TIL. 
THE FLORIDA REEFS. 
ЁтовтрА 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. Extensive 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 line 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 gradually deepens as one 
passes westward from the mainland. The flats, as well as the 
whole of the tract of surface between the mainland and the 
Tortugas, dip slightly to the westward. These shoals are liter- 
