270 



GEOLOGY OF THE BRISTOL COAL-FIELD. 



At Portishead the beds may be seen dipping S,E., and again 

 pointing to the centre of the Coal basin. It is, however, at the 

 south edge of the basin that we have the greatest local develope- 

 ment of the Devonian beds, which attain a great altitude, forming 

 the magnificent range of the Mendip Hills, the cause of this 

 being a powerful upheaval of volcanic basalt. 



Downhead 

 d Common, 



Pig. 1 1 . — Section near Downhead. 



Tothill Souse. 



Leigh'upon- 

 Mcndip. 



a Basalt. — h Devonian.— c Lower Hmestone shales.— <? Carboniferous 

 limestone. 



The beds of Eed Sandstone and Conglomerate form exceedingly- 

 good building material, which resist the weather without damage 

 for many years. The church towers in many villages and towns 

 in the county of Somerset are striking examples. From a small 

 quarry on the bank of the Avon was obtained the stone with 

 which the buttresses of the Suspension Bridge were built. 



At Portishead a very good section of the Conglomerate beds 

 may be studied on the shore near the Hotel, at Woodhill Bay. 

 It was here that Dr. Martyn found the scales of Holoptychius, 

 noticed for the first time in this locality. Since then remains of 

 Coccosteus, Fucoids, &c., have been collected. Dr. Martyn 

 gathered his specimens in a bed of Conglomerate that projects 

 out of the beach, but the same bed has been traced to its position 

 on the rocks, as shewn in the following list of the beds. 



