200 
THE VOYAGE OF H.M.S. CHALLENGER. 
likewise necessarily come from the decomposition of certain silicates under the action of 
the ncid in the analyses, for instance, from the zeolites, the altered volcanic minerals, 
glasses, and lapiUi. A part of the water shown in the column under the head of loss on 
ujnition must also be regarded as belonging to the constitution of clay, another part to 
hydrated oxide of iron, and to the decomposed volcanic rocks present in the deposit, 
especially hydrated glasses, of which palagonite is the most constant, and a third part 
must be referred to the organic substances present. 
The percentage of oxide of iron in the soluble portion is very variable, but is occasionally 
present in greater quantity than the alumina, and shows, as the microscopic examination 
had already done, that we are dealing with a ferruginous clay. The ferric hydrate is 
the pigment that colours the clay in the majority of cases, as the hydrated peroxide of 
manganese does in some special regions. We can indeed by the aid of acids remove these 
two substances, and thus decolorise the clay. Although in clay iron replaces alumina in 
many cases, still it appears from the results of microscopic examination that here at least a 
considerable portion of the hydroxide of iron exists in a free state in the Eed Clay, or 
intimately mixed with manganese as is generally the case in Deep-Sea Deposits. 
The carbonate of calcium is present for the most part in the form of shells and 
skeletons of organisms ; the proportion is, however, reduced to a minimum in the Red 
Clays and Radiolarian Oozes, if a comparison be made with most of the other Deep-Sea 
Dejx)sit8. As to the carbonate of magnesium indicated in all the analyses, it might be 
that a part came from the debris of organisms where it would be in isomorphic mixture 
with carbonate of calcium, but the percentage of carbonate of magnesium appears to be 
proportionally too high to that of the carbonate of calcium to admit this interpretation. 
It seems therefore probable that the carbonate of magnesium comes from the sulphate 
of magnesium contained in the sea-water acting on the carbonate of calcium and replacing 
a part of this carbonate which forms the debris of organisms deposited on the floor of the 
ocean. In this case we should have an incipient dolomitisation. 
The sulphate of lime present in all the analyses doubtless comes from the 
sulphate of lime contained in sea- water, which impregnates the deposit when freshly 
collected, and is precipitated with great facility from sea-water when the deposit is placed 
in alcohol; large crystals of this substance are formed in this way in the bottles in which 
the deposits were preserved with a small quantity of spirit. It seems well to remember 
that in the inter[)retation of all analyses of Deep-Sea Deposits, we have always to take 
account of the facility with which certain sea-salts are retained in clayey matter. One of 
the striking properties of this substance is to absorb with avidity and to retain sea-water 
and its salts ; to such an extent is this the case that they are difficult to remove even after 
repeat<Kl washings. In the case of a Globigcrina Ooze we have made some experiments 
bearing on this point, and have shown that after repeated washings with cold water the 
deposit retained more than 1 per cent, of alkalies not combined with sedimentary materials. 
