54 Proceedings of Royal Society of Edinburgh. [sess. 
unchanged ; when, however, an oxide is produced, as in the case 
with tin and mercury salts, it forms merely a superficial coating. 
From the results of numerous experiments, which it is unneces- 
sary to record here, we have good grounds for assuming that 
carbonate of lime, either in a massive or comminuted condition, or 
in solution, carries out the most important function of withdrawing 
metallic and other bodies from sea-water, which may he said to hold 
(often in minute amount) almost every elementary substance in 
solution, and fixing these in a concentrated condition. 
The geological significance attaching to this property of carbonate 
of lime is apparent, as, without question, many metallic ores owe 
their origin to this source. 
Manganese Deposits in Marine Muds. By Robert Irvine, 
F.C.S., and John Gibson, Ph.D. 
(Read January 9, 1891.) 
Two theories have been put forward in order to explain the 
formation of manganese deposits in marine muds, and more 
particularly with regard to manganese nodules : one by Murray, 
in a paper read before this Society in 1876, the other by Buchanan 
in 1888. Murray assumes the gradual oxidation of carbonate of 
manganese resulting ultimately in the formation of hydrated 
peroxide of manganese. Buchanan first propounded his theory in 
1880, and subsequently, in a paper read before this Society in 
December 1890, argues as follows : — 
“ The principal agent in the comminution of the mineral matter 
found at the bottom of both deep and shallow seas and oceans is 
the ground fauna of the sea, which depends for its subsistence on 
the organic matter which it can extract from the mud. 
“ In order to fit them for collecting their nutriment in this way, 
the animals have been fitted with different forms of masticating or 
milling apparatus, so as to thoroughly deal with the matter which 
they pass through their bodies. It has been shown that most 
silicates are decomposed to a certain extent when ground or pul- 
verised under water ; so that the mere mastication of the sand or 
mud in presence of pure water would have a decomposing action on 
