DEVELOPMENT OF THE REEF 239 



ocean. It may, perhaps, rise from ocean depths, in places 

 where the oceanic currents deposit more abundant sediment, 

 or it may build on some already prepared elevation; we can 

 at any rate assert that, wherever its formation starts, it will 

 certainly end at the Limiting Line of Sedimentation. Great 

 numbers of these banks are known, and their very number does 

 not warrant their mention. They may be built, perhaps, on 

 volcanic elevations (Murray), on denuded volcanic islands 

 (Wharton), or on denuded remains of ancient land (Stanley 

 Gardiner) ; or they may be built on inequalities of the ocean 

 floor not included under these heads, or be raised from ocean 

 bottom by sedimentation alone. I do not know that we have 

 much evidence to show what they may be ; and I do not think 

 that we have any reason to believe that they are all volcanic. 



For the explanation of the development of coral structures 

 I do not think it is to be greatly deplored that we cannot 

 assign a common origin to them all, for in their development 

 they are all essentially sedimentation products, and so have to 

 adhere to the laws governing its deposition. 



The Bathymetrical Limits of the Keef - building 

 CoEALS. — It is next natural to inquire at what point, below 

 the level of the waves, may the growth of the reef-building 

 corals start ; and on this matter again our evidence cannot be 

 said to be complete. Dana collected all the evidence at his 

 disposal, and placed the limit at 20 fathoms; Darwin, from 

 his data, set it at from 20 to 30 fathoms; and Stanley 

 Gardiner, from a series of dredgings in the Maldives and 

 Laccadives, has confirmed Darwin's estimate. By the careful 

 work of Basset-Smith upon the Tizard and Macclesfield banks 

 {Ann. Mag. Nat. Hist, 1890, p. 353), the knowledge of the 

 distribution of the reef-builders has been greatly increased, 

 and the bathymetrical range has been found to be far more 

 extensive than was supposed. In a series of dredgings, in 

 depths between 31 and 45 fathoms, Basset-Smith found 19 

 species of living corals, and 12 of these sj^ecies are typically 

 reef species. The most remarkable result is the certain know- 

 ledge that a species of Favia was flourishing at a depth of 



