FLORIDA REEFS. 



even an extensive interruption in the succession of the keys before we reach 

 the Tortugas. These last, however, as well as the bank west of these keys, 

 belono- none the less to the main range of keys, from which they are only 

 separated bv a more extensive and deeper depression. West of Sand Key 

 the reef itself becomes gradually less elevated, until it is finally lost where 

 the ship channel, south of the Marquesas, expands into the broad depression, 

 separating that group of keys and shoals from the Tortugas. 



In order to understand fully not only the topography, but also the mode 

 of formation of all these keys and reefs, it must be remembered that the 

 risino" reefs, which form more or less continuous walls, reaching at unequal 

 heights nearly to the surface, or above the level of the waters, are only 

 a particular modification of those formations growing upon coral grounds 

 under special circumstances. It has been ascertained, whenever similar 

 investigations have been made, that living corals do not occur in depths 

 exceeding twenty fiithoms, tbat the reef-building species prosper from 

 a depth of about twelve fathoms nearly to the surface, and that different 

 species follow each other at successive heights. Now, if we keep in mind 

 these facts, we shall see that all the coral-bound islands of the West Indies, 

 as well as of the main-land of Central America, constitute an extensive coial 

 field, divided by broad, deep channels, over which the coral reefs extend, 

 with different features, according to the depths in which they occur and the 

 chano-es which their own growth has gradually introduced uj)on the locali- 

 ties where they are found, influenced and modified to some extent also by 

 the direction of the prevailing currents and the action of the tides. 



The formation of the main range of keys in their primitive condition as 

 a reef, — for, as we shall see hereafter, they have been a submarine reef 

 before they rose as islands above the level of the ocean, — the formation 

 of this range, we repeat, at gradually greater distances from the main-land, 

 as we follow their course from east to west, has been simply owing to the 

 depth of the bottom from' which the reef has risen. It has followed the line 

 of ten or twelve fathoms' depth ; and if there is so wide an interruption 

 between the Marquesas and the Tortugas, it is because the ground is deeper 

 over that space. Again, if the Pine Islands have a nortlnvesterly direction, 

 while the main range runs more from east to west, it is no doubt because 

 the body of water emptying from the northern part of the gulf, along the 

 western shores of the peninsula, has, for a time, run chiefly over that field, 

 while the tract of mud flats between the keys and the main-land was filling 



