338 
THE VOYAGE OF H.M.S. CHALLENGER. 
the interpretation that seems the most probable. As a general rule an attempt will be 
made to explain the facts by reference to similar phenomena taking place on the land 
surfaces, or in shallow water, which have been for a long time under the direct observa- 
tion of geologists and chemists. In sea- water the sulphates are deoxidised by carbon and 
hydrogen, — one of the greatest chemical changes which occurs in the sea ; in fresh water, 
where sulphates are absent or present in small amount, this reaction cannot take place. 
It is probable that the reactions follow a nearly similar order in the shallow waters of 
the ocean and in the abysmal regions, but at the same time the intensity of these 
reactions, and their subsequent results, may be considerably modified in those deep- 
water deposits where there is a great pressure, an absence of mechanical action and of 
solar rays. 
Tlie chemical products under consideration will be discussed under the following heads : 
— I. Clay; II. l\langanese Nodules ; III. Zeolites; IV. Phosphatic and other concretions. 
I. Clay. 
The fundamental basis of all clayey deposits, whether in geological formations or the 
deposits of modern oceans, is the hydrated silicate of alumina — Al203,2Si02,2H20, 
which is derived from the decomposition of all the aluminous silicates in rock masses 
under the action of water, and especially of water containing carbonic acid. The silicates 
of pota.sh, soda, lime, protoxides of iron and manganese are thus decomposed at ordinary 
temperatures, and these silicates — the felspars, pyroxenes, amphiboles, for instance — also 
contain more or less alumina and magnesia. The first-mentioned bases: — the potash, 
.soda, lime, and protoxides of iron and manganese — are transformed into carbonates 
and, dissohdng in the water, may be carried away in solution, silica being at the 
same time set free ; the silicates of alumina and magnesia, being much less soluble, 
remain behind as a residue, are transformed into hydrated silicates, and give rise 
on the one hand to clay, and on the other to talc. As all the eruptive and meta- 
morf)hic rocks are composed for the most part of aluminous silicates, they all undergo 
these changes resulting in the production of hydrated silicate of alumina, and it follows 
that these rocks and minerals are the original source of all the clayey material so vridely 
distributed in recent and past geological formations. 
Although hydrated silicate of alumina may occur, in nature, in a pure state in the 
form of crj’stals, they arc exceedingly rare. It usually occurs in an amorphous condition 
and mixed with many foreign substances. Even kaolin, which is usually regarded as 
pure clay, always contains more or less debris derived from the rock from which it 
originated. Kaolin, and clays a})proaching kaolin in composition, have always been 
transported suspended in water from their place of origin, and thus when deposited may, 
in sjKJcial circumstances, be freed from many of the extraneous particles with which they 
