Coal Basins. 599 



vividly before the mind than figures, or words, viz. one 

 hundred and thirty-four millions, one hundred and 

 twenty-five thousand, one hundred and sixty-six tons. 



Besides coal and iron, the Coal-measures yield quan- 

 tities of clays, which are of considerable value. The 

 chief of these is fire-clay, which is used so largely 

 in the manufacture of crucibles and fire bricks, and in 

 furnaces. 



If we look at the geological map of England, we 

 see that large patches are coloured black. These are 

 the Coal-measure districts of Great Britain. Some 

 of these coalfields, as for instance, the coalfields 

 of South Wales and the Forest of Dean, lie obviously 

 in basin-shaped forms, and the coal-beds and other 

 strata crop to the surface all round the basin. But in 

 other parts of England, the coal-formation does not 

 occur in obvious basins, but seems merely to form a 

 portion of the ordinary surface of the country. 

 Nevertheless, the basin-shaped form of the Coal- 

 measures is often continued under the overlying 

 Permian and New Red formations, one half or more of 

 these basins being hidden from view, and buried 

 under hundreds of feet of more recent strata that lie 

 uncomformably upon them. The reason of this is that 

 the Carboniferous strata were disturbed and thrown 

 into anticlinal and synclinal folds before the beginning 

 of Permian and New Red Sandstone times, as shown in 

 fig. 115, p. 601. 



The coalfields marked No. 1 now show at the surface. 

 Strata marked 2 separate them. These consist of Car- 

 boniferous Limestone lying in an anticlinal curve, as 

 in Derbyshire, and part of the original coalfield shown 

 by the dotted lines 3, in old times covered 2. The 

 remaining parts of this original coalfield on the east 



