Fusion products of breccia and mylonite along the Alpine Fault Zone are locally hyalomylonite. Chemical data from this hyalomylonite and those from the Himalayas suggest that they are not the products of preferential melting of a low—melting point fraction, nor in most cases are they produced by total melting. The evidence suggests that progressive melting of the country rock occurred, probably in the presence of water, until the fault was lubricated. The composition of the melt is controlled by the rock mineralogy, temperature, and the time between initial melting and final chilling. A friction temperature of 750°C is estimated to have developed in parts of the Alpine Fault Zone; stresses on the order of 250 b could produce this temperature.