THE FLORIDA REEFS. 53 
ally studded with mangrove islands, often arranged without any 
apparent regularity, either forming continuous ranges or small 
archipelagos, broken by narrow and shallow channels. The 
vegetation of these low islands is most luxuriant, and consists 
mainly of mangroves. 
Upon the flats which have reached the surface of the sea the 
young mangrove plants drift in immense quantities. They are 
fusiform bodies of about six inches in length, resembling a 
cigar; they float vertically, and when once stranded soon ын 
tliair way into the soft mud of the flats, and take root, sending 
out shoots in all directions. The new stem rises rapidly, send- 
ing down new shoots to the ground from higher points, forming 
thus an arch of roots from which spread the branches of the 
mangrove trees. Around such a nucleus additional sand and mud 
soon collect, and gradually build up extensive islands, covered 
with a thick tangle of mangroves and other plants. The man- 
grove islands on the flats to the north of Key West are specially 
noteworthy. (Fig. 35.) The keys proper are all similar in struc- 
ture, and form an extensive chain of low islands, rising nowhere 
more than about twelve feet above the level of the sea. Start- 
ing from north of Cape Florida, they form an immense crescent 
extending as far west as the Tortugas. The keys usually con- 
sist of the accumulation of dead corals, or of coral rock or coral 
sand cemented into a greater or less degree of compactness. 
The principal ones are long, narrow islands, varying in width 
from a mile to less than a quarter of a mile, and in length from 
mere patches to such islands as Key Largo, Indian Key, or Key 
West, the longest of which extends about fifteen miles. The 
keys are separated by shalļow channels; they all slope very 
gradually to the north into the mud flats, and present their 
steepest face to the south on the shores which skirt the ship 
channel that separates them from the r oef proper. 
The reef proper forms a curve similar to that of the keys, 
never receding from them more than from three to fifteen miles. 
The channel аа formed, which separates the reef from the 
keys, is navigable for small vessels the whole length of the reef 
as far a a Cape Florida. "The reef reaches the surface of the sea 
at only a few points, as at Carysfort, Alligator Reef, and Ten- 
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