Abstract: The final product of evolution of acid and alkaline magmas is the highviscosity, low-eutectic point melts. The rocks formed by them show a well-developed structure of plastic flowage deformation. In order to distinguish between lavas with this structure and ignimbrites, the author proposes theterms rhyolite of plastic deformation, glassy rocks of plastic deformation and trachyte of plastic deformation for the former;while the latter specifically fers to rocksformed by fusion of pyroclasts, with the exception of the "ignimbrite"formed by plastic deformation and liquation of melts.