vi.] ON COEAL AND COEAL REEFS. 133 



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 evidence, suffice to 

 prove the general constancy of the operations of Nature 

 in this world, through periods of almost inconceivable^ 

 duration. 



