40(5 W. TOPLEY AND G. A. LEB0UR ON THE INTRUSIVE 



19. On the Intrusive Character of the Whin Sill of Northumber- 

 land. By W. Toplet, Esq., F.G.S., Assoc. Inst. C.E., Geological 

 Survey of England and Wales, and G. A. Lebotjr, Esq., F.G.S., 

 Lecturer in Geological Surveying at the University-of-Durham 

 College of Physical Science, Newcastle-on-Tyne. (Head 

 December 6, 1876.) 



(Communicated to the Society by permission of the Director-General of 

 the Geological Survey.) 



[Plate XVIII.} 



Contents. 

 i. Introduction. 



2. The Carboniferous-Limestone Series of Northumberland. 



3. Bibliography. 



4. Stratigraphical Eelations, 



5. Mineral Characters. 

 (>. Age and Origin. 



7. Conclusion. 



1. Introduction. — The basaltic rocks of the north of England occur 

 in two forms— as sheets lying amongst the sedimentary strata, and 

 as dykes cutting through them. The intrusive character of the 

 latter is, of course, undisputed ; but there is much uncertainty in the 

 minds of many geologists as to the character of the former. 



In this paper we purpose to show, from the detailed work of the 

 Geological Survey, that in Northumberland there can be no doubt 

 whatever as to the intrusive nature of the bed or beds of basalt 

 known as the Whin Sill. This conclusion can be established both 

 by the evidence of individual sections and by the line of outcrop of 

 the trap. 



The question of the nature of the Whin Sill is of importance in 

 three ways : — 1st, as a point of theoretical geology, bearing upon 

 the history of volcanic action in Britain ; 2nd, in reference to the 

 classification of the Carboniferous System ; 3rd, in its bearing upon 

 practical mining. 



The Whin Sill is best known in Teesdale, especially in the two 

 fine waterfalls of High Eorce and Cauldron Snout. It also appears 

 along the face of the Penine escarpment, and is beautifully exposed 

 in the " Nicks " which furrow the face of that range. Although 

 this district is known to us, we will not stay to describe it, be- 

 cause it has already much engaged the attention of geologists, 

 especially Professors Sedgwick and Phillips. It is generally sup- 

 posed that there is but little evidence of intrusion there ; but this is 

 not the case ; the evidence is plain if one only looks for it, and it 

 was sufficient to satisfy so close an observer and so clear a reasoner 

 as Professor Sedgwick. 



One of the finest sections along the Penine escarpment is that at 



