120 GEOLOGY OF THE BRISTOL COALFIELD. 



sea leyel, the Jflalness only being broken by isolated hills that 

 have withstood the action of the wayes which in former days 

 washed away and denuded the surface. Across the lower part of 

 the map is seen a ridge of hills very different from those already 

 mentioned, and much higher. As we shall notice further on, 

 these were raised by the violence of Tolcanic action from 

 underneath. 



The watershed of the country of the Bristol Coal-field is 

 drained by several rivers running westerly into the Bristol 

 Channel. The Avon receiving in its course the Frome and Trym, 

 divides Gloucestershire from Somersetshire. The most southerly 

 high lands are drained by the Axe, that rises near "Wells, and is 

 joined by the tributary that emerges in so singular a manner 

 from the Cheddar rocks, after passing under-ground for a 

 considerable distance. 



The western sea-bord is so low that in many places it is below 

 the spring-tides which would periodically flow over its surface if 

 the inhabitants did not keep in good repair strong sea banks 

 many miles in extent. Sometimes a stone wall ten feet high has 

 been required for the same purpose. "From these low levels the 

 hills rise abruptly till they reach a considerable elevation. 



The following is a list of the altitudes of a few : — 



Feet. Feet. 



