Many studies of individual examples of hydrothermal alteration have been made, but a general summary has not appeared since 1915. The changes due to hydrothermal alteration are textural, mineralogical, and chemical.

As a rule the primary texture is destroyed, roughly in proportion to the intensity of alteration, but only in extreme examples is it entirely gone. Many of the rocks are softened and bleached and consist of a felt-like or messy aggregate of secondary minerals. Less commonly the rock remains fresh, owing to the development of anhydrous minerals such as albite.

The most common secondary minerals, approximately in order of abundance, are: sericite, quartz, pyrite, carbonates, chlorite, epidote, alunite, adularia, albite, various sulphides and numerous less common, but at places characteristic, minerals.

Chemical changes are extensive and vary widely. An average has been made of all available pairs of analyses divided into acid, intermediate, and basic types, as well as a combined average. Comparison of altered with fresh rock shows a gain in potassa and water, a loss in iron, magnesia, lime, and soda, and little change in silica and alumina during alteration.

Diagrams are used to bring out significant features of various groups of analyses as well as of the averages prepared.

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