ON THE DISCOVERY 0¥ SILURIAN BEDS IN TEESDALE. 27 



2. Discovery of Silurian Beds in Teesdale. By W. Gunn, Esq., 

 F.G.S., of H.M. Geol. Survey, and C. T. Clough, Esq., B.A., 

 E.G.S., of H.M. Geol. Survey. (Read June 20, 1877.) 



(Communicated by permission of the Director-General of the Geological 

 Survey of the United Kingdom.) 



Our colleague Mr. J. It. Dakyns, a few months back, put the ques- 

 tion to geologists, " Is there a base to the Carboniferous rocks in 

 Teesdale?" (Geol. Mag., Decade ii. vol. iv. p. 58), and briefly men- 

 tioned the reasons which had induced him to ask the question. 

 Some time previously to the visit of Mr. Dakyns to Teesdale men- 

 tioned in the article referred to, Mr. Howell, of the Geological Sur- 

 vey, had made the same suggestion to one of us. 



We have already, in the Geol. Mag. (torn. cit. p. 139), stated that 

 there is no doubt that this suggestion is correct. In the present 

 paper we offer to the Society the detailed proofs of this. 



It will be advisable in the first place to say something about the 

 physical structure of the dale, in order that the mode of occurrence 

 of the older beds may be better understood. 



It is generally well known in the dale that there is a great dis- 

 turbance, going by the name of the Burtreeford Dyke (from Burtree- 

 ford, in Weardale, where the disturbance is well seen), entering the 

 dale near Langdon Head and running diagonally across it roughly 

 from north to south, and that this disturbance lifts the beds up to 

 the west very considerably. As far as we have been able to make 

 out in this district, the dyke does not appear to consist so much of 

 an actual break as of a sharp run-up of the beds, with possibly local 

 breaks here and there as well. The total displacement caused by 

 this disturbance is sometimes thought to be indicated by the differ- 

 ence in height, about 350 feet, between the Whin Scars of Cronkley 

 Fell on the west of the dyke and the Whin Scars of Forcegarth 

 Hill on the east of the dyke. But it is not safe to assume that the 

 Whin Sill behaves as a regular sill, and occurs everywhere on the 

 same geological horizon ; and in this case especially the assumption 

 would lead one very far astray. We have been able to make out 

 that between Forcegarth Hill and Cronkley Fell the whin has 

 changed its position very greatly, that the whin of Forcegarth Hill 

 occurs about the horizon of the Single-Post Limestone of Westgarth 

 Forster's section, and that on Cronkley Fell it is on the horizon of 

 the Bundle or Melmerby-Scar Limestone, which in this district may 

 be taken as being about 400 feet below the Single Post ; so that it is 

 evident that the displacement caused by the dyke is not at all com- 

 pletely indicated by the difference in height of the Cronkley and 

 Forcegarth- Whin Sears ; for to this difference must be added the 

 400 feet of beds which come in between the two limestones in 

 question. 



