1/4 CARBONIFEROUS. 



not necessarily the areas of deposition, but are caused by 

 disturbances and by denudation of the Coal-measures, which 

 in many tracts were no doubt formerly connected. It is 

 seldom, perhaps never, that we obtain the full thickness of 

 the Coal-measures, because great denudation has in nearly all 

 cases affected the strata : thus, although the Lancashire coal- 

 field is 6600 feet in thickness, the strata have never been seen 

 to graduate upwards into the New Red Sandstone, and, con- 

 sequently, their upper boundary is unascertained.^ In some 

 instances, where the New Red rocks have been supposed to 

 rest conformably upon the Coal-measures, the red rocks have 

 proved to be Coal-measure sandstones stained red by infil- 

 trations of iron-oxide. 



In South Wales the total thickness of the Coal-measures 

 has been reckoned at from 7000 to 8000 feet, or more. 

 Estimating the increase of sediment at two feet a century, 

 and admitting with Mr. C. Maclaren that it might take lOOO 

 years to form a bed of coal one yard in thickness. Prof Hull 

 has calculated that the deposits forming the South Wales 

 Coal-field (including the Millstone Grit) might have been 

 accumulated in 640,000 years.^ This estimate is, of course, 

 very vague. 



Coal. — This 'fossil fuel' is not, strictly speaking, a mineral, 

 being of organic origin ; but it is nevertheless frequently classed 

 as such amongst the Hydrocarbons. Chemically, in addition 

 to carbon (about 75 to 94 per cent.) and hydrogen, coal con- 

 tains nitrogen and oxygen in variable proportions. Sulphur 

 is almost always present in the form of iron-pyrites, etc., 

 and silicate of alumina occurs in small quantities, constituting 

 the ashes after coal is burnt. Coal is composed of vegetable 

 matter, which through chemical change and pressure, as well 

 as from original decomposition, has lost most of its structure. 

 Numerous spores and spore-cases of Lycopodiaceous plants, 

 however, and sometimes woody structure, may be here and 

 there detected. Amongst the conspicuous kinds of plants 

 which helped to form it are Ferns, Horse-tails {Eqinsctacecs), 

 Giant Club Mosses [Lycopodiacea;), and Conifers. 



Sir William Logan first pointed out in 1840, that every 

 coal-seam in the Great South Wales Coal-field rested upon 

 a bed of clay, called 'underclay,' which was penetrated by 

 the roots known as Stiguiaria, and evidently formed the old 

 terrestrial soil upon which the plants originally grew.^ This 



' E. W. Binney, Q. J. ii. 26. 



2 Coal-fields of Great Britain, edit. 4, p. 71. 



3 Proc. G. .S. iii. 275 ; T. G. S. (2}, vi. 491. 



