THE TORTUGAS AND FLORIDA REEFS. 119 



been produced, not by the uplifting of the continent, but by the gradual rising of the 

 bank itself into suitable depths in consequence of the accumulation of animal ib'bm 

 upon it. That level once attained, reef-building corals would first establish themselves 

 on such spots as were most favorably situated, with reference to currents and pre- 



g winds, both of 



healthy growth, and thus the reef 



would be begun 



How far the growth of corals is affected by such local conditions is perhaps 

 nowhere better seen than in the smaller West Indian islands. On the eastern 

 side, exposed to the prevailing trade winds and washed by the great equatorial 

 currents, the corals flourish, while on the lee side they do not exist at all. The 

 whole eastern coast of Honduras, of Yucatan, and of Venezuela, exposed to the 

 same action and washed beside by the Gulf Stream, is studded with coral reels. 

 To the action of the Gulf Stream on the south coast of Jamaica and of Cuba we 

 must ascribe the presence of extensive coral reefs, and to the same cause is un- 

 doubtedly due the great San Pedro Bank. The fringing reef which skirts nearly 

 the whole northern coast of Cuba is in a less flourishing condition than the Florida 

 reef on the opposite shore, which is reached not only by the main current of the 

 Gulf Stream, but also by the prevailing winds. For a similar reason, corals are 

 found alive only on the edges of the Great Bahama, where they are subjected to the 

 beneficent action either of currents or of winds, driving the silt clear of the growing 

 corals, and bringing an abundant supply of food. The same causes which have formed 

 the great mud banks to the northward and westward of the Florida Reef have, in the 

 case of the Bahama Banks, formed the immense sand flats and shallows which are 

 fringed by living corals on the east and west. They owe their existence, on the one side, 

 to the wash of the northerly trend of the great equatorial current and to the action of 

 the trades ; on the other, to the clearing action of the Gulf Stream. It must also 

 be remembered that the Bahama Plateau was originally joined to Florida, as part of 

 the great fold which built up the framework of that peninsula, and that it was also 

 connected at one time with the island of Cuba. It was also united with the reefs, 

 now elevated to eleven hundred feet, which joined the eastern and western islands 

 in more recent geological times, and formed, before the Tertiary, the two extremities 

 of Cuba. On the southern side the reefs are still in Ml activity, while on parts of the 

 northern coast, in the vicinity of Havana, they have been elevated to a height of no 

 less than one thousand or eleven hundred feet, while the present barrier reef of the 

 north shore of Cuba forms an immense reef, extending nearly without break from 

 Cape San Antonio to the eastern edge of the old Bahama Channel. The Bahama 







