192 CARBONIFEROUS, 



the Mendips beneath the flats of Sedgemoor there is very little 

 doubt that another Coal-tract exists. (See Fig. 24, p. 160.) The 

 eastern limits of the Coal-basin to the north of the Mendips may- 

 be pretty well defined, but those of the probable basin to the 

 south are not known. 



The productive Coal-measures are divided into an upper and lower 

 series, separated by a middle unproductive series, which includes 

 the Pennant Grit. The middle series is largely composed of grit- 

 beds, which are much used for building-purposes. Grits are 

 exposed at Yate Common, Iron Acton, Mangotsfield, Hanham, 

 Brislington, Nailsea, Temple Cloud, etc. The Pennant Grit and 

 the Francomb Stone belong to this division, which is sometimes 

 termed the Pennant Grit Series. In the Bristol district the 

 Holmes Rock, formerly classed as Millstone Grit, and the Doxall 

 Stone, are included in the lower series of Coal-measures. 



Although the Radstock coal-district is much concealed by 

 Secondary strata, yet owing to their attenuation pits are sunk 

 through the Lias, and at Clan Down, even through Inferior Oolite. 

 The heaps of refuse, or "tips," thrown out from the mines form 

 very marked features in the country near High Littleton, Paulton, 

 and Radstock. The depths of the Coal-pits vary from 500 to 2000 

 feet. Few Coal-pits are so free from fire-damp. At Radstock the 

 colliers work with naked candles, but in some parts, as in the 

 Nettlebridge Valley, there is a liability to fire-damp. The seams of 

 coal worked are often very thin, being near Radstock from one to 

 three feet in thickness. 



The Radstock district is noted for the variety and preservation 

 of the fossil plants.^ 



Among the remarkable features of this Coal-field are the masses 

 of Carboniferous Limestone at Luckington and Vobster (see p. 161); 

 the Radstock slide fault, in which the downthrow is as much as 

 180 feet in places, and the 'overthrust' 1050 feet; while in the 

 Nettlebridge Valley the beds are greatly disturbed, in some places 

 the coal seams are vertical, and in others so twisted and faulted 

 that the same seam of coal has been penetrated three times in one 

 shaft. These disturbances are somewhat similar to those met with 

 in the Belarian Coal-fields. 



13. NORTH WALES COAL-FIELDS. 



The Coal-measures of North Wales are separated by an extensive 

 fault into two tracts, respectively called the Flintshire and Denbigh- 

 shire Coal-fields. Probably they are connected underground with 

 the North Staffordshire Coal-field. (See Fig. 3, p. 25.) 



In Flintshire the beds extend along the borders of the Dee, by 

 Mostyn and Flint, and further south from Mold to Hawarden. 

 In Denbighshire the beds extend from Minera, west of Wrexham, 

 by Ruabon, and Chirk to Oswestry in Shropshire. 



1 See also E. Wethered, Proc. Cotteswold Club, vii. 73. 



