38 Proceedings of Royal Society of Edinburgh. [sess. 
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 modifying 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 con- 
tinually 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 views himself to the case of 
marine muds. At page 256, after noticing that it is due to the 
milling action of the gizzards of worms that the supply of 
exceedingly finely divided mineral matter, which is removed from 
the surfaces of every field by every shower of rain, is constantly 
renewed, he adds in a note : “ This conclusion reminds me of the 
vast amount of extremely fine chalky mud which is formed within 
the lagoons of many atolls, where the sea is tranquil and waves 
cannot triturate the blocks of coral. The mud must, so I believe, be 
attributed to the innumerable annelids and others animals which 
burrow into the dead coral, and to the fishes, Holothurians, &c., 
which browse on the living corals.” Darwin further gives an 
