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On Changes of Level in the Pacific Ocean. 

 By J. D. Dana, Esq. 



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

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

 deposits, in the character of the igneous rocks, and in the 

 features of the surface. The points of evidence are as 

 follow : — 



A. Evidences of Elevation. 



1. Patches of coral reef, or deposits of shells and mud 

 from the reefs, above the level where they are at present 

 forming. — The coral-reef rock has been shewn 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 groiv above ordinary low tide, is about one sixth 

 the height of the tide, though it seldom attains this height. 



Beach accumulations of large 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 com- 

 monly without a distinguishable fragment of coral or shell ; 

 and in most of these particulars they are distinct from reef 

 rocks. (Pp. 369, 370, vol. xi.) 



2. Sedimentary deposits or layers of rolled stones inter- 

 stratified among the igneous layers. 



3. Compactness of the igneous rocks. — The great un- 

 certainty of this kind of evidence has been shewn 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. 



