44 Papers from the Marine Biological Laboratory at Tortugas. 



CONCLUSION. 



The observations so far available are too few, and the area they cover 

 too small, to attempt to make any broad generalization at present. How- 

 ever, it can be stated with a fair degree of certainty that the very extensive 

 chalky mud flats forming the Great Bahama Bank and those which are 

 found in places in the neighborhood of the Florida Keys are now being 

 precipitated by the action of the BacteriiLm calcis on the calcium salts 

 present in solution in sea-water. From this the suggestion is obvious that 

 the Bacterium calcis, or other bacteria having a similar action, may have 

 been an important factor in the formation of various chalk strata, in addi- 

 tion to the part played by the shells of foraminifera and other organisms in 

 the formation of these rocks. Dr. T. Wayland Vaughan has also suggested 

 that the Miami oolite and other oolitic rocks may owe their origin to the 

 occurrence of some diagenic change in the precipitate of very finely divided 

 particles of calcium carbonate produced in this way by bacterial action. 

 If this view as to the formation of chalk and oolite rocks is correct, it would 

 seem probable that these strata must have been deposited in comparatively 

 shallow seas whose temperature approximated to that of tropical seas at 

 the present time. 



It has also been shown that bacterial denitrification is far more rapid 

 and complete in the tropical seas around Jamaica, Tortugas, and Andres 

 than in the temperate waters of the Bay of Biscay and the English Channel, 

 and hence an explanation is provided of the relative scarcity of plankton 

 and algal growth in the former localities, in accordance with the terms of 

 Brandt's hypothesis. 



The distribution of the bacteria, both as to numbers and species, has 

 been shown to vary at different localities and at dififerent depths, but there 

 are at present too few observations to enable any conclusions or generaliza- 

 tions to be drawn. 



As it now stands, the investigation can at most be considered to offer 

 a mere indication of the part played by bacterial growth in the metabolism 

 of the sea. To obtain a real insight into the question, it would be necessary 

 to make more extensive bacterial and chemical observations in tropical, 

 temperate, and arctic waters, to study the bacteriology of other areas where 

 calcium carbonate is being precipitated from the sea, and to make further 

 investigations in the laboratory into the chemistry of the reactions that can 

 be brought about by various species of marine bacteria. 



