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 tlie 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 invustigation of this point, which did not come within the limits of my survej', would be of 

 considerable practical importance, inasmuch as it may lead to the discovery of a basis of coral rock, 

 affording a far more solid foundation for the construction of the lighthouses wanted along that coast, 

 than the loose shore-detritus. 



