AGASSIZ: BAHAMAS. 179 



to be learned regarding the barrier and fringing reefs and the great 

 lagoon of New Caledonia. We need additional information regarding 

 the thickness of coral reefs to complete the evidence of their great 

 thickness as due to subsidence. The thickness of such elevated reefs 

 as may without question be considered true coral reefs is not as great 

 as it has been supposed by various -observers (see Dana, p. 27-i). The 

 more recent examinations of the thickness of the elevated coral reefs in 

 the Solomon Islands by Guppy, in the Barbados by Harrison and Jukes- 

 Browne, in Cuba by R. T. Hill 1 and myself, all tend to prove that ele- 

 vated coral reefs nowhere have attained a thickness of more than two 

 hundred to two hundred and fifty feet. This shows that a thickness of 

 two thousand feet observed in Cuba by Professor 0. W. Crosby, of twelve 

 hundred feet near Matanzas reported by myself, and of two thousand feet 

 of the white limestones of Jamaica, are not the thickness of coral reef 

 builders, but of marine limestones of very different character ; and that 

 the true reef-building corals have merely built a sheet of moderate 

 thickness upon these older limestones, which have been mistaken for 

 limestones built up by reef-building corals. The reefs of the weather side 

 of the island of Oahu form quite a thin sheet overlying the volcanic rocks. 



Similar conclusions have been reached by the recent examinations of 

 dolomitic limestones, which are of great thickness (over two thousand 

 feet), and were until recently unhesitatingly considered as coral reef 

 limestones. They have now been shown by Miss Ogilvie and by Rothpletz 

 to be marine limestones, while the true coral reef builders formed de- 

 posits of very moderate thickness, a coral growth, as it were, not thicker 

 than one hundred and fifty feet. It is very evident that, in the exam- 

 ination of the coral deposits of former geological periods, great care must 

 be taken to distinguish between coral growths forming a comparatively 

 thin veneer and marine limestones of great thickness in which corals are 

 occasionally found. 



The discussion of Langenbeck on the coral reefs of former geological 

 pei'iods is subject to revision after a re-examination of the localities, in 

 accordance with the. recent explorations of the dolomite region, and of 

 our more accurate knowledge of the distribution and thickness of the 

 West Indian reefs. The fossil reefs of the Jura were among the earliest 

 known. Attention was first called to them by Thurman (1859) and 

 Gressly (1860), but they have not been examined recently with refer- 

 ence to the points now in dispute regarding the formation of coral reefs. 2 



1 Amer Journ. of Science, September, 1894. 



2 See Zschokke, F. Die Korallenriffe im Schweizerischen Jura. Aarau, 1890. 



