STRUCTURE OF CORAL ISLANDS. 173 



Florida reefs and the opposite shores or reefs along the 

 Bahamas and Cuba, called the Straits of Florida. A few para- 

 graphs on these straits by Mr. de Pourtales are cited from 

 his memoir referred to on page 167. The places described will 

 be found on the map at the close of this volume. 



" In transverse sections of the channel, the greatest depth 

 is nearest its southern or eastern shore, and in a longitudinal 

 section the depth diminishes in passing toward the north, 

 finding its minimum in the narrowest part between Cape 

 Florida and the Bemini Islands, after which it increases 

 again. In a transverse section between Key West and Ha- 

 vana, the greatest depth is 853 fathoms ; between Sombrero 

 Light and Elbow or Double-Headed Shot Key, on the Sak 

 Key Bank, 500 fathoms ; between Carysfort reef and Orange 

 Key, on the Great Bahama Bank, 475 fathoms ; and between 

 Cape Florida and the Bemini Islands, 370 fathoms. In fol- 

 lowing a cross section from the emerged coral reef called the 

 Florida Keys, the so-called Hawk Channel is first crossed, 

 limited outside by the living coral reef. Its greatest depth 

 is seldom more than six or seven fathoms, generally much less 

 toward its northern extremity; it is often interrupted by 

 shoals, and so-called heads of five coral, and its bottom con- 

 sists of calcareous mud from decomposed corals and corallines. 

 Next comes the reef, rising nearly to low-water mark, but by 

 no means continuous. It extends from Cape Florida, south 

 and west, to a short distance from beyond Key West, and 

 seems to be slowly increasing in that direction. 



" Although the deep blue colour of the water after passing 

 the reef seems to indicate a very abrupt slope, there is in no 

 part of it anything to compare with the sudden deepening on 

 the edge of the coral reefs of the Pacific Ocean, or even of 

 the Baham.as or the coast of Cuba. The distance from the 

 reef to the loo-fathom line is not less than three miles, and 

 often as much as six. In this space the bottom consists 

 of calcareous mud, and is not particularly rich in animal 

 life. From ninety or a hundred fathoms to two hundred and 

 fifty or three hundred, the bottom slopes rather gently in the 



