Morey & Bowen — Melting Potash Feldspar. 17 



Plainly, then, in the early separation of leucite, itself a 

 consequence of the incongruent melting of orthoclase, lies 

 the key to the origin of these nephelite syenites. 



This conclusion opens up the whole question as to 

 whether many other nephelite syenites may not have been 

 formed in a similar way. Leucite may have formed at 

 a certain stage and the evidence of its formation, after 

 its breakdown into orthoclase and nephelite, may not 

 always have been preserved in the form of the pseudo- 

 leucite structures. Even in the example described by 

 Shancl the nephelite of the pseudo-leucites has suffered 

 a further change to muscovite and a zeolite. Through 

 such further changes, not necessarily exactly of this kind, 

 the evidence of the former existence of leucite may fre- 

 quently, perhaps, have been entirely destroyed. More- 

 over, upon the change of leucite into orthoclase and 

 nephelite the course of crystallization will follow lines 

 upon which our work throws no light and it may be 

 possible that even the more common, highly sodic, 

 nephelite syenites could form as further differentiates. 



Examining nephelite syenites and their associates .with 

 these considerations in mind, we find a considerable 

 amount of evidence that the formation of leucite may have 

 occurred as an intermediate step in their genesis. 



The Himausak batholith in Greenland is stratified in 

 a manner closely related to that shown by the Loch 

 Borolan mass. At the top is a quartzose phase, arfved- 

 sonite granite, which passes downward into quartz-free 

 syenite and finally into sodalite and nephelite syenites of 

 great variety. 14 In one of these, the so-called lujavrites, 

 which are the lowest exposed rocks of the mass, there 

 are large crystals of analcite which Us sing presents 

 reasons for believing were formerly leucite. 15 The 

 nature of the stratification and the existence of these 

 pseudo-leucites (?) is so strikingly similar to the condi- 

 tions at Loch Borolan that one must consider the possi- 

 bility that the early separation of leucite has been a 

 factor controlling the differentiation of the mass (see 

 fig. 5). 



14 X. V. Ussing, Geology of the Country around Julianehaab, Greenland, 

 Med. om Gronland, vol. 28, p. 322, fig. 29,' 1911. 



15 Op. eit. pp. 164, 165. 



Am. Jour. Sci.— Fifth Series, Vol. IV, No. 19.— July, 1922. 

 2 



