GEOLOGY DIAGRAM 20 219 



Sometimes the sido of the volcano opens, and discharges incan- 

 descent lava, like the molten metal that "flows from a furnace. 

 This lava forma a true river of fire, which flows upon the surface 

 of the ground, and consumes everything in its track, but it 

 generally advances very slowly. There are no longer any vol- 

 canoes in England or France, but there were formerly many. 

 The best known of these extinct volcanoes are those of Auvergne, 

 in France, which no longer discharge smoke, or flames or lava, 



f' 



Extinct volcanoes of Auvergne. 



but which were formerly active, and discharged lava- torrents 

 which can still be traced. 



J&arthqualcea are tolerably frequent in the neighbourhood of 

 volcanoes, though they may also occur in countries which are 

 not volcanic. These are shakings of the ground which, when 

 sufficiently violent, cause great catastrophes by overturning 

 houses. But earthquakes are fortunately very uncommon, and 

 by no means severe in England, for the very reason that there 

 are no volcanoes nearer than Iceland and Italy. 



AmospTieric air. The air which surrounds us, .and which we 

 breathe, is also a mineral substance in a gaseous state. We have 

 mentioned its composition and properties at pages 1 1 and 12. 



INDUSTRIAL MINERALS, DIAGRAM 19. 



After having briefly described Ihe most remarkable pheno- 

 mena of the earth, we will notice the principal useful substances 

 which man has found in it. Those which he obtains from the 



