54 



THREE CRUISES OF THE " BLAKE. 



nessee Reef. At other points it forms extensive shoals, which 

 are covered with a few feet of water. In other localities again, 

 the pounding of the breakers on the edges of the reefs has ac- 

 cumulated dead corals which form small keys along the line of 

 the main reef, as at Sand Key, Sombrero, and the Samboes. 

 There are passages of greater or less depth across several parts 

 of the reef, giving access from the Gulf Stream to the interior 

 ship channel. The main entrances to Key West Harbor are 

 formed by such channels. The depth of water on the reef, like 

 that of the mud flats, increases as one passes from Cape Florida 

 towards Sand Key and the Marquesas. 



The small keys of the reef proper are built of accumulations 

 of larger coral boulders forming the foundation pieces of corals, 

 and fragments of shell and coral sand arranged according to 

 their size in the interstices, and heaped up by the action of the 

 waves, the tides, and the winds. Similar agencies must have 

 formed the keys proper, for they consist of the same coral rock 

 and sand, acted upon for a much greater length of time by the 

 storms. 



A walk along the sea-face of any one of the keys will show 

 its coast line to be in incessant movement. In exposed places 



Fig. 36. — Coral Breccia. 



Fig. 37. — Coral Or.lite. 



the larger fragments broken off by the breakers from the 

 coral rock of the shores are split into smaller fragments, which 

 in their turn are changed to pebbles, and then finally become 

 cemented into coarser or finer sand or impalpable powder. The 

 cementation of these fragments at different stages gives us the 



