304 CORALS AND CORAL ISLANDS. 
CHAPTER V. 
ON CHANGES OF LEVEL IN THE PACIFIC OCEAN.1 
I. EVIDENCES OF CHANGE OF LEVEL. 
It has been shown that atolls, and to a large extent other 
coral reefs, are registers of change of level. From the evi- 
dence thus afforded, the bottom of a large part of the Pacific 
Ocean is proved to have undergone great oscillations in recent 
geological time. In this direction, then, we find the grandest 
teachings of coral formations. In treating the subject we 
necessarily bring into connection with it evidences of change 
of level from other sources. The proofs of change of level 
here considered are the following : — 
A. Evidences of elevation. 
1. The existence on coral or other islands of patches of 
coral reef, and deposits of shells and sand from the reefs, 
above the level where they are at present forming. 
The coral reef-rock has been shown occasionally to in- 
crease, by growth of coral, to a height of four to six inches 
above low-tide level when the tide is but three feet, and to 
twice this height with a tide of six feet. It may, therefore, 
be stated as a general fact, that the limit to which coral may 
grow above ordinary low tide is about one sixth the height of 
the tide, though it seldom attains this height. Its existence 
1 The conclusions and nearly all the facts presented in this chapter are from 
the author’s Exploring Expedition Report, in which the subject is illustrated by a 
colored chart of the Pacific Ocean. 
