392 THE BOTTOM OF THE SEA. 



ture of the sea is 60° Fahr. A difference of tem- 

 perature, amounting to 17° Fahr., therefore arrests 

 the growth of the coral reefs. 



The Persian Gulf, the Red Sea, and that part of 

 the Indian Ocean comprised between Africa and 

 Sumatra, are also very rich in coral. We must, 

 however, remark that these seas have the highest 

 temperature of any on tlie globe. 



For the same reason coral flourishes in the seas 

 of the West Indies, and on the eastern coast ot 

 Florida. The researches of Agassiz shovv that the 

 entire peninsula of Florida is formed of rocks 

 belonging to the present epoch, and that these 

 rocks are principally composed of coral reefs and 

 marine shells. The southern and western coasts of 

 Florida are surrounded by an immense number of 

 islands, separated one from another by very narrow 

 channels. These islands are in many instances 

 connected at low-tide, or even sometimes only sepa- 

 rated irom dry land by flat marshes. 



These islands, designated " Keys," form concentric 

 lines fronting the continental shore, from which they 

 do not extend a great distance, the most remote 

 being ten leagues off. They rise little more than 

 from six to twelve feet above the level of the sea ; 

 and, like the land itself, are composed of coral, both 

 in masses and in sand thrown up by the sea, the 



