A HISTORY OF DURHAM 



higher and better known bed is usually also known as the Little Lime- 

 stone. This one is about 1 8 feet thick. 



Another group of shale and sandstone, then 



No. 200. The Ninth or Jew Limestone. Amongst the lead-miners 

 an idea (without foundation in many cases) has long prevailed that 

 profitable mining could not be carried on beneath this bed. Several of 

 the most paying lead deposits have been worked to the west in lower 

 strata. The Jew Limestone is about 24 feet thick. 



Some eight or nine alternations of shale and sandstone occur 

 beneath 



No. 190. The Tyne-bottom Limestone. This is one of the best 

 known named limestones in the series, but the name has often been 

 misapplied. This is due to the fact that over a considerable tract of 

 country the bed properly so called lies next above the great sheet of 

 basalt known as the Great Whin Sill in the north of England and to 

 the consequent inference quite a mistaken one that the first limestone 

 above this intrusive and horizon-shifting mass of igneous rock must 

 everywhere be the same. Many miners still refuse to regard the Whin 

 Sill as intrusive because of the supposed constant position (as they think) 

 of the Tyne-bottom Limestone above it, arguing in a vicious circle 

 thus : The Tyne-bottom Limestone is next above the Whin Sill at A, 

 the limestone lying upon the Whin Sill at B or C must therefore be the 

 Tyne-bottom Limestone also, and the Whin Sill has therefore not 

 changed its horizon and is not intrusive. An instructive example of 

 bad logic and worse geology. That the lower courses of the lime- 

 stone are commonly baked, and the shales which often lie between it 

 and the basalt indurated into porcellanite or ' whetstone ' by the heat 

 of the once molten sheet, is evidence of intrusion which they do not 

 take into consideration. For some four miles the river South Tyne 

 runs upon this limestone, hence its name. In Durham it is one of the 

 lowest of the Bernician limestones to crop out at the surface ' to the 

 day,' as it is termed locally. It is usually about 24 feet thick. Shales 

 and sandstones follow as usual, then comes 



No. 1 86. The Eighth or Single Post Limestone. This is a thin but 

 very constant bed, about 6 feet in thickness only. Single Post means 

 single course, i.e. the bed consists of one layer or course of limestone, 

 most of the thicker limestones comprising several posts individually 

 seldom so thick as this. The word * post,' as met with in records of 

 mining sections, more often means ' sandstone,' the latter word being 

 in practice very commonly omitted from the full description which 

 should be Sandstone Post or Freestone Post = Sandstone Bed or Course. 



Next come shale and sandstone, then 



No. 181. The Cockle-shell Limestone. A still thinner but well- 

 known bed, seldom exceeding 3 feet in thickness. It is usually full 

 of Productus giganteus, the ' cockle-shell ' of the miners, but though it 

 takes its name from this circumstance it must not be supposed that 

 this fossil is in any degree specially characteristic of this horizon. It is 



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