246 



SCIENCE 



[N. S. Vol. XLIII. No. 1103 



ing give them still further interest and help 

 to confirm an idea recently expressed by T. 

 W. Vaughan^ in his geologic research on the 

 origin of the islands in the waters in which 

 these species of Halophila grow, viz., that 

 South Florida and the adjacent Keys have 

 undergone a recent depression. The material 

 of which these islands is formed was deposited 

 in Pliocene times, this material being con- 

 tributed by various agencies, coral reefs and 

 calcareous mud precipitated by denitrifying 

 bacteria, etc. The land thus formed suffered 

 a series of oscillations which Vaughan tabu- 

 lates and which the writer here reproduces 

 from Dr. Vaughan's paper. 



Oscillations of South Florida 



T? f_ i Uplift. 



Keeent ^ Depression (modern reefs). 



TDepression (Pleistocene reefs, part 



_, . , J of wliieh stood as mueh as IS 



Pleistocene- i ^^^^ ^^^^.^ ^^^ jg^^l)^ 



l-Uplift. 

 _,,. /Depression (some coral reefs but 



Pliocene— j ^^ well-developed reefs). 



Vaughan says in part concerning these 

 movements : 



Pliocene deposition was followed by uplift 

 which was succeeded by depression during the 

 Pleistocene subsidence along a curve from the 

 eastern side of Biscayne Bay, first trending south 

 and then bending west, a barrier coral reef flour- 

 ished, separated by a channel from the main bank 

 on which the Miami oolite was forming or had 

 formed in strongly agitated waters. West of the 

 coral reef, on an extensive flat in shoal water, the 

 Key West oolite was formed, while still further 

 to the westward the Tortugas were outlined under 

 the influence of water and currents. 



This period of events was succeeded by the ele- 

 vation of the entire key region to more than fifty 

 feet above its previous level. This uplift was suc- 

 ceeded by a depression, lowering the surface thirty 

 feet or more, establishing the same relation of 

 the sea level to the land that now prevails. Sub- 

 sequent to the beginning of the last depression the 

 present barrier reef has developed seaward of the 

 keys on a platform already prepared for it; the 

 Marquesas have been formed by winds and cur- 



9 Vaughan, Thomas Wayland, U. S. Geological 

 Survey, "Building of the Marquesas and Tortugas 

 Atolls and A Sketch of the Florida Reef Tract," 

 Pub. No. 183, Carnegie Inst, of Wash., D. C, 1914. 



rents and coral reefs have reestablished them- 

 selves in the Tortugas. 



From this it is seen that the land in the 

 region in which these two species of Halo- 

 phila have been collected has in Eecent times 

 undergone a considerable depression. The 

 writer therefore assumes that these plants 

 grew, in earlier times, in much shallower 

 water and in depths at which they are more 

 generally found to-day. Small^" mentions the 

 genus as occurring in " shallow water," Bal- 

 four collected H. ovalis on Rodriguez on the 

 reefs surrounding the island " on shoals just 

 uncovered at low water " and H. sUpulacea in 

 " slightly deeper places where it is sub- 

 merged." Baron d'Eggers,^^ who collected H. 

 Baillonis Aschers. in Saint Thomas harbor, 

 says: 



The plant grows in coarse sand at a depth of 

 two to four fathoms, here and there. 



The habitat of H. Beccarii Aschers., too, has 

 been well described by Beccari^^ himself in his 

 book, where he says: 



Toward dusk on the bank of the Bintulu I 

 caught some curious crustaceans and a small water 

 snake. It was only after I had been wading 

 about for some time that I discovered that the 

 soft substance under my feet was not mud but a 

 sheet of vegetation composed of a minute sub- 

 merged plant hidden by a thin layer of fine slush 

 so that it was not easily distinguished at first. I 

 afterward found that in some places it was un- 

 covered and quite exposed. 



From these references it is seen that the 

 genus is usually found in comparatively shal- 

 low water. The sea grasses in general, accord- 

 ing to Ascherson,^^ are found in depths not 

 over ten meters but Lorenz^* reports Posidonia 

 Oceanica (L.) Del. as occurring in the Gulf 

 of Quarnero at a depth of eighteen to thirty- 



10 Small, J. K., loc. eit. 



11 d'Eggers, Baron, "Flora of St. Croix and the 

 Virgin Islands," Smitkson. Inst. Bull. Nat. Mus., 

 No. 13, p. 98, Wash., D. C, 1879. 



12 Beceari, Odoardo, ' ' Wanderings in the Great 

 Forests of Borneo," London, p. 262. 



13 Ascherson, P., loc. cit. 



i* Lorenz, Th., ' ' Physik. Verhaltnisse und Ver- 

 teilung der Organismen im Quarn Golfe," p. 249, 

 Wien, 1863. 



