The geological structure of Dura Den is more than ordinarily interesting, presenting, as it does within a limited distance, and in close juxtaposition, the two series of the old red sandstone and carboniferous systems, an included mass of overlying trap, a greenstone dyke, and a vein of galena. The whole length of the dell, with its windings, from the ruins of the castle resting on the conglomerate red, to the outgoing on the south into Ceres basin of the coal formation, does not exceed a mile and a half. The rocks overhang the road which passes through the valley, the sandstone in some places rising precipitously into bold mural cliffs of a hundred feet in height, and presenting colored and well-defined sections of the different layers of which it is composed. These constitute the fish beds of the yellow sandstone group, lying toward the northern extremity of the den, and consist of beds of variegated marls, intermixed with friable arenaceous bands, and hard, compact, fine-grained building stone. The carboniferous series are separated from those of the yellow sandstone by the greenstone dyke referred to, which immediately, and inconveniently for sight of the junction, interposes betwixt the two systems. The lower beds of the independent coal formation are here thrown up to an angle of 26°, the yellow sandstone adjacent being nearly horizontal, and in no place exceeding an inclination of eight or ten degrees. The coal beds have been lifted up by and repose anticlinally upon the trap, where the cutting for the road has exposed the outcrop of the seams; and thus, in a narrow space and lying on the surface, we may mark the outgoing and the incoming of a vast revolutionary epoch, organic and inorganic, in the earth’s history. The strata, consisting of alternating bands of coal, shale, ironstone, and sandstone, assume toward the head of the valley a nearly horizontal position, abutting against a mass of trap which separates the lower from the upper workable beds of the bituminous mineral in the Ceres basin.