Early in the investigation of epigenetic mineral deposits it was recognized that hot solutions were important and that wall-rocks were commonly highly altered. Physical changes in the wall-rock are conspicuous especially bleaching, development of porosity, permeability, and textural changes. The changes, however, are varied and some rocks become dark colored, others dense and impervious. Changes in mineralogy are complex, but less commonly only recrystallization may occur. The list of minerals and mineral groups formed by hydrothermal alteration comprises 68 names and doubtless will be expanded as research continues. The most characteristic minerals include sericite, quartz, chlorite, sulfides, epidote, zoisite, clinozoisite, leucoxene, clay minerals, calcite, and other carbonates. Minerals formed at an early stage may alter to other minerals as the process continues. Some minerals, such as biotite, may be susceptible to alteration but on occasion form as an alteration product. Quartz is the only abundant primary mineral that commonly resists destruction by hydrothermal processes. The chemical composition of rocks and of hydrothermal solutions forms chemical systems of extreme complexity. Therefore, the chemical reactions and ultimate chemical changes are varied. Substances such as H 2 O, SiO 2 , S, Co 2 , K, Na, Ca, Mg, B, F, and so on are introduced into the rocks or transferred from one place to another. The composition of the wall-rock exerts a strong influence on the early stage alteration whereas the composition of the solution is dominant in the late stage. The nature of the process of hydrothermal alteration is such that zones of alteration are a normal result. These are not uniform but naturally depend on the composition of the wall-rock and of the solutions, with changes in the solutions during the process having a strong influence. As a result of intense alteration, dissimilar rocks, particularly igneous rocks, tend to develop a uniform product consisting in some districts, for example, of quartz and sericite. A conspicuous feature of the alteration of the wall-rocks of some veins is that there is little relation to the composition of the vein. Thus gold quartz veins in California have a highly carbonatized wall-rock. The nature of wall-rock alteration has an important bearing on inferences as to the source of the hydrothemal solutions because of the complex chemical additions in some examples.