[en] Fracture widths were measured and percentages of secondary mineral infilling types were estimated on 3194 primary fractures in basalt core in order to characterize the distribution of width and the nature of the secondary infillings. The core samples were from the Rocky Coulee, Cohassett, McCoy, and Umtanum flows in core holes RRL-2, 6, 14, and DC-16A. Fracture width was characterized by calculating statistics of width samples, constructing cumulative frequency plots, and comparing the percentages of fractures with various predominating infilling types (including voids) among basalt flows, infraflow structures, and fracture width groups. Basalt flows, infraflow structures, and core holes cannot be differentiated consistently on the basis of fracture width. There is only one population of widths, and that population can be characterized by a log-normal distribution, and arithmetic mean of 0.23mm, and a standard deviation of 0.49mm. 6 refs., 13 figs., 12 tabs