MATERIALS OF THE EARTH 51 



Conditions favoring metamorphism. Besides water, two factors 

 favor the metamorphism of rocks, viz., heat and pressure. Their 

 action gives rise to three general cases, but these blend indefinitely: 

 (1) great heat without exceptional pressure, (2) exceptional pressure 

 without great heat, and (3) great heat and great pressure con- 

 joined. Exceptional heat arises especially from the intrusion of 

 hot lavas, and from pressure. Exceptional pressure arises chiefly 

 from the weight of overlying rocks, and from lateral thrust due to 

 the shrinkage of the globe. Thrust usually gives heat as well as 

 pressure. The water in the rocks not only works changes in them, 

 but greatly facilitates the chemical and mineralogical changes 

 favored by heat and pressure. 



Metamorphism by heat. When a mass of lava is poured out 

 upon the surface, it bakes the mantle-rock which it overflows. 

 The extent of the baking depends on the mass of lava and its tem- 

 perature. The nature of the effect is much the same as in the bak- 

 ing of brick. It consists of dehydration of the material, induration 

 of the loose matter by welding due to the partial fusion of the 

 particles, and the development of new compounds. The time in- 

 volved is short, the pressure trivial, and the water action limited. 

 If the heat were sufficiently intense, the loose material over which 

 lava flows would be fused; but complete fusion does not usually 

 take place when lava spreads out on the surface. 



Intrusions of lava (p. 15) heat the surface above themselves 

 as well as that below. In this case the heat of the lava can only 

 escape through the neighboring rock, and the temperature effects 

 for a given mass of lava are more considerable. Furthermore, the 

 time during which the adjacent rock is hot, and therefore the time 

 during which thermal waters are operative is usually longer than 

 in the case of extruded lavas, and the effects are chemical and 

 crystalline changes, rather than mere baking. The resulting 

 changes are greater the greater the mass of the lava and the higher 

 its temperature. When a vent or fissure is the passageway for 

 lavas that continue to come to the surface for a long time, as in the 

 case of long-lived volcanoes, the rocks which form the walls of the 

 vent are heated for long, and this gives rise to metamorphism 

 through heat, without very unusual pressure, though usually with 



