124 THE TORTUGAS AND FLORIDA REEFS. 



(Plate III.) shows at a glance the mound-shaped mass which forms the base, rising 

 from the general twenty-five to thirty fathoms level, with its abrupt side facing the 

 east on the extremities 1-10' * of the section lines. There are also seen the deep 

 furrows, more or less broad, which have been scooped out of this mass by the 

 action of the currents, such as those passing through the Southwest Channel. (See 

 lines 1—1' to 6-6', with the secondary channels cut between the Loggerhead Bank 

 and the Brilliant and White Shoals, Bush, Garden, and Long Keys ; lines 2-2', 3- 

 4-4', 5-5', as well as the primary channels formed by the Northwest Channel and' the 

 secondary between Middle and East Key, on line 7-7' and 8-8'.) With this prelimi- 

 nary examination of the relief of the Tortugas, we can now pass to the examination 

 of the distribution of the more important species of corals, and see how far we are able 

 to explain the peculiar formation of the Tortugas from causes still in operation at the 

 present day (Plate II.). 



The corals which give to the reefs their peculiar physiognomy are the extensive 

 patches of Madrepora (principally M. cervicomis), the clusters of the two common 

 species of Porites (P. furcata and P. clavaria) more or less covering the shallow 

 tracts of coarse sand, and Mcmndrim areoluta growing between the more or less 

 extensive patches of marine lawns formed by a species of Thalassia, with occasional 

 patches of Anadyomene. In other parts of the reefs large Holothurians (Miilleria) 

 lie scattered on the bottom, or in somewhat deeper regions we find pockets filled 

 with large Diadematidoe. Immense masses of Nullipores {TJdotea Halimedea) and 

 Corallines grow on the shallowest flats on the tops of the branches of Madrepores 

 which have died from exposure to the air, either from growing up to the surface 

 and becoming exposed by extreme low tides, or from the action of strong winds 

 blowing the water from the flats. The destructive effect of an extremely low tide 

 on a growing reef is well shown on the flats to the southward of Fort Jefferson, 



where the upper part of the branches of a certain size reaching up to a given 

 level are frequently killed off by low tides. Exposure to the action of the sun 

 even for a very short time is sufficient to kill them. The extreme sensitiveness of 

 all corals to atmospheric action is well known, so that it becomes plain, as has 

 been stated already by Darwin, Dana, and others, that no coral reef can grow 

 above the level of the lowest tides, and that all subsequent additions of material 

 must be due to accumulation of sediment transported by the action of the tides 



Loggerhead Bank 

 l face. Although 



posed 



