Descriptions of several occurrences of propylitized andesites are reviewed, including the original descriptions of "propylites." The principal explanations that have been offered for propylitization are: (a) dynamometamorphism; (b) autohydration; (c) origin through post-volcanic activity, generally by vein-forming solutions. The general geology of the portion of the Comstock district pertinent to the discussion is reviewed. The unaltered andesites are pyroxene-amphibole andesites. There are three principal types of alteration, deuteric, propylitic, and zeolitic. The deuteric is probably due to water vapor contained in the rock at the time of its crystallization. The propylitic appears to be due to heated solutions given off by a later intrusive. The zeolitic is apparently associated with vein-forming agencies.