296 



STRATIGKAPHICAL GEOLOGY 



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In North Staffordshire these mea- 

 sures are 2400 feet thick if the top 

 of them is taken at the marine band 

 above the Mansfield coal, but Dr. 

 Kidston is inclined to place it at 

 the Ash coal, still higher up. They 

 are probably quite as thick in Lanca- 

 shire if the same horizon is taken 

 as the line of division. In Flint 

 they are considered to be about 1000 

 feet thick and 1800 feet at Wigan, 

 in Lancashire, where they are well 

 exposed in the cuttings of the Wigan 

 and Liverpool railway. Oarbonicola 

 acuta, G. robusta, and C. turgida are 

 common in these measures. 



On the eastern side of the area 

 there is a similar development of 

 Lower Coal-measures, but the thick- 

 nesses assignable to them depend 

 upon the horizon which is taken to 

 be their upper limit. In Nottingham 

 and Leicester they are said to be about 

 1000 feet thick, but when the series 

 recurs in Warwickshire they are 

 found to be absent, and must there- 

 fore have thinned out against the 

 ridge of ancient rocks which underlie 

 the Charnwood and Nuneaton dis- 

 trict. Neither are they found in 

 the South Staffordshire coalfield. 



Middle Coal - measures. 

 This group has a wider extension 

 than those below. It is also a more 

 varied series, consisting of grey shales 

 and grey or white sandstones with 

 many seams of coal and of ironstone 

 (carbonate of iron), this association 

 of coals and ironstones in the same 

 series of beds making it commercially 

 the most important group in the 

 whole series. 



It is believed to attain its greatest 

 thickness in Lancashire, where if 



