GEOLOGT OF THE BRISTOL COALFIELD. 121 



The climate of the district varies very considerably on account 

 of the great difference in boil, the variety of elevation, and the 

 shelter given from the winds passing through the vallies by the 

 hill sides. Difficulty of drainage is also a potent cause of 

 climatic difference, and has a most powerful influence on the 

 health of the inhabitants, much more so than people generally 

 think. 



The whole of the district drawn on the Map is Palaeozoic, 

 bearing patches of Secondary deposits on its surface. 'No stratifi- 

 cation has been largely disturbed by volcanic forces that have 

 been exerted at different periods. The last evidently produced 

 an anticlinal, running irregularly from H. E. to S. AY., and in its 

 course giving rise to faults at nearly right angles. Sometimes the 

 eruptive lava passed through and appeared on the surface of the 

 ground; and sometimes only rose far enough to cause an elevated 

 disturbance. Before commencing a description of the several 

 geological formations, we had better stay and consider these 

 disturbing influences by an examination of the igneous mass 

 itself. 



These eruptive rocks have had the general names of Trap and 

 Greenstone. We soon find from lithological scrutiny and 

 chemical analysis, that they are composed really of many distinct 

 mineral substances. We must always remember that these 

 igneous rocks were not ejected by an aerial volcano, as Etna or 

 Vesuvius, but were actually molten masses from the interior of 

 the earth, and protruded through the bed of the sea, and always 

 U7tder ivater. For this reason these igneous rocks have characters 

 that differ from the lavas of aerial craters on account of the great 

 pressure under which they were ejected. As the molton matter 

 burst through, the beds themselves were half melted and changed. 

 If through beds of sand, for instance, we often see a gradual 

 sequence from loose sand to a completely solid rock of quartz. 

 The best localities for observation are the Tortworth district, at 

 Woodford, or Damory Bridge, at Charfield, Avening, Broad-field 

 Down, Weston-super-Mare, and Uphill. The oldest of these rocks 



