Mr. Witham on the Greenstone Dyke, fyc. 343 



No. XVIII. — On the Effects produced by the Greenstone Dyke upon the 

 Coal, in passing over Cockfield Fell, in the County of Durham, by H. T. M. 

 Witham, Esq., V. P., F. G. S., $c. 



Although innumerable examples of the changes caused by the action of 

 volcanic dykes have been already described by various authors, yet I have 

 read of none whose influence has been more powerful or more extensive 

 than that produced by the great Cleveland or Bolam Dyke upon the coal 

 which lies under Cockfield Fell. 



This colossal dyke is too well known to geologists to require here any 

 particular remarks. The distance of its course, running from S. E. to 

 N. W., is computed to be about 70 miles, and how much further it may 

 go in either direction is as yet a matter of uncertainty. The breadth 

 differs, but from its extent, generally, the mass of volcanic matter injected 

 must have been enormous. Cockfield Fell is about two miles to the north 

 east of Staindrop, a market town in the county of Durham. 



Under this common several seams of coal are worked. The coal, with 

 the other strata, is thrown up to the south six yards by the dyke. In 

 working towards it, when within fifty yards, the coal begins to change. 

 It first looses the calcareous spar, which occurs in the joints and faces, 

 and begins to look dull, grows tender and short ; and also looses its 

 quality for burning. As it comes nearer it assumes the appearance 

 of half burnt cinder, and approaching still nearer the volcanic mass, it 

 grows less and less in thickness, becoming a pretty hard cinder, and 

 only two feet six inches thick. Eight yards further it is converted into real 

 cinder, and more immediately in contact with the basalt it becomes, by de_ 

 grees, a black substance called by the miners dawk or swad, resembling 

 soot caked together, the seam of coal being reduced to nine inches in 

 height. There is a large portion of pyrites lodged in the roof of that part 



