218 J. P. Iddings — Origin of Quartz in Basalt. 



an increase of pressure against and from the retaining walls, 

 which pressure we have assumed tends to decrease the mobility 

 of the molecules. And of course the actual mobility or 

 liquidity of the magma will depend on the relation between 

 these two tendencies, which, if balanced, will result in the 

 storing up of potential mobility, which would show itself should 

 the pressure be relieved. 



Heated, hydrated magma underpressure • unstable rigidity. 

 — We may then imagine a heated, hydrated magma within the 

 earth under such a pressure that it has become solid, the 

 rigidity being an unstable one ; and we may further assume 

 that this solidification is either to an amorphous, glassy mass, 

 or to a more or less crystallized one, that is, to one which is 

 wholly crystallized to an aggregation of minerals, or to one 

 which is made up of crystallized minerals and rigid amorphous 

 material ; a glassy, porphyritic mass. Under these conditions 

 the solid magma will possess a potential mobility or liquidity, 

 which will exert itself to melt the solid mass, if the pressure is 

 relieved or lessened, as may happen when the fracturing of the 

 earth's crust ruptures the retaining walls and permits the escape 

 of this pent-up body, stored with gigantic expansive energy. 



Refusion. — The melting up of an unstable, solidified mass 

 by the heat inherent in it would undoubtedly proceed differ- 

 ently in the different minerals or the glass composing it, some 

 fusing more rapidly than others. The difference would be the 

 greatest between anhydrous refractory minerals, like quartz, 

 and hydrated glass, through which they might be scattered 

 porphyritically. If the refusion is complete, nothing will 

 remain to indicate a former state of solidification. If the 

 process of fusion is checked by the cooling of the magma in 

 consequence of its eruption through cooler rock masses, there 

 might remain in the cooling magma remnants of the minerals 

 previously existing in it. 



Final consolidation. — Moreover it is evident that the nature 

 of the minerals of final consolidation will be affected by the 

 nature and amount of the minerals formed at the time of 

 unstable consolidation which remain unmelted. If these are 

 highly acid the minerals of final consolidation will be jDropor- 

 tionately basic, and vice versa. 



Application to quartz-bearing basalt. — Applying the fore- 

 going general considerations to the occurrence of porphyritic 

 quartz grains in basalts, it seems reasonable to suggest that the 

 production of extremely acid and basic silicate minerals in 

 deep-seated magmas may have been brought about, like their 

 production in certain magmas after they have reached the sur- 

 face, by the influence of absorbed water acting imder favorable 

 conditions of pressure and teuvperature, which combined to 



