142 BULLETIN OF THE 



presence of foraminiferal limestones of concretions of manganese, up to 

 a height of nearly nine hundred feet, (the limit is usually, according to 

 Guppy, five to six hundred feet,) would indicate a total elevation of 

 more than twelve or fifteen hundred feet, and there appears to be no 

 reason, from what we know of the formation of barrier and of fringing 

 reefs, and of their extension seaward, why the thickness of the reef lime- 

 stone should be limited to one hundred and fifty or two hundred feet 

 even in an area of elevation. 



Guppy infers that corals may begin to build at greater depths than 

 those usually assigned, as some of the elevated reefs in the Solomon 

 Islands " rest upon partially consolidated calcareous ooze, which is not 

 found in depths under fifty fathoms on the outer slope of the present 

 reef ; that in the case of reefs with a gradual slope, where the lower 

 margin of the band of detritus lies within the zone of reef-building corals, 

 a line of barrier reefs will be ultimately formed beyond this band, with a 

 deep channel inside ; but if the band is formed on a steep slope, and 

 reaches beyond the limit of reef-building corals, no such barrier reef will 

 be found on account of the silt." 



It is not necessary, as is supposed by Guppy,^ in his account of the 

 Coral Reefs of the Solomon Islands, to have an upheaval to bring corals 

 within the constructive power of the breakers. Their natural growth is 

 quite sufficient to raise them beyond that point. He gives for the forma- 

 tion of barrier reefs, and as an explanation of the existence of a lagoon 

 inside of the reef, the same explanation as is given by Leconte, — that the 

 outer growth of the corals is in the direction of clear water, while it is 

 limited inland by the silt and muddy character of the water of the 

 barrier reef channel. He is also inclined to attribute the cause of con- 

 secutive barrier reefs to elevation. This certainly has not been the 

 cause in Florida. The reefs have grown up from the bottom wherever 

 the platform had attained the proper level for coral gi'owth. 



Bourne, who examined the Diego Garcia atoll ^ and the coral forma- 

 tions of the Indian Ocean, came to the conclusion that the whole charac- 

 ter of the Chagos group is very much opposed to the theory that atolls 

 and barrier reefs are formed during subsidence. There are several atolls 

 rising above the waves, that of Peros Banhos being fifty-five miles in 

 circuit, and composed of numerous small islets placed upon a ring-shaped 



1 Guppy, Solomon Islands, Calcareous Formation of the Solomon Group, Proc. 

 R. S. Edinb., XXXII. Part III., .1885. 



2 The Atoll of Diego Garcia and the Coral Formations of the Indian Ocean, by 

 J. C. Bourne, Nature, March 1, 1888, p. 414; April 5, 1888, p 646. 



