Foote and Bradley — Chemical Composition of Nephelite. 441 



It has been shown by Bayley* and by Morozewiczf that rocks 

 occur where both nephelite and very pure albite must have been 

 formed simultaneously. Morozewicz in particular has shown 

 that in the rock investigated by him, albite crystallized 

 throughout the entire time when nephelite was forming. The 

 nephelite in both rocks was analyzed. It is probable that the 

 material was not as carefully purified as in the cases considered 

 in our former article, since the object was to show that neph- 

 elite was present and not to derive its formula. Still, the 

 results should show approximately the limiting ratio for silica, 

 when nephelite is saturated. The ratios calculated from their 

 analyses are as follows : 



Bayley Morozewicz 



(anal, by Clarke) 



Si0 2 2-15 2-15 



A1 2 3 1-00 1-00 



(Na 2 ,K 2 )0 -94 -95 



The ratios between alumina and alkalies is not as sharp as it 

 should be, perhaps. If, instead of calling alumina 1, the error 

 is allowed to rest equally on alumina and alkalies, which|seems 

 fair, the ratios become : 



Bayley Morozewicz 



(anal, by Clarke) 



Si0 2 2*21 2-21 



A1 2 3 1*03 1-03 



(Na 2 ,K 2 )0 -97 '98 



Whichever way ./the ratios are calculated, the value for silica 

 comes very close to 2*2, which represents the limiting ratio. 

 The value which we obtained before was 2'21. Since the two 

 values are practically identical, there seems to be no reason 

 whatever for modifying our original statement regarding this 

 ratio. Judging purely by the evidence available, we cannot 

 agree with Bowen ' that natural nephelites are probably never 

 saturated.' 



It is to be hoped that the problem may be attacked from 

 the synthetic standpoint in an adequate way. By this 

 means, it will perhaps be shown whether the saturation limit 

 changes appreciably with the temperature of formation, and 

 also whether the potassium content affects this limit to any 

 great extent. 



Chemical and Mineralogical Laboratories of the 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., February, 1912. 



* Bull. Geol. Soc. Amer. , iii, 231 , 1892. f Min. Petr. Mitt. , xxi, 238, 1902. 



