156 Generation of Sulphur in 



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 sul- 

 phides 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, in localities 

 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 exercise a controlling or modi- 

 fying influence on one another. For, although marine animals 

 are much less sensitive to variation 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. 



While dealing with this subject it is proper to refer to 

 Darwin's book on Vegetable Mould and Earthworms, which was 

 published in 1881. His masterly investigations, in the kindred 

 department of the part played by earthworms in the formation 

 of the terrestrial soil, strengthened me much in my belief in 

 the soundness of the views above developed as to the formation 

 of marine muds. Indeed, to a certain extent he extends his 



