PERTHITIC FELDSPARS. 149 



of the solid reached this temperature at yi there would occur a sharp 

 change in the composition of the solid crystals and we should have 

 formed a crystal phase of the composition represented by the point 

 ?/5, a microcline mixed-crystal, and simultaneously a certain amount 

 of an albite mixed-crystal would separate out. The composition of 

 the microcline crystal would then change along the dotted line until N 

 was reached; the albite phase would change along gM. If there 

 was no sharp inversion point then the mixed-crystals, 7/3, would persist 

 until the point ye was reached when an unmixing would begin with the 

 formation of a microcline mixed-crystal whose composition would 

 then change along the line rN until N was eventually reached, and an 

 albite mixed-crystal of a composition represented by corresponding 

 points on the line qM. The composition of the albite would eventually 

 be that of the point M. The albite phase set free by such an unmix- 

 ing, inasmuch as it takes place in the solid state, would not in all 

 probability escape to any extent from the original crystal, but rather 

 would withdraw in some more or less regular manner and crystallize 

 within the microcline. As the two phases are closely similar in crystal 

 structure and volume, the unmixing is perhaps in the nature of a 

 parallel shifting of the units of crystal structure throughout the space 

 of the original crystal, and the two finally become fixed in parallel 

 orientation as the familiar perthitic intergrowth. We can easily 

 imagine conditions, such as rapid cooling, under which the inversion 

 and unmixing process would not take place, or take place imperfectly, 

 and there might then result homogeneous orthoclase crystals contain- 

 ing all of the 20% of albite, or a crystal containing anyway less of the 

 intergrown albite phase than that which would have formed had 

 the equilibrium been completely established throughout. The fair 

 degree of constancy in composition exhibited by the two mixed-crystal 

 phases as deduced in the present investigation on the pegmatitic 

 perthites, indicates that the unmixing approaches in these, at least, 

 a fairly definite end point. 



If we start with a melt of a composition represented by the line z, 

 crystallization would begin when the temperature had fallen to the 

 curve AE with the formation of a crystal of the composition s^. This 

 would change continuously along the lower curve until the point i was 

 reached when crystals of composition q would also begin to form, and 

 these two mixed-crystals would then continue to grow until the 

 solidification was complete. A similar procedure would obtain for 

 a mixture, say of composition w, the only difference being that there 

 would be formed a larger proportion of crystals of composition q rela- 



