PREFA TOR Y NOTE. xix 



seems a more natural one, for many of the phenomena at 

 least, than that of the subsidence of the foundation to which 

 the great vertical thickness of barrier-reefs has been hitherto 

 referred.' He, however, acknowledges that it is 'difficult 

 to account for the great depth of some of the lagoons — forty 

 fathoms — on any other theory than that of subsidence.' 

 This explanation, however, appears to be negatived in some 

 measure by the observations of some American geologists, 

 among whom the names of W. H. Dall and A. Heilprin may 

 be specially mentioned. Thus, the former states 1 that 'the 

 coral formation observed by Agassiz in the region in the keys 

 must be of very limited scope, as it has not been identified 

 from the mainland of Florida by any modern geologist ;' and 

 the latter also notes 2 that ' no observed facts sustain the coral 

 theory of formation as propounded by Agassiz. They prove, 

 on the contrary, that the coral tract of Florida is confined to 

 a border region on the south and south-east, and there are 

 no tertiary reefs whatever.' 



More recent still than the observations of Agassiz are those 

 of Guppy, 3 who has spent several years among the islands of the 

 Solomon Archipelago. These observations, as far as they con- 

 cern the theories in question, may be briefly summarised. The 

 islands, by the masses of coral limestone which have been 

 found on them, indicate elevation ; these upheaved reefs are 

 situated upon a basis of volcanic mud having the same character 

 as that dredged up by the Challenger Expedition from around 

 volcanic islands ; and this mud envelops ' anciently sub- 

 merged volcanic peaks.' He also states that corals thrive 

 best in the breaker-wash, and do not flourish in the 'break of 

 the tide-swell;' that the de:ached reefs, which are submerged 

 round these islands, represent the earliest stage in reef-forma- 

 tion, and that when they have in their growth upward reached 

 a height of from 4 to 8 fathoms distant from the surface, 



1 Amer. Jotirn. Set., 3rd ser., xxxiv. p. 161. 



2 Trans. Wagner. Inst. Set., May, 1887. 



3 Trans. Roy. Soc. Edinb., xxxii. p. 545; Proc. Roy. Soc. Edinb., 

 xiii. p. 857. 



