ELEVATION AND SUBSIDENCE OF LAND. 147 



live. Thus, in whatever situation the skeleton of a 

 Madrepore or Millepore may be found, it is certain 

 that it must have grown within thirty fathoms of the 

 surface of the ocean. When it coats the summit of 

 the lofty mountain of Tahiti, where Mr. Stutchbury 

 found a regular stratum of semi-fossil coral at 5000 

 and 7000 feet above the level of the sea, it must have 

 been lifted up by the elevation of the rock on which 

 it was originally deposited. If it is brought up from 

 the depth of 200 or 300 fathoms, as at Cardoo Atoll, 

 or Keeling Atoll, it must have been dragged down to 

 that depth by a gradual subsidence of the foundation 

 on which the living Madrepore once nourished. It is 

 by these movements of upheaval and subsidence of the 

 earth's crust that Mr. Darwin explains the different 

 forms which the coral reefs assume. Elizabeth Island, 

 which is eighty feet high, is entirely composed of 

 coral rock. The coral animals, thus progressively 

 lifted up above their element, are compelled to carry 

 on their operations more and more remote from the 

 former theatre of their constructive energies, but can- 

 not extend deeper than their allotted thirty fathoms ; 

 the direction of their submarine masonry is therefore 

 centrifugal and descending. Where the land that 

 supports them is, on the contrary, in a state of sub- 

 mergence, they are compelled to build their edifices 

 progressively higher, and in a narrower circuit; in 

 other words, their growth is centripetal and ascend- 

 ing; the terms f ascending' and ' descending/ of 

 course, only applying in this case to the relation of 

 the coral builders to the unstable land, not to the level 

 of the unchanging sea. 



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