FLORIDA REEFS. 27 



width throughout, with 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 Mands, 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 until 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 

 with Gorgonia, among which rise a number of coral heads. 

 • These heads are hemispheres of Porites or Astrsea. In the shoaler parts 

 Maeandrinas 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 



