Morey & Boiuen — Melting Potash Feldspar. 21 



and finally becoming' typical lencite crystals. The point 

 at abont ±200° is therefore not the true melting-point of 

 orthoclase bnt is the temperature at which it melts incon- 

 gruently, breaking np into liquid and lencite. The exact 

 temperature of this decomposition we have determined 

 as somewhat lower than 1200°, namely, about 1170°. The 

 temperature of final disappearance of leucite is about 

 1530% so that the interval of incongruent melting is 

 remarkably large, viz., 360°. Three natural potash feld- 

 spars, microcline from North Carolina, sanidine from 

 Laacher See, and adularia from St. Gotthard show the 

 same kind of behavior, though in these the upper limit 

 of melting (disappearance of leucite) is lowered some- 

 what through the presence of foreign matter. 



This incongruent melting of orthoclase is of particular 

 importance in petrogenic theory because it shows plainly 

 how, by fractional crystallization, a homogeneous liquid 

 could form a differentiated mass consisting of orthoclase 

 and leucite in one part and of orthoclase and free silica 

 in another. It shows, too, that leucite can form from a 

 liquid containing an adequate amount of silica to form 

 orthoclase and that a mass may have leucite as early 

 crystals (phenocrysts) together with free silica as late 

 crystals (groundmass). These considerations explain 

 the occurrence of such a rock as the leucite granite por- 

 phyry of Brazil and such a differentiated mass as the 

 syenite laccolith at Loch Borolan, Scotland. It is to be 

 noted that both these occurrences show pseudo-leucites, 

 formed secondarily after leucite, and consisting, as do 

 the leucite s of intrusive rocks in general, of an inter- 

 growth of orthoclase and nephelite (or secondary 

 products after nephelite). This regular behavior of leu- 

 cite in breaking up into orthoclase and nephelite suggests 

 that the early separation of leucite, with a subsequent 

 change of that nature, may afford a key to the origin of 

 many nephelite rocks as well as leucite rocks. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, 

 March, 1922. 



