Chap. XX.] 
DEEP-SEA DEPOSITS. 
on the death of the plant ; and also that the precipitation may have been 
brought about bv the action of calcareous sediments, when on the sea 
floor, or when passing through the water on the way to the bottom, the 
process being one of substitution by which calcium carbonate passes into 
solution with the precipitation of a corresponding amount of manga- 
nese oxide. He does not. however, regard either of these, or any other 
theories that have been advanced, as adequately explaining the formation 
of these submarine manganese-ores. 
I do not intend to discuss here the various theories outlined above ; 
but I will give what seems to me a fairly satisfactory explanation, for 
the consideration of those who have had practical acquaintance with 
deep-sea deposits of manganese-ores. Now according to the researches of 
Dittmari, the waters of the ocean contain both dissolved oxygen and 
carbon dioxide. Even at the greatest depths there is a small quantity 
of oxygen in solution^, though the amount is much smaller than in the 
surface layers. Dittmar takes the presence of this small amount of oxy- 
gen in the depths of the ocean as a proof that the water is not 
absolutely stagnant3. but subject to a slow circulation. The amount of 
carbon dioxide in sea-water seems to be greater than that of the oxygen ; 
but it is usually less than is required to keep all the carbonate of 
calcium present in sea-water in the form of bicarbonate*. Now 
calcareous sediments are of frequent occurrence on the sea-bottom, and 
in such cases it may be supposed that the manganese-ores contained in 
the sediment have at least in part been formed by the replacement 
method considered by Penrose. But it must be remembered that a 
large proportion of the sea-bottom is covered with red clay containing 
abundance of manganese-ores, both in the form of concretions and as 
incrustations on the remains of marine organisms. A certain proportion 
of the manganese-ore in these red clays may have been formed bv the 
replacement by calcareous matter of the manganese salts contained ia 
the adjacent water ; for these red clays do contain a certain amoimt of 
calcareous matter, this being in the form of such corals and thick sheUs 
as either live at, or have been able to reach, the great depths at which, 
these red clays are found, without suffering the fate of the thinner- 
shelled organisms of being dissolved on the way down. But the amount 
1 Challenger Reports, Physics and Chemistry, I, (1884). 
2 Loc. cil., p. 226. 
3 Loc. cit., p. 225. 
4 Loc. cit., p. 220. 
H M 
