FLORIDA BANKS. 209 
The banks continue’ westward of Key West to the Mar- 
quesas Key, which is atoll-like in form and probably in ori- 
gin. But it is so situated at present within the sand-made 
area that there are no growing corals “on its weather-side.” 
Farther west a true atoll—that of the Tortugas — stands 
apart from the bank, a channel of eleven to sixteen fathoms 
intervening between it and the Marquesas reef. Branching 
corals and others are growing in abundance about it, which 
are the source of the coral fragments that are thrown on the 
beach. The bank may perhaps receive some light calcareous 
silt from the reefs to the eastward ; but the intervening chan- 
nel through which the tide sweeps must be filled before drift- 
ings from the eastward will reach effectively the Tortugas. 
Whether a bend in the south end of the peninsula of 
Florida determined the direction of drift of the marine cur- 
rents in the region, and the bend in the form of the skirting 
reef, or whether the direction of current drift alone gave di- 
rection to the belt of reef, is not determined by any positive 
facts. The east-and-west trend of Cuba favors the former of 
these explanations; and the position of the Tortugas leads 
to the conclusion that it was probably an atoll off the ex- 
tremity of the Florida Bank. An artesian boring in the 
centre of the Tortugas with a diamond drill that would sup- 
ply a large core for study, would give the facts for positive 
conclusions as to the nature of the basement and the depth 
to which it consists of coral-reef rock. 
No good evidence of elevation of the Florida Bank has 
been reported. The height of the islands and cays is not 
satisfactory evidence, as it is so generally due to sand-drifts. 
At the same time the opinion that there had been a rise of 
half a dozen feet or so may be found to have much in its 
favor. 
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