METAMORPHISM 291 



developed in shales and other sedimentary rooks out of materials 

 already present, or with such additions as ground-water may make. 

 Such changes take place even under ordinary conditions of heat and 

 pressure, through the help of ground-water. 



Change in composition. Besides simple deposition in pores and 

 cracks, mineral matter in solution may enter into combination with 

 other mineral matter, giving rise to new and in some cases to more 

 complex and more compact mineral substances. The changes 

 effected in this way go on slowly, but in the long course of time, they 

 may go so far that none of the original rock material remains in its 

 original condition all having entered into new combinations. 

 Soapstone or steatite, serpentine, Moritic and talcose rocks, all of which 

 occur in large bodies, were developed primarily through the chemi- 

 cal rearrangement of the mineral matter of some older rock, with the 

 addition of some mineral matter brought in by ground-water, and 

 with the subtraction of some soluble matter. Their metamorphism 

 is largely chemical. 



Other conditions favoring metamorphism. Besides water, heat 

 and pressure favor the metamorphism of rocks. Their action gives 

 rise to three general cases, but these three blend indefinitely: (i) 

 great heat without exceptional pressure, (2) exceptional pressure 

 without great heat, and (3) great heat and great pressure acting to- 

 gether. Exceptional heat arises especially from the intrusion of 

 lavas, and from pressure. Exceptional pressure arises chiefly from 

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

 age of the earth. Thrust generates heat as well as pressure. 

 The water in the rocks greatly facilitates the chemical and mineral- 

 ogical changes favored by heat and pressure. 



Metamorphism by heat. When lava is poured out on the sur- 

 face, it hakes the mantle-rock which it overflows. The extent of 

 the baking depends on the mass and temperature of the lava. The 

 nature of the effect is much the same as in the baking of brick. It 

 consists in the dehydration of the material, its induration by welding 

 due to partial fusion, and the development of new compounds. The 

 time involved is short, the pressure slight, and the water action 

 limited. If the heat were great enough, the loose material over 

 which lava flows would be fused; but complete fusion does not usu- 

 ally take place when lava spreads out on the surface. 



Intrusions of lava (p. 228) heat the surface above as well as that 

 below. The heat of the lava can escape only through the neigh- 



