DISCOVERY OE SILUBIA.N BEDS IN TEESDALE. 29 



most prominent set, striking nearly east and west, looks like clea- 

 vage. Much of the mass is greatly cut up by quartz veins, and has 

 a very crushed and disturbed appearance, giving one the idea that 

 the dip is far from constant in direction or amount — that the beds 

 are, in fact, much contorted. In places the laminae of soft shale are 

 seen to be crossed in various directions by dark grey or blackish 

 bands, which, if they at all indicate the bedding, show that the above 

 idea is correct. Even if we suppose that the beds are horizontal, 

 there must be at least a thickness of 65 or 70 feet. Considerable 

 time has been spent in searching for organic remains, but hitherto 

 without result. 



Several igneous dykes, four at least, pass through this bed. The 

 most westerly of these is seen very clearly crossing the river in an 

 E.N.E direction, with a width of about ten yards and a southerly 

 hade of 50° from the horizontal. Two others have a similar direc- 

 tion and hade, but are somewhat narrower. The direction of the 

 most easterly dyke is rather uncertain, but appears to be about 

 S.E. by E. 



Our colleague Mr. F. Rutley has examined for us several speci- 

 mens of these dykes (which do not vary much in character) ; and he 

 gives the following description of one of them : — 



"Ina microscopic section of this rock mica crystals (magnesium 

 mica ?), in part decomposed into a yellowish-white substance, are 

 seen in considerable quantity. Crystals of apatite, decomposed gar- 

 nets, patches of pyrites, and some small rhombohedral crystals 

 which are probably siderite, are also present. The last-named 

 crystals, although sometimes irregularly distributed, are sometimes 

 aggregated in strings, or along minute fissures. In this section 

 some of the clearly discernible crystals of felspar exhibit triclinic 

 banding. The rock seems to be intermediate between minette and 

 kersanton." 



It has long been evident to the dalesmen that this Pencil-mill bed 

 is not simple unaltered Carboniferous shale ; and two general theories 

 have prevailed as to what it is. Both of these assume it to be 

 Carboniferous shale, but altered : the one supposes the alteration to 

 have been effected by the Whin Sill ; the other that the alteration 

 was effected by the dykes there seen, and that these dykes have 

 some connexion with whin which is now and then seen occurring 

 roughly along the line of the Burtreeford Dyke. 



The first of these theories is at once seen to be untenable, owing 

 to the great vertical distance between the base of the whin on 

 Cronkley Scar and the top of the Pencil-bed, which is about 165 

 feet ; and it is in this district beyond parallel for the metamor- 

 phism effected by the Whin Sill to extend to any thing like so great 

 a distance. At the High Force the limestone at the bottom of the 

 section is only 20 feet below the whin-base, and yet it shows no 

 certain sign of any alteration. 



Neither is the second theory satisfactory. In the first place, the 

 general line of the Burtreeford Dyke probably passes several hundred 

 yards to the east, and all the igneous rocks which are seen in con- 



