SURVEY OF CORAL REEFS. 485 



Stream and its prevailing currents, was of 

 great practical service to the Coast Survey. 

 It was especially valuable in determining how 

 far the soil now building up from accumula- 

 tions of mud and coral debris was likely to 

 remain for a long time shifting and uncertain, 

 and how far and in what localities it might be 

 relied upon as affording a stable foundation. 

 When, at the meeting of the American Asso- 

 ciation in the following spring, Agassiz gave 

 an account of his late exploration, Dr. Bache, 

 who was present, said that for the first time 

 he understood the bearing of the whole sub- 

 ject, though he had so long been trying to 

 unravel it. 



The following letter was written immedi- 

 ately after Agassiz's return. 



TO SIR CHARLES LYELL. 



CAMBRIDGE, April 26, 1851. 



... I have spent a large part of the win- 

 ter in Florida, with a view of studying the 

 coral reefs. I have found that they consti- 

 tute a new class of reefs, distinct from those 

 described by Darwin and Dana under the 

 name of fringing reefs, barrier reefs, and 

 atolls. I have lately read a paper upon that 

 subject before the American Academy, which 



