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clay, sand, and gravel, while further concentration is due to this capillary action. In the case of nitre, indeed, the saline material is very often, if not generally, formed in soils or guano beds and undergoes its first concentration by this capillary action.

In the various chemical changes mentioned above, the class of salts that remains, whether oxides, carbonates, haloid compounds, etc., varies with the nature of the bases affected. Thus, iron sulphides and copper sulphides are both oxidized and form sulphates. But here the similarity of their behavior ends, for the iron sulphate probably passes then into a basic sulphate and then into a hydrous sesquioxide, while the copper sulphate takes up carbonic dioxide and water and forms basic carbonates. The iron sulphate might, under certain conditions, form a carbonate in a similar manner, but this compound would be very unstable under the conditions existing in the alteration of sulphide deposits and would quickly go into the form of the hydrous sesquioxide, while the carbonate of copper is stable under existing conditions and remains.

In the same way, if silver sulphide and iron sulphide are both oxidized and then affected by waters carrying common salt or other chlorides in solution, the silver is converted to chloride, which is insoluble and remains; while the chorides of iron are much less liable to be formed, as they are soluble, and some of them unstable, compounds, and even if they were formed, they would be leached out or oxidized. Hence, though chloride of silver is a common product of alteration in silver deposits, chloride of iron is never found, at least to any extent, as a product of alteration of iron deposits.

Again, it is frequently found that unaltered auriferous iron pyrites contains a certain amount of silver, while the altered part often carries almost none. In such cases, the gold has remained stable during the alteration, while the silver, in the absence of a chloride or other reagents to convert it to an insoluble compound, has been dissolved and carried away in solution by the acid materials generated during alteration.

Hence, the materials in surface waters affect different bases