126 THE TORTUGAS AND FLOKIDA EEEFS. 



upon the exposed slopes of the reefs destroy, even on calm days, large quantities 

 of corals which have been weakened by the borings of Mollusks, Annelids, Echino- 

 derms, and Sponges. On windy or stormy days the powdered fragments are driven 

 far and wide, turning all the surrounding water to chalk color for a considerable 

 distance from the reef. It is not an uncommon thing after a blow to come upon 

 this water discolored by the fine calcareous silt it holds in suspense to a distance 

 of six to ten miles from the outer reef. I have seen between two and three inches 

 of fine silt deposited in the interval between two tides after a prolonged storm. 

 The limitation of coral reef growth to shallow depths may be due to the fact that 

 the ooze held in suspense rapidly sinks to the bottom from the top, the surface water 

 remaining clear. The rapidity with which the corals are choked readily explains 

 why they must of necessity have a limited vertical distribution depending upon 

 local causes. This is well shown along the sections of Tortugas off the Marquesas, 

 along the line of the main reef. We find corals living and flourishing at a much 

 greater depth, and there seems to be no simpler explanation of the limited bathy- 

 metrical range than that of the baneful action of the silt held in suspense near all 

 reefs. That the silt is carried on the bottom by currents and waves is well known, 

 and on the bottom of the Gulf Stream to the north of the Straits of Bemini as far 

 as Cape Hatteras, we have a huge muddy river carrying its silt to the steep slope 

 south of Hatteras, depositing occasionally a few patches of green sand along the 

 sides of its course, while the upper waters are perfectly clear and of the deepest 

 blue. 



Corals alone cannot supply all the sand we know to be carried by the Gulf 

 Stream. We must add to this the silt, mud, and sand which comes from pelagic 

 animals, and which is distributed by the winds and waves, to be spread uniformly 

 over large areas, as is well shown from the distribution of the immense mass of cal- 

 careous ooze over the whole of the bottom between Florida and Cuba. This undoubt- 

 edly owes its origin in part to the silt the Gulf Stream brings from the southeastern 

 edge and slope of the Yucatan Bank, in addition to the accumulation due to the 

 pelagic fauna which it sweeps along its course. The amount of work done by the 

 animals living upon the reef in preparation for the grinding process of the breakers 

 is very great. All writers upon the reefs have referred to the destructive agency of 

 boring Mollusks, Annelids, and Echinoderms riddling the coral branches and heads 

 with holes, and preparing the way for their fracture into larger or smaller fragments. 



The Echinoderms living upon the flats seem to live almost exclusively upon the 

 organic matter and Foraminifera they find mixed with the coral sand, upon which they 



