272 ANNUAL OF SCIENTIFIC DISCOVERY. 



depth to the surface. The formation of the two latter classes of reefs 

 has been ascribed to the subsidence of the bottom, combined with the 

 upward growth of the corals. The range of living, reef-building corals 

 has been ascertained to be limited between a depth of sixteen to twenty 

 fathoms, and a few inches below low- water mark ; for, unless con- 

 stantly submerged, they die ; but they are frequently found dead at 

 enormous depths, forming walls of coral rocks very precipitous. 



In Florida, we have no barrier reef, but a series of concentric reefs 

 enclosing parallel channels, formed without the slightest indication of 

 submergence or upheaval. There are the outer reef, the Florida keys, 

 and the shore bluffs, with the main channel, south of the keys ; the 

 mud flats, between the keys and the main land, with a slight depth of 

 water, often not more than two feet ; and flat, low islands, on which 

 there is an extensive growth of mangroves. The keys rise from ten to 

 twelve, seldom thirteen, feet above the level of the ocean. Near the 

 shore there are mud and coral sand accumulations, which are evidently 

 the results of the decomposition of the solid parts of the corals them- 

 selves. Beyond the keys, the channel is from five to six, and seldom 

 more, fathoms deep. Its boundaries are frequently indicated by small 

 islands or shoals, some of which form very dangerous reefs, such as 

 Carysfort reef. It is within this channel that the wreckers take up 

 their abode, being safely sheltered from the strong gales which blow 

 frequently outside, behind the walls of the outer reef and the bar 

 islands, rising for a few feet above the level of the ocean. No coast, 

 said Prof. A., has a more secure and safe navigation than this, if it be 

 properly understood ; every twenty miles there is the broadest and 

 salest harbor to run into. But, at present, it is perhaps more danger- 

 ous to know of these harbors than to be ignorant of their existence ; 

 for the lights and signals along the shore are located without reference 

 to using these places of refuge. 



Adverting to the geological and zoological character of the general 

 reef, the Professor remarked that it was important to ascertain whether, 

 as reported by some, the reef consisted of dead corals only ; or, as 

 others maintained, was composed of living corals, still growing and 

 extending ; or whether, as it had been asserted by others, it consisted 

 only of oolitic rocks, containing no indications of corals whatever. All 

 these statements are found to be consistent with the qualification that the 

 three classes occupy different localities. On the outer reef, from Cape 

 Florida to Key West, in from ten to twelve fathoms up to the surface 

 of the water, living corals are found, greatly differing, however, in con- 

 stitution the Madrepores (Madrcpora palmata} especially rising to 

 the very surface, while the commonly so called brain coral (Mcan- 

 drina) occurs in the lower, and the Astr&a in still lower levels. Speci- 

 mens of the different species were exhibited. The Madrepora palmata 

 form extensive fields of powerful stems, branching and expanding near 

 the surface into large flats, resting upon strong bases, and presenting 

 the appearance of leaves spread out. The Professor characterized these 

 fields as a wonderful spectacle. This extensive growth does not occur, 

 however, further than one or two fathoms below the surface. Lower 

 down a number of other species are found. 



