VEGETABLE SOIL DIAGRAM 19. 219 



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

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

 This lava forms 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, 



Extinct Volcanoes of Auvergne. 



but which were formerly active, and discharged lava-torrents 

 which can still be traced. 



Earthquakes 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. 



Atmospheric 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 11 and 12. 



ARABLE OR VEGETABLE SOIL. 



This is so called because it is the soil most favourable to the 

 growth of plants useful to man. It is formed naturally of 

 sand and clay, or the detritus of rocks mixed with an uncertain 

 proportion of organic matter yielded by all the animal and vege- 

 table substances which rot on its surface. The presence of these 



