THE FELDSPARS : THEIR DETERMINATION. 41 



are usually to be found in the slide, and their maximum angle is taken 

 as the index of the feldspar. In actual practice, Professor Pumpelly took 

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

 gave angles between 32' and 36° ; as labradorite, those between 36' and 

 62° ; and as anorthite those over 62°. 



AVhen more than one feldspar is present in the slide, only that 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 this 

 method to distinguish between oligoclase and albite.* 



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

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

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



That the work of both Levy and Pumpelly was independent and original 

 ■with both, can be inferred from the fact that the latter asked me early in 

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

 order to aid his experimental work 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 uiu'cliable 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. X 



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

 well as of other minerals, is the micro-chemical method of Dr. E. Boficky, 

 which consists essentially in subjecting the specimen to the action of fluo- 

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



» Proc. Am. Acad. Sci., 1S78, xiii. 253-309; Geol. Wise, 18S0, iii. 30. 

 t Ann. Mines, 1877 (7), xii. 392-lf)9 ; Comptes Rendus, 1878, Ixxxvi. 31:6-31:8. 

 + Proc. Nat. Mus., 1S81, pp. 134-13G. 



G 



