CORAL-REEFS. 109 



bottom, is far more important in indicating the depth at 

 which the larger kinds of coral nourish than almost any 

 number of separate observations on the depth, at which 

 certain species have been dredged up. For we can under- 

 stand the gradation, only as a prolonged struggle against 

 unfavourable conditions. If a person were to find the 

 soil clothed with turf on the banks of a stream of water, 

 but on going to some distance on one side of it, he 

 observed the blades of grass growing thinner and thinner, 

 with intervening patches of sand, until he entered a 

 desert of sand, he would safely conclude, especially if 

 changes of the same kind were noticed in other places, that 

 the presence of the water was absolutely necessary to the 

 formation of a thick bed of turf : so may we conclude, with 

 the same feeling of certainty, that thick beds of coral 

 are formed only at small depths beneath the surface of the 

 sea. 



I have endeavoured to collect every fact, which might 

 either invalidate or corroborate this conclusion. Capt. 

 Moresby, whose opportunities for observation during his 

 survey of the Maldiva and Chagos Archipelagoes have been 

 unrivalled, informs me, that the upper part or zone of the 

 steep-sided reefs, on the inner and outer coasts of the atolls 

 in both groups, invariably consists of coral, and the lower 

 parts of sand. At seven or eight fathoms depth, the 

 bottom is formed, as could be seen through the clear 

 water, of great living masses of coral, which at about ten 

 fathoms generally stand some way apart from each other, 

 with patches of white sand between them, and at a little 

 greater depth these patches become united into a smooth 

 steep slope, without any coral. Capt. Moresby, also, 

 informs me in support of his statement, that he found only 

 decayed coral on the Padua Bank (northern part of the 



