524 N. L. BOWEN 



magmas, but it is hoped that some principles of general significance 

 may be thereby brought to light. 



As a beginning we shall refer to equilibrium in the plagioclase 

 feldspars which is given in Figure i. In the figure we note that 

 liquid of composition AbiAuj begins to crystallize with the separa- 

 tion of crystals of composition about AbiAn4. As the cooling pro- 

 ceeds, if perfect equilibrium obtains, the crystals will be made over 

 by the liquid so that the composition of the crystals changes along 

 the curve ACB. It is plain, then, that if one had a mass of liquid 

 AbiAni at 1450°, that is just saturated, and added to this mass some 

 crystalline material of the composition AbiAn4 (approx.) (already 

 heated to 1450°) which we shall now call foreign inclusions, the 

 liquid would, if perfect equilibrium were attainable, make over these 

 inclusions as the temperature fell so that their composition followed 

 the curve ACB. How much of this work the liquid will be able to 

 accomplish in any individual case will depend on such factors as the 

 size of the inclusions, their permeability to the liquid, and the rate 

 of cooling, but the tendency is very plain. We can thus have a 

 liquid exerting a marked influence upon inclusions even though 

 these are precisely of the composition in equilibrium with the liquid, 

 provided the solid is a member of a solid solution series. It should 

 be noted, however, that this action is without effect on the course 

 of the liquid. The composition of the liquid follows the curve ADB 

 whether the inclusions are present or not. The career of the liquid 

 may, however, be brought sooner to a close as a result of reaction 

 with the inclusions, that is, the liquid may be entirely used up at a 

 somewhat higher temperature. 



We may now examine the case of adding to a plagioclase liquid 

 some solid plagioclase more calcic than that with which the liquid is 

 in equilibrium. To 50 g. liquid AbjAui at 1450° (just saturated) 

 let us add 50 g. solid AbjAug, already heated to 1450°. Equilibrium 

 will be established, if the temperature is kept constant, only when 

 the solid is completely changed to that with which the liquid is in 

 equilibrium, AbiAn4 (approx.). Since the total composition is 

 represented by the point K we can easily determine the proportions 

 of liquid and crystals. They will be 17 per cent liquid of composi- 

 tion AbiAui and 83 per cent plagioclase of composition AbiAn4 



