318 UOUALS AND CORAL ISLANDS. 



CHAPTER V. 



ON OHANG-ES OF LEVEL EST THE EAOIEIO 



OCEAN. 



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 increase, 

 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 on an 

 island at a higher level would be proof of an elevation of the 

 land. 



