228 



VULCANISM 



I. INTRUSIONS 



Fluid rock forced into fissures and solidified there forms dikes 

 (Fig. 221); forced into chimney-like passages it forms pipes or 

 plugs; insinuated between beds of other sorts of rock, it forms 

 sills; and accumulated in considerable bodies which arch the strata 

 up over them, it forms laccoliths (Fig. 222). If it breaks and lifts 

 its cover, instead of arching it up, it is a bysmalith. Some laccoliths 

 and bysmaliths are large enough to make good-sized mountains, 

 of mound-like form. The Henry Mountains of Utah are laccoliths. 

 Still more massive intrusions of igneous rock are sometimes called 

 batholiths. The very great bodies of granite in Canada and along 

 the axes of some of our western mountains are examples. The 

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

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

 trusions are usually seen only after erosion has removed the rocks 

 which overlay them. 



There appear to be cases where intrusions come so near the 



surface as to develop explosive phe- 

 nomena at the surface. At any 

 rate, it is certain that occasional 

 violent explosions take place where 

 no lava comes to the surface. The 

 explosion may be due to an intrusion 

 of lava, or it may be due to the pene- 

 tration of surface-waters to hot rocks 

 that have remained uncooled from 

 previous volcanic action. A case 



Fig. 222. Ideal cross-section of 

 a laccolith with accompanying 

 sheet and dikes. (Gilbert, U. S. 

 Geol. Surv.) 



of this kind occurred in Japan in 1888, where there was a sudden 

 and violent explosion which blew away a considerable part of the 

 side of a volcanic mountain which had not been in eruption for at 

 least a thousand years. The explosion filled the air with ashes and 

 debris like a violent volcanic eruption. There was but one eruption, 

 and within a few hours the cloud of dust had disappeared and the 

 phenomenon was ended. No lava was extruded. 



Intruded igneous rock changes the rock into which it is forced. 

 Thin dikes and sills produce little effect, but greater masses alter 

 the adjacent rock notably. The metamorphism is effected by (i) 

 the heat, (2) the pressure incident to the intrusion, and (3) the 

 chemical changes stimulated by the heat, water, and gases issuing 

 from the lava, and by pressure in the presence of ground-water. 



