The Hett Dike has a direction of E.N.E., and passes through the Coal Measures, Millstone Grit, and Mountain Limestone. It may possibly have its origin from the Whin Sill. It is seen in Eggleston Common, and where it crosses Eggleston Burn it is nearly vertical, and 33 feet wide. It is traceable eastward to Hett and Quarrington, passing through the Crow Trees colliery and dividing and charring the Five-quarter and High Main Coal Seams; but it does not penetrate the overlying Magnesian Limestone. As it extends eastward and approaches the surface it diminishes in width, and at Crow Trees it is only 6½ feet.
The Cockfield Dike, from its great length, is one of the most important, extending from north-west to south-east about 70 miles, and passing through Mountain Limestone, Millstone Grit, Coal Measures, and New Red Sandstone. It varies in width from 17 feet to 60 feet. At Cockfield the coal in contact with it is charred, and the strata are upcast 18 feet on the south.
No basaltic dike has yet been seen among the Magnesian Limestone strata.
We know not the vomitaries of these augitic igneous rocks. Along the whole range of the Whin Sill no crateriform hollows or cones appear: the molten matter, therefore, had probably been ejected through long lines opened by deeply-seated forces acting in the general direction of north to south. The vertical dikes of basalt have a direction transverse to that of the Whin Sill; and though, as Philips remarks, "geographically related to it," are never seen in junction with it. They are too small to have been the vomitaries of the Whin Sill; and supposing they are of the same age, they do not help us much to determine the period of eruption. None of them, excepting the Cleveland Dike, which is at the southern extremity of our district, pass through beds more recent than the Coal Measures. The Whin Sill, however, as Philips has shown, is anterior to the east and west veins of Tynedale, for it is divided by these veins of fissure, and, as these fissures have resulted from the Penine fault, the Whin Sill is older than the Triassic beds. Subsequently, then, to the Carboniferous, and prior to the Triassic era, this district was convulsed
