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208 CORALS AND CORAL ISLANDS. 
that — “the greater number, if not all the Keys, rest upon a 
foundation of corals. At Sand Key large rugged masses of 
dead coral are seen bordering the Key on the windward side, 
and rising above low water; similar masses may be seen at 
Sambo Key, and at other places along the outer reef. But 
the Keys within this barrier present better opportunities for 
studying the foundation upon which they rest. At Key 
Vacca corals rise to a height of four feet above high water, 
and present not the slightest evidence of disturbance, beyond 
the upward movement which raised them to their present 
position. The rocky mass of coral along the margin of the 
Key is undermined by the waves, and otherwise worn into sin- 
gularly rugged shapes, with sharp projecting points. Even at 
some distance from the water, bunches of coral project above 
the surface wherever the overlying sand is washed away. 
“In the shallow water off many of the Keys very beauti- 
ful patches of Alga, interspersed with living corals, are seen 
within six or eight inches of the surface. Off Indian and 
Plantation Keys dark knobs of coral are visible upon the 
white mud of the bottom, which render the navigation 
amongst these Keys dangerous. On lower Matacumba I 
traced the rugged coral rocks for a mile in extent; I also 
found them on Conch Key, as I did indeed on nearly every 
island that I examined, where a section could be found on 
the shore, from which the overlymg sands were washed.” 
What is the actual thickness of the Florida coral-reef 
formation, and what it rests upon, no one knows, as no at- 
tempts to ascertain by borings have been made. The Missis- 
sippi silt makes no encroachments on the region; for it is 
carried westward by the tidal currents of the Gulf, and does 
~ not reach eastward to within five hundred miles of the Flor- 
ida reef region. 
