86 THREE CRUISES OF THE " BLAKE." 



each other to a certain extent from the action of the stronsr 

 winds opposed to the prevailing trade-winds, present a more 

 normal growth than that of the North Key Bank, which is par- 

 ticularly exposed to the full fury of the northers ; they must 

 counteract to a great extent the action of the trade-winds. The 

 distribution of the broken ground, the position of the masses of 

 Madrei:)ora 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 counterbalancing 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 op- 

 posite takes place during the prevalence of northers. The out- 

 line 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 water on the flats and to their position. The coarser mate- 

 rials covering the flats and shore lines, and made up of large- 

 sized fragments, are gradually changing to the coarse sand 

 which forms the flats nearer the outer edge of the reefs ; and 

 this, in its turn, is changed into the fine silt which fills the 

 channels, and eventually limits the growth of the corals to re- 

 gions where they can find permanent lodging, and are not im- 

 mediately 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 coarser or finer, according to the exposure of the beach, and 

 the finest sand is found in the most sheltered places, where the 

 silt has free chance to settle. (See Fig. 53 for a view of a char- 

 acteristic coral sand beach at Key West.) The scarcity of 

 fossils in the coral limestones of the reef has already been dwelt 



