130 CRUISE OF THE BARRERA 



the land. Whatever silts are brought down by the 

 small and unimportant rivers appear to be caught 

 by the shore swamps through which the rivers 

 sluggishly meander before reaching the sea. Over 

 all the bottom there are evidences of deposit of a 

 certain amount of oolitic mud resulting from the 

 activities of the denitrifying bacteria that release 

 the calcium in sea-water solution into infinitely 

 fine particles calcium-carbonate. The amount 

 here of this oolitic "mush" is exceedingly small as 

 compared with the vast areas covered by it in 

 southern Florida and the Bahamas. 



Over much of the bottom is a growth of Thalassa 

 and other grasses and weeds, especially within the 

 bays. In the fairway are extensive areas of clean 

 sand-bottom interspersed with patches of marine 

 vegetation. Except within a belt adjacent to 

 the reef there are but few gardens of sponge and 

 gorgonians. 



The last feature of the region, as already noted, 

 is the reef itself on the outer edge of the island 

 shelf. It varies in width from a few yards to a 

 half-mile, and its continuity is broken by numerous 

 channels across. Some of these afford ample way 

 for entrance of vessels of ordinary draft. 



