522 G. H. DREW. 



possible suggestion that the odour of sulphuretted hydrogen noticeable 

 in the deeper layers of the mud-flats might be due to the reduction of 

 the Calcium sulphate to a sulphide and subsequent decomposition of 

 the sulphide by bacterial action. 



These observations have shown that on the chalky mud-flats of the 

 Great Bahama Bank the B. calc.is is found in enormous numbers, and 

 also that this bacterium is capable of precipitating Calcium carbonate 

 from fluid media containing soluble Calcium salts. It would seem a 

 fair deduction that these mud-flats have been precipitated by the action 

 of the B. calcis on the soluble Calcium salts carried into the sea by 

 drainage from the land, where extensive and rapid weathering of the 

 limestone rock is in progress. 



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. However, 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 neighbourhood of 

 the Florida Keys, are now being precipitated by the action of the 

 Bacterium calcis on the Calcium salts present in solution in sea -water. 

 From this the suggestion is obvious that the Bacteriuvi calcis, or other 

 bacteria having a similar action, may have been an important factor in 

 the formation of various chalk strata, in addition 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 diagenetic 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 round Jamaica, the Dry 

 Tortugas, and the Bahamas, than in the temperate waters of the Bay 

 of Biscay and the English Channel, and hence an explanation is pro- 

 vided of the relative scarcity of plankton and algal growth in the 

 former localities, in accordance with the terms of Brandt's (2) hypo- 

 thesis. 



