in Yorkshire and Durham. 27 
In the quarries which they are now excavating near Bolam, Botam quarry. 
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 overlying 
columnar mass are highly instructive, and will be described in 
their proper place*. The old excavations, in the direction of 
Houghton-le-side, shew that the trap is there confined to a 
fissure nearly forty feet wide, which, with a slight undulation 
in its direction, bears to a point about S.E. by E. 
There is another locality, the mention of which must not be Sandstone on 
omitted, though I think it probable that it is not in the line aan 
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 Wackerfield- 
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 farther to the east. The exca- 
vations in that place would not deserve any particular attention, 
were it not for the important fact, that at their western termina- 
tion 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 Gaundlass 
Mill to Houghton-le-side, a distance of about ten miles, the dyke 
of trap is uninterrupted—(2) That it may be connected with other 
dykes, which appear still farther to the north-west nearly in the 
* See Plate II. Fig. 4. 
D2 
