542 G. A. Lebour — Limits of the Yoredale Bocks. 



so many other beds taken for it. The reason of this is simply that, 

 with the rooted belief that the Great Whin Sill was a regularly inter- 

 bedded trap-flow, whatever calcareous band chanced to be found 

 next above it was instantly supposed to be the Tyne-bottom Lime- 

 stone. This identification was sometimes right and sometimes wrong 

 (more often the latter), and it necessarily led to great confusion re- 

 specting the horizons of the beds above. 1 In the faulted district 

 which lies to the North of Alston it would be a difficult thing to 

 trace with any certainty a line of boundary depending on an ill- 

 characterized bed such as this ; but in this case the difficulty becomes 

 almost an impossibility if we bring the following considerations to 

 bear upon the subject. 



It is no part of the object of this paper to describe in detail the 

 beds which form the so-called Yoredale Series in Northumberland ; 

 but it is necessary, for a proper understanding of my argument, that 

 I should refer to a paper in which I have shown that the several 

 beds of limestone which make up the calcareous element of the series 

 in the Alston District increase in number in its northern extension, 

 that not only limestones of average thickness, but also grits, shales, 

 and coals appear, and sometimes disappear, as intercalated beds of 

 greater or less continuity. 2 



The Yoredales, which in the Alston District are about 500 feet thick, 

 in about twenty miles of northerly trend increase to some 2000 feet, 

 only a few of the more marked beds being traceable throughout. 

 Among these the Tyne-bottom Limestone has no place. No one but 

 a Wernerian miner, relying on the golden rule of thumb, could at 

 the end of the twenty miles point out with any degree of certainty 

 the bed which is indeed the "Tyne-bottom." It may be there, or it 

 may have thinned out, as many others have been proved to do. 

 At any rate, here is no fit base for a great stratigraphical division. 

 In the Pennine Escarpment, however unphilosophical and unnatural 

 the line might be shown to be, it is a convenient one. Here in mid- 

 LNbrthurnberland it has not even that recommendation. No doubt a 

 nearly correct approximate horizon could easily be found marking 

 the level of the Tyne-bottom bed. But what would such an approxi- 

 mate line divide ? 



Above it, from 1000 to 2000 feet of grits, shales and limestones, 

 and thin coals ; below it, a much greater mass of grits, shales, 

 limestones, and thin coals, similar in every respect to the series above. 

 Similar lithologically, similar in the individual thicknesses of its 

 beds (for the thick limestones of the Scar Series have disappeared 

 by this time), and similar in fossils. Of what use can such a divi- 

 sion be ? 



Nearly all the organisms which it was supposed characterized the 

 Yoredales in Northumberland have now been found in the lower 



1 The Scar beds are not supposed to be worth much for lead-mining purposes, 

 and are therefore, -with a few striking exceptions, seldom worked into. 



2 On the "Little Limestone" and its Accompanying Coal in South Northumber- 

 land, by G. A. Lebour, Trans. N. Engl. Inst. Min. Eng. 1875, vol. xxiv.p. 1, etseq. 



