File:The Quarterly journal of the Geological Society of London (12736568934).jpg

CHARACTER OF THE WHIN SILL OF NORTHUMBERLAND.
415
the escarpment of basalt, on the summit of which is built the Great
lloman Wall, deserves special mention, first, as being an almost
unique example of its kind, and,'secondly, because, oddly enough, it
has been (very inaccurately, it is true) figured by Hutton in the
very paper in which he proved to his satisfaction the contempo-
raneity of the Whin.
A bed of limestone with an accompanying shale, which has for
some distance from the west run regularly below the basalt and
parallel to it, is here suddenly forced upwards almost vertically into
the Whin, by which it is baked and altered in a very evident
manner. Now where this limestone underlies the W T hin Sill the
latter is capped by sandstone ; but immediately after the disturbance
just described the limestone and shale are seen overlying the Whin
in place of the sandstone, the outcrop of which, however, is in
nowise disturbed by the occurrence, and can easily be traced run-
ning still parallel to but further above the trap-sheet.
This is merely a case of the Whin Sill being seen actually in the
act .flagrante delicto) of shifting its horizon ; but it has this
further interest, that it proves that at this point the molten mass
was forcing itself from east to west, the change having necessarily
been from a lower to a higher horizon. This may have been but a
local direction due to the mass having to circumvent obstacles in
its progress, or to some other cause ; but there are some reasons
(.?. g. the bifurcation to the N.E., the generally greater thickness
in that direction, &c.) which would lead us to imagine that this is
but a part of a general truth with regard to the position of the
original focus of the Great- Whin eruption.
Although, as a rule, it may be said that the upper surface of the
Whin is tolerably smooth, yet there are many instances of isolated
masses, both great and small, having been thrust up from the main
sheet to a greater or less distance through the overlying beds. The
eastern branch, important as it is, must, we think, be looked upon
as an extreme case of this kind ; and between it and small bosses a
few yards only in diameter, appearing as little islands through thin
layers of limestone capping the Whin, of which there are very nu-
merous examples on the dip-slope of Gunnerton Crags (fig. 5), at Great
Fig. 5. — Section at Gunnerton Crags, showing the upper surface of
the main or western branch of the Whin Sill protruding through
the overlying Limestone,
B. Whin Sill.
GAL.