197 



The Basaltic Bocks of Northumberland. By the 

 late George Tate, F.G.S., &c. 



The igneous rocks of Northumberland may be arranged in 

 two groups, of different age and mineral composition — the 

 felspathic and the augitic, or the porphyries and basalts ; the 

 former being of much greater age than the latter. Of the 

 porphyry and syenite of the Cheviots I gave an account in 

 1867* ; and in this paper I shall attempt to describe the 

 character, the range, and the relations of the basaltic rocks 

 of the county. William Hutton, in 1831, gave an account 

 of The Stratiform Basalt of the North of England-^ ; but 

 his description of the northern portion of the range is but 

 slight, and not always accurate, and he leaves unnoticed the 

 important sections exposed along the coast. Having ex- 

 amined most of the basaltic dykes, and also the basaltic 

 Whin Sill, from its most northern outbreak at Kyloe Crags 

 to its south-western extremity in the county near Glen- 

 whelt, with the exception of a portion between Kirkwhelp- 

 ington and the North Tyne, I give the result of my own 

 independent observations, excepting when it is otherwise 

 stated. 



Any one with even a limited knowledge of rocks must 

 perceive, that the external appearance and the structural 

 and mineral characters of the pillared masses of basalt at 

 Kyloe Crags, Dunstanburgh, and House Steads, are different 

 from those of the stratified rocks, such as limestone and 

 sandstone, with which they are associated. The basalt, 

 when fresh fractured, presents a dark grey colour and crystal- 

 line structure. It is chiefly composed of augite and felspar ; 

 and having a proportion of protoxide of iron, is generally 

 magnetic ; it is here and there, as at Budle, amygdaloidal, 

 and sometimes, as at Dunsheugh, porphyritic. From the 

 quantity of augite in its composition, it is both hard and 

 tough, and hence its value as one of the best of road-stones, 

 the coarser varieties being best adapted for this purpose. It 

 decomposes slowly, but what soil results from decomposition 

 is very fertile, and produces a number of rare indigenous 

 plants. When in great mass it has a columnar structure, the 

 pillars being rude prisms, irregularly jointed; and a few 

 even approach the hexagonal form seen in Fingal's cave. 



* Proceedings of Berwickshire Nat. Club, vol. v., p. 359-370. 

 t Trans, of Nat. Hist. Soc. of Northumberland and Durham, vol. ii., p. 

 187-214. 



