1882.] 127 Crosby. 



of a series of reefs, and compare it with the reef approaching it 

 most nearly in altitude, and that with the next in descending 

 order, and so on down to the modern reef, the evidence is per- 

 fectly conclusive ; for the differences at each step are of the same 

 kind and in the same direction. Notwithstanding its solidity, 

 the limestone of El Yunque weathers in the same irregular, ragged 

 and cavernous way as that of the lower reefs. 



The erosion which has swept away, for a distance of many 

 miles up and down the coast, all but this solitary remnant of a 

 reef more than a thousand feet thick, must certainly have dimin- 

 ished the height of El Yunque by as much as two hundred feet ; 

 and we may safely conclude that the original altitude of this 

 reef was not less than two thousand feet. On the island of 

 Jamaica precisely similar reefs have been observed at an elevation 

 of three thousand feet ; and Mr. Sawkins, in his report on the 

 geology of that island, says that the reef -limestone has a maximum 

 thickness of not less than two thousand feet, and that the oldest 

 of it was formed after the close of the Tertiary period. 



The concentric coral-reefs of Florida, it is well known, have 

 their summits all on nearly the same level, which is but little 

 above the level of the sea ; constituting an important exception 

 to what for the last forty years has been regarded as the general 

 rule, viz., that extensive coral-reefs are formed on areas of subsi- 

 dence. Mr. Alexander Agassiz points out * that the great Alceran 

 Reef, on the Yucatan Bank, which is atoll-like in form and stands 

 on an area of elevation, is another exception to Darwin's theory ; 

 and he also adds both the ancient and modern reefs of Cuba to 

 the list. 



That Cuba has, in recent geological times, been an area of 

 extensive elevation, the reefs fringing its mountains to a height 

 of nearly two thousand feet afford indisputable evidence. But 

 does it necessarily follow that these reefs were formed while the 

 land was actually rising ? It is a fact well known to geologists 

 that all great movements of the earth's crust are oscillatory. 

 Thus, during the Paleozoic era, the middle portion, especially, of 

 the Appalachian system in North America was an area of pro- 

 found subsidence ; but, although the general tendency was 



i Bulletin of the Mus. Comp. Zool., Vol. v, No. 1, p. 2. 



