President's Address. 25 



of the encircling reef is made to be about 2000 feet.^ Pro- 

 fessor Dana by one estimate puts it at 1150, and by another 

 at 1750, feet. He assumes that in general the thickness of 

 solid coral must be considerable, though he admits that cal- 

 culations based on the seaward continuation of the slope of 

 the land are liable to error from many causes.^ Even if we 

 admit (what cannot be proved) that the calcareous mass of 

 any coral-reef does attain a thickness of many hundred feet, 

 it would not necessarily consist wholly of solid coral.^ Pro- 

 fessor Agassiz has followed the growth of a reef on a platform 

 of calcareous organic detritus, and he has found elevated 

 coral-reefs which rest on such a platform. Mr Murray's 

 observations explain how a reef may grow outward on a 

 talus of its own debris. There appears to be no reason, 

 indeed, why a calcareous mass of almost indefinite thickness 

 might not be formed without the aid of subsidence. Its 

 upper zone might be directly due to coral growth, while the 

 larger part of the mass would be composed of an aggregate 

 of coral debris, mixed with the remains of mollusks, ecliino- 

 derms, and other calcareous organisms. So rapid is the 

 destruction of organic structure, through the solution and 

 redeposit of carbonate of lime by infiltrating water, that a 

 special and careful search might be required to determine 

 the actual limits of the true reef and of its calcareous plat- 

 form, and even such a search might not be successful.* 



After a full consideration of this second difficulty, I feel 

 compelled to admit that no valid argument in favour of sub- 



1 Coral-Reefs, 2d edit., p. 65. 



2 Corals and Coral Islands, 2d English edit., 1875, p. 126. 



^ Professor Dana cites examples of raised coral-reefs 250 to 300 feet above 

 sea-level ; but we do not yet know how much of the rock is solid coral, and 

 how much may be fonned of aggregated organic cUbris. 



[^ Since this Address was read, and the chief portion of it appeared in 

 Nature (Nov. 29 and Dec. 6, 1883), an interesting letter by Dr Gupx^y an- 

 nounces that he has found elevated coral-reefs at heights of 100 to 1100 or 

 1200 feet among the Solomon Islands ; that the coral-rock itself forms a com- 

 paratively thin crust, and is underlaid by a mass of impure earthy limestone, 

 abounding in foraminifera and other pelagic organisms, such as pteropods 

 {^Nature, 3d Jan. 1884). This is precisely the structure that the observations 

 of Mr Murray and Professor Agassiz would lead us to expect. — Note added 

 \2th January 1884.] 



