20 Morey & Bowen — Melting Potash Feldspar. 



way even in the dry melt. The discovery of this fact 

 should not, however, lead lis to reject the effects of water 

 as important in promoting such behavior of the polysili- 

 cates. 23 Nor need we abandon the suggestion that in the 

 preparation in the liquid of f eldspathoid molecules, under 

 the influence of water, there enters the possibility, with 

 appropriate fractional crystallization, of the formation 

 of feldspathoid-bearing rocks. However, these reactions 

 represent, as the writer has always admitted, 24 a consid- 

 erable extrapolation from any facts that have yet received 

 laboratory demonstration. It is with considerable satis- 

 faction, therefore, that we announce a laboratory demon- 

 stration of the fact that a mass consisting in one part of 

 feldspar and quartz and in another of feldspar and feld- 

 spathoid can form from a single homogeneous liquid. 

 The method of formation of these contrasted parts, which 

 may be referred to as subalkaline and alkaline, respec- 

 tively, is the method of fractional crystallization, which 

 has also been shown fairly definitely to be adequate for 

 the production of all varieties of subalkaline rocks from 

 one liquid. 25 



Summary. 



A pure synthetic orthoclase was prepared by crystal- 

 lizing glass of the composition KAlSi 3 8 in a bomb with 

 water vapor. This material is particularly suitable for 

 the determination of the melting-point of pure orthoclase 

 and was used for that purpose. The temperature ordi- 

 narily given as the melting-point of orthoclase is about 

 1200° and has been determined on natural crystals. 

 When our artificial crystals were held at 1200° for a week 

 they gave a product which had the appearance of a glass, 

 megascopically, but which, examined under the micro- 

 scope, showed a structure described as a very fine cross- 

 lining. At higher temperatures this structure became 

 more distinct, taking the successive forms shown in iig. 1, 



22 N. L. Bowen, The later stages of the evolution of the igneous rocks, J. 

 Geol., Suppl. to vol. 23, 60. 1915. 



23 In dry melts albite shows no tendency to decompose in a manner analo- 

 gous to that shown by orthoclase. 



24 N. L. Bowen, Crystallization-differentiation in magmas, J. Geol. 27, 395. 

 1919. 



25 N. L. Bowen, The later stages of the evolution of the igneous rocks, 

 J. Geol., Suppl. to vol. 23. 1915. 



