Ch. VI. DISTRIBUTION OF CORAL-REEFS. 171 



change of any kind in the outer submarine slope of an 

 island would influence the height to which the living 

 corals on the margin would be constantly bathed by the 

 surf, and to which they would consequently be able to 

 grow. Again, it seems possible that if during one season 

 of the year the currents of the sea and the prevalent 

 winds coincided in direction, the waves would then reach 

 to a higher level and the corals grow higher, than at 

 another season when the currents and the winds did not 

 coincide in direction. The result would be that the 

 corals which during the one season had grown to their 

 Ml height, would at the other season expose their dead 

 summits, and give the appearance of the land having 

 been slightly elevated. I have referred to these possi- 

 bilities merely to show how difficult it must ever be to 

 judge whether low coral formations have really been 

 raised to a height of only two or three feet, as Dana 

 believes to have been the case with several groups of 

 atolls. To me it seems more probable that all the above- 

 mentioned appearances merely indicate that the atolls 

 in question have long remained at the same level. If, 

 however, the conclusion arrived at by so excellent an 

 observer as Professor Dana, should hereafter be con- 

 firmed, the question will arise, seeing how immense an 

 area has been thus affected, whether those geologists are 

 not right who believe that the level of the ocean is 

 subject to secular changes from astronomical causes. 



