in. 



THE FLORIDA REEFS. 



Florida projects between the Gulf of Mexico and the Atlan- 

 tic as a broad, low peninsula, not strictly limited to the land 

 which rises above the level of the sea. ExtensiAe shoals on its 

 southern extremity reach between the mainland and the line of 

 keys, and stretch nearly as far west as the Tortugas. Taken in 

 connection with the immense bank to the westward of the pe- 

 ninsula, they form the continuation of the mainland, below the 

 surface, to about the hundred-fathom line, — the mass of the 

 Florida plateau proper. To this must be added the narrower 

 coast-shelf of the eastern face of the peninsula. This shelf has, 

 however, a very different character ; for on the eastern side of 

 the peninsula, the calcareous or coral sand gives way to the 

 siliceous sand characteristic of the eastern Atlantic coast south 

 of Cape Hatteras. 



The shore Hne of the southern extremity of the peninsula is 

 ill-defined, and, with the exception of the short stretch between 

 Cape Sable and Cape Florida, marked by bluffs of coral lime- 

 stone, is similar to that of the Everglades. It consists mainly 

 of innumerable low islands separated by narrow channels from 

 the mainland, to which it becomes united by flats, often bare 

 at low water. Similar flats, but far more extensive, stretch west- 

 ward from the mainland, back of the keys as far as to the 

 northward of Key West and the Marquesas. (Fig. 34.) 



Portions of these flats are dry at low tide, and are separated 

 by occasional patches, more or less extensive, of deep water. 

 The water which covers these flats graduaUy deepens as one 

 passes westward from the mainland. The flats, as well as the 

 whole of the tract of surface between the mamland and the 

 Tortugas, dip sHghtly to the westward. These shoals are liter- 



