480 G. H. DREW. 



The main contentions raised in this paper are — 



(1) That in the seas of the American tropics bacteria exist which 

 are actively precipitating Calcium carbonate from the Calcium 

 salts present in solution in sea-water. It is suggested that this 

 bacterial action has been a very considerable factor in the formation of 

 chalk and many other varieties of sedimentary rock, chiefly, or in 

 part, composed of Calcium carbonate. It is also contended that the 

 vast deposits of chalky mud now being formed to the West of the 

 Bahamas, and in the neighbourhood of some of the Florida Keys, are 

 being precipitated by bacterial agency, and that a similar process 

 plays an important part in the cementation of fragments of coral and 

 other detritus into compact coralline rock. 



(2) That the destruction of Nitrates by bacterial action in the seas 

 of the American Tropics is far in excess of that occurring in Tem- 

 perate waters. Hence an explanation is afforded of the relative 

 scarcity of plant life (and consequently of animal life) in Tropical as 

 compared to Temperate seas, in accordance with the terms of 

 Brandt's (2) hypothesis. 



Preliminary notes on this work have already been published in the 

 Tortugas Laboratory Eeports for 1911 and 1912 (4 and 6) and in the 

 Journal of the Marine Biological Association (5). The chrono- 

 logical sequence of the investigations will be followed in the account 

 given here of the experimental work. 



GENERAL CONSIDERATIONS AND PREVIOUS WORK. 



It is generally conceded that the plankton of Tropical and sub- 

 Tropical seas is far less in quantity than that found in colder waters.* 



The zoo-plankton depends ultimately for its food on the phyto- 

 plankton, hence any factor limiting the growth of the phyto-plankton, 

 which was capable of exercising its influence in Tropical and not in 

 Temperate or Arctic waters, might offer an explanation of this pheno- 

 menon. It has been shown by various investigators that this factor is 

 not temperature, light, or salinity, and it has been suggested that the 

 explanation may lie in the relative deficiency in Tropical seas of the 

 Nitrates or nitrogenous compounds which are so essential for all plant 

 life. A matter of common observation in support of this view is the 

 remarkable scarcity of Algal growth in the shallow waters of Tropical 

 shores as compared with that in Temperate regions, and the fact that 

 in the Tropics, wherever sewage or other nitrogenous waste is poured 

 into the sea, a free growth of Algae is found. 



* For the most recent work, and full discussion of this subject, see "The Depths of 

 the Ocean," by Murray and Hjort (13), p. 386 et scq., London, 1912. 



