HOW AN OYSTEE EEEF OBTAINS SOIL. 151 



the air at low tide longer than it is hidden by the water of the high tide, 

 oysters will cease to grow at the centre, while still flourishing around the 

 edges. The dead shells, becoming brittle, are soon broken to pieces by 

 the waves — a work in which the breakers are aided by boring mollusks 

 and worms, whose excavations cause large pieces of the consolidated reef 

 to break off under every storm. These, tossed about in the water, are 

 soon crushed, pulverized, and thrown upon the crest of the reef. The 

 topmost growth of closely crowded shells, filled in with their wave- 

 crushed fragments, speedily becomes cemented, by decomposition of some 

 of the materials in the sea-water, into a compact deposit. Further action 

 of the waves powders this into shell-sand (or, more properly speaking, 

 into sheN-dust), and produces the basis of a most fertile soil. 



To this end, now that the solid reef stands fairly at or a little above 

 the surface of the water, comes speedy and efficient help. Opposing the 

 flow of the currents, the old oyster-bar intercepts and furnishes lodgment 

 to all sorts of drifting sea-wrack, receives a growth of the algse and grasses 

 which frequent half-submerged levels, and is all the time built up at the 

 crest by the washing upon it of fragments broken from its edges. It is 

 not long, therefore, before a sort of shelly soil is formed, weeds and herb- 

 age reach it by means of favoring winds or the* droppings of birds, and 

 finally some floating mangrove stem or seed takes root there, and man- 

 ages to get so firm a foothold that the storms do not tear it away. 



This done, the far-reaching and tangled roots of the bush form an 

 eddy which deposits sand and floating stuff until more mangroves have 

 room to root themselves, and the bar ceases to be a " reef :" it has become 

 a " mangrove key." 



Now the mangrove (of which there are several kinds) is a very curious 

 tree, and one particularly well fitted for its circumstances. Its seeds 

 germinate in the calyx of the flower, and, before they drop, grow to be 

 little brown stems, six or seven inches long and about as thick as one's 

 finger, with small rootlets at one end. Such mangrove seedlings float 

 about all this coast and among the islands in such quantities that, as 

 Professor Agassiz put it, one would suppose some vessel laden with Ha- 

 vana cigars had been wrecked there, and its precious cargo scattered in 

 the ocean. 



" In consequence of their shape, and the development of the root, one 

 end is a little heavier than the other, so that they float unevenly, with the 

 loaded end a little lower than the lighter onp. When they are brought 

 by the tide against such a cap of soil as I have described, they become 

 stranded upon it by their heavier ends; the rootlets attach themselves 



