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-water, 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 
carbon dioxide, but he seems to think that 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 especially 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 unprotected . . . 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 greater 
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 Н. М. $. “Chal- 
lenger.” Physics and Chemistry, I., р. 213. London, 1884. 
2 Nature, June 30, 1898, LVIII., р. 201. 
8 On the distribution of diatoms in polar seas, see An introduction to the study 
of seaweeds, by George Murray, p. 19, 197. London, 1895. 
4 Nature, 1889, XXXIX., р. 424. 
