22 FLORIDA REEFS. 
around a submarine hill. The group consists of a circle of flat, low islands, 
not far apart from one another, divided by deep channels and enclosing 
a sheet of water, — a circular harbor as it were. Instead of steep walls like 
the Pacific lagoons, these islands have extensive shoals; but the channels 
and the harbor are deep, safe, and accessible, so that the fort now rising 
upon Garden Key will have especial advantages as a maritime station. 
These islands are not built of the debris of coral animals. The rock consists 
chiefly of the remains of corallines. This confirms my belief that they have 
grown up around a submarine rising ground ; for the coralline or limestone 
Alga3, especially that kind of coralline of which the Tortugas rock is chiefly 
composed, do not prosper in deep water, but thrive on shoal flats. The reef- 
building corals have combined with this extensive coral vegetation to bring 
the Tortugas to their present condition. It seems, at first, astonishing that 
a sea-plant should build these extensive rocky islands. But we must 
remember that the corallines absorb lime and form limestone secretions no 
less than the stony corals themselves. Indeed, the coralline rocks of the 
Tortugas are no more an anomalj' than the coal beds : in the former, the 
rock is built of the limestone secretions of the plants ; in the latter, of their 
woody or celkdose tissue. 
Coral Reefs. 
After examining a growing coral reef, so full of life, so fresh in appear- 
ance, so free from heterogeneous materials, in which the corals adhere so 
firmly to the ground, or, if they rise near the surface, seem to defy the 
violence of the ocean, standing uninjured amid the heaviest breakers, an 
observer cannot but wonder Avhy, in the next reef, the summit of which 
begins to rise above the level of the water, the scene is so completely 
changed. Huge fragments of corals, large stems, broken at their base, 
gigantic boulders, like hemispheres of Porites and Maaandrina, lie scat- 
tered about in the greatest confusion, — flung pell-mell among the frag- 
ments of more delicate forms, and heaped upon those vigorous madi'epores 
which reach the surface of the sea. 
The question at once arises, how is it that even the stoutest corals, 
resting with broad base upon the ground, and doubly secure from their 
spreading proportions, become so easily a prey to the action of the same sea 
which they met shortly before with such effectual resistance ? The solution 
of this enigma is to be found in the mode of growth of the corals them- 
