G. Siavalas, E. Tsompanidou, S. Kalaitzidis, A. Bouzinos, and Christanis Κ., “EARLY STAGES OF LIGNITE FORMATION IN PTOLEMAIS BASIN: A COAL-PETROGRAPHIC APPROACH”, geosociety, vol. 36, no. 1, pp. 334–341, Jan. 2004.