1890 - 91 .] Mr Irvine and Dr Gibson on Manganese Deposits. 55 
the silicates which it contains. This action is much assisted, in the 
case of marine animals, by the fact that the water which they pass 
through their bodies along with the sand is charged with sulphates. 
These are easily reduced to sulphides by the action of the organic 
matter of the secretions of the animals. The resulting sulphide at 
once suffers double decomposition with any oxide of iron or 
manganese which is present as such in the mud, or may be being 
set at liberty from silicates under the decomposing influence of 
trituration under water. The sulphides of manganese and iron so 
formed are, in course of nature, extruded by the animals, and if 
exposed to the sea-water on the surface of the mud are quickly 
oxidised, the manganese taking priority. The mud below the 
surface layer, where ground life is abundant, remains blue, being 
protected by the oxidation of what is above it. 
“ At the bottom of the ocean the mineral matter is thus exposed to 
a reducing process due to the life of the animals which inhabit it, 
and to an oxidising process due to the oxygen dissolved in the 
water. Other things being equal, the redness or blueness of a mud 
or clay depends on the relative activity of these processes. They 
also require a controlling or modifying influence on one another. 
For, although marine animals are much less sensitive to variations 
in the amount of oxygen in their atmosphere than terrestrial 
animals, it is certain that there must be a limit to the deficiency of 
oxygen which each animal can support; and when this limit is 
approached, its reducing activity is diminished, or it may be 
extinguished. The water in the course of circulation is being 
continually renewed, and, meeting with a diminished amount of 
freshly-reduced matter, it is able to push the oxidation of the mud 
to a greater depth. It is easily conceivable that in many of the 
deep parts of the ocean the amount of ground life may be so limited 
that the water has no difficulty in oxidising at once its ejecta; and 
these conditions would be favourable to the formation of a red 
clay or chocolate mud, according to the preponderance of iron or 
manganese.” 
In a word, that the animals passing sand or mud through their 
bodies with sea-water tend to reduce the sulphates present in the 
sea-water, and the alkaline sulphides so formed cause the formation 
