376 ELEMENTARY GEOLOGY 
morphism, is not difficult to find, for this substance is 
always present in the rocks. 
The heat that aids im the alteration may be of 
volcanic origin, either because of neighborhood to a 
voleanic vent, or else to some buried mass of intruded 
lava. Or to some degree it may be the heat originally 
in the earth. At present the burial of the rocks to 
depths of many thousands of feet (some have been 
buried beneath more than 40,000 feet of strata), must 
cause a great increase in heat.’ Perhaps in the earlier 
ages of the earth, burial less deep than this would 
produce higher temperatures, for then the earth was 
not so cold near the surface. 
The folding of mountains, with the consequent 
slipping of the rocks over one another, also causes 
heat, just as we may heat two stones by rubbing them 
together; such heat, thus generated by mountain 
growth, may aid or even cause metamorphism. Then, 
finally, heat may result from chemical change as 
water passes down into the earth, Just as we may heat 
a solution by adding some substance which induces a 
reaction. 
From most of these causes, pressure may also result 
(Figs. 225 and 228). The intrusion of rock masses 
exerts pressure upon the strata on all sides of the 
intruded material. Chemical change, resulting from 
1 For the temperature increases about 1° for every 50 to 60 feet of depth. 
