I. I ntroduction . T he region in which this volcanic series is developed occupies an extensive area in the extreme west of Pembrokeshire on the southern side of St. Bride's Bay, and includes the islands of Midland, Skomer, Grassholm, and the Smalls. Skomer is by far the largest of the islands, and is situated about two-thirds of a mile from that part of the mainland which is known as Wooltack. It has a length, from east to west, of about 2 miles, a breadth of a little over a mile, and an area of 722 acres. Between it and the mainland lies the small island of Midland, with an area of 21½ acres, which is separated from Skomer by the Little Sound, only about 100 yards in width, and from the mainland by the broader stretch of water known as Jack Sound. Grassholm Island lies 8 miles to the west of Skomer and covers an area slightly less than that of Midland, while the Smalls, which occupy only three-quarters of an acre, lie 8 miles to the west of Grassholm. Skomer Island is a detached portion of that plateau which is so well marked in the west of Pembrokeshire: it rises more or less abruptly to an elevation of 200 feet; but, on account of the smaller size of Grassholm, the highest levels attained by this island fall some way short of the usual plateau-horizon, while the Smalls rise only a few feet above the level of high-water at spring-tides. All these