PREFACE 
TO 
THE SECOND EDITION. 
The first 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-Reefs. 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 
