4o8 /. E. L. VOGT 



complete equiKbrium between the solid and the liquid phase as in 

 the case of most deep-seated rocks: 



Start End 



Abi Ans 1521° 1465° 



Abi An2 1490° 1372° 



Abi Aril 1450° 1287" 



With incomplete equihbrium the crystallization of the final melt 

 would have taken place at a somewhat lower temperature. 



In anorthosites , with only 4-10 per cent of pyroxene, olivine, 

 iron ore etc., the last mentioned minerals, according to Bowen's 

 diagram for the system Ab : An : diopside, will occasion only a 

 relatively small reduction of the temperature for the beginning of 

 the crystalHzation of the plagioclase. The relatively small quantity 

 of end-magma in these rocks, will at a late stage of the soHdification 

 crystallize along a eutectic boundary-line (or plane) at a much 

 lower temperature, about 1225-1200°. 



For the various anorthosites poor in foreign minerals (calculated 

 at I atmosphere pressure and assuming water-free melts) we will 

 have the beginning of crystallization in the bytownite rocks at 

 about 1475-1500°; in the relatively An-rich labradorite-rocks, 

 with Aba An3— Abi An^, at about 1450°; and in labradorite-rocks, 

 with about Abi Ani at about 1400°. Following this, the main part 

 of the plagioclase will crystallize at a little lower temperature, 

 down to or a Httle below 1300°. Finally, comes the crystalHzation 

 of a small quantity of end-magma at a somewhat lower temperature, 

 about 1200°. 



For dunite, consisting chiefly of an iron-poor oHvine (most 

 frequently about 0.09 Fe2Si04 . 0.91 Mg2Si04, sometimes even 

 with as little iron sihcate as 0.075 Fca Si04 . 0.925 Mg2Si04) with 

 a small percentage of picotite, chromite, bronzite, etc., the crystal- 

 lization' must 'have started at as high a temperature as 1500°. 

 With very Uttle admixture of foreign minerals and at the same time 

 with an olivine especially poor in iron, the temperature may have 

 been a Httle higher, as 1550-1600°. The main part of the crystal- 

 lization wiU take place at a temperature higher than 1400°, and 



' Cf. this Journal for 1921, Fig. 22, p. 522. 



