198 Mr. Tate on Basaltic Rocks. 



Such characters, along with effects produced on stratified 

 rocks in contact with it, evidence that this basalt is of igne- 

 ous origin, and that at a distant period it was like lava in a 

 molten state ; but on cooling under pressure acquired a 

 crystalline structure and a jointed and pillared form. Such 

 characteristics will be noted as we pass along in our survey 

 of the Northumberland basalts. 



The basaltic rocks are similar in character throughout the 

 whole of Northumberland, excepting where they are in con- 

 tact with stratified rocks ; but there is a difference in the 

 mode of their occurrence ; there is the Basaltic Whin Sill, 

 which has a long range and is in some parts intercalated 

 with sedimentary beds ; and there are also basaltic dykes, 

 which cut nearly perpendicularly through stratified rocks. 

 Each requires a separate notice. 



BASALTIC WHIN SILL. 



The Basaltic Whin Sill is the most remarkable rock in 

 the North of England, on account of its long and tortuous 

 range, and of its relation to and effect upon the strata it 

 traverses, and among which it has been intruded. Though 

 locally called a Sill, because sometimes appearing as a 

 stratum ; yet it is not a true stratum, for its thickness varies 

 very much, from two feet to nearly two hundred feet, and 

 the parallelism of its upper and under surfaces is preserved 

 only for short distances ; so that though its extension in the 

 line of direction is great, yet its extension in the line of dip 

 is inconsiderable. It appears at all heights from the sea- 

 level up to one thousand feet above it. Following its wind- 

 ings, its course is more than eighty miles long, from Kyloe 

 on the north to Glenwhelt at the south-west. The range, 

 however, is not continuous and unbroken ; but there is a 

 succession of craggy eminences, rising high in bold and 

 picturesque cliffs above the general level of the country, with 

 intervals between, in which no basalt is visible ; and even 

 where there is a continuous range for several miles, as along 

 the line of the Roman Wall, yet is the outline broken by 

 gaps, locally called nicks, in the rock, which there appears 

 in an alternate succession of lofty cliffs, steep and rugged, 

 with deep hollows between. 



The most northern outbreak of the Sill is at Kyloe Crags, 

 half-a-mile W.S.W. of Kyloe Church, in a fine mural cliff, 

 500 feet above the sea level, extending for a mile E.S.E. to 

 Bogle Houses, with a cliff-face to the south-west. The basalt 



