FLORIDA REEFS. 27 
width throughout, Avitli the exception of those few places where the 
reef widens, or the mud flats from the keys encroach upon it. Its nar- 
rowest passages are between Looe Key and the Pine Islands, between 
Pickle's and French Reefs, and between Key Rodriguez and Tavernier. 
Is is also somewhat narrowed between Alligator Reef and Indian Key, 
and is widest off Key "West. Its depth varies also slightly, being shoaler 
in its eastern range than to the west. The shallowest part is between 
Pickle's Reef and Key Rodriguez, and between Looe Key and Pine Islands. 
But if we do not take into account those spots where the depth is reduced 
from local circumstances, we may say that, as a whole, the ship-channel 
begins to the east, with a depth of about two fathoms between Fowey Rocks 
and Soldier Key, increasing gradually thence imtil it reaches three fathoms 
between Pacific Reef and Old Rhodes, then becomes again slightly reduced 
between Carysfort Reef and Key Largo ; after which, with the exception of 
the shoals between Pickle's Reef and Key Rodriguez, it deepens again to 
three, four, five, or even six fathoms, until, between Looe Key and Pine 
Islands, it shoals once more to fourteen feet. Farther on, it increases again 
to five, six, and seven fathoms, the average depth between Key West and 
the Reef being five or six fathoms ; and still beyond, more towards the west, 
sinks to eight, nine, and ten fathoms between the western extremity of the 
Marquesas and the western end of the reef, where it spreads into the great 
depression separating the Tortugas from the Marquesas. The character 
of the bottom varies in different parts, as do also the living beings which 
it supports. Where it is the most shoal, as between Fowey Rocks, Triumph 
Reef, and Long Reef, on one side, and Soldier Key and the Ragged Keys on 
the other, the bottom consists of coral sand, overgrown with what is called 
the country grass ; that is to say, a variety of the limestone algae mingled 
Avith Gorgonia, among which rise a number of coral heads. 
These heads are hemispheres of Porites or Astrsea. In the shoaler parts 
Mfeandrinas and Millepora alcicornis also occur ; they are more numerous 
near the reef than near the keys, and are, indeed, the inner expansion of the 
reef itself where the corals are less continuous, forming isolated patches 
rising out of coral sands. 
To the west of Long Reef, especially between Carysfort and Key Largo, 
the coral sand rises here and there in the form of shoal sand-banks, inter- 
mixed with coral heads, — an arrangement which is probably owing to the 
more rapid currents flowing in that part of the channel, which is precisely 
