The Geolog System is a computer-oriented method of recording geological field observations, primarily from drill holes, in porphyry-type and other mineral deposits. The System also facilitates manual analysis and interpretation of the data.Data are compiled on GEOLOG, an 80-column format describing principal zone (cap, supergene, hypogene, fresh rock, fault), interval, rock type, fractures, alteration mineral assemblages, limonites, and ore mineral assemblages. Prescribed headings on GEOLOG are keyed to logical word abbreviations and simple codes to give concise, consistent, and readable descriptions. Because column names are easily modified and various codes and scales are available, GEOLOG is flexible; it therefore facilitates description of many types of deposit, accommodating a variety of logging techniques that may emphasize theoretical examination, routine survey, or rapid reconnaissance. Collecting data with Geolog System appears better than existing or standard logging techniques because information is recorded to be: keypunched directly; retrieved readily; consistent with respect to routine description of presence or absence of particular features; specific by use of prescribed headings, codes and scales; related to an existing porphyry deposit model. Time and effort involved in collecting drill data on GEOLOG are comparable to common logging procedures.Interpretation of the large amount of complex data on GEOLOG is aided by using a computer to provide manipulation of data and appropriate output. Because GEOLOG is keyed to recently published models of porphyry deposits that show ore location with respect to zoning of mode and degree of dispersion of mineralization, alteration mineral assemblages, and ore mineral assemblages, application of the System has important future use in predicting ore locations from even preliminary data.