2 FLORIDA REEFS. 
whole length of the eastern shore, are probably a direct continuation of 
the keys, covered with drifted sand.* This is certainly the case with the 
range of keys extending from the main-land to Cape Florida, which limits 
to the east the bay of Miami, their formation being of coral rock, but 
covered by silicious drift-sand. 
As to the southernmost extremity of the main-land proper, it is very 
difficult to determine its outlines, as it consists of innumerable islands, 
sometimes separated by narrow channels, and sometimes assuming the 
character of real islands only at high water, being mostly connected with 
the main-land by very shallow flats. This is especially the case along the 
southwestern extremity of the peninsula. The outline of the southern 
shore, however, between Cape Florida and Cape Sable, is better defined, — 
presenting, in almost unbroken continuity, steep bluffs of the same coral 
limestone which forms the bottom of the everglades, and may be traced, 
without interruption, along the Miami from the seashore to the everglades. 
South of the main-land, between it and the range of keys, there are 
extensive flats, which, even at high water, are but slightly covered, and 
which the retreat of the tide lays bare, leaving only narrow and shallow 
channels between the dry flats, with occasional depressions of greater depth. 
These mud flats extend not only between the main-land and the keys as 
far as Cape Sable, but may be traced to the north along the western 
shores of the continent, and to the west along the northern shores of the 
keys, not only as far as Key West and the Marquesas, but even to the 
Tortugas. 
There is, however, this remark to be made, — that to the west the mud 
flats become covered, by degrees, with deeper and deeper water ; or, in 
other words, that these low grounds, extending between the main-land and 
the main range of keys, dip slightly to the west, being gradually lost 
in the shoals extending north of the Marquesas and the Tortugas, along 
the western shore of the peninsula. These flats are interspersed with 
innumerable low islands, known in the country by the generic appellation 
of the Mangrove Islands, respecting which we shall give further details 
hereafter. 
The shoals between Cape Sable, Cape Florida, and the main range of 
* A direct investigation of tbis point, which did not come within tlie limits of my survey, would be of 
considerable practical importance, inasmuch as it may lead to tlie discovery of a basis of coral rock, 
aifording a far more solid foundation for the construction of the lighthouses wanted along that coast, 
than the loose shore-detritus. 
