72 THREE CRUISES OF THE " BLAKE." 



interesting reef as a sort of epitome of the mode of formation 

 of the great Florida Reef and of the Bahamas Bank. Taken 

 in connection with its position on the Yucatan Bank, it will 

 jjerhaps explain the mode of formation of the greater part of 

 the Florida peninsula as connected with the bank lying to the 

 westward of the mainland and to the northward of the Florida 

 reefs. 



The specimens thus far obtained from the rock composing the 

 Yucatan plateau consist of the same limestone as those of the 

 great Florida Bank ; and there are forming upon it, though to 

 a much more limited extent, patches of reefs near the thirty and 

 twenty fathom curve, such as the Shoals, the Triangles, Cay Are- 

 nas, the Areas, the Madagascar, English, Alacran, and the 

 fringing reefs of the eastern edge of Yucatan, that extend along 

 the Mosquito coast to the central part of Central America. 



It is evident from the above that we need not refer the atoll- 

 shaped form of this reef (Fig. 44) to the subsidence of the Yu- 

 catan Bank as a whole, since the action of the prevailing winds 

 and currents would account for all the existing phenomena. 

 The decay of the animals living upon the great plateau, added 

 to the deposition of all the animal life brought to it by the cur- 

 rents, would explain a gradually increasing elevation of the sur- 

 face till the level was reached at which reef-building corals 

 could flourish, and at which a reef would naturally be formed. 

 Darwin has noted the close resemblance between encirclingr bar- 

 rier reefs and atolls. It seems to me that the structure of the 

 Marquesas (Fig. 44) and of Alacran proves conclusively that 

 not one point of difference exists between a barrier reef and an 

 atoll. Darwin has also called attention to the fact that in 

 shallow seas, such as the Persian Gulf and parts of the East 

 Indian Archipelago, the reefs lose their fringing character and 

 appear as irregularly scattered patches, often covering a con- 

 siderable area ; and he also observes that many reefs of the 

 West Indies have been formed in like manner upon large and 

 level banks lying a little beneath the surface, — banks which 

 he believes to have been caused by the accumulation of sedi- 

 ment. Such patches of reef-building corals would seem, from 

 their analogy with the Tortugas, to be the beginning of more 

 extensive reefs. 



