20 N. L. Bowew — Genetic Features of 



perature of final consolidation is unknown. Neither 

 nephelite nor diopside appears, at least not at any tem- 

 perature where the identification of phases is possible. 

 They mutually destroy each other in the reaction which 

 produces melilite. 



A mixture of 70 per cent nephelite and 30 per cent 

 diopside begins to crystallize at 1235° with separation of 

 nephelite. At about 1180° the nephelite is joined by 

 olivine and melilite, and down to temperatures where the 

 crystals formed are no longer identifiable the mass 

 consists of nephelite, melilite, forsterite and liquid. No 

 diopside appears, it being used up by reaction with 

 nephelite to form melilite. 



Nature of the Melilites. 



The melilites formed from the mixtures studied do not 

 correspond with the end member, akermanite, whose 

 equilibrium relations are given in Ag. 1, but are strictly 

 analogous to natural igneous melilites. Some of the 

 crystals, particularly those formed at higher temper- 

 atures, are positive like akermanite but of much weaker 

 birefringence. With increasing richness in nephelite in 

 the total mixture the melilites separated become isotropic 

 and finally pass over into negative melilites. The decrease 

 of birefringence is accompanied by a slight decrease 

 of refringence which is about 1.630 in the sensibly 

 isotropic member. The series is quite different, there- 

 fore, from the akermanite-gehlenite series of Ferguson 

 and Buddington in which the refringence increases as the 

 birefringence decreases on passing away from aker- 

 manite. The present artificial melilites are, however, 

 readily explained and correlated with natural melilites 

 in the light of the recent work of Buddington. 15 The 

 relation to Buddington 's mixtures is brought out clearly 

 by adding nephelite directly to akermanite and crys- 

 tallizing the fused mixture at a low temperature. When 

 10 per cent nephelite is added to akermanite the product 

 is nearly homogenous melilite of somewhat lower refrin- 

 gence and distinctly lower birefringence than akermanite. 

 There is, however, a very little forsterite in excess and 



15 These melilites fall under the subdivision humboldtilite as used by 

 Buddington. Following paper, this number, p. 75. 



