The problems of identification of the fine-grained hydrothermal alteration minerals are fairly well advanced. The fields of formation at which these minerals were formed are not so well known. Some common hydrothermal minerals have been arranged on a chart, the abscissa of which represents the pH of the depositing solutions, while the ordinate represents temperature. The chart is presented with a multiple purpose---i.e., to present in readily usable form the known information regarding the field of formation of minerals; to serve as a stimulus to students to attack the hydrothermal problem from a laboratory viewpoint, and to bring out further information from individuals who may have better data. It is expected that the chart will change from time to time with the accumulation of new information.