GEOLOGY 



later. Here it will suffice to say that A is a treeless moorland tract in 

 which bogs and crags abound, B an area of wooded and, here and there, 

 gorge-like valleys or ' denes ' with good open arable land between them, 

 C a broad zone of grass-covered billowy down-like ground bounded by 

 a marked rounded scarp on its western side and by bold sea-cliffs to the 

 east, and D a thick-soiled ruddy quarter devoid of striking features. 



It is needless to add that both A and B, and in a minor degree C 

 also, are now much disfigured by the mining operations which have been 

 for so long a time carried on within their limits. 



SILURIAN SYSTEM 



The most ancient deposits to be seen in the county probably, but 

 by no means certainly, belong to the Stockdale Shale group of the 

 Silurian System. Only the upturned edges of these beds are visible, and 

 that too only in a very small inlier laid bare by the erosive action of the 

 Upper Tees close to the fine basaltic crags of Cronkley Scar, above the 

 High Force, at the old Pencil Mill. Long ago the late Professor John 

 Phillips had noticed these rocks and had noted their resemblance to the 

 ' Grauwacke ' of the older Palaeozoic formations, but without assigning 

 any geological date to them.' It was not however till 1875 that the 

 exposure was carefully studied by Messrs. Gunn, Clough and Dakyns, and 

 the approximate age of the strata ascertained." The natives had for 

 centuries used the soft clay-slate of which the beds consist for slate- 

 pencils, and the name of the old mill standing by the river at the point 

 of their outcrop testifies to this. The uptilted position of the layers 

 and their denudation before the deposition of the lowest over-lying 

 Carboniferous material sufficiently prove the pre-Carboniferous age of 

 the pencil beds ; their lithological characters are those of the Stockdale 

 Shales as they occur in the Lake District. Some dykes of mica-trap 

 (minette) accompany them here as in their typical area of development, 

 and so far give confirmatory (though in the absence of fossils still incon- 

 clusive) evidence as to their age. 



CARBONIFEROUS SYSTEM 



There are no Old Red Sandstone or Devonian rocks cropping out 

 in the county. The feebly developed brecciated deposits which occur 

 at and towards the base of the Carboniferous Series in the Pencil Mill 

 inlier already mentioned do not even represent the true basement beds of 

 the system, since they are merely the fragmental shore accumulations of 

 a portion of the Lower Carboniferous considerably younger and higher 

 than the oldest and lowest horizon of that period. This is a point not 

 always clearly understood. There is a base to the Carboniferous in Dur- 

 ham but it is not the base of the system. Of anything corresponding 

 to and truly contemporaneous with the chocolate-coloured breccias 

 which occur in pockets on the face of the Pennine escarpment not 



' Illustrations of the Geology of Yorkshire, pt. 2, 1836, p. 78. 



' Stuart. Joum. Geol. Soc. xxxiv. and Geol. Mag. (December 1 1), iv. 58, 59, 139, 140. 



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