30 FLORIDA REEFS. 
Ragged 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 
