ON TITI? ORIGIN OF MOUNTAIN EzVNGES. 153 



the solid crust in. tlio disturbed regions," and thus tipper 

 ends of the injected fissures would be recognised by geolo- 

 gists as dykes of igneous rock. 



iinb tlio formation of a mountain I'idge by such compres- 

 sion in a solid crust I'cstiiig on a liquid inter-stratum would, 

 by the laws of hydrostatic equilibrium, involve certain con- 

 sequences, i'irst, the thickening of the crust would not 

 only take place upward, so as to give rise to the visible 

 mountain mass, but also downwards, so as to give rise to 

 solid roots of the mountains projecting into the liquid inter- 

 stratum. Mr. Fisher calculates that two-fifths of the 

 thickening would appear as a mountain chain, while three- 

 fifths would go to form invisible mountain roots. "The 

 existence of these roots of tlio mountains is not a mere 

 matter of speculation. They have been felt by the plumb- 

 line in the following raanner. The great mass of the Hima- 

 laya mountains was, dui'ing the Indian Trigonometrical 

 Survey, found to attract tlu) plumb-line. But upon its 

 being calculated how much attraction ought to bo attributed 

 to this mountainous mass, it was found that, though they 

 attracted the plumb-line, yet they ought to have attractcnl 

 it more than they did. Sir G. B. Airy explained this 

 anomaly by the existence of downward protuber-ances of 

 a lighter crust into a lieavier substratum ; this is exactly 

 the same supposition to which our reasoning has led ns." 

 " Again, the existence of roots of the mountains ought to be 

 revealed by phenomena of underground temperature ; and 

 wo find such to be the case." A thickened crust in mountain 

 regions ought to be accompanied by a diminished increment 

 of temperature for a given increment of depth. Dui'ing the 

 construction of the St. Gotha,rd and Mont Cenis tunnels, the 

 increment was only about half the average ; namely, 1° i\ih. 

 for 100 feet, instead of 1° Fah. for from 60 to GO feet. 



