G. A. Lebour — Limits of the Yoredale Rocks. 541 



persistently between the same beds in any other part of it, even if 

 the examination of its outcrop in the latter region have revealed no 

 change of horizon. Slight variations of level, however, seem to be 

 admitted by some of the upholders of the interbedded character of 

 the Whin Sill, even south of Northumberland. 1 



In arguing this point in conversation, I have sometimes been met 

 with the objection that the Great Whin Sill was not really taken as 

 the boundary between the Yoredale Series and the Scar Limestone 

 Series, but that the bed of limestone lying upon it in the Alston 

 District and along the Pennine Escarpment was taken as the bottom 

 Yoredale bed — this limestone being well known in the lead-mining 

 districts as the " Tyne-bottom Limestone " — without reference to the 

 trap, whose absence or presence had but an accidental connexion 

 with the line of division. I believe that this ingenious way of 

 shuffling out of the difficulty is of quite recent invention — dating, in 

 fact, from the time when the true nature of the Whin Sill was con- 

 clusively shown. That it is so will be made apparent by a glance 

 at the most authoritative geological maps of England which have 

 appeared of late years. In them the Yoredales and the Scar, or 

 Carboniferous Limestone proper, divisions will be found very clearly 

 defined by a continuous red divisional line of Basalt — the Great 

 Whin Sill — without a hint of any other demarcation being sought. 2 

 Again, passages such as the following, which testify similarly to the 

 general acceptance of the trap-sheet as a base-line, might be multi- 

 plied with ease, viz. "Yoredale Eocks. — In this series we include 

 all the strata from the Fell-top Limestone inclusive to the Great 

 Whin Sill." 3 



My position then, with regard to the Great Whin Sill, is this : 

 that being undoubtedly intrusive and subject to change of level, it is 

 totally unfit to serve as the boundary line between two great divi- 

 sions of the Carboniferous rocks. 



Setting aside the great trap-sheet therefore, and seeking for a less 

 fickle base for the Yoredale rocks in the North, we have offered 

 us merely a bed of Limestone — the Tyne-bottom Limestone of the 

 Alston miners. 



This "Tyne-bottom Limestone" is the tenth calcareous bed men- 

 tioned by Westgarth Forster (in descending order) in his section of 

 the strata of the Alston Moor District. It has no lithological cha- 

 racter to distinguish it from the other limestones, either higher in 

 the Yoredales above it, or in the Scar Series below ; its thickness 

 (about 22 feet) is not constant, even over a limited district ; its 

 fossils are useless for purposes of identification. Indeed, no bed 

 perhaps in the entire Carboniferous Limestone Series in the North of 

 England is more difficult of identification — certainly none has had 



1 Nicholson, op. jam. cit. f p. 78. 



2 The geological map which accompanies the Coal Commission Report is an 

 exception, although in that map the assumed doubtful line of boundary between 

 Yoredale and Scar is more out than usual, owing, doubtless, to the general character 

 of the map. 



3 A Synopsis of the Geology of Durham and Part of Northumberland, by 

 Eichard Howse and J. W. Kirkby, Newcastle-on-Tyne ; 1863, p. 22. 



DECADE II.— YOL. II. NO XI. 35 



