At high confining pressure the coefficient of friction, μ, for granite depends on the relative displacement of the surfaces. For ground surfaces, μ reaches a maximum after about 0.1 cm and then decreases to nearly a constant value after 0.5 cm of sliding has occurred. Features on the surfaces after sliding suggest that the maximum is reached when intimate contact is first established. Also, this maximum value is the same as the initial μ for perfectly mated rough surfaces. The decrease in μ from the maximum is probably caused by rolling on wear particles between the surfaces, μ decreases with an increase in normal stress, owing to a finite shear strength at zero pressure of interlocking irregularities on the surfaces. Water reduces the frictional shear strength of granite by about 400 bars, independent of the normal stress across the sliding surfaces. Brittle fracture of surface asperities may be the controlling mechanism during the frictional sliding of brittle materials such as granite. Up to the highest pressures investigated, sliding movement between the surfaces occurred with violent stick-slip. Stick-slip along a pre-existing fault may be a source of crustal earthquakes. The ‘brittle-ductile’ transition pressure in silicate rocks may simply be the pressure at which the frictional shear strength is equal to the fracture shear strength. In the Coulomb theory it is assumed that the strength of a rock is determined by μ and the cohesive strength. The theory does not hold for westerly granite. According to the effective stress theory, the stress required for one block of rock to slide on another in the presence of pore fluid of pressure p is given by τ = μ(σn - p). The theory holds for granite if μ is the coefficient of friction for sliding on water-saturated surfaces and if allowances are made for the fact that μ may be a function of the effective stress across the surfaces.