THE GENESIS OF THE HIGH RANK COALS

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Abstract
Summary: Confirmation of previous work is afforded by two bores at Prestwich which prove 4,000 ft. of coal-bearing strata in which, as indicated by 220 coal-seam analyses, coal seams display marked decrease in water content but no apparent decrease in volatiles with depth. By comparing Jurassic coals and Coal Measures coals of the same rank which lie in adjoining fields in north-east England it is shown that there is no necessary relationship between the age and the rank of coal seams. In the High Rank Series (coals with volatiles of less than 31% d.a.f.) rank is determined by and increases with the intensity of the orogenic forces and natural heat treatment to which the coal has been subjected. This, stated as the Roberts-White Theory, is illustrated by examples: (a) from Lancashire and (b) from Staffordshire where metallurgical coking coals of less than 30% volatiles occur, but only in the downfolded synclines; (c) from the Alston Block where a devolatilisation from high volatile bituminous coals to carbonaceous coals is related to large scale thrusting and to the magmatic mineralisation; (d) from the main South Wales coalfield where progressive devolatilisation from bituminous coals to anthracite is towards an inclined plane (probably a thrust) that lies beneath the coalfield; (e) from highly compressed strata in Pembrokeshire and (f) from similar strata near Bideford, Devon, where coal seams are wholly anthracitic and not infrequently reduced to culm. It is shown that comparison between coal seams in the High Rank Series is most accurate when the Fuel Ratio is used. Also in this Series accurate direct mathematical comparison of coal seam volatiles is possible only on the basis of a standard amount of Fixed Carbon. A 50% Fixed Carbon basis is chosen to fulfil these requirements. In the case of the main South Wales coalfield loss of coal seam volatiles is related to the distance of the seam from an inclined plane beneath the coalfield, and on the 50% Fixed Carbon basis the relationship can be expressed by the following exponential equation: v 2 = v 1 exp ( - 2.8 × 10 - 4 y ) in which v 2 and v 1 are the volatiles of a lower and an upper seam respectively and y is the vertical distance between the seams.

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