THE FLORIDA REEFS. 81 
same length 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 
Kastern 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 through 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 Kast 
Key Bank, till we finally come to the low elevation forming the 
southeast slope of Hast 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 through them has been 
prevented. The presence of a few large heads of mueandrinas 
and astreans, 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 
