18 Proceedings of the Royal Physical Society. 



of the warm currents. East of the Mosquito coast in Central 

 America, one of these banks may be said to stretch com- 

 pletely across to Jamaica. Similar banks rise off the Yucatan 

 coast ; likewise on the windward side of the islands, where 

 the ocean-currents first reach them. 



That these banks lie upon volcanic ridges and peaks can 

 hardly be doubted ; though we have no means of telling what 

 depth of recent limestone may have accumulated upon them. 

 Among the islands recent volcanic masses rise high above 

 sea-level, in Martinique reaching a height of more than 4000 

 feet. And, as usual in volcanic regions, there are numerous 

 proofs of recent upheaval, such as the Basse Terre of Guada- 

 loupe, the successive terraces of recent limestone in Barbadoes, 

 and the upraised coral-reefs of Cuba, which lie at a height of 

 1100 feet above sea-level. 



The West Indian seas have long been famous for their 

 coral-reefs. Professor Agassiz insists that the distribution 

 of these reefs is determined by the direction of the food- 

 bearing ocean currents. They flourish on the windward side 

 of the islands and along the whole eastern coast of Honduras, 

 Venezuela, and Yucatan ; but on the leeward shores they do 

 not exist at all. Cuba is fringed both on the north and south 

 side with reefs ; but the southern reefs, directly bathed by 

 the Gulf Stream and exposed to the prevailing winds, are 

 more flourishing than the northern reefs, which are, to some 

 extent, cut off from the equatorial current by banks and 

 islands. 



The depth at which corals will flourish in these seas has 

 been found to be rather less than that which has been ascer- 

 tained to be in general their downward limit. Professor 

 Agassiz concludes that they do not thrive below a depth of 

 six or seven fathoms in the Florida seas, though on the 

 outer reef, directly exposed to the open currents and pre- 

 valent winds, they descend in scattered heads to about ten 

 fathoms. 



Each successive stage in the growth of an atoll seems to be 

 laid open for study in the prolongation of the Florida Eeefs. 

 The map of that region (Fig. 2) shows a remarkable broken 

 chain of islets and strips of land, running parallel with the 



