Page:The Journal of geology (IA journalofgeology21894univers).pdf/313

The process of hydration, when the materials affected do not require oxidation before they can become hydrated, may extend down indefinitely below the limit of oxidation; but when oxidation is necessary before hydration is possible, the latter process of course can extend no deeper than oxidation. Thus the silicate of alumina in feldspar may become hydrated and form kaolin without the intervention of oxygen. This is brought about by the action of carbonic acid and water, which react on the feldspar and form alkaline carbonates, kaolin and free or hydrous silica. Theoretically, therefore, kaolinization ought to go on to any depth than can be reached by water and its almost universal accompaniment, carbonic acid. In this case, however, the base in question is already in its peroxide condition (Al₂ O₃), but when a base is not in this condition, it frequently requires oxidation previous to hydration. Thus sulphide of iron does not become hydrated until it is peroxidized, and this mineral, therefore, requires oxidation previous to hydration.^[1]

The various materials other than oxygen in surface waters have a very important effect on the mineral matter with which they come in contact, and their action sometimes takes place before that of oxidation, though it often requires at least a partial previous oxidation. The effect is both to form new chemical compounds with the materials involved, and to dissolve and bodily remove certain materials. As with oxygen, however, so with these other agents of alteration, they are more active above the drainage level of the country than below it, and an additional reason for this is that many of the materials affected require a primary oxidation before they enter into other chemical combinations. Thus sulphide of lead is oxidized to sulphate of lead before it can take up carbonic acid and form carbonate of lead; while on the other hand, carbonate of lime can be converted to sulphate of lime (gypsum) by the action of sulphuric acid or certain sulphates without any change in the degree of oxidation of the lime.