368 GEOLOGY 



from (3) the chemical changes stimulated by the heat, water, and 

 gases issuing from the lava, and by pressure in the presence of 

 ground-water. 



The total amount of lava which has risen toward but not to the 

 surface far exceeds all that has flowed out at the surface. Intru- 



Fig. 289. A dike two feet wide, cutting through sandstone. The exposed 



Eart of the dike is on land scarcely above the water. Arran, coast of 

 cotland. (H. M. Geol. Surv.) 



sions are usually seen only after erosion has removed the rocks 

 which overlay them. 



There appear to be certain cases where the intrusion comes so 

 near the surface as to develop explosive phenomena without the 

 extrusion of lava. From the nature of the case this is an inference 

 rather than a demonstration. It is certain, however, that occasional 

 violent explosions take place where no lava comes to the surface. 

 The explosion may be due to the intrusion of lava, or it may be 



