14 Morey & Bowen — Melting Potash Feldspar. 



spar. To be sure, if perfect opportunity for reaction 

 were presented, either quartz or leucite should be absent, 

 but it is easy to conceive of conditions under which such 

 opportunity would fail. 



Cross describes a rock from Wyoming which contains 

 leucite. Bulk analysis of this rock shows, however, that 

 the silica present is entirely adequate to have formed 

 orthoclase with the potash and alumina present. It 

 seems necessary, therefore, to assume that the glassy 

 groundmass is highly siliceous. 11 Such an assumption 

 would be entirely justified in the light of our results. 



There is one occurrence that appears to illustrate in a 

 convincing manner the separation of leucite under the 

 influence of gravity and the consequent formation of a 

 quartzose differentiate in those parts from which the 

 leucite has moved. Near Loch Borolan in Scotland there 

 is a differentiated laccolith which is described by Shand 

 as being made up of the following in stratiform arrange- 

 ment, as stated below, and shown in fig. 4: 12 



I. Quartz syenites (nordmarkite with 12 per cent, quartz and 



other more quartzose types). 

 II. Transition zone of quartz-free syenites. 



III. Feldspathoid-bearing syenites. 



IV. Probable ultra-basic zone (noted in one locality). 



The syenites of III are in part demonstrably pseudo- 

 leucite bearing. If it can be imagined that the laccolithic 

 chamber was filled with a magma very rich in alkaline 

 feldspars, and with not more than a moderate excess of 

 free silica, this magma might, as our results show, begin 

 to crystallize with separation of leucite. The actual pro- 

 portions of the various rock types in the composite mass 

 (see fig. 4) show that the general liquid would be very 

 rich in feldspar, principally orthoclase, and excessively 

 poor in the molecules that go to make up the heavy 

 minerals. The density of such a liquid as a glass at 

 ordinary temperatures would probably be not far from 

 that of rhyolitic obsidian (2.37). 13 The density of leucite 



11 W. Cross, this Journal, 4, 122 and 132. 1897. 



12 S. J; Shand, Trans. Edin. Geol. Soc, 9, pt. Ill, p. 202, 1909, and pt. V, 

 p. 376, 1910. See also Home and Teall, Trans. Boy. Soc. Edin., 37 pt. 1, 

 p. 163, 1892. 



13 H. S. Washington, Khyolites of Lipari, this Journal, 50, 449. 1920. 



