Cii. VI. DISTRIBUTION OF CORAL-REEFS. 
173 
time its former angle relatively to the upper bank of 
living corals. Now it seems highly probable that a 
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 3 r ear 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 
full height, w r ould 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 he 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 seemsmore 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. 
