THE TORTUGAS AND FLORIDA REEFS. 109 



rather than by the remains of an older anterior reef. At the Tortugas, the contrary 

 seems to be the case ; but this perhaps is due to the fact that the strong currents 

 which sweep over the reefs, and have excavated the Southwest Channel, have also 

 established conditions favorable to the growth of corals on both sides of this channel, 

 and that the two lines of keys are due to this cause. Had the currents run only 

 from the Southeast through the Northwest Passage, larger keys, separated by channels 

 running north and south, would then have been formed. 



I shall first show by an examination of the Tortugas how 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 recon- 

 struct the past history of the peninsula in its southern portion. Beginning with an 

 account of the formation of the present Reef, based upon the knowledge obtained 

 by a careful surve}- of the Tortugas, I shall then proceed to the elucidation of the 

 structure of the peninsula itself. 



The Tortugas (Plate I.) are situated at the very extremity of the slope upon which 

 the line of the Florida Pteefs 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 Mar- 

 quesas to Cape Florida. There is as yet nothing at the Tortugas corresponding to 

 the extensive mud flat stretching uninterruptedly a few feet below the surface of the 

 water, to the northward of the line of keys (Plates X., XI.). The northern part of 

 this flat, from Cape Sable to Key Biscayne, is fringed on the southeast face by the 

 line of narrow keys reaching from Cape Florida to Bahia Honda (Plate VI.). In the 

 oldest part of the reef, the bay to the north of the keys, the waters of which once 

 undoubtedly covered the whole space between Pine Keys and Cape Sable, has little 

 by little been filled up and transformed for the greater part into the wide shallow 

 mud flats now extending over that area. Next comes, from the Pine Keys to Re- 

 becca Shoal, a comparatively more recent portion of the reef, in which the northern 

 extremities of the kej^s rise somewhat higher above the general level of the mud 

 flats. These two adjoining regions of flats and kej's run parallel to the main reef, 

 at a distance of from one to nine miles from the outer line of reefs ; the reef that 

 is, par excellence, over the whole surface of which the living corals still prosper. Far- 

 ther yet to the westward, at a distance of fifteen miles from the western extremity 

 of the outer parts of the reef, rise the Tortugas. In this group a condition of things 

 prevails at the present day, which must have been repeated over and over again from 



