30 W. GTJNN AND C. T. CLOTJGH ON THE 



nexion with the Burtrecford Dyke are totally different in character 

 from those of the Pencil-mill. We shall, moreover, subsequently 

 adduce reasons to show that the Pencil-mill dykes have effected 

 little or no change in the surrounding beds. 



Thus it has been necessary to reject the above two theories. We 

 are, in fact, obliged to conclude that the Pencil-mill bed is not 

 Carboniferous at all. Below we give the main considerations which 

 have led us to this conclusion : — ■ 



(a) Character of the Bed. — A comparison of this pencil-bed with 

 various other pencil-beds which are known to be merely Carbonife- 

 rous shale altered by proximity to whin shows a great difference 

 between them. Even in hand specimens the difference is usually 

 well marked ; we do not know of any approach in altered Carboni- 

 ferous shales to the extremely tine and homogeneous texture of this 

 bed, nor to the greenish grey or purple-red tint which sometimes 

 prevails in it. Probably on this evidence alone any one who had 

 been well acquainted both with altered Carboniferous shale and with 

 the various Silurian shales would have been led to suspect that the 

 bed was not Carboniferous. More than a year ago a specimen of it 

 was sent to the Rev. T. G. Bonney ; and he remarked that it had a 

 wonderful resemblance to some of the Silurian shales. On the other 

 hand, there can be no doubt about its great resemblance to the Pale 

 Slates or Stockdale Shales that are seen to overlie the Coniston 

 Limestone in the neighbourhood of Dufton. Mr. W. T. Aveline, 

 who came to examine this bed, pronounced at once in favour of its 

 being of Silurian age, and thought it most resembled the Pale 

 Slates. 



(b) The Character of the Dykes. — ~No mica-trap dykes are known 

 in the Carboniferous beds of this district, or of any part of the north 

 of England, while such dykes are common in the Silurian districts 

 of the Lake-country (vide Geol. Survey Memoir on Quarter Sheet 

 98 N.E., pp. 16, 17). 



(c) Beds not altered by the DyJces. — All the evidence attainable 

 tends to show that it is not the dykes which traverse the bed that 

 have altered it. The bed can be traced 230 yards from the nearest 

 visible dyke ; and probably the nearest point that a dyke passes 

 through cannot be less than 130 yards from this place ; and yet here 

 the bed has just the same character that it has close to the dykes. 

 Surely, however great the altering power of these dykes might be, 

 we should expect at this distance to find some decrease in this 

 alteration. Moreover the dykes of this kind which occur in the 

 Lake-country do not usually effect much alteration in the sur- 

 rounding beds. (Geol. Survey Memoir on Quarter Sheet 98 S.E. 



(d) Character of the Veins. — In this district, veins when passing 

 through Carboniferous shales are generally mainly filled with what 

 the miners call " douk," soft dark clay evidently formed from the 

 shale on either side of the vein. Now, as we have said, there are 

 very many veins cutting through the pencil-bed, and the vein-stuff 



