174 CORALS AND CORAL ISLANDS. 



shape of a rough rocky floor, without great inequalities ; this 

 formation obtains its greatest breadth, of about eighteen miles, 

 a little to the east of Sombrero Light, and tapers off to the 

 west, where it ends in about the same longitude as the end of 

 the reef; toward the east and north it approaches nearer the 

 reef, and ends gradually between Carysfort reef and Cape 

 Florida. This bottom, which is called ' Pourtales Plateau ' in 

 Prof Agassiz's report (see map), is very rich in deep-sea corals, 

 the greatest number of those described in these pages [the 

 memoir here cited from] having been dredged on this ground. 



''Outside of the rocky bottom the Globigerina mud pre- 

 vails and fills the trough of the channel. 



" On the Cuba shore the bottom is rocky and the slope 

 very abrupt, particularly for the first four or five hundred 

 fathoms. Along the Salt Key and Bahama Banks, the slope 

 is also exceedingly abrupt, but the underlying rock is often 

 covered with mud." 



Prof Agassiz observes that the rocky bottom of the Pour- 

 tales Plateau is a true coral-rag — in other words, ordinary coral 

 reef-rock — being made up of an agglomeration of fragments of 

 corals and sand, cemented into a solid limestone. 



Bahama Islands. — The Bahamas (the western margin of 

 which is shown on the map of the Florida Reefs) are coral 

 reefs and reef islands, essentially like atoll reefs. The northern 

 end of the group lies opposite southern Florida, and from this 

 point they stretch off to the west of south-west in a double 

 series, nearly parallel to the trend of Cuba and San Domingo, 

 and terminate properly in Turk's Island and some other 

 reefs north of the latter, — the whole length above 600 miles. 

 The loo-fathom line of soundings extends around the 

 two northern ranges of reefs and islands, which, therefore, 

 make up one bank, the Little Bahama Bank ; and another 

 similar line embraces the next six islands as parts of a 

 second bank, called the Great Bahama Bank, whose whole 

 length is about 300 miles. New Providence Island, the site 

 of the seat of government of the group, Nassau, is the middle 

 one of the three northern islands of this bank. The relation of 



