216 Prof. Sedgwick on some Trap Dykes 
the village; and its further 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 men- 
tioned, that it has generally been assumed as the prolongation 
of them. 
Bolam Quarry. 
In the quarries which they are now excavating near Bolam, 
the vertical dyke is unusually contracted in its dimensions; but 
on reaching the surface, it undergoes a great lateral extension, 
especially on the south-west side, so that the works are con- 
ducted in a perpendicular face of columnar trap more than 
two hundred feet wide. ‘The changes produced by this over- 
lying columnar mass are highly instructive, and will be de- 
scribed in their proper place. The old excavations, in the 
direction of Houghton-le-side, show that the trap is there con- 
fined to a fissure nearly forty feet wide, which, with a slight 
undulation in its direction, bears to a point about S.E. by E. 
Sandstone on the Trap. 
There is another locality, the mention of which must not be 
omitted, though I think it probable that it is not in the line of 
the great dyke. In this opinion I may, however, have been 
misled by the maps of the district, in which many of the places 
are laid down entirely out of their true bearings. At Wacker- 
field-lane-end, half a mile W.N.W. of Hilton, a mass of trap 
appears to range east and west, and may therefore join the 
leading dyke which intersects the country still further to the 
east. The excavations in that place would not deserve any 
particular attention, were it not for the important fact, that at 
their western termination horizontal beds of sandstone are 
seen to rest immediately upon the upper surface of the dyke. 
I have been informed that masses of trap occur on the north- 
east side of the quarries of Bolam; but I had no opportunity 
of examining them with a view of ascertaining their probable 
connexion with the principal dyke. 
From all these facts we may infer—(1) ‘That from Gaund- 
lass Mill to Houghton-le-side, a distance of about ten miles, 
the dyke of trap is uninterrupted—(2) That it may be con- 
nected with other dykes, which appear still further to the 
north-west nearly in the same line of direction, and through’ 
them 
