plateaux, are stupendous monuments of 

 the part which the propulsive force of 

 the vapours in the subterranean magma 

 has played in the past history of the 

 globe. The most noted of them, the great 

 Whin-Sill of the north of England, 

 averages from 80 to 100 feet in thickness 

 and has been injected, possibly at successive 

 intervals, between and across the Car- 

 boniferous strata, over an area of probably 

 not less than 1000 square miles. With 

 what type of earthquake the extravasation 

 of such great sheets of molten material 

 would be accompanied may be left to the 

 imagination. 



It is remarkable that John Michell, 

 who recognised the influence of elastic 

 compression in generating the vibratory 

 movement in an earthquake, should not 

 have advanced still farther, and have per- 

 ceived that the explosive shock to which 

 he attributed the earthquake must of 

 itself give rise to a wave of elastic com- 

 pression in the earth, which starting out 

 38 



