GEOLOGY 



out among the limestones above the chief sheet. Notwithstanding the 

 changes of horizon, the baking and consequent metamorphism of the 

 shales and limestones above as well as beneath the Great Whin Sill — 

 phenomena which render the contemporaneity of the sheet an impossi- 

 bility, it is strange that the lead miners as a rule still decline to regard it 

 as contemporaneous, and the bed of limestone which happens to be next 

 above it is always, by them, called the Tyne-bottom Limestone (see 

 p. 6), as has been mentioned before. Some very fine pectolitc has been 

 found in joint cracks in the Whin Sill near Middleton. 



The Cockfield or Bolam Dyke is, next to the Whin Sill, the most 

 remarkable mass of igneous rock in the county. It is a continuation of 

 the well known Cleveland Dyke, which to the south of the Tees is seen 

 cutting through the Jurassic rocks, and, though it does not every- 

 where come to the surface, it can be traced north-west beyond the 

 county boundaries as far as Armathwaite where it crosses the Eden 

 with a thickness of 54 feet. At Cockfield its thickness is very vari- 

 able, 15 to 66 feet. It is the longest known dyke in Britain, being 

 some 1 10 miles in length (and possibly nearly 200 miles). At Bolam 

 it spreads out laterally in the form of a sill baking coal seams and shales 

 above and below in the same manner as, elsewhere, it bakes and alters 

 them to right and left of its course. The stone of this dyke is often 

 known as 'Old Roger' on Tees-side. 



The Hett Dyke runs across the coalfield from Quarrington Hill 

 (on the Magnesian Limestone escarpment) to Tudhoe and Hett. It 

 resembles the Whin Sill in composition, and is quite unlike the Cleve- 

 land Dyke petrologically. At Brancepeth, about 300 yards from a 

 branch of this dyke, coked or ' cindered ' coal occurs over an area of 

 about 50 square yards. This is an unusual distance for contact meta- 

 morphism of this kind to be felt, but there is in north Durham a long 

 and broad zone running nearly across the coalfield several square miles 

 in area, where the coal generally has the appearance of having been 

 altered by ' whinstone,' although no dyke or sheet can be pointed to as 

 the cause of this — the coal is however rendered unsaleable by the change 

 it has undergone, whatever this may be due to. The Hett Dyke can 

 be seen near the confluence of the Bedburn Beck and the Wear, and 

 thence runs to Egglestone Moor. 



The Hebburn Dyke runs from near Cleadon to the Tyne, which it 

 crosses at Hebburn. It is known in Northumberland as the Walker 

 Dyke. It may possibly be represented by the amazing number of 

 basaltic blocks on the sea-beach at Whitburn, but it is not actually seen 

 anywhere piercing Permian rocks. 



There are a few other dykes in the county very similar in character 

 to the above. All these are probably of Tertiary age, though this 

 must always remain doubtful. All of them as well as the Whin Sill are 

 infinitely younger than the Minette dykes (mica-trap) which have 

 already been referred to (p. 3) as cutting through the older Palaeozoic 

 beds of Cronkley in Upper Teesdale. 



23 



