THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 521 



magma, that is, the excess of temperature above that at which 

 crystallization begins. Somewhere within the earth there is material 

 (whether a rigid liquid or a crystalline solid we need not here con- 

 sider) which is capable of becoming fluent liquid upon release of 

 pressure. At great depths there may be excessively hot material 

 capable of giving excessively hot liquid, but this is hardly concerned 

 with matter that ever comes to the light of day as an igneous rock. 

 There must be a transition zone representing a passage from stable 

 crustal material incapable of becoming magma, through a zone 

 capable of giving magma with some suspended crystalline matter, 

 to a zone capable of giving a completely molten magma. It is from 

 the zone giving rise to magma with some suspended crystals, rather 

 than from the zone giving a simple liquid, that we must expect the 

 great upwellings of magma to come, because of the advanced posi- 

 tion of the partly crystalline material with respect to an action pro- 

 ducing such upwelling and because it may be readily mobile when 

 the amount of suspended crystals is small. As the magma rises in 

 the crust several factors may combine to reduce the amount of 

 crystals. Among these factors is the increased solubility of the 

 crystals resulting from the lowered pressure, for the solution of sili- 

 cates usually takes place with increase of volume. Besides this 

 there may be an actual addition of heat as a result of the Joule- 

 Thomson effect^ and of possible exothermic reactions ensuant upon 

 reduced pressure. It is probable that, in the usual case, the magma 

 still retains some crystals even when it rises to shallow depths within 

 the crust for, though the intensity of the above actions is thereby 

 increased, it is also passing into colder and colder surroundings and 

 must lose a corresponding amount of heat. It is possible that in 

 some cases a magma may have lost all its crystals by the time it has 

 risen to shallow depths and perhaps may have a temperature above 

 the saturation temperature under the conditions there prevailing. 

 However, even if we make the liberal assumption that it is super- 

 heated 100° (say its temperature is 1100° and the saturation tem-' 

 perature 1000°) and that this condition is acquired when it has 

 reached a level 5 km. below the surface, it should be noted that 

 the mere immersion of blocks of country rock amounting to about 



^L. H. Adams, Bull. Geol. Soc. Amer., Vol. XXXIII (1922), p. i44- 



