Morey & Boiven — Melting Potash Feldspar. 19 



An intimate relation between lencite and nephelite 

 rocks is observed in many fields. A striking example is 

 shown by the Magnet Cove complex. In particular it 

 is noteworthy that a lencite porphyry and a f oyaite, asso- 

 ciated there, have nearly identical chemical composition, 

 at least in some specimens. Washington has called atten- 

 tion, also, to a similar relationship of a lencite-rich rock 

 (lencite phonolite) of the Sabatinian district, Italy, with 

 a nephelite syenite of Beemerville, N. J. 20 



In volcanic fields the frequent intimate association of 

 trachyte, leucite-trachyte and phonolite is suggestive in 

 this connection. We may mention only the Laacher See 

 area from which region came one of the feldspars (sani- 

 dine) which we used in our investigation and which shows 

 in a typical way the decomposition into lencite and liquid. 



On the whole, then, there is a considerable body of evi- 

 dence pointing to the importance of the incongruerit melt- 

 ing of orthoclase as a factor not merely in the formation 

 of leucite rocks but of other feldspathoid-bearing rocks 

 as well. It is not intended here to set this up as the sole 

 factor involved in the formation of feldspathoid rocks. 

 There are many indications that some leucitic rocks are 

 formed as a result of differentiation along lines which 

 produce a liquid rich in mica molecules and that then the 

 extrusion of the liquid places it in surroundings where 

 it is unable to retain the water necessary for the forma- 

 tion of mica, with the result that leucite is formed just 

 as it is when mica is melted in an open crucible. 21 The 

 more basic leucite rocks, leucite basalts, etc. may perhaps 

 have formed in some such way. 



In former papers the writer has presented reasons for 

 believing that in the reactions which are revealed by the 

 presence, side by side in a rock, of alkaline feldspar, mica, 

 and quartz, there is evidence of the breakdown of the 

 polysilicate (feldspar) molecules into less siliceous 

 (feldspathoid) molecules and free silica. This was 

 believed to be due to the hydrolyzing action of water in 

 the magma. 22 It was not then anticipated that it would 

 be found that potash feldspar breaks down in a similar 



20 H. S. Washington, Igneous complex of Magnet Cove, Arkansas, Bull. 

 Geol. Soc. Amer. 11, 399, 1900; and Roman Comagmatie Eegion, Carnegie 

 Institution of Washington, Pub. No. 57, 1906, p. 47. 



21 H. S. Washington, The formation of leucite in igneous rocks. J. Geol., 

 15, 379, 1907; and N. L. Bowen, The later stages of the evolution of the 

 igneous rocks, J. Geol. Suppl. to vol. 23, 60, 1915. 



