VOLCANOES 349 
The intrusion of these great masses of lava into the 
earth, adds heat to parts of the cool crust. As a result 
of this the rocks are often baked and metamorphosed 
by contact with the intrusion; and as steam reaches 
out into the surrounding strata, many interesting 
changes are caused. Many hot springs and mineral 
veins no doubt owe their origin to these supplies of 
intruded heat. A great mass of lava, blanketed by 
cold rocks, which are poor conductors of heat, will 
take many centu- 
ries, and probably 
many thousands of 
years to cool. Dur- 
ing all this time the 
neighboring strata 
are heated, and 
water with very high temperature is constantly passing 
through them, so that profound changes may result 
before the intruded rock cools. 
Finally, voleanic eruptions are frequently responsi- 
ble for earthquake shocks and earthquake water waves 
in the ocean. Even the ineffectual attempts of lava 
to reach the surface, when it forms dykes, etc., also 
produce earthquakes; and hence, in this indirect way, 
voleanoes do much destruction on the earth. <A vol- 
canic eruption is usually preceded and accompanied 
by such shocks. 
Fic. 213. 
Diagram to illustrate laccolite. 
