86 THREE CRUISES OF THE ‘“ BLAKE.” 
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 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 
Madrepora 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, Kast 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 
