As long ago as 1816, Brewster showed that many crystals have a lamellar structure developed in them by pressure, and his observations became the starting point of a long series of very valuable researches. Reuseh proposed that the planes along which such structures are developed by mechanical means should be called "Gleitflächen" (gliding-planes), and he, Pfaff, and other mineralogists have investigated a number of curious cases of the kind. Baumhauer in 1879 showed that the lamellar structure could be brought into existence, in the ease of calcite, by simply inserting s knife-blade in a definite direction into cleavage fragments of the mineral, and his observations were confirmed in the following year by Brezina.