408 Vf. TOPLEY AND G. A. LEBOUR ON THE INTRUSIVE 



studied in Scotland or Northumberland, it is very doubtful whether 

 such divisions would ever have been made in those areas ; thick beds 

 of sandstone (often coarse and pebbly), and bands of shale, extend 

 right throngh the series. In the higher part (the Coal Measures) 

 there are good seams of coal ; in the lower part (the Limestone 

 Series) there are bands of limestone and seams of coal ; in the 

 middle part (the Millstone Grit) there is no limestone, and rarely any 

 coal. 



The term " Yoredale Rocks " was introduced by Professor Phillips 

 to designate a series of beds lying between the " Main " or " Great 

 Limestone," and the " Tyne-Bottom Limestone," succeeded below 

 by the " Scar-Limestone Series." The typical section is that in 

 Upper Wensleydale. The Millstone Grit of this author included 

 all beds between the Great Limestone and the Coal Measures, whether 

 they contained limestones with marine fossils or not. They do in- 

 clude such limestones even in North Yorkshire ; in Northumberland 

 there are from three to six well-marked limestones within this 

 distance. 



The Whin Sill of Teesdale, Alston, &c. lies just below the Tyne- 

 Bottom Limestone, or is always supposed to do so. If this band of 

 trap occupied a constant horizon, it might serve as a convenient 

 boundary, though it would not necessarily be a natural one. It has 

 generally been assumed that such is the case ; and the base of the 

 Yoredales has been drawn accordingly in geological maps. 



One important result of recognizing the intrusive character and 

 varying position of the Whin Sill is this, that it is no longer to be 

 relied upon as a boundary*. Not only does it shift about, in places 

 lower, more often higher than the supposed base-line, but it some- 

 times lies above the Great Limestone itself ; that is to say, the bed 

 of trap which is supposed to mark the base of the Yoredale Series 

 sometimes lies above the limestone which forms the top of that 

 series. It is very doubtful whether the so-called " Tyne-Bottom 

 Limestone " of the miners is always the same bed ; and certainly 

 this limestone cannot be traced northwards through Northumber- 

 land. As in this country there is no definite base for the Yoredales, 

 so there is no reason, supplied by the characters of the rocks, for 

 drawing a line here at all. Professor Phillips himself saw this diffi- 

 culty ; for he says : — "In this progressive change of character to the 

 northward, we lose by degrees the distinction of lower scar lime- 

 stone ; and it becomes not only difficult to draw the line for its 

 upper boundary, but doubtful whether it is proper to make such an 

 attempt. In the northern parts of Northumberland it appears 



* For a further discussion of this question see G. A. Lebour (< On the Limits 

 of the Yoredale Series in the North of England," Geol. Mag. Dec. 2, vol. ii. 

 p. 539, 1875. In this paper the term "Bernician Series " is proposed for the 

 beds lying between the Millstone Grit and the Tuedian Beds or Calciferons 

 Sandstones (the "Valentian Series" of Prof . A. Geikie, MS.). Subsequently to 

 the publication of this paper it was found that the term "Bernician " had been 

 proposed as a division of the Carboniferous Limestone by Dr. S. P. "Woodward 

 in 1856; it was adopted by Dr. Karl Mayer in 1874. (See G. A. Lebour, Geol. 

 Mag.. Dec. 2, vol. iii p. 19.) 



