138 ON CORAL AND CORAL REEFS 



branching corals, and a profusion of CyatJwphyllia, or 

 cup-corals." 1 



Thus, in all the great periods of the earth's history 

 of which we know anything, a part of the then living 

 matter has had the form of polypes, competent to sepa- 

 rate from the water of the sea the carbonate of lime 

 necessary for their own skeletons. Grain by grain, and 

 particle by particle, they have built up vast masses of 

 rock, the thickness of which is measured by hundreds 

 of feet, and their area by thousands of square miles. 

 The slow oscillations of the crust of the earth, pro- 

 ducing great changes in the distribution of land and 

 water, have often obliged the living matter of the coral- 

 builders to shift the locality of its operations ; and, by 

 variation and adaptation to these modifications of con- 

 dition, its forms have as often changed. The work it 

 has done in the past is, for the most part, swept away, 

 but fragments remain, and, if there were no other evi- 

 dence, suffice to prove the general constancy of the 

 operations of Nature in this world, through periods of 

 almost inconceivable duration. 



1 Dana, Manual of Geology, p. 272. 



