Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/37

 distance of 20 yards from the dyke; but in others to that of 3 or 4 yards only.

From the southern side of the main dyke two others branch off, one to the south-east, the other to the south-west. The latter is called from its breadth the 70 yard dyke, and is filled with a body of hard and soft sandstone. This intersects the upper or Beaumont seam, which is not thrown out of its level by the interruption. The seam however decreases in thickness from the distance of 15 or 16 yards, and the coal first becomes sooty, and at length assumes the appearance of coak. This phenomenon is unknown elsewhere except in the vicinity of basaltic dykes.

The south eastern branch is only 20 yards in breadth, and hard white sandstone together with other rocky fragments fill the cavity, and are in part cemented together by calcareous spar. Although the strata are thrown up only 20 feet on the north-eastern side of the vein, yet great confusion has taken place in its vicinity, and much water was found to issue from it.

From the northern side of this part of the main dyke many small slips extend, some of which alter the level of the Newbiggin coal-seam without affecting that of the Kenton seam lying only 13 fathoms above it.

The Birtley, Tantoby or Tanfield dyke is next in magnitude and length after the main dyke. From Tatlield on the Wear it ranges towards the west, passing through Leefield, Ouston, Birtley Fell, and Urpeth collieries: thence in the direction of Beamish hall it traverses Tanfield Moor, and crosses the Derwent near Derwent-coat Forge. In Tanfield Moor colliery it is in all an upcast on the northern side of 40 fathoms; but instead of consisting of one strong