMUDA ISLANDS. 



An alternate movement of elevation and subsidence is no 

 more strange over an oceanic area than it is on the continental 

 borders. Yet we have here almost everywhere evidences of a 

 differential movement, and no geologist has for a moment ex- 

 pressed surprise at the manifestation. What then is the 

 anomaly of the occurrence of such movements in a coralline sea ? 

 How is the conception of subsidence antagonized by the facts 

 of elevation? If we conceive of an atoll, with a deep lagoon, 

 once having been formed through subsidence, what is to pre- 

 vent a succeeding elevation from lifting parts of this atoll, or 

 for that matter, the entire atoll-ring, above the water? We 

 could still have the lagoon of subsidence retained, and yet as a 

 last record of movement we would have merely the evidence 

 of elevation. Because a certain structure is formed through 

 subsidence it does not follow that this subsidence should not 

 be followed by elevation. This is but the order of things we 

 find everywhere expressed in the history of continental masses. 

 Indeed it would be but natural to look for local oscillations in 

 regions of extensive movement. Mr. Bourne lays great stress 

 upon the evidences of elevation (of a few feet) which are pre- 

 sented by Diego Garcia, and claims them to be conclusive 

 against "the idea of any subsidence being in progress, as Mr. 

 Darwin fancied to be the case in the Keeling atoll"*. I con- 

 fess that I can find nothing in this evidence which would pre- 

 clude an assumption of subsidence sufficiently recent to have 

 produced the characteristic atoll form. We have in the elevated 

 beach-rock of the Bermudas unequivocal evidences of elevation, 

 but equally conclusive are the evidences of the subsidence 

 which followed this elevation. In other words we have here 

 the conditions of Diego Garcia simply reversed. Again, in re- 

 gions where, as in that represented by the great Ghagos Bank, 

 it might be assumed that " drowned " atolls have been formed 

 as the result of too rapid subsidence, a change of movement 

 would be all but certain to develop reefs of elevation in combi- 

 nation with those which are assumed to bear in their structure 



*Loc. cit., p. 446. 



