Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/44

28 Little Swinburn there is only one range towards Gunnerton. At Whelpington, on the Wansbeck, two beds of Basalt are seen, separated from each other by a bed of metamorphosed shale one foot thick, the basalt being below limestone and resting on sandstone. It crosses the North Tyne below Haughton Strother, and appears on the line of the Roman wall at Limestone Corner, and then, after a curve southward, it joins the line of the wall again eastward of Shield-on-the-Wall, thence running westward, as far as Thirlwall, in a succession of high-pillared craggs, with deep gaps or "nicks" between, and with cliff faces to the north and north-west, and reaching, at Winsheals, an elevation of 1000 feet above the sea level. About a mile to the west of Glenwelt, it crosses the turnpike road, beyond which it is little seen in our district till we reach the upper part of Teesdale. Though the basaltic sills in Durham are similar to those in Northumberland, and are intruded among the Mountain Limestone in a similar manner, yet it is not certain that all belong to the same eruption. Near the borders, between Northumberland and Cumberland, basalt forms the bed of a burn, in Knaresdale, for about 400 yards, and also of Gildergate Burn for about 100 yards; but it appears more in Cumberland, in the streams which carry off the drainage of the western side of Cross Fell and Hartside. At Wear Head a basaltic sill appears broken by the Burtree Ford basaltic dike; but further down the Wear, near Stanhope, there is another basaltic sill, which Sir Walter Trevelyan thinks is different from the Great Sill, and situated among beds higher in the Mountain Limestone series. From Unthank Bridge this little sill forms the bed of the river for some hundred yards, and is traceable westward, as far as Westerhope, thinning out in that direction; it is 20 feet thick in Rookhope Burn, where it is intruded between limestone beds, the lower one being metamorphosed.

The Great Basaltic Whin Sill attains its maximum thickness in the Tees, where it is above 200 feet thick, giving a picturesque character to the wild scenery of Upper Teesdale. It is seen above the Weal, an extensive pool formed by a natural dam