28 A NEW FLORA OF 



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 sand- 

 stone. It crosses the North Tyne below Haughton Strother, and 

 appears on the line of the Eoman 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 siUs 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 Eookhope Burn, where it is 

 intruded between limestone beds, the lower one being metamor- 

 phosed.* 



The Great Basaltic Whin Sill attains its maximum thickness 

 in the Tees, where it is above 200 feet thick, giving a pictur- 

 esque character to the wild scenery of Upper Teesdale. It is 

 seen above the Weal, an extensive pool formed by a natural dam 



* Trans. Nat, Hist. Soc. of Northumberland, &c. Vol. I, p. 58. 



