30 FLORIDA REEFS. 



Eao-ged Keys and Soldier Key, the shoals are more distinct from one another, 

 divided by broader and deeper channels than occur in any other part of the 

 tract lying between the main keys and the main-land. This is explained by 

 the freer influence of the Atlantic tides on this part of the reef, as com- 

 pared with its more western range. West of a line extending from Black 

 Point to Elliott's Key, the whole space between the main-land and the main 

 keys is occupied by an almost uninterrupted mud flat, covered by a shallow 

 sheet of water varying from four to five feet in depth, though in some occa- 

 sional depressions it may measure seven feet or thereabouts. The mo- 

 notony of this great stretch of mud flat is broken by the innumerable man- 

 grove islands already mentioned, as well as by the shoaler flats left dry at 

 very low water. What has been said of the shoals off" Cape Florida applies 

 equally to those here mentioned, and needs no repetition. The mode of 

 formation of the Mangrove Islands, however, is both peculiar and interest- 

 ing, and demands a few words of explanation. The mangroves are among 

 the most important geological agents in this region ; but for them the loose 

 sand and mud would remain an ever-shifting ground of movable particles. 

 To understand this we must know something of the mode of growth of the 

 mangrove seed. Like that of all viviparous plants it germinates upon the 

 parent stock, the new plant attaining a length of some six inches before it 

 drops from the old tree. As these trees grow down to' the water's edge, and 

 as at high tide the interior of the Mangrove Islands is submerged, the new 

 plants are of course dropped into the water, and are swept about and scat- 

 tered by its movements. Like brownish-green sticks, fusiform in shape, 

 they float about in great numbers, with the heavier end, where the root is to 

 be, slightly sunk below the surface. Floating thus, they suggest the idea 

 that a ship-load of cigars has been wrecked upon the reef, and swept inland 

 by the tides. Presently these fusiform bodies are stranded along the edge 

 of some mud flat, touching ground at last with their heavier loaded end. 

 Their hold is at first very loose, but made to turn, by the rising and falling 

 tides, like a rod upon a pivot, they soon work their way into the soft mud 

 and plant themselves firmly. Immediately the long, rapidly-growing roots 

 begin to shoot out and soon form a close screen, giving stability to all the 

 loose materials about them and holding the mud and sand in place. So 

 spreading is the root that a young mangrove, not more than two feet in 

 height, will send its roots out in all directions over an area of some six feet. 

 As it rises it constantly sends down new roots to reach the ground from 



