Pseudotachylites have been recognized for over a century. However many problems concerning their formation remain in dispute. Pseudotachylites are traditionally interpreted as an indicator of high-velocity slip O10 cm/s) and hence the earthquake activity (Spray), the presence of pseudotachylites can not only be indicative of meteorite impacts, but also be used to infer the past deformation history and behavior of a fault zone or fault shear zone, especially the deformation mechanism of a fault zone in the deep crust while an earthquake occurs. In recent years, pseudotachylites, which are widely known as earthquake faulting fossils, have been paid great attention to by the researchers with the deepened researches on "active faults". Pseudotachylites can be discovered in different depths of the crust and can be divided into five types (under a microscope) according to micro-lites in the matrix. There are two contrasted kinds of pseudotachylites in the crust, one having been formed by frictional fusing of rocks in a fault zone and the other by extreme mylonitization of the rocks and injection of the finely pulverized material into fractures (Philpotts, 1964). The bulk compositions of pseudotachylites and their wall rocks show that they are generated chiefly by the preferential selective melting of minerals of the lowest melting points in the wall rocks. The latest experiment (Spray, 1995) shows that comminution is a prerequisite to frictional melting. Depending on the velocity-shear stress - displacement relations prevailing during the frictional slip, the rocks produced in seismogenic zones can be dominated by comminuted wall rocks or fragments-melt mixes (pseudotachylites).