in the Pacific y ^c. 103 



provided the depth was not too great, the polypes would soon 

 by the dififusion of their gemmules, extend themselves over 

 this portion of the reef, the same as elsewhere. Assuming 

 the depth of the channel at that period to have been from 

 twenty to forty feet, and the upward increase of the coral 

 therein, to have been ever since equally rapid with that on 

 the reef, it is clear that the original difference of level, be- 

 tween this latter and the bed of the channel would remain 

 unchanged to this hour, were there no tide whatever setting 

 in or out. Even supposing that before the action of the fresh 

 water ceased, the subsidence had been so great that this dif- 

 ference of level amounted to between two and three hundred 

 feet, (though it is very doubtful whether such could ever 

 have been the case, since the amount of elevation from the 

 deposition of alluvial matter, would in all probability be suffi- 

 cient to counterbalance the depression by subsidence,) this 

 would not invalidate the explanation here suggested. 



In all the Paumotus that I have seen, these channels are 

 very narrow, often but a few feet, and rarely exceeding forty 

 or fifty yards in width. Even in the semi-encircling reefs of 

 Tahiti, Samoa and Hawaii, they are seldom more than a few 

 rods across. Supposing then the bed of a channel to have 

 been at any period so far below the surface as to preclude the 

 formation of coral upon it ; still, in process of time, the lateral 

 increment would form a bridge across, at the depth best adapt- 

 ed to the requirements of the zoophytes. During the same 

 period its width would be considerably contracted at the sur- 

 face, and except in large openings, might be wholly closed up. 

 Instances of these bridsfed channels are numerous throughout 

 the coral islands of Polynesia. It frequently happens that 

 the approaching shelves have not yet come in contact, and a 

 crevice from only a few inches to a yard in breadth, is left. 

 On looking down this, the bottom and sides of the ancient 

 channel, are seen as distinctly through the transparent water, 

 as if within reach of the hand. Such crevices are unusually 

 frequent at Rose Island. An examination of these has led 

 me to conclude that the existing entrances into lagoons, are 

 attributable only to their original magnitude having been such 



