PREFACE 



THE SECOND EDITION. 



The fikst edition of this book appeared in 1842, and 

 since then only one important work on the same 

 subject has appeared, namely, in 1872, by Professor 

 Dana, on Corals and Coral-Eeefs. In this work he 

 justly says that I have not laid sufficient weight on 

 the mean temperature of the sea, in determining the 

 distribution of coral-reefs ; but neither a low tempera- 

 ture nor the presence of mud-banks accounts, as it 

 appears to me, for the absence of coral-reefs through- 

 out certain areas; and we must look to some more 

 recondite cause. Professor Dana, also, insists that 

 volcanic action prevents the growth of coral-reefs 

 much more effectually than I had supposed ; but 

 how the heat or poisonous exhalations from a volcano 



