416 W. TOPLEY AND G. A. LEBOUR ON THE INTRUSIVE 



Bavington, at West Hills, and elsewhere, there is every gradation 

 of size and form. It may be noted that although these " extra " 

 outbursts, so to speak, are found occasionally in connexion with the 

 main or western branch of the Whin, yet they are markedly more 

 frequent to the dip of the eastern branch. In other words, these 

 tokens of ill-restrained energy are commonest where the ejecting force 

 acted most vertically. And they are moreover more common in the 

 eastern than in the western portion of the Whin Sill's range, or 

 where there was least pressure of overlying rock. 



That notwithstanding its great thickness at its outcrop the Great 

 Whin Sill should not have reached the denudation-line along its 

 entire course, is perhaps somewhat surprising. But although in a 

 few doubtful cases, which may be the result of faulting, the clear 

 unbroken continuity of the beds above and below across the lines 

 of the Whinless gaps can leave no doubt in the minds of field- 

 observers that these breaks are distinct facts, that they are but 

 surface breaks no one, we suppose, will dispute. 



In its course through South-west and Mid Northumberland the 

 Whin Sill runs more or less in the same direction as the larger 

 faults. It is perhaps owing to this fact that the Whin is here 

 seldom seen to be faulted. At the Stone-Croft lead-mine, near 

 Hay don Bridge, the veins fault the Whin considerably. It is also 

 faulted at Elf Hills, on the north of Hartington, at Wards Hill, and 

 in many places to the north of Alnwick. We do not know of any 

 perfectly clear case, although we have met with some doubtful ones, 

 of the Whin being unaffected by a fault which throws the beds on 

 both sides of it. 



It may frequently be noticed that rocks adjacent to the Whin 

 Sill are altered by it unequally. Sandstone is but little altered, 

 shale very much so ; limestone is sometimes but little changed, 

 while at other times it is rendered crystalline. Recent experiments, 

 in which one of us has taken part, have proved that shale is a very 

 bad conductor of heat ; limestone and sandstone are far better con- 

 ductors*. We can therefore well understand how it is that shale 

 should plainly show the effects of the Whin, when limestone and 

 sandstone show but little alteration. It has been denied, by some 

 who disbelieve in the intrusion of the Whin Sill, that alteration of 

 the overlying beds has occurred. But of this there is abundant 

 proof in the district which we have described f. The complete list 

 of places where this may be observed is too long for insertion here ; 

 but we may mention the following as localities where the facts are 

 particularly well shown : — Milking Gap, Sewing Shields, and Tep- 

 permoor on the line of the Roman Wall ; West Hills and Three 



* Prof. A. S. Herschel and G. A. Lebour, " On the Thermal Conductivities 

 of certain Eocks," Eep. Brit. Assoc, for 1873 (Appendix), p. 223; for 1874, 

 p. 128 ; for 1875, p. 54. 



t. Prof. Sedgwick, fifty years back, gave most conclusive evidence of this in 

 Teesdale ; and the sections described by him may still be examined ; but sub- 

 sequent writers have mostly ignored, or endeavoured to explain away, his ob- 

 servations. (See especially pp. 162, 177, 178, and 182 of his second paper 

 already referred to.) 



