Most hydrothermally altered rocks are readily grouped into 2 principal facies, the argillic and phyllic (mica) facies. The argillic facies comprises the propylitic, montmorillonitic, and kaolinitic types, and the phyllic facies the muscovitic and biotitic types. The characteristic mineral assemblages of the propylitic type of alteration are conveniently represented on a standard ACF diagram, and those of the other types on AKF diagrams to which water (H) has been added. Generally each type of alteration characterizes a subzone of alteration that bears consistent relationships to neighboring zones and to a presumed source of heat and altering fluids. These relationships also are in full accord with the inferred physical-chemical requirements of the corresponding facies. Thus, the rocks of the outermost, propylite, subzone generally are enriched in CaO, MgO, Na 2 O, H 2 O and CO 2 . The rocks in the intermediate, montmorillonite, subzone are only slightly changed in bulk composition, but those of the inner, kaolinite, subzone are impoverished in the very constituents that have been added to the propylite subzone. The rocks of the innermost, phyllic, zone also are impoverished in CaO, MgO, and Na 2 O, but are enriched in K 2 O. However, their outer parts (muscovite subzone) are more impoverished in CaO, MgO, and Na 2 O, than their inner parts (biotite subzone). The upper limits of thermal stability for the different facies at 1000 bars water pressure are fixed by the breakdown temperatures of individual minerals - epidote at about 460 degrees C., kaolinite at 400 degrees C., and montmorillonite at 440 degrees C. for the argillic facies, and muscovite at about 620 degrees C. for the phyllic facies. Biotite of the phyllic facies is stable well into the melting range of granitic rocks.