slowly tending to fill in the channel, where small patches 

 of reef heads are formed. The living reefs do not ex- 

 tend north of Fowey Rocks, not only because of the 

 drop in temperature as one travels north, but mainly, 

 it has been suggested, because of a southward drift of 

 siliceous sands which kill or restrict coral growth by 

 silting action. 



The Marquesas and Tortugas reefs, which have been 

 called atolls are not properly so, but are ring-shaped 

 reefs built on shoals of sediment on the shallow banks. 

 They are not associated directly with subsidence, nor 

 are they fringed by deep water, in the way true atolls 

 are. 



To the west of Florida the shallow muddy bottoms 

 do not offer a foothold for corals. Nevertheless recent 

 diving expeditions carried out by biologists of the 

 Marine Laboratory have shov/n the presence of a con- 

 siderable reef coral growth in depths of from 50 to 1 50 

 feet, in the Gulf of Mexico, west of Tarpon Springs, 

 Florida. 



The Florida reefs are best studied some miles south 

 of Miami and are easily examined through a glass 

 bottomed bucket from a small boat in fair weather. 

 Many of the smaller coral species may be found close 

 inshore by those willing to wade a short distance at 

 numerous places in the Florida Keys. Some are found 

 as far north as the shores of Elliot Key or on the bars 

 south of Biscayne Key. In small patches, reef corals 

 are to be found considerably north of Miami, but only 

 offshore and in relatively deep water. 



'Bahamas. The same general process that underlies 

 the origin of the Florida reefs is probably true of the 

 Bahamas. The living reefs, however, are much better 



39 



