The deformation behavior of quartz and feldspar has been studied in Westerly granite deformed dry at a constant strain rate of 10-6/s, confining pressures of 1.5-15 kbar, and temperatures of 25¿-1000¿C. Samples deformed at lower temperatures and pressures show throughgoing faults; those deformed at intermediate conditions show a combination of grain-scale faults and plastic deformation; and those deformed at higher temperatures and pressures show plastic deformation with no faults of any scale. On a grain scale the deformation is inhomogeneous at all conditions because of the polyphase nature of the material. Detailed petrographic and transmission electron microscope (TEM) observations have been made of the deformed speciments. The fault gouge consists of very fine grained material which verges on being amorphous, but no evidence of melt was seen. In the regions away from fault zones, there is a transition from dominantly microcracking to dominantly dislocation glide and climb; this transition is primarily a function of temperature. Dislocation motion is thermally activated and is probably almost unaffected by pressure over the range investigated. Thus at low temperature the strain rate that can be produced by dislocation motion is limited, and the difference between this and the imposed strain rate must occur by microcracking. The way in which the microcracking accomplishes the deformation depends on pressure. At low pressures (<5 kbar) the microcracks link up to form a throughgoing fault after very low strain; at higher pressures (7.5-15 kbar) they produce only grain-scale faults, ''deformation bands,'' and undulatory extinction, and no throughgoing faults are formed after 15-20% shortening. At the laboratory strain rate of 10-6/s the transition from dominantly microcracking to dominantly dislocation motion occurs at approximately 300¿-400¿C for quartz and 550¿-650¿C for feldspar. When one allows for the slower natural strain rates and the presence of water, this grain-scale brittle-ductile transition may correspond to the limiting depth of earthquakes on strike slip faults.