I. Introduction. In 1909, one of us described the geology of the area around Plynlimon and Pont Erwyd, and a certain succession was then established. Subsequently, an area of 70 to 80 square miles north-west of Plynlimon has been mapped in detail, and the succession previously established proves to hold good, but some new light has been thrown upon the structure and its behaviour. The present communication deals with the geology of an area of about 25 square miles, near the town of Machynlleth, where the succession, and especially the structure, present novel features which are described in some detail. The district lies on the border of the three counties of Merionethshire, Montgomeryshire, and Cardiganshire, and is included in parts of the 1-inch Ordnance maps, N.S. 149 & 163, and O.S. 59 N.E., S.E. The 6-inch maps, Montgomery 25 S.E., S.W. & Montgomery 32 N.E., N.W., cover the area—with the exception of a narrow strip in the north-west, which is contained within Merioneth 46 S.E. The district is drained by the River Dovey and its tributaries, of which the narrow glen called the Llyfnant Valley, lying near the southern margin, is the most picturesque. Near the town of Machynlleth, which stands on the eastern margin of the Dovey Valley, the floor of the valley expands from its normal width of about three-eighths of a mile to nearly a mile: below this point it becomes constricted, and afterwards opens out into a broad estuary, flanked on the south-east by extensive marshes.