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



Series, from which, however, it is separated by a sheet of basaltic 

 trap Avell known in the North of England as the Great Whin 

 Sill. It is to this lower limit that I wish to call particular attention. . 



Granting that no stratigraphical boundary could be more con- 

 venient than one marked by a non-intrusive, continuous, evenly- 

 interbedded mass of trap, especially when we remember that, even 

 in its typical localities, the Scar Limestone Series is (setting fossil 

 evidence aside) only distinguished from the upper group by the 

 thickness of its beds of limestone — granting this, it must yet be 

 obvious that each one of these conditions is essential to the arrange- 

 ment. If the basalt be shown to be intrusive, that is, injected 

 among, and not merely over, hardened beds, if it be discontinuous 

 in any portion of its course, if it shift its horizon to higher or lower 

 portions of the Carboniferous mass, then this trap utterly fails as 

 a natural divisional line. 



Now in a paper read at the Bradford Meeting of the British 

 Association in 1873 x by my friend Mr. W. Topley, F.G.S., and 

 myself, which has yet only been printed in abstract, we showed, 

 I believe quite conclusively even to the late Prof. Phillips, who had 

 formerly upheld the opposite view, that in its career across Nor- 

 thumberland to the North Sea the Great Whin Sill was distinctly 

 and undoubtedly intrusive, that it was occasionally discontinuous, 

 and that it was subject to changes of level so important as in some 

 cases to carry it to a position above that very " Main " or " Great " 

 Limestone which, as is stated in the quotation above, is one of the 

 highest beds in the Yoredale Series. It is needless to repeat the 

 detailed descriptions by which we established these facts, as the 

 paper will probably before long be published in full ; but if any 

 scepticism remain on the subject, I would call attention to a 

 section" which, in the words of one very competent to form an 

 opinion, " satisfactorily clears up even to the most fastidious person 

 the intrusive character of the rock." 3 



It is true that the Whin Sill in Durham and along the great 

 Pennine Escarpment is wonderfully regular, and can, for those portions 

 of its extent, be looked upon as, for all practical purposes, the 

 equivalent of a truly interbedded or contemporaneous trap, although 

 even there the signs of its intrusive nature were sufficient to con- 

 vince so experienced an eye as that of the late Prof. Sedgwick. 

 But surely the mere fact that north of Alston Moor its horizon 

 cannot be depended on for any distance along its outcrop, is sufficient 

 to throw the gravest doubt on the Basaltic sheet's continued geo- 

 logical horizon to the clip or eastern side. There is no reason to 

 believe that an intrusive sheet of trap in which great and frequent 

 shifts of level in one part of its area have been proved, would lie 



1 Oiv the "Whin Sill of Northumberland, by "W. Topley, F.G.S., and G. A. 

 Lebour, F.G.S. (Abstract in Report of the British Association for 1873, London, 

 1874, part 2, p. 92). 



2 On the ' Great ' and ' Four-Fathom ' Limestones and their associated beds 

 in South Northumberland, by G. A. Lebour, F.G.S. ,_etc, Trans. N. Engl. Inst, of 

 Min. Eng., 1S75, vol. xxiv. p. 140, and fig. i. pi. xxxiii. 



3 Mr. B. Hwse, in discussion on above paper, ibid. p. 147. 



