CHAPTER V. 



ON CHANGES OF LEVEL IN THE PACIFIC OCEAN. 



Evidences of change of level in the Pacific are to be looked 

 for in the height or condition of the coral reef formations or de- 

 posits ; in the character of the igneous rocks ; and in the features 

 of the surface. The points of evidence are as follows : — 



A. Evidences of elevation. 



1. Patches of coral reef or 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. 



Beach accumulations of lanre masses seldom exceed eight feet 

 above high tide, and the finer fragments and sand may raise the 

 deposit to ten feet. But with the wind and waves combined, or 

 on prominent points where these agents may act from opposite 

 directions, such accumulations may be thirty to forty feet in 

 height. These are drift deposits, finely laminated, generally 

 with a sandy texture, and commonly without a distinguishable 

 fragment of coral or shell ; and in most of these particulars they 

 are distinct from reef-rocks. (See pages 17 and 32.) 



2. Sedimentary deposits, or layers of rolled stones interstrati- 

 fied among the igneous layers. 



3. Compactness of the igneous rocks. — The great uncertainty 

 of this kind of evidence has been shown in another place. 



B. Evidences of Subsidence. 



1. The existence of wide and deep channels between an island 

 and any of its coral reefs ; or in other words, the existence of 

 barrier reefs. 



2. Lagoon Islands or Atolls. 



3. Submerged atolls. 



4. Deep bay-indentations in coasts as the terminations of val- 

 leys. — -In the remarks upon the valleys of the Pacific Islands, it 

 has been shown that they were in general formed by the waters 

 of the land, unaided by the sea \ that the sea tends only to level 



