578 



GEOGRAPHICAL BOUNDARIES 



[Ch. XXIII. 





fc> 



intermediate 



evidence that the subterranean fire is at work continuously, 



om 



gaseous vapours, especially, carbonic acid gas, are disengaged 



om 



impr 



mineral ma 



as 



are discharged by volcanos 



during eruptions. 



When 



communication 



fissure, whereas vents which are linearly arranged imply a 

 lono* line of dislocation in the earth's crust, analogous to 

 those great lines of rending, upheaval, or dislocation to which 



mountain 



We 



the side of a great Volcanic cone is rent, an open and straight 



sometimes formed many 



miles 



produced, at the bottom of which incandescent lava was 



seen. 



Here 



of eruption are thrown up in succession at points where the 

 caseous matter obtains the freest access to the surface, and 



& # - 



lias power to force up lava and scoriae. 



What 



small 



•m chains thousands or nines m leng 



The distances to which trap dikes or the lava which once 

 filled the lower parts of vertical rents can 



sometimes 



monument 



Whin Sill, for exam 



north of Yorkshire and part of Northumberland 



miles 



remote 



part of a deep fissure extending upwards to the surface of 

 the earth's crust, whether covered by the sea or atmosphere. 

 M. Alexis Perry, in his History of Earthquakes, has shown 

 that violent subterranean movements are most frequent along 

 the axes of mountain-chains. 



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