1. Milner, H. B., Sedimentpetrography, 1, 200 (George Allen and Unwin, London, 1962).
2. Ralston, O. C., Electrostatic Separation of Mixed Granular Solids (Elsevier Publishing Co., Amsterdam, 1961).
3. Maijer, C., Geological investigation of the Amarante Region (Northern Portugal) with special reference to the mineralogy of the cassiterite-bearing albite pegmatites, thesis, Univ. Amsterdam, 1965.