25 
coloured black by the ferrous sulphide, whilst the greater part of the sand was 
free of ferrous sulphide, thus evidence of the parallelism mentioned between 
organic matter and formation of ferrous sulphide. The clay bottom in the 
Kattegat (samples from Anholt) does not contain sulphide of iron; but if we allow 
snch a sample to stand in the glass-tubes above-described for taking up the 
samples, we see after some time that round, black patches of sulphide of iron 
begin to form a little distance under the surface. It is clear, therefore, that a 
reduced supply of oxygen here leads to the production of ferrous sulphide. 
The occurrence of ferrous sulphide in the sand on our coasts of the North 
Sea has been shown by Warming (Kgl.-D. Vid. S. Skr. 7 R II, 1904). 
We find in the literature various notices regarding the fact, that arms of 
the sea (fjords in Norway) and whole inland seas (Black Sea) can become unin- 
habitable to animals owing to the lack of oxygen. This has usually been explained 
by saying, that the animals use up the oxygen in the water, when it is not 
sufficiently renewed hy inlets and outlets. These explanations certainly only contain 
a part of the truth. It is scarcely the animals which have brought the Black Sea 
into the condition in the deep water, which is described by Zerno&v (Inter. Revue 
d. ges. Hydrobiol. og Hydrogeogr. Il., pp. 99—123, 1909, III p. 226, 1910). The 
hydrogen sulphide region of this Sea is rather due to the extremely rich plant 
production, which is great enough, along with the slow renewal of the water- 
masses, to form a bottom-soil containing a large amount of organic matter. In 
this way the possibility is created for a rich bacteria life, production of hydrogen 
sulphide and so on, which certainly contribute to a very high degree to deprive 
the water of oxygen. The Black Sea is briefly an example of the conditions which 
have been described above for our inner fjords, only in a much more advanced 
stage. Curiously enough, so far as known to me, similar conditions are not met 
with everywhere in freshwater where plant materials are deposited in great quan- 
tities: here the preserving action of the humie acid probably has some influence 
(peat-bogs). We may therefore, to a certain extent, regard the large oceans as the 
lungs of the sea, which supply the water-masses of the inner seas with oxygen 
and remove the superfluous organic matter. 
IV. Organic matter in the sea-water. 
In the introduction to the chapter on the deposition of organic matter 
on the sea-bottom, we mentioned how the plants of the Zostera belt were carried 
away from their original grounds and at last in more or less finely divided condi- 
tion deposited on the sea-hottom. It is the water of the sea which achieves the 
transport. It is of interest now to follow the organic matter on the way from the 
place of production to the place of deposition, namely in the sea-water itself; 
4 
