78 THE STORY OF LIFE IN THE SEAS. 



have to be one of greater length than is possible 

 in this book. 



The living edge of the Coral-reef is in some 

 cases situated on the top of a submarine preci- 

 pice of very considerable height, and in many 

 places the sounding-line goes down to a depth of 

 five or six hundred feet a few yards beyond the 

 limits of the reef. The practical difficulties in the 

 way of determining the character of the Fauna 

 of any sea-bottom that shelves in this manner 

 are very great, but where it is partly composed 

 of massive lumps of solid Coral they are at 

 present insurmountable. Every time the dredge 

 or trawl reaches the bottom it becomes entangled 

 in the Coral branches, and is liable to be seriously 

 torn, or even lost. Swabs and iron hooks and 

 fish traps may yield some scraps of information, 

 but speaking generally, the Fauna of these steep 

 slopes is scarcely known at all. 



The most important question, from the geo- 

 logical point of view, that has to be determined is 

 the depth of water in which reef-building Corals 

 can live and thrive. This is still a matter of uncer- 

 tainty owing to the practical difficulties met with 

 in the attempts to investigate it. Darwin esti- 

 mated that the limit of vigorous coral growth 

 was between 20 and 30 fathoms, but in recent 

 years, owing to the discovery of luxurious Coral 

 patches in 44 fathoms on the Tizard and Maccles- 

 field banks, there is a pretty general opinion that 

 his estimate is too low. 



Whatever the exact limit may be, it is quite 

 clear that in many parts of the world the sea- 

 bottom quite close to the outer edge of the reef 



