336 GEOLOGY OF THE BRISTOL COAL-FIELD. 



Another Coalfield is on the south side of Diindry, about six miles 

 in length and two in width, furnishirg material at Pensford, Glutton, 

 Paulton, Eadstock, Ac. A third division is that of Bedminster and 

 Ashton, all of it being covered by the Lias. 



A small Coal-basin is at Nailsea. It was formerly supposed to be 

 merely a continuation of the Bedminster beds, but an examination 

 of the surrounding Limestone is against this idea On every side 

 the Limestone dips towards the centre of the basin, except on the 

 west, where all the Limestone has been washed away by the waves 

 of the sea. The deposit is small — only about three miles by one 

 and a half. Here the Coal is worked both in the Pennant and 

 Lower measures. 



The borings and sinkings for the Severn Tuunel, prove that the 

 Coal beds and Pennant grits extend under the Severn into 

 Monmouthshire. Some thin seams of good Coal were extracted 

 when making the shaft at Portskewet, containing numerous 

 specimens of fossil ferns. 



The entire Coal series may be well divided into four distinct 

 parts, each lying within the other, and all in a basin- shaped cavity. 

 The first being deposited immediately on the upper shales of the 

 Carboniferous Limestone. 



1. Millstone Grit. ... 1000 feet in thickness. 



2. Lower Coal Measures 2000 „ 



3. Pennant Sandstone ... 1725 ,, 



4. Upper Coal Measures... 3000 ,, 



7725 



Millstone Geit. 

 The close of the Carboniferous period was marked by the surface 

 of the land gradually rising, and a sandy deposit, formed by the 

 disintegration of the Devonian rocks, so that the upper shales ai-e 

 easily distinguishable from the aluminous shales at the base of the 

 Carboniferous rocks. 



