of Edinburgh, Session 1883-84. 
523 
observe in considerable quantities in the clay of the centre of tbe 
Pacific, have been formed at the expense of the decomposing vol- 
canic matters spread out upon the bed of that ocean. 
In connection with this formation of zeolites, reference may be 
made to a chemical process whose principal seat is the red clay areas, 
and which gives rise to nodules of manganiferous iron. This sub- 
stance is almost universally distributed in oceanic sediments, yet it is 
not so much of the areas of its abundance that we intend to speak as 
to the fact of its occurrence in the red clay, because this association 
tends to show a common relation of origin. It is exactly in those 
regions where there is an accumulation of pyroxenic lavas in decom- 
position, containing silicates with a base of manganese and iron, 
such for example as augite, hornblende, olivine, magnetite, and basic 
glasses, that manganese nodules occur in greatest numbers. In the 
regions where the sedimentary action, mechanical and organic, is, as 
it were, suspended, and where, as will appear in the sequel, every- 
thing shows an extreme slowness of deposition, — in these calm waters 
favourable to chemical reactions, ferro-manganiferous substances 
form concretions around organic and inorganic centres. 
These concentrations of ferric and manganic oxides, mixed with 
argillaceous materials, whose form and dimensions are extremely vari- 
able, belong generally to the earthy variety or wad, but pass some- 
times, though rarely, into varieties of hydrated oxide of manganese 
with distinct indications of radially fibrous crystallisation. The in- 
terpretation to which we are led, in order to explain this formation of 
manganese nodules, is the same as that which is admitted in explana* 
tion of the formation of coatings of this material on the surface of 
terrestrial rocks. These salts of manganese and iron, dissolved in 
water by carbonic acid, then precipitated in the form of carbonate of 
protoxide of iron and manganese, become oxidised, and give rise in 
the calm and deep oceanic regions to more or less pure ferro-man- 
ganiferous concretions. At the same time it must be admitted 
that rivers may bring to the ocean a contribution of these same 
substances. 
Among the bodies which, in certain regions where red clay 
predominates, serve as centres for these manganiferous nodules, are 
the remains of vertebrates. These remains are the hardest parts of 
the skeleton — tympanic bones of whales, beaks of Ziphius, teeth of 
