TIIE TORTUGAS AXD FLORIDA REEFS. 123 



key. This bank we have called 



(Plates III. and IV.) Loggerhead B 



Between this and the Garden Key and Long Key Bank, there are a few shoals 

 running more or less parallel with Loggerhead Bank, the largest of which are Brilliant 

 and White Shoals. Garden Key and Long Key Bank form a rectangular shoal of 

 nearly the 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 Channel, 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, with 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 Southwest Channel. 



An examination of the sections of the Tortugas from the west to the east, on 

 the lines parallel to A A', B B r , etc. (Plate IV.), shows the gradual rise of the 

 mound forming the Tortugas, as we pass from the west side of the Loggerhead Bank 



along the line A A! to a line passing through the southwest slope of the same bank, 

 B B, till we come across the main bank of the group on the line C C, and again tail 

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

 line D D', till we finally come to the low elevation forming the southeast slope of 

 East Key Bank along the line E E\ The action of the Southeast and Northwest 

 Channels in keeping open the passage between Long Key and Loggerhead Bank, 

 and that between North Key Bank, is well shown on the sections B B' and C C,' as 

 well as the secondary channels separating Bird Key, Garden Key, and Long Key. 

 These channels are undoubtedly the last traces of the deeper and wider channels, 

 probably once running parallel to the Southeast Channel. They have little by little 

 been filled up after the sand flats of Bush Key began to form, thus preventing the free 

 circulation of the tides through these channels. The presence of a few large heads 

 of Maeandrinas and Astrseans, as well as the luxuriant growth of Madrepora cervkorm 



low-water mark 



sides of these channels, now changed into sand flats 



(see Plate II. ), seems to indicate formerly a more active tidal circulation through them 

 than is now taking place. An examination of the cross-sections in 



the direction of 



the prevailing winds, and in the direction of the tides along the lines 1-T to 10-10 



