58 THREE CRUISES OF THE’ “ BLAKE.” 
far the explanations given by Agassiz, Le Conte, 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 
TORTUGAS. 
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Fig. 38. — The Tortugas 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 extending from the Rebecca 
Shoal and Marquesas to Cape Florida. There is as yet nothing 
at the Tortugas corresponding to the extensive mud flat stretch- 
