Data are presented from a series of experiments on layers of granular quartz gouge in the double-direct-shear geometry at a normal stress of 25 MPa. The apparent friction of a layer, defined as the ratio of the applied shear and normal stresses, shows considerable variability depending on the thickness of the layer and the particle size distribution of the gouge. Measurements of layer thickness during the experiments also show that the layers thin as shearing proceeds. Layer thinning is attributed to the processes of extrusion and densification, and by analyzing these processes the observed variations in apparent friction are reconciled with a constant coefficient of internal friction for quartz gouge. For thinning due only to extrusion, the principal assumption made is that the deviatoric stress and plastic strain rate are coaxial. When densification is also admitted, a simple flow law with one adjustable parameter is required to relate the volumetric and shear strain rates. The results and analysis show that (1) extrusion and densification must be considered when interpreting the measured frictional properties of gouge layers; (2) shear localization is inhibited in these experiments; and (3) the apparent friction of a natural fault zone may be less than sin ϕ, where tan ϕ is the coefficient of internal friction of gouge in the fault zone.