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 work 

 their 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 shallow channels ; they all slope very 

 graduaUy 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 reef 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 thus formed, which separates the reef from the 

 keys, is navigable for small vessels the whole length of the reef 

 as far as Cape Florida. The reef reaches the surface of the sea 

 at only a few points, as at Carysfort, Alligator Reef, and Ten- 



