16 Morey & Bowen — Melting Potash Feldspar. 



at ordinary temperatures is 2.46. At higher tempera- 

 tures, then, there would probably be a definite though 

 small margin of density in favor of leucite crystals, 

 especially since the liquid would, no doubt, contain a 

 considerable amount of volatile substances, whereas the 

 obsidian noted above is nearly free from these. Presum- 

 ably the leucite crystals could settle under the influence 

 of gravity; indeed, the general arrangement of zones 

 can scarcely leave room for doubt that they did and gave 

 a lower zone (III) much enriched in that mineral. When 

 the time of reaction of the leucite with liquid arrived 

 there would be an amount' of leucite above that requisite 

 for the reaction and some would be left in excess. The 

 excess leucite was, during the further crystallization of 

 the mass, transformed into pseudo-leucite (orthoclase + 

 nephelite) which is the normal fate of natural leucites 

 when cooled slowly under deep-seated conditions. On 

 the other hand, in the zone from which the leucites were 

 entirely removed (I) there were no crystals to react with 

 the liquid (which would be the natural analogue of our 

 liquid B, fig. 3) and it crystallized appropriately with an 

 excess of free silica. 



We are thus able not merely to accept Shand's inter- 

 pretation of the differentiation as gravitative, but we may 

 go further and connect the differentiation with the course 

 of crystallization ; in short, we may state that the differ- 

 entiation was due to fractional crystallization under the 

 influence of gravity. One may still accept the possibility 

 that the magma was affected by absorption of limestone, 

 but the formation of both a quartzose and a f eldspathoid- 

 bearing portion from a homogeneous mass shows plainly 

 that desilication of feldspar molecules by limestone was 

 not essential to the production of feldspathoids. The 

 fact that leucite can exist under certain conditions in equi- 

 librium with a liquid containing excess silica is the secret 

 of the coexistence of two such differentiates. 



Bearing of the Results on the Origin of Nephelite 



Syenites. 



Now the rocks of zone III, while undoubtedly at one 

 time leucite rocks, in part, at least, are now simply 

 nephelite syenites. This is the result of the usual change 

 of the leucite to orthoclase and nephelite, already noted. 



