The Interior of the Earth 



only 52 per cent., and for those at sea level the per- 

 centage drops to 47. The content of iron varies 

 in inverse ratio. In fact the lavas issuing from 

 the crater of a volcano must not be considered 

 to have arrived there unaltered from the centre 

 of the earth. They have necessarily melted 

 and carried off part of the lining of the passages 

 through which they have been flowing for many 

 miles. Moreover, it would be childish to imagine 

 the internal fluid mass rounded into a perfect 

 sphere; the corrugations of the crust must have 

 made themselves felt inside, more perhaps than 

 outside, where various causes tend to plane and 

 level the surface, so that some volcanoes may 

 be fed from relatively superficial regions, while 

 others have their source at greater depths, and 

 draw their lavas from remoter and consequently 

 differently-constituted strata. 



It seems, therefore, impossible that each 

 volcano should be provided with a corresponding 

 independent liquid cavity. It becomes almost 

 necessary to admit the existence of vast liquid 

 lakes beneath each of the chief volcanic regions. 

 Moreover, 320 volcanoes are known to have been 

 active during the last three centuries, and are 



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