THE BEHAVIOR OF INCLUSIONS IN IGNEOUS MAGMAS 525 



(approx.). We have now 66 g. An and 17 g. Ab in the crystalline 

 state. We had formerly only 45 g. anorthite and 5 g. ablite. Equi- 

 librium will therefore be established with evolution of heat to the. 

 amount of 21 X 104.2 + 12 X48.5 cal. In order to keep the tempera- 

 ture constant, then, we should have to abstract 2770 cal. If, on the 

 other hand, no heat were abstracted the temperature would rise 

 somewhat and equilibrium would be established at a slightly higher 

 temperature. For the particular case we have assumed we may 

 readily calculate that equilibrium would be established at about 

 1465°, when the mass would consist of about 60 per cent crystals of 

 composition AbjAnj. Even when the reaction takes place adia- 

 batically, there is an increase in the proportion of crystals. The 

 reaction is in no sense a solution of foreign material. Rather by a 

 making over of the foreign material it becomes no longer foreign 

 but identical with the crystalline matter with which the liquid is in 

 equilibrium. Moreover, if the originally foreign matter is more 

 calcic than the crystals with which the liquid is in equilibrium the 

 reaction is an exothermic one. How much of this reaction would 

 take place in any individual case cannot be predicted. It will 

 depend upon rate of cooling and other factors that readily suggest 

 themselves, but there is plainly a tendency toward such a reaction 

 and the reaction is exothermic. 



Let us now examine somewhat more minutely into the cause of 

 this exothermic reaction and we shall find that it is due to a general 

 principle and is not dependent upon the particular properties of the 

 plagioclase series discussed. During the crystallization of a plagio- 

 clase mixture a small decrement of temperature results in the reac- 

 tion: 



plagioclase+liquid = a little more plagioclase of somewhat more sodic com- 

 position. 



Since this is an equilibrium reaction taking place with falling temper- 

 ature, it must be exothermic. When we add the plagioclase AbiAup 

 to liquid AbiAuj we merely integrate this reaction over the tempera- 

 ture range i5oo°-i45o° and the composition range AbiAn9-AbiAn4. 

 Thus we could start with a liquid of composition AbiAug, permit it 

 to crystallize until at 1 500° the crystals would be of the composition 

 AbjAug, filter off the crystals, permit the liquid to cool to 1450°, 



