182 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



there are in many places several reefs overlapping each other, it seems 

 probable that the process on the Brazilian coast is an approximately con- 

 tinuous one to which this method of derivation is hardly applicable. 



Carbon dioxide from sea-icater, but not of eruptive origin. — Analyses 

 of sea-water show that it contains carbon dioxide in considerable but 

 varying quantities. It is most abundant in the cold waters of polar 

 regions and of the deepest oceans. 



Dittmar says the atmosphere and the decay of organisms supply this 

 cai'bon dioxide, but he seems to tliiuk tliat most of it issues, like sub- 

 marine springs, beneath the ocean waters.^ 



Knudsen thinks that "predominance of animal or vegetable life in 

 any part of the sea causes the variations in the amount of contained 

 oxygen or carbonic acid." ^ 



In studying this matter we should not lose sight of the fact that 

 marine animals having lime carbonate skeletons do not derive that lime 

 carbonate directly from the water, but they must get it through the 

 agency or interposition of the plants they use for food ; that is, the 

 plants take it from the water and deliver it to the animals. We need 

 not especial!}' concern ourselves, however, with the source of supply ; for 

 present purposes the fact of prime importance is this difference in car- 

 bon dioxide contents between the polar and tropical sea-waters, and the 

 general tendency of the carbonates to accumulate at the equator.^ 



"Whatever may be its cause, there is then a general tendency for car- 

 bon dioxide to be taken up about the poles and liberated in the tropics. 

 Dr. John Murray pointed out in 1889 how the organic matter in the 

 ocean is attacked and dissolved by sea-water. "As soon as life loses its 

 hold on the coral structures, and wherever these dead carbonate of lime 

 remains are uupi'otected . . . they are silently, surely, and steadily re- 

 moved in solution."* The methods by which the sea-water gets access 

 to the calcareous particles can be seen along coral reefs and in sim- 

 ilar shallow seas during a gale. The waters are affected to a gi-eater 

 depth than usual, and the softer debris over the bottom is stirred up and 

 mixed with the water, giving it a milky appearance. Under such cir- 

 cumstances, the calcareous particles are exposed to the water through 

 its entire depth. 



1 Report on the scientific results of the exploring voyage of H. M. S. " Chal- 

 lenger." Physics and Chemistry, I., p. 213. London, 1884. 



2 Nature, June 30, 1898, LYIIL, p. 201. 



' On the distribution of diatoms in polar seas, see An introduction to the study 

 of seaweeds, by George Murray, p. 19, 197. London, 1895. 

 * Nature, 1889, XXXIX., p. 424. 



