AGASSIZ : BAHAMAS. 187 



tween the Mira por vos Bank and Crooked Island Bank, or in the passage 

 between the outlymg banks to the south of the Bermudas. 



Granting that the deep chainiels have been formed by subsidence, it 

 does not follow that the present distribution and existence of the corals 

 on the summits of the banks is due to the same cause. 



In fact, we may apply, only inverting the argument, the reasoning of 

 Professor Dana against a theory of elevation as explaining the origin of 

 barriers and atolls : " Coral reefs may, like sea beaches, be made at dif- 

 ferent heights on the slopes of rising land; but this is not tlie result of 

 elevation, which is implied ; for barrier reefs and atolls are the objects 

 whose origin is to be accounted for." Substitute subsidence for rising 

 land, and remembering that reef corals will not grow at a greater depth 

 than twenty fathoms, we eliminate subsidence as a factor, unless we are 

 prepared to accept and imagine a synchronism between the growth of 

 corals and subsidence in a great number of the districts in \Yhich they 

 flourish, — of which we have no proof. 



