264 N. L. Bowen — The Ternary System, etc. 



the experimental determination of the three-phase-boundaries 

 in the system being investigated are then presented. 



The course of crystallization in typical mixtures is described 

 and the value of the three-phrase-boundaries pointed out, 

 especially their usefulness in determining the composition of 

 mix-crystals separating at any temperature. In considering 

 crystallization it is shown that crystallization may proceed 

 according to two different methods ; first, that in which the 

 liquid is, at any temperature, in equilibrium with all the crys- 

 tals and all parts of the crystals occurring in it; and second, 

 that in which the liquid is in equilibrium at any temperature 

 only with the crystals separating at that temperature. In a 

 general way, crystallization of the second type is favored by 

 quick cooling. 



The importance of distinguishing between the two types of 

 crystallization is great in the present system. It is shown that 

 the difference between the conclusions arrived at in the present- 

 work and those arrived at in an earlier investigation of the 

 pyroxene series carried out at this laboratory is due largely to 

 the fact that in the earlier work crystallization of the second 

 type occurred in many of the mixtures, whereas, if equilibrium 

 is to be studied, crystallization of the first type must be 

 obtained. 



In the optical part of the paper the optical properties of the 

 various crystalline phases are given. The properties of the 

 series of monoclinic pyroxenes extending from diopside to 

 enstatite vary continuously with composition. The artificial 

 pyroxenes are compared with the enstatite-augites of Wahl. 



In considering the bearing of the results on petrol ogic pro- 

 blems attention is called to the resorption of the olivine, for- 

 sterite, in the artificial mixtures and its probable relation to 

 resorption of olivines in natural rocks. The consequences of 

 the possible settling of crystals in a fluid magma (crystalliza- 

 tion-differentiation) is discussed in the light of the facts known 

 concerning the artificial mixtures. 



Geophysical Laboratory 



Carnegie Institution of Washington, 

 Washington, D. C, June 23, 1914. 



