MOUNTAIN FORMATION 



out a trench. As he digs along the trench, the earth 

 that he excavates he throws on to either side of the 

 trench, so that a ridge appeal's on each side of the 

 excavation. The result is the same in the case of 

 the continuous lava explosions in deep seas, especially 

 in those deep seas like the waters off the west coast 

 of South America, where a great range of mountains 

 runs parallel to an ocean that is of great depth only 

 a short distance from the land. A trough or trench 

 is cut downwards by successive explosions and expulsions 

 of lava. As the trough is arched downwards like a 

 broad letter U, the steam pressure from beneath cannot 

 easily force it upwards. What will therefore happen? 

 Imagine what would happen in a steam saucepan or 

 kettle if the vapour could not get out at the top or 

 lid. It would tend to blow out at the sides. Or if you 

 think of a slab of dough rising under the effect of yeast. 

 Suppose the baker presses a flat board on the top of 

 the rising dough, and presses down on it so that it 

 cannot force its way upwards. It will then naturally 

 spread out to the sides. Similarly the rising yeasty lava 

 under the curved ocean bed has to force its way side- 

 ways under the crust. It forces its way partly towards 

 the land where the mountains run along the coast as 

 in the case of the Andes or farther out underneath the 

 ocean. Generally the movement of the lava will be 

 towards the mountains till the trough gets broad and 

 deep and the mountains very far away, and so high 

 that their weight offers unexpectedly great resistance 

 to the underground stream of lava. Then the release 



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