THE TORTUGAS AND FLORIDA REEFS. 127 



feed and which fills their digestive cavity. Their action, however, while undoubtedly 

 an important one in that they reduce the sand to a smaller size, is yet very slight as 

 compared to the action of the breakers upon the sea face of the reef. Darwin. Dana, 

 and others have referred to this agency as one among those at work in triturating the 

 corals. By some observers, these animals are supposed to be simpl}'^ on the living 

 coral. This is certainly not the case either with Holothurians and Diadematidte, or 

 with Clypeasteroids : livingon flats, they swallow the sand as they find it. But with 

 Cidaris and Echinometra, which dig out holes in the rock, the case is different. 

 H. H. Guppy has also observed the Holothurians full of sand on the flats of the 

 reefs of the Solomon Islands. 



The Loggerhead, Bird, and Bush Keys Banks, which protect each other to a certain 

 extent from the action of the strong winds opposed to the prevailing trade winds, 

 present a more normal growth than that of the Ea.st, North Key Bank, which is par- 

 ticularly exposed to the full fury of the northers, which must counteract to a great 

 extent the action of the trade winds. This can be seen on sections (Plate III.) 7-7', 

 8-8', 9-9', which show a nearly equal slope on the eastern and western sides of the 

 bank. The distribution of the broken ground, the position of the masses of Madre- 

 pora cervicornis, and the trend of the sand flats, all alike show the conflicting action 

 to which the two slopes of this great bank have been subjected. This counterbalan- 

 cing action of the northers and of the trade winds is also well shown by its effect on 

 the position of the islands themselves. During the prevalence of southeasterly winds, 

 East Key, Sand Key, and Middle Key extend bodily to the westward, the materials 

 for their growth being washed from the eastern shores. The opposite takes place 

 during the prevalence of northers. The outline of Loggerhead Key is also constantly 

 shifting, and, according to the officers of the Lighthouse Board, none of the landmarks 

 furnished by these islands can be relied upon in the location of buoys. 



What takes place upon the shores of the islands also takes place, of course, upon 

 the flats. Owing to the action of the winds and waves, the whole mass of the surface 

 of the reef is kept in more or less active movement, according to the depth of the flats 

 and to their position. The coarser materials covering the flats and shore lines, made 

 up of large-sized fragments, are gradually changing to the coarse sand forming the 

 flats nearer the outer edge of the reefs ; and these, in their turn, are changed into 

 the fine silt which fills the channels and eventually limits the growth of the corals to 

 regions where they can find permanent lodging, and are not immediately under the 

 influence of this shifting sand and silt. The quality of the sand forming the beaches 

 at different points on the keys and flats depends entirely on its position. It will be 



