FLORIDA REEFS. 37 



All the key-s, all the islands along the whole extent of the reef, and all 

 that part of the penmsula which consists of coral formations, have been 

 formed by subaerial deposits accumulated under tidal agencies. The con- 

 trast between these subaerial deposits and. the rock of submarine origin is 

 striking;:. Near the shore the torrential character of the stratification is still 

 evident ; but at some distance from the islands and the main-land the strata 

 are more regular, and contain, also, more and better-preserved fossil remains. 

 The depth at which coral reefs start from some given foundation is stated 

 by different authors to be from twelve to twenty fathoms. It would seem 

 that the Florida reef has sprung up in somewhat shallower waters, for be- 

 low ten or twelve fathoms there are no indications of living coral growth. 

 The outer reef ends off the Marquesas in a depth of about ten fathoms. 

 The ship-channel itself is only some ten fathoms in depth there, and very 

 few coral heads are found in it. Add to this, that the depression between 

 the Marquesas and Tortugas, though for the greater part not exceeding 

 twelve or thirteen fathoms, is entirely unobstructed by coral heads, and that 

 in the ship-channel there are very few coral heads noticed below a depth of 

 six or seven fathoms. We are therefore inclined to believe that, in lati- 

 tudes bordering on the Tropics, the normal depth for the foundation of 

 a coral reef is from ten to twelve fathoms, and that, if they spring up from 

 greater depths, it must be in the Tropics proper. 



JPh//swai Changes in tJie Gulf Stream. 



There are several questions of the deepest scientific interest, which may 

 be advanced by a due consideration of the facts observed upon the reefs of 

 Florida. There we have a peninsula — a narrow, flat strip of land, project- 

 ino; for about five des-rees from the main-land — between the Atlantic Ocean 

 and the Gulf of Mexico, and forming an effective barrier between the waters 

 of the two seas, which otherwise, even by the change of a few feet in the 

 relative level of the intervening peninsula, would communicate freely with 

 one another ; and this peninsula we now know to have been added to the 

 continent, step by step, in a southerly direction. 



We know that the time cannot be far behind us when the present reef, with 

 its few keys, did not exist, and when the channel, therefore, was broader, 

 and the Gulf Stream flowed directly along the main range of keys. We 

 know, further, that at some earlier period the keys themselves were not yet 



