GEOPHYSICAL LABORATORY. 161 



as components, nor in terms of any other ternary system. This is due to the 

 reaction which takes place between anorthite (CaAl2Si208) and forsterite 

 (Mg2Si04) with the formation of spinel (MgAl204) as a prunary phase. The 

 composition of the liquid and the general relations of that part of the system 

 where spinel occurs can be expressed no more simply than in terms of the four- 

 component system CaO-MgO-Al203-Si02. The greater part of the mixtures, 

 however, behaves as if the system were a true ternary one, and that part has 

 been made the subject of detailed examination. 



The solid phases occurring in the system are the following: anorthite 

 (CaAl2Si208) , forsterite (Mg2Si04), cristobalite (Si02), tridymite (Si02), clino- 

 enstatite (MgSiOs), and spinel (MgAl204). 



The binary systems. — The system forsterite-silica is described in a previous 

 paper. The system anorthite-silica is a true binary system with a eutectic 

 point of composition anorthite 52 : silica 48 at the temperature 1353° ±2°. 

 The system forsterite-anorthite is not a true two-component system, spinel 

 being formed as a primary phase in mixtures between forsterite 14 : anorthite 

 86, at 1444° ±2°, and forsterite 45 : anorthite 55, at 1465° ±2°. Outside the 

 interval between these mixtures the prunary phases are forsterite or anorthite, 

 respectively, and the crystallization takes place as in normal two-component 

 systems until the composition of the liquid has reached one of the two points 

 mentioned. 



The ternary system contains two quintuple points: (1) The point where 

 the fields of anorthite, forsterite, and clinoenstatite meet ; composition, anor- 

 thite 55, forsterite 25.5, silica 19.5; temperature 1260° ±2°. This point is not 

 a ternary eutectic but a reaction point. (2) The ternary eutectic anorthite, 

 clinoenstatite, silica; composition, anorthite 50.5, forsterite 16.5, silica 33; 

 temperature 1222° ±2°. 



A discussion is given of the course of crystallization of typical mixtures on 

 cooling under equilibrium. It appears that forsterite (Mg2Si04) occurs as a 

 primary phase under the normal crystallization of certain mixtures in spite 

 of the fact that these mixtures contain silica in excess over the ratio in which 

 Si02 and MgO form MgSiOg. The composition of these mixtures is expressed 

 by points lying between the conjugation line and the field boundaries, for- 

 sterite : clinoenstatite and forsterite : anorthite. At a later stage of crystalli- 

 zation there is a reaction between the forsterite thus formed and the remaining 

 liquid, this reaction always resulting in complete solution of the forsterite 

 and a simultaneous crystallization of clinoenstatite. When the reaction is 

 over, the crystallization continues toward the ternary eutectic anorthite-clino- 

 enstatite-silica, in various ways, according to the initial composition of the 

 mixture. In other mixtures which have forsterite as a primary phase, namely, 

 those whose composition is expressed by points on the forsterite side of the 

 conjugation line, forsterite is only partly dissolved and the crystallization is 

 now complete at the reaction-point (the first quintuple point referred to). 



The bearing of the equilibrium relations here indicated on petrological 

 problems is discussed. 



(6) Das ternare System: Anortliit-Forsterit-Silicium-2-Oxyd. Olaf Andersen. Neues 



Jahr. Min. Geol. (in press). 



A German translation of "The system anorthite-f orsterite-silica " (Am. J. 

 Sci. (4), 39, 407-454, 1915). Reviewed under No. 5 above. 



(7) Some notes on the theory of the Rayleia;h-Zeiss interferometer. L. H. Adams. J. 



Wash. Acad. Sci., 5, 265-276 (1915). 



In the course of some work on the freezing-point of dilute aqueous solutions, 

 in which a Zeiss interferometer was employed as a means of determining the 



