TflE FELDSPARS: THEIR DP:TERM[NATI0\. 41 



are usually to be found in the slide, and tlieir niaxiniiiin angle is taken 

 as the index of the feldspar. In aetual t)riictice, Professor Punipelly took 

 as oligoclase those feldspars of which several individuals in a rock section 

 gave angles between 32' and 3G' ; as labradorite, those between 3G'- and 

 62° ; and as anorthite those over G2\ 



When more than one feldspar is present in the slide, only tliat one can 

 be distinguished which has the highest angle ; and this may be the minor 

 or subordinate feldspar. It is even possible for a single crystal, only, of one 

 feldspar, to change the conclusion as to the rest of the feldspars in the sec- 

 tion. Then, again, the sections examined may be so cut as to give a lower 

 angle than they should ; therefore the observer concludes he has a different 

 feldspar from the one actually present. It is scarcely possible by thi.s 

 method to distinguish between oligoclase and albite.* 



Professor Pumpelly was, however, anticipated in order of time, in tiie 

 publication of this method, by M. Michel Levy, who discussed the subject 

 mathematically, and applied the principles to many different minerals.! 



That the work of both Levy and Piuupelly w'as independent and original 

 with both, can be inferred from the fict that the latter asked nie early in 

 the year 1876 to undertake a mathematical discussion of this subject, in 

 order to aid his experimental worlv which he was then upon. The mathe- 

 matical portion the present writer had then neither time nor inclination to 

 perform, but the practical work of Professor Pumpelly resulted in that 

 method of determination which has been given before. Schuster's results 

 would, however, appear to render such determinations of but little value at 

 present. 



Dr. George W. Hawes in 1881, showed that the common method of 

 distinguishing triclinic from monoclinic feldspars was unreliable in certain 

 cases; for labradorite from St. Paul's Island and Canada, anorthite from 

 New Hampshire, and oligoclase from Bodenmais, exhibited none of these 

 supposed distinguishing features, i. e. striation in common and polarized 

 light, t 



Amongst the methods used for the determination of the feldspars, as 

 well as of other minerals, is the micro-chemical method of Dr. K. Boiicky, 

 which consists essentially in subjecting the specimen to the action of tluo 

 silicic acid, hydrotluoric acid gas, chlorine gas, etc. ; and microscopically 



* Proc. Am. Acad. Sci., 1878, xiii. 253-300; Gcol. AVisc, ISSO, iii. 30. 

 t Anil. Mines, 1877 (7), xii. 392-409; Comptes Rendus, 1S7S, Ixxxvi. 3i6-;US. 

 + Proc. Nat. Mus., 1881, pp. 134-136. 



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