22 Proceedings of the Boyal Physical Society, 



older stages of development among coral-reefs may be traced. 

 By the westward drift of the calcareous sand and silt, the 

 lagoons have been converted into flats, and these in succession 

 have been turned into more or less continuous dry land. There 

 is no evidence of subsidence. The area seems to have remained 

 stationary for a long period, or, if there has been movement 

 at all, it has been in an upward direction. Should the present 

 condition of things be prolonged, there will be a further 

 extension of the Florida coast-line. By the heaping-up of the 

 shells of dead organisms in the track of the counter-current, 

 the submarine bank will continue to be brought up within 

 the depth at which reef-building corals can grow. Successive 

 clumps of reef-builders, springing up and growing outward, 

 will build atoll-shaped reefs. The abrading action of the 

 waves upon these reefs will furnish detritus to be drifted into 

 the lagoons and channels, which will eventually be silted up 

 into dry land. 



An interesting indication of the progress of these changes 

 is furnished by the terrestial fauna and flora of the Florida 

 reefs. The plants of the mainland are found likewise on the 

 reefs, but become fewer in number as they are followed 

 southward, until, on the Tortugas — the last addition to dry 

 land — the flora consists of a few bay-cedars, a hop-vine with 

 a thick white flower, Bermuda-grass, and a solitary mangrove 

 tree. One of the species of land-shell common at Key- West 

 has found its way to the Tortugas. No terrestial reptiles 

 have yet reached that furthest atoll, though at Key- West, less 

 than 100 statute miles to the east, many of the frogs, toads, 

 lizards, and snakes, common to the southern mainland, have 

 already established themselves. 



It will be observed that the conclusions arrived at by Pro- 

 fessor Agassiz, from his own independent researches, entirely 

 confirm those previously announced by Mr Murray. That 

 two observers, who have enjoyed exceptional advantages in 

 the investigation of this subject, should come to practical 

 agreement, must be admitted to be a strong argument in 

 favour of the views which they have adoj)ted. 



Putting together all the data which have here been sum- 

 marised, I think we are driven to admit that barrier-reefs 



