FLORIDA BANKS. 205 
West. They are broad reef-ground flats more or less swept 
by the tides; the Biscayne Bay, two to eight miles wide, is 
on the east side of Florida, and the larger Florida Bay on the 
south. The width of the bank rapidly increases to the west- 
ward, becoming thirty miles off Cape Sable. No subdivision 
into a fringing and barrier reef is suggested by its features. 
“The whole tract between Cape Sable and the Keys as far as 
Cape Florida, at least as far as Soldier Key, is so shoal that 
it is inaccessible except for very small vessels.” This shoal 
region, as Professor Agassiz’s Memoir (1851) states, is literally 
studded with mangrove islands, either In continuous ranges 
of great extent, or making “innumerable small islets, so inti- 
mately interwoven and separated by so narrow and shallow 
channels as to be almost impenetrable.” The mangrove 
bushes, which seem to be “striding out in the mud” in conse- 
quence of the many root-making stems that grow downward 
from the branches, serve to entangle the sand, sea-weed, and 
drift-wood that float by, and thus aid in the process of emer- 
gence. The material of the great mud-flats of the area on 
the bottom of the shoal water is the finest of coral sand; 
but it is darkened in color by the carbonaceous results of 
animal and vegetable decomposition, so that much of it looks 
like ordinary silt or mud. 
Along the western side of Florida the coast-flats extend 
from Cape Sable northward for fifty to sixty miles, with a 
mean width of nearly ten miles; but the sands of the flats 
are partly siliceous. Outside of the bank the waters are 
deeper, but the bottom continues to be largely calcareous.' 
The bed of coral rock at Tampa Bay containing silicified 
corals has been supposed to be of recent origin; but it is 
1 The “Three Cruises of the Blake,” of Mr. A. Agassiz, has, at page 286 of 
the first volume, a colored map of much interest illustrating the nature of the 
sea-bottom in the Florida region. 
