In some districts, at least, competent rocks are more hospitable hosts to ore than incompetent ones, and surely this is what would be expected from their mode of failure in fracturing. "Competent" as the term is used here, refers to rocks that are relatively strong but, when they do fail, break as though they were brittle material. "Incompetent" refers to rocks which are weak and have a tendency to deform plastically or by flow. Under most conditions, quartzites, conglomerates, and fresh igneous rocks are competent. Incompetent are shales, slates, schists, and limestones; also igne¬ous rocks that have been altered to sericite, chlorite or serpentine. These generalizations, however, are subject to some modifications with varying circumstances. In the first place, competence is a relative matter. A limestone between shale beds is likely to be-have as a competent rock; an identical limestone between beds of quartzite is likely to be-have incompetently. Further-more, the manner of failure depends in some degree on the manner in which the rocks were deformed. A limestone under light load may behave as a competent rock, but under high confining pressure, especially in the presence of solvent and with the rate of deformation slow enough to give time for recrystallization, it may behave as very incompetent material.

Competent rocks, in addition to their tendency to fail by fracture rather than by shear, have an advantage in that they yield to fracture in such a way as to provide permeable channelways. Their strength tends to -prevent fractures from squeezing shut, and, if they do succumb to failure adjacent to fracture-walls, it is by spalling to produce a jumble of fragments which presents large surfaces to ore depositing solutions. When competent rocks shatter, they produce either a net-work of interconnected cracks or a permeable breccia free from gouge. Furthermore, since the shearing angle (see p. 295) decreases with in-creasing brittleness, a shear-fracture passing from semi-plastic into brittle rock is deflected toward the plane of maximum normal stress and therefore toward an attitude more favorable to opening by the movement that initiated the shear. An example of the contrast in mode of failure between competent and incompetent material is to be seen at Woods Point in Victoria (Figure 65), where two complementary sets of reverse faults form gold-bearing veins within a broad dike but disappear in the adjoining incompetent slates.

In spite of the superior mode of failure of competent rocks, it is not everywhere true that they are the most hospitable to ore deposition. At Bendigo (Victoria), incompetent slate beds have localized slipping: On the limbs of the anticlines where they overlie competent quartzite beds, they form the sites of quartz veins, locally known as "backs," some of which carry much visible gold. Similarly in the Porcupine District (Ontario) highly incompetent beds of carbonaceous slate have been replaced by gold-bearing quartz to form exceptionally rich veins, apparently because they became localized zones of intense shear¬ing. In contrast, some of the thickest and most massive flows of dacite are poor ore-carriers because they were too strong to break. However, among the formations in this district that conform to neither of these extremes, the usual rules of competency hold fairly well; the most consistent ore-bearing zone is a series of competent greenstone flows, each thin enough to fail, but brittle enough to form good fractures. The poorest horizons are thick incompetent pillow lavas that yielded along myriads of tight minor shears but afforded no continuous localized open channelways.

Of the districts listed in Ore Deposits as Kelated to Structural Features there are 21 in which rocks more competent than their neighbors are the favored ore-carriers, as against seven in which weak or incompe¬tent rocks are the favorable ones. (In 17 other districts listed, it is not clear whether the favored rock, if any, was more competent or less so than the adjoining formations.) If this is a fair sampling it would indicate that the odds are 3 to 1 in favor of competent rocks. But the exceptions are so numerous that the generalization is not of much help in predicting in advance which rocks are likely to be ore-carriers.