the Great Whin Sill, Northumberland. 



253 



with which this appears to blend, the numbers being greatest 

 immediately over the feeder, and falling off on either side of it (Figure). 

 The general spherical cliaracter of the inclusions and the lack of sharp 

 boundaries between them and the surrounding rock are explicable as 

 the results of two opposing tendencies — surface-tension, promoting 

 a spherical drop-liUe form, and diifusion, tending to the blending of 

 the magmas and the establishment of uniform composition. 



There are two questions arising out of the hypothesis which require 

 consideration ; the first is relative to the feeders of the Whin Sill, 

 the second concerns the differentiation-process. 



With regard to the first there appears to be no positive evidence. 

 Thus Professor Lebour and his collaborators write: "Up to the 

 ])resent time no whin dyke has ever been proved to belong to, cut 

 through, or come in contact with, the Great Whin Sill." ^ And 

 again : " The rt^lation which the whin dykes of the district bear to 

 the Whin Sill is an interesting question ; but it is one upon which 

 little can be said. There is no certain case in Northumi)erland of 



Section at Snook Point. 



Illustrating disposition of Whin Sill and inclusions and the hypothetical 

 dyke-feeder. (Below the heavy contmuous line the section is diagrammatic ; 

 the portion which can be observed lies above this line.) 

 1. Great Limestone. 



'2. Fine-grained whin, without amygdules, altered at contact 

 Whin Sill, with hmestone. 



about -! 3. Bedded whin, with amygdules and inclusions. 

 25 feet thick. 4. Bedded whin, with amygdules. 



,.5. Columnar whin, altered at contact with sedimentary beds. 



6. Carboniferous sandstone. 



7. Hypothetical dyke-feeder of whin, containing inclusions 



(black dots). 



a whin dyke intersecting the Wliin Sill." - So far as the writer is 

 aware these woi'ds adequately express the state of our knowledge on 

 the subject at the present day ; visual proof of connexion at Snook 

 Point is not to be expected, owing to the limited exposure on the 

 loreshore and the thick cover of blown sand on the adjacent land. 

 With respect to the differentiation-process, it would appear that 



^ G. A. Lebour & Mark Fryer, " On the Harkess Kocks, near Bamburgh " : 

 North of England Institute of Mining and Mechanical Engineers, 

 vol. xxvi, p. 1, 1877. 



^ W. Topley & G. A. Lebour, " On the Intrusive Character of the Whin Sill 

 of Northumberland " : Q..J.G.S., vol. xxxiii, p. 406, 1877. 



