conceivably give rise to a minor earth- 

 quake. Michell therefore suggested what 

 may be a vera causa in volcanic areas. 



Although his conjecture that the travel- 

 ling of the earthquake waves along the 

 surface of the earth is due to the propul- 

 sion of concomitant waves of vapour 

 between the strata underneath, has never 

 been accepted, we should remember that 

 it included the first recognition or at least 

 premonitory suggestion of the potency of 

 highly heated aqueous vapour as a sub- 

 terranean dynamical agency in geological 

 operations. Whether, if suddenly gene- 

 rated at a local centre, in the manner he sup- 

 posed, superheated steam could force its 

 way between the strata for some distance, 

 as he imagined, may be doubted. But 

 since his time, and in a manner never 

 dreamt of by Michell, aqueous vapour has 

 been ascertained to play a stupendous part 

 in volcanic activity. We now know that 

 the internal magma which underlies vol- 

 canic regions contains a vast volume of 

 36 



