GEOLOGY OF PART OF SOUTH DURHAM. 193 



originally one great area of Coal Measures, which, by various sub- 

 sequent disturbances and denudations, has been reduced to the 

 existing series of isolated patches called coal-fields. In the case 

 of the two former coal-fields, we consider their isolation has taken 

 place by the elevation of the Coal Measures and other Carbo- 

 niferous strata, formerly and still partially intervening, into a 

 great anticline, with an axis east and west, which, by the wast- 

 ing action of denudative forces, has been in a great measure ob- 

 literated. The immense amount of strata thus removed in this 

 intervening region will be better realised by reference to fig. 18, 

 where the dotted line a a indicates the position of the basal beds 

 of the Coal Measures in the anticline. By estimating the Coal 

 Measures at their maximum thickness in Durham, the thickness 

 of the Carboniferous rocks denuded at Middleton Tyas, Moulton, 

 and Barton would be between three thousand and four thousand 

 feet. That this elevation and succeeding wearing down of North 

 Yorkshire strata was not confined to the west of the district 

 seems more than probable ; only to the east the denuded forma- 

 tions have been in part replaced by thick deposits of Trias and 

 other rocks. 



From these remarks it will be seen that it is our opinion that 

 the limits of the Durham coal-field have almost been reached, — 

 that there are no good grounds for anticipating its further deve- 

 lopment to the south. It may be that in the south-east some 

 faulted fragments of coal strata, like the piece brought in by the 

 Wiggles worth fault, at Cockfield, will be found to extend the 

 eoal-field beneath the Magnesian Limestone and Trias a little 

 further towards the Tees ; but that any workable coal should be 

 found in the grit country south of the high land about Cock- 

 field, Bolam, and Houghton-le-Side, we certainly see no reasons 

 for supposing. 



COCKFIELD DYKE. 



The most westerly point where we have seen this dyke is 

 about half a mile west of Wooley Hill, where it is quarried for 

 road-metal. It is about twelve yards wide, and worked thirty 

 feet deep. The sides of the excavation are formed of sandstone 



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