CHARACTER OF THE WHIN SILL OF NORTHUMBERLAND. 421 



varied from 3500 to 6000 feet, the difference would probably be very 

 important. 



The relation 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 Northumberland of a 

 Whin dyke intersecting the Whin Sill ; but to the south of Alnwick, 

 near Shield Dykes, there is an instance which strongly suggests this. 

 The Whin Dykes are generally dykes and nothing more. In only 

 a few cases have they been proved to send out lateral branches 

 amongst the strata. A brief enumeration of some localities in which 

 this has been observed is given by Mr. I. L. Bell in a paper on the 

 chemical alteration of Whin at and near its contact with sedimen- 

 tary rocks *. 



The Whin Dyke which runs from near the coast, past Kadcliffe 

 Colliery, Acklington and Cartington, into the Porphyrite of the Che- 

 viots, seems to overflow at Clennel, just before entering the Cheviot 

 country. There are some curious beds of Whin in a shaft at Shil- 

 bottle Colliery, south-east of Alnwick ; Mr. Tate supposed them to 

 be overflows from a Whin dyke which occurs close by ; but whether 

 this is so, or whether they belong to the Whin Sill, cannot at present 

 be proved. 



7. Conclusion. — The foregoing paper has touched upon several 

 collateral subjects ; but the main point has been to establish the in- 

 trusive character of the Whin Sill. That this has been injected 

 between the strata, after their deposition and consolidation, is, we 

 think, now sufficiently evident. 



The exact geological date at which the intrusion took place can- 

 not be determined. Northumberland offers no conclusive evidence 

 upon the subject ; but so far as the evidence in this and other dis- 

 tricts goes, it seems probable that the intrusion took place at the 

 close of the Carboniferous period. 



Of the relations of the Whin Dykes of Northumberland to the 

 Whin Sill we have no certain knowledge. The dykes may be of 

 different ages ; and some of them probably belong to the same period 

 as the majority of the long trap dykes of the south of Scotland, 

 which Prof. A. Geikie has shown to be of Tertiary age. 



EXPLANATION OF PLATE XVIII. 



Sections of the Carboniferous-Limestone series of Northumberland, showing the 



positions of the Whin Sill. 



Discussion. 

 Mr. Warington W. Smyth said that he had gone over the dis- 

 trict referred to in the paper with Mr. Blackwell, the geologist who 

 first determined the intrusive character of the green-and-white rock 



* Proc. Roy. Soc, vol. xxiii. p. 543, 1875. See also a notice by Mr A L 

 Stevenson, Trans. N. of Engl. Inst. Eng. vol. xxiii. p. 160, 1874 Mr Bell 

 refers to the Whm Sill as a "bedded trap;" but probably he does not attach to 

 that term the idea ot contemporaneity. 



