Cockfield Fell 
dyke. 
26 Professor Sevewick on Trap Dykes 
Proceeding some miles farther to the S.E. we come to the 
north-western termination of Cockfield Fell dyke, which is seen 
in a quarry by the side of the brook which runs past Gaundlass 
Mill. In that single locality it assumes a compound form, bemg 
made up of three distinct and nearly vertical masses of trap alter- 
nating with a variety of indurated slate-clay. The following is 
a transverse horizontal section of the whole dyke. (1) On the 
south-west side, common coal shale, which, as it approaches. the 
dyke, becomes much indurated and has a vertical cleavage. In 
this state it is provincially termed pencil. (2) Trap one yard. 
(3) Pencil about four or five yards, but of variable thickness and 
much shattered. (4) Trap two yards. (5) Pencil half a yard. 
(6) Trap about seven yards. (7) Coal shale resembling No. (1). 
These entangled masses of coal shale are probably not pro- 
longed far beyond the quarry, as they are seen in no other 
section. 
The dyke afterwards ranges through the coal works which are 
opened in Cockfield Fell about half a mile to the north of the 
village; and its farther progress in a direction about E.S.E. is 
marked in Blackburn quarry and Crag-wood. Near the former 
place it is intersected by a cross course, and heaved several yards 
out of the line of its direction. To the S.E. of Crag-wood, it 
would perhaps be impossible to trace it at the surface; but the 
vein of trap which runs along the high ridge of coal strata 
between Bolam and Houghton-le-side, agrees so well in character 
and direction with the masses above-mentioned, that it has 
generally been assumed as the prolongation of them. 
as I in vain endeavoured to discover the traces of a dyke farther down the river. If this con- 
jecture be right, it will be necessary to remove the dyke (which in the map accompanying 
Mr. Winch’s Memoir is made to cross the Tees below Egglestone) to a place considerably to 
the N. W. of its present position. When so represented, it will be seen, by an inspection of 
the map, that the basalt in Teesdale and tbe neighbourhood of Cockfield Fell are much more 
nearly in a straight line than they have been represented. 
