J.D. Dana on Changes of Level in the Pacifie Ocean. 159 
clusion either one way or the other, as is done by Darwin, from 
the fact that the reefs are small or wholly wanting, until the pos- 
sible operation of the several causes limiting their distribution 
has been duly considered. 
The influence of volcanoes in preventing the growth of zoo- 
phytes, extends only so far as the submaritie action may heat the 
water, and it may, therefore, be confined within a few miles of a 
Volcanic island, or to certain parts only of its shores. 
_ There are three epochs of changes in elevation which may be 
distinguished and separately considered. 1. The subsidence in- 
dicated by atolls and barrier reefs. 2. Elevations during more 
a rocky island that lies buried beneath the waves. Through 
the equatorial latitudes, such marks of subsidence abound, from 
the Eastern Paumotu to the Western Carolines, a distance of 
toup, Samoa, and the Salomon Islands to the Pelews, it will 
form nearly a straight boundary trending N. 70° W., running be- 
measure the thickness of the coral constituting it, would inform 
y 
