The hydrothermal alteration, which was accompanied by the introduction of copper sulphides in the disseminated copper deposits, is considerably more complex than is generally recognized.Detailed microscopic study of selected porphyry specimens from the important deposits in the United States, together with a review of published data, leads to the recognition of four main types of alteration which are designated as follows: 1. Quartz-orthoclase; 2. Quartz-sericite-pyrite; 3. Sericitic and argillic alteration with quartz subordinate; 4. Biotitic alteration.No district is a simple example of one kind of alteration but each district shows a dominance of one or two of the above types. Most districts show a rather long list of alteration products when studied in detail.The argillic alteration has been little emphasized in earlier work, but is now known to be very important in several districts of which Morenci is an outstanding example. The clay minerals occur in the sulphide ore and are characteristically earlier than sericite.The more important clay minerals include hydromuscovite, kaolinite, allophane, halloysite, and beidellite. Less common are potash clay, montmorillonite, and dickite.Available chemical data indicate that calcium and sodium are generally removed from the rock to a significant degree, and potassium remains constant or increases. Silica is gained in a few deposits but commonly shows minor changes. Quartz, no doubt, is often formed by the freeing of silica during alteration.The relation of copper content to the various kinds or stages of alteration is indicated in a general way pending detailed studies in all districts. The problem differs depending upon whether the deposits are those where enrichment is important or where they are primary deposits of commercial grade. In enriched deposits the distribution of pyrite with respect to the alteration is of considerable importance. In primary ores the evidence indicates that chalcopyrite was introduced with the sericitic alteration and less commonly with biotite. Pyrite often accompanies sericite but at San Manuel the highly pyritic alteration accompanies a dominantly hydromuscovite alteration, whereas chalcopyrite plus some pyrite occurs with intense sericitic alteration.