This study focuses on the petrography of the lignite deposit at Prosilio, which is in the northern part of the larger basin of Prosilio-Polyracho-Trigoniko. The western part of this basin, which is in the center of our study, belongs to the larger graben structure that contains many other well-known lignite basins in Greece (e.g., Florina-Ptolemaida-Kozani-Servia-Elassona). The sediments in the basin are mainly limnic of Neogene age, surrounded by strata of Mesozoic or older age. The results in this study originated from core samples taken extensively within the lignite beds and were augmented with surface samplings from a recent mine face at Prosilio. The study consists of a macroscopic description of the lignite beds, followed by a microscopic description based on 16 polished samples. The results were interpreted on the basis of the groundwater index (GWI), vegetation index (VI), gelification index (GI) and tissue preservation idex (TPI). The above indices provided a clue as to the conditions of peat formation in the swamp. Specifically, the ratio of VI/GWI helped determine the hydrogeology and the availability of nutrients in the swamp. The ratio of TPI/GI indicated the paleoenvironmental conditions in relation to the water table. The above ratios suggested that the initial (parent) vegetation was that of open water. The conditions then changed in an alternating manner towards telmatic and limnotelmatic, with a few minor and local exceptions, although the dominance of the latter appeared to have prevailed.