THE FLORIDA REEFS, 81 



same len^h as that of Loggerhead, with an average width of 

 nearly two miles, the great sand flats of this shoal being those 

 of the Long and Bush Key tract. The Southwest Channel, 

 with a depth varying from ten to twelve fathoms, separates 

 Loggerhead Bank from the Bird, Garden, and Long Key Bank. 

 This, in its turn, is separated from the still greater North, 

 Northeast, East, and Middle Key Bank by the Southeast Chan- 

 nel, with a depth of about nine fathoms, while the Northwest 

 Channel separates Loggerhead Bank from the North Key Bank, 

 with an average depth of from seven to ten fathoms. The 

 Eastern Bank is irregularly horseshoe-shaped, convex to the 

 east, and partly surrounds a great interior bay, which has an 

 average depth of about seven fathoms. The flood tides run 

 from the south through the Southwest and South channels in 

 a northeasterly direction, the ebb tide flowing in the opposite 

 direction. The strongest tidal current passes throug^h the South- 

 west Channel. 



An examination of sections of the Tortugas from the west to 

 the east shows the gradual rise of the mound forming the Tor- 

 tugas, as we pass from the west side of the Loggerhead Bank to 

 a line extending through the southwest slope of the same bank, 

 and across to the main bank of the group ; the mound falls 

 slowly on the eastern slope as we cut across the east end of East 

 Key Bank, till we finaUy come to the low elevation forming the 

 southeast slope of East Key Bank. The action of the tides 

 through the Southeast and Northwest channels is well shown 

 in the fact that they keep open the passages between Long Key 

 and Loggerhead banks, and between the former and North 

 Key Bank, and also the secondary channels separating Bird 

 Key, Garden Key, and Long Key. These are undoubtedly the 

 last traces of the deeper and wider channels, probably once run- 

 ning parallel to the Southeast Channel. They have gradually 

 been filled up since the sand flats of Bush Key began to form, 

 so that the free circulation of the tides throuo-h them has been 

 prevented. The presence of a few large heads of mseandrinas 

 and astrseans, as well as the luxuriant growth of Madrepora 

 prolifera (Fig. 47) near low-water mark, on the two sides of 

 these channels, now changed into sand flats, seems to indicate a 



