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Sect. VI.] 



GEOLOGY. 



193 



for instance, by stumps of trees, the fouudation-posts of 

 sheds, by wells or graves or other works of art, now stand- 

 ing beneath the level of high- water mark, and which there 

 was good reason to believe must have once stood above 

 its level. The observer must bear in mind that cocoa-nut 

 trees and mangroves will grow in salt-water. If such 

 evidence be found, inquiry ought to be made whether 

 earthquakes have been felt. On the other hand, all 

 masses of coral standing so much above the level of the 

 sea that they could not have been thrown up by the 

 breakers during gales of wind, at a period when the reef 

 had not grown so far out seaward, should be investigated 

 and their height measured. There is reason to believe 

 that some coral-reefs have been thought to have been 

 upraised, owing to the effect of the lateral or horizontal 

 extension of the reefs having been overlooked ; for the 

 necessary result of this outward growth is gradually 



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to break the force of the waves. 



so that the rock 



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now further removed from the outer breakers, become 

 worn to a less height than formerly, and the 



more 



mland corals not being any longer constantly washed by 

 the surf, cease to live at a level at which they once flou- 

 rished. It is indispensable that specimens of all upraised 

 corals, and especially of the shells generally associated 

 with thorn, should be collected ; for there can be no doubt 

 that ancient strata containing corals, have in some in- 

 stances been confounded with recent coral-rock. The 

 importance of ascertaining whether coral-reefs have un- 

 dergone, or are undergoing, any change of level, depends 

 on the belief that all the characteristic differences be- 



Atolls and Encircling reefs on the one hand, and 



tween 



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