Significant fine-textured micro-fracture patterns exist in the wall rocks near faults in the Pachuca district, Mexico, and such patterns are absent from the same rock distant from the faults. The chances are 9 to 1 that strong micro-fracturing comes from within 20 m. of a fault. Thus, the micro-fracturing is due to stresses operative during faulting. Hence thin sections disclose the presence of faults. In drill cores it is often difficult to distinguish fault material from ordinary broken core, because gouge that indicates faulting may be washed away by drill water. Thin sections, however, will indicate whether the crushed material represents a fault or not; likewise the presence of fault veins can be detected.