64 Washington and Wright — feldspar from Linosa. 



Again, assuming the possibility of existence of soda anorth- 

 ite, it would presumably depend on the conditions controlling 

 during crystallization, whether the sodium-alumino orthosili- 

 cate would crystallize as nephelite in separate individuals, or 

 as soda anurthite in mixed crystals with albite and anorthite. 

 With identical chemical composition of the rock, we would 

 have in the one case a nephelite tephrite, and in the other a 

 feldspar basalt, but the norms of both would be the same and 

 would show normative nephelite, since nephelite and soda 

 anorthite are normatively the same molecule. We might 

 explain in this way the anomaly of certain noncrystalline 

 rocks containing plagioclase, the norms of which show considera- 

 ble nephelite, though none of this mineral is present in the 

 mode ; and this may be applied to the normatively nephelitic 

 feldspar basalts of Linosa itself. In some cases, of course, the 

 discrepancy is to be attributed to the readjustments of the 

 normative molecules due to the formation of other modal 

 minerals, and the case is mentioned as illustrating some of the 

 petrographic possibilities consequent on the existence of soda 

 anorthite. 



The points brought out in the preceding paragraphs indicate 

 the importance and necessity of the verification of the assumed 

 existence of soda anorthite, before any modification is called 

 for of our ideas in regard to the composition and constitution 

 of the soda-lime feldspars, based as these are on the large 

 amount of accurate work by Tschermak, Schuster, Fouque, 

 Michel Levy, Fedorow, and others. The very general agree- 

 ment of the observations of all these workers with the theory 

 that only mixtures of NaAlSi 3 8 (albite) and CaAl 2 Si 2 O s 

 (anorthite) are involved is strong evidence in favor of its gen- 

 eral truth, and points to the conclusion that, if present at all, 

 the amount of soda anorthite must be very small in most feld- 

 spars so far examined. On the other hand, the occasional very 

 notable divergences from the figures demanded by the pre- 

 mises and the variability of the feldspars optically, indicate 

 the possibility of the presence of some modifying constituents 

 one of which might well be the molecule suggested by us. 



The Formation of Soda Anorthite. 



Fouque and Michel Levy* succeeded in 1880 in producing 

 oligoclase, labradorite, and anorthite containing strontium, 

 barium and lead, instead of calcium, but they did not appar- 

 ently attempt the formation of soda anorthite. 



* Synthese des Mineraux et des Koches, Paris, 1883, p. 145. 



