PREFACE TO RE-ISSUE 



During tlie short period that has elapsed since the appearance 

 of the first issue of this book some contributions have been 

 made to our knowledge of submarine conditions which have 

 a very direct bearing upon the debated question of the 

 batbymetrical limit of the reef-building corals. Firstly, the 

 " Michael Sars " North Atlantic deep-sea expedition has 

 determined that light exerts a great influence on the Helland 

 Hansen photometer at depths of 100 metres, and some in- 

 fluence even at 1000 metres; in the clear waters of tropical 

 t)ceans this distance would almost certainly be greatly ex- 

 ceeded, so that it is safe to assume that some light asserts its 

 influence very far below the limit at which reef-building 

 corals live. 



Secondly, a discussion which took place in the columns of 

 Nature towards the end of 1910 produced the very welcome 

 contribution of Mr. A. K. Hunt, who so long ago as 1882 had 

 conducted researches regarding the Limiting Line of Sedimen- 

 tation in wave-stirred areas, and deteriiiined the laws which 

 govern its presence. 



c Both these contributions afford valuable additions to the 

 subject-matter of chapter xxi.," and furnish very real support 

 for the conclusions arrived at. 



