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. 



L. Limestone. 



