58 



THREE CRUISES OF THE " BLAKE. 



far the explanations given by Agassiz, Le Coute, and Hunt are 

 satisfactory as regards the formation of the group of islands 

 making the extremity of the reef, and shall then attempt, by 

 the help of the dredging- operations of the " Blake " along the 

 Florida Bank, to reconstruct the past history of the peninsula 

 in its southern portion. Beginning with an account of the for- 

 mation of the present reef, based upon the knowledge obtained 

 by a careful survey of the Tortugas, I shall then proceed to the 

 elucidation of the structure of the peninsula itself. 



The Tortugas (Fig. 38) are situated at the very extremity of 



Fig. 38. — The Tortug-as from Coast Survey Charts. 



the slope upon which the line of the Florida reefs has been built 

 up. They form the most recent of the cluster of Florida reefs, 

 and have not as yet been transformed into the normal coral reef 

 characteristic of the whole line extendino- from the Rebecca 

 Shoal and Marquesas to Cape Florida. There is as yet nothing 

 at the Tortugas corresponding to the extensive mud flat stretch- 



