Plagioclase Feldspars. 599 



in excellent agreement with the results of the temperature 

 method. In Table III and fig. 2 the results are shown. 



In the theoretical discussion equations are developed which 

 express the concentrations on the liquiclus and solidus at any 

 temperature in terms of the melting temperatures and latent 

 heats of melting of the two components. The experimental 

 results therefore make possible the calculation of the latent 

 heat of melting of anorthite and of albite. The calculated 

 values are 104-2 cal. per gm. for anorthite and 48*5 cal. per gm. 

 for albite and these values remain practically constant for all 

 ranges of composition (see Table IY). The calculated latent 

 heat of anorthite is in excellent agreement with the figure 

 found by direct measurement, 105 cal. per gm. ~No direct 

 determinations of the latent heat of albite have been made. 



It is shown that if these values of the latent heats are taken 

 and liquidus and solidus curves calculated, the resulting curves 

 (see fig. 3) pass very close to the experimentally determined 

 temperatures (within the limits of error of the temperature 

 measurements). It is also shown that values of the latent heats 

 differing from these by as little as 10 per cent will not give a 

 like result. (Table Y.) 



This extreme agreement with the requirements of theory 

 and its bearing on certain theoretical questions is discussed. 



The geological significance of the complete solid solution of 

 the feldspars is considered, as well as the extent to which zon- 

 ing may occur under favorable conditions and the consequent 

 very great range of temperature through which plagioclase 

 may crystallize. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 



Washington, D. C, March, 1913. 



