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 
Ni 
ws: 
(or aE, у 
А 
sas 
Fig. 36. — Coral Breccia. Fig. 37. — Coral Обе. 
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 
