FLORIDA REEFS. 25 



island formed chiefly of broken shells and coral fragments thrown up on the 

 edge of the reef, a new feature begins. The higher ledges approach each 

 other again, and the depressions between them are shallower than those to 

 the east. We have here in close succession from east to west the American 

 Shoal under six feet of water, the Pelican Reef with about four feet of water, 

 and East, Middle and West Sambos, three little keys of a few acres in ex- 

 tent, consisting chiefly, like Looe Key, of broken shells and coral fragments, 

 forming so light a crust upon the edge of the reef that storms occasionally 

 wash away the whole accumulation. Between West Sambo and Sand Key, 

 another depression on the reef, with a depth of water varying from four to 

 five fathoms, forms the main channel leading into Key West. Toward Sand 

 Key the reef widens considerably, but the corals are less crowded upon it. 

 Large accumulations of coral sand intervene between the living heads, and 

 detached masses and fragments of corals are thrown up in quantities upon 

 the ridge, forming extensive shoals. To the east of Sand Key these shoals are 

 known as the Eastern Dry Rocks and Rock Key ; to the west, as the Western 

 Dry Rocks. Sand Key itself is a small island also consisting chiefly of coral 

 fragments and coral sand heaped above the field of large coral boulders 

 stretching almost uninterruptedly from Eastern Dry Rocks to Western Dry 

 Rocks, This part of the reef contrasts remarkably with the Sambos, and, taken 

 together, they explain the process by which a living reef is transformed into 

 a dry key. At the Sambos as at Carysfort we have a field of Madrepora 

 palmata mixed with a few stocks of Millepora alcicornis and bunches of Gor- 

 gonia rising as a level ledge to the surface of the water at low tide. Upon 

 the most prominent points of this ledge, coral boulders and sand are accumu- 

 lated, every such accumulation forming the nucleus for a key. Comparing 

 such incipient keys with Sand Key, we there find that the collection of 

 coral boulders and sand has increased so much as to cover entirely the sum- 

 mit of the once living reef, spreading also far down upon its slopes. Before 

 the storm of 1846, vegetation was spreading over Sand Key, and a light- 

 house was built upon it. In the hurricane of the 11th of October, the 

 lighthouse was washed away with the mass of coral boulders on which it 

 stood. This mass of loose materials had in fact been mistaken for con- 

 tinuous coral rock sufficiently solid to support the structure based upon it. 

 This unfortunate event has shown that the loose cap resting upon the 

 more solid reef should always be removed before laying the foundation 

 of a lighthouse. The solid rock is not to be reached at the level of low- 



