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Pseudotachylite is a rock mass, generally in stringers and veins or small fracture fillings formed by melting on fault plane during very rapid movement and very rapid chilling giving rise to glassy material which may get devitrified producing spherulitic structures through later geological history. Has sharp contacts with country rocks and dark appearence (cf. obsidian) owing to no crystallization but only rapid chilling.

In many mylonite zones there is evidence that the rocks have melted, and extremely fine-grained material comprises dikes and sills that run out from the mylonite zone and intrude the neighboring rocks. These fine-grained rocks are called pseudotachylites [Shand (1916)]