Morey & Boiven — Melting Potash Feldspar. IS 



of ortkoclase, the early formed leucite crystals might 

 collect locally and if so they would not be completely 

 used up by reaction with the liquid and a localized leucite- 

 bearing mass would result. 



Xow this early separation of leucite from a mixture of 

 the composition of orthoclase cannot be immediately neg- 

 atived by the addition of other components. Excess silica 

 rapidly neutralizes the tendency, of course, but other sub- 

 stances cannot have a comparable effect. Our microcline 

 from North Carolina shows that 25 per cent foreign 

 material (mainly albite) is insufficient to neutralize this 

 tendency, in fact the interval in which leucite appears is 

 as much as 270°, so that it would plainly require a con- 

 siderably larger amount of albite to bring about the dis- 

 appearance of the leucite field. Therefore, in magmas 

 rich in orthoclase, and even in those containing mucli 

 albite as well, if at the same time they contain not more 

 than a small excess of free silica, it is to be expected that 

 this early separation of leucite may occur. The leucite 

 crystals should disappear with falling temperature but 

 the two factors noted in the foregoing may intervene to 

 bring about their persistence. The factors are the armor- 

 ing of the leucite crystals or their local collection, or 

 indeed both. 



Evidence of the Existence of Similar Relations in Natural 

 Leucite Rocks. 



Certain described rocks give evidence of the occurrence, 

 under natural conditions, of the phenomena noted. 

 Hussak describes a rock from Brazil consisting of pheno- 

 crysts of leucite (now pseudoleucite) in a groundmass of 

 quartz and feldspar, which rock he terms a leucite granite 

 porphyry. Evidently strongly influenced by the dictum 

 of Rosenbusch and Zirkel that leucite and quartz cannot 

 occur together, Hussak suggests the possibility that the 

 leucites are remnants of fragments of a foreign rock, 

 caught up by the granitic dike. In his final conclusions, 

 however, lie inclines towards the opinion that it is an 

 ordinary igneous rock from which the mineral combina- 

 tion quartz-leucite-orthoclase has crystallized. 10 Our 

 results show plainly that such a mineral combination is 

 possible, particularly with the relations he notes, namely, 

 phenocrysts of leucite in a groundmass of quartz and f eld- 



10 E. Hussak, Xeues Jalirb., 1, 27. 1900. 



