206 CORALS AND CORAL ISLANDS. 
proved, by the observations of Dr. E. C. Stearns and Prof. A. 
Heilprin, to be Tertiary and probably Miocene. At Tampa 
Bay and along the coast south there is shell-rock, or coquina, 
in process of formation in some places. The large accumu- 
lations of worm-like tubes on this coast, which have been 
referred to Serpule, have been recently found by Mr. W. H. 
Dall to be the tubes of a Mollusk, — the Gastropod Vermetus 
nigricans, Whose worm-like spirals are much like those of 
sea-worms.! 
Along the outer borders of the great Florida Bank thea 
waters abound in growing corals. They cover its margin, 
and also the bottom adjoining at depths of one to seven or 
eight fathoms, this being the ordinary limit of reef-forming 
corals in the Florida seas. Over this marginal belt of grow- 
ing corals, toward its outer limit, there are many spots of 
half-emerged reef and some islets; and these reefs make the 
outer boundary of a navigable channel carrying four to seven 
fathoms of water. This belt of growing corals is ordinarily 
called the “ Florida Reefs.” 
The islands and keys of the bank are almost wholly of 
wind-drift origm. The sands of the outer margin of the reef, 
where thrown up by the waves, have been taken up by the 
winds and deposited in long, high drifts a mile or so back. 
Key Largo, one of the eastern of the drift ridges, is thirty 
miles long and between one and two miles wide. Key West 
is near the southwestern extremity of the bank, and has the 
rather unusual height of fifteen feet above tide-level. Hast 
of Key West for thirty-five miles the Keys are cut into strips 
that trend north and south, —the Pine Islands, — owing to 
passages that have been kept open by the tides. 
The drift-sands of the Keys are consolidated into a coral 
1 Dall, American Journal of Science, 1887, xxxiv., 161. 
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