62 SECTIONAL ADDRESSES 
a coal seam which extended across the Midland Province. This is the 
famous Alton, Halifax Hard, Bullion, Upper Foot, or Crabtree Coal Marine 
Band. Near the top of the Productive Measures the Mansfield, Sharlstone, 
Dukinfield, or Speedwell Marine Band is similarly continuous in all Pennine 
coalfields, and the thickness of measures between the two affords a trust- 
worthy indication of the aggregate amplitude of negative movement in the 
various districts during the Coal Measure period. By measurement of 
this distance we recognise in South-east Lancashire, where the intervening 
thickness exceeds 4,000 ft., the regional centre of the Coal Measure collect- 
ing dish. About that centre in all directions the thickness of sedi- 
ment accommodated diminishes to 2,000 ft. in less than 50 miles. By 
plottings of isohypses of sediment between successive coal seams, we can, 
with labour and persistence, prove the local variations in the amplitude 
of depression to any degree of accuracy we choose. Each coal seam grew 
at water-level during a waiting period, but individual coals are not 
sufficiently persistent, and only exceptional groups of coals have a coalfield- 
wide distribution. For comparative studies of variations of the rate and 
amount of movement as between one coalfield and another, we therefore 
depend upon the modern method of identification and correlation of Coal 
Measure horizons by interstratified non-marine lamellibranch zones.” 
Consideration of the lowest group, the Halifax Coal Measures, shows 
them thickest in North or Central Lancashire, where also the Millstone 
Grits are thickest. There more than 1,000 ft. of Coal Measures underlie 
the Arley Mine. Equivalent measures § at outcrops in Yorkshire, Derby- 
shire and North Staffordshire are less than half that thickness. The 
Ovalis zone, 600 to 1,000 ft. thick in Yorkshire, is more than 1,500 ft. 
thick in Central Lancashire. It thins eastwards across Yorkshire, and 
to the south-east across Derbyshire to: Nottingham. ‘The Modiolaris 
zone, the main coal-bearing belt, maintains through Yorkshire and 
Derbyshire a wonderfully constant thickness, about 1,000 ft., along the 
strip of country where Park Gate and Barnsley coals are wrought. ‘This 
zone is fully 1,200 ft. thick about Oldham, but thins southwards through 
Cheshire into North Staffordshire, and more rapidly westwards across 
Lancashire. 
Variation of thickness in the Similis-Pulchra zone is much more rapid. 
This zone attains its maximum thickness in the Pennines south-east of 
Manchester. East of the Pennines a plotting of isohypses for the sedi- 
ments between the Barnsley Coal and the Mansfield Marine Band proves 
a thinning from over 1,000 ft. at outcrop to less than 500 ft. in the most 
easterly of working pits, which rate of thinning, if continued, would give 
the Barnsley Bed the Mansfield Marine Band for its roof within a very 
few miles east of the Trent. Beds between the Mansfield and the Shafton 
7 D. A. Wray and A. E. Trueman, ‘ The Non-marine Lamellibranchs of the 
Upper Carboniferous of Yorkshire and their zonal sequence,’ H.M.G.S. Summary 
of Progress for 1930, Pt. III, p. 70 (1931). 
7 A. E. Trueman, ‘A suggested correlation of the Coal Measures of England 
and Wales,’ Proc. South Wales Inst. Eng., vol. 49, p. 63 (1933). 
8 D. A. Wray, L. Slater and G. E. Eddy, ‘ The Correlation of the Arley Mine 
of Lancahsire with the Better Bed Coal of Yorkshire,’ H.M.G.S, Summary of 
Progress for 1930, Pt. II, p. 1 (1931). 
