466 C. T. CLOITGH ON THE SECTION IT 



51. The Section at the High Fobce, Teesdale. By C. T. Clottgh, 

 Esq., B.A., F.G.S., Scholar of St. John's College, Cambridge, of 

 the Geol. Survey of England and Wales. (Eead June 21, 1876.) 



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

 of Great Britain.) 



It has been my duty, in the course of Survey work, to examine the 

 ground near the High Force. In doing this, several facts have come 

 to light which will, perhaps, be not uninteresting, as they refer to 

 a somewhat classical district, and correct ideas which have been 

 hitherto erroneously held on the authority of Phillips's section at the 

 High Eorce. These ideas have lately acquired a special importance 

 from their bearing on the subject of "selective metamorphism ;" and in 

 correcting them I may be helping, in some small degree, to clear the 

 way for the formation of a theory to satisfactorily account for those 

 instances of selective metamorphism which stand undoubted. 



In Phillips's < Geology of Yorkshire ' (1836), part ii. pp. 78, 79, the 

 following passage occurs in connexion with the section at the High 

 Force: — " Shale or plate is so much altered at the High Force in the 

 relation of the joints that most persons mistake a part of the pris- 

 matic masses, really composed of metamorphic shale, for trap, and 

 suppose the latter to rest on limestone. The true series is as follows, 

 proceeding downwards : — 



" a. Basalt, rudely prismatic, grey with lichen. 

 b. Thin plate, not very much indurated. 

 e. Bed of plate, subprismatic. 



d. Beds of plate, laminated. 



e. Thin limestone bed, with a superficial layer of pyrites. 

 /. Bed of hard pyritous limestone. 



g. Several beds of common dark limestone, with white shells and corals." 



(Among the Dalesmen the word "plate" is equivalent to "shale;" 

 and the basalt referred to is part of the great " whin sill " of Tees- 

 dale, &c.) 



On reading over this section it naturally strikes one at once as a 

 very odd thing that there should be a subprismatic bed of plate, c, 

 below the thin plate not very much indurated, b — in other 

 words, that the plate which lies nearest to the altering cause 

 should be less altered than the plate which is further from it. Here, 

 indeed, if the beds b and c had really been of the character described 

 above, we should have a most striking instance of selective meta- 

 morphism. I think I shall be able to show, however, that the above 

 description is not correct — that the bed c is not altered plate, 

 but basalt, that an examination of the general rock-structure of 

 the beds a and c proves conclusively that the only differences 

 between them are such as should naturally be looked for in beds of 

 such different thicknesses, and that this conclusion is fully borne 



