aqueous and other vapours which are held 

 absorbed or dissolved in its molten mass 

 under the immense pressure that pre- 

 vails deep below the surface. When this 

 pressure is lessened as the lava mounts 

 in the throat of a vent, the imprisoned 

 vapours escape with explosive violence, 

 blowing the liquid material into the 

 finest dust, which may fill the air and 

 darken the sky over a wide stretch of 

 country, while at the same time the still 

 steaming lava may issue incopiousstreams 

 from the crater. Moreover, geologists 

 have long been familiar with the fact that 

 besides being erupted at the surface by 

 the enormous expansive force of these 

 absorbed vapours, the magma has in 

 many places forced its way horizontally 

 through the terrestrial crust, often be- 

 tween the planes of stratification, over 

 wide areas and for long distances. The 

 Intrusive Sheets or Sills, so abundant in 

 the British Isles, dating from Archaean 

 time up to that of the Tertiary basalt- 



37 



