Washington and Wright— Feldspar from Linosa. 53 



material used for the chemical analyses, and was found to be 

 2-692 at 3° and 2*693 at 17°. The latter may, therefore, be 

 considered to be the specific gravity of the mineral, or, rather, 

 correcting for about 0*75 weight per cent, or 0*35 volume per 

 cent, of included magnetite, the true specific gravity of the 

 mineral would be 2-684. The hardness is slightly less than 

 that of adnlaria, but not far from 6. 



Except for ferruginous stains derived from the inclosing 

 basaltic scoria on the exterior portions of the crystals, and the 

 very small and rare inclusions of magnetite to be described 

 later, and which did not interfere with the optic determina- 

 tions, the material is remarkably fresh, colorless and trans- 

 parent, and admirably adapted for optic work. Zonal structure 

 was not evident. 



The refractive indices were measured in sodium light on an 

 Abbe-Pulfrich total refractometer, the probable error of the 

 values being less than ± *001, with the following results: 

 a Na = l-5549, Na =l-5587, y Na =l'5634, 

 y— a=0'0085, y — £ = 0*0047, £-a=:0-0038 



According to these values the measured crystal plate was 

 optically positive and 2V Na =82° 48' '. On a second, less per- 

 fect plate slightly higher values were obtained, but the observed 

 differences were only a little greater than the probable error. 



The optic axial angle was measured directly on a Wulfing 

 axial angle apparatus in sodium light, the plates being im- 

 mersed in a liquid of the refractive index 1-559, the observed 

 readings giving, therefore, at once the true optic axial angle 

 2V Na . Five plates normal to the bisectrix c and one plate 

 normal to a were measured. The values thus obtained did not 

 agree well, and several of the plates were remeasured with 

 practically the same results.* The probable error of the values 

 in the following table is certainly less than ±30'. 



Crystal 









Optical 



Extinction 



plate 



2V Na 





Dispersion 



character 



angle,! a *a 



1 



78° 0' 





p>v 



+ 



28-0° 



2 



78° 29' 









+ 



12-0° 



3 



84° 29' 





p>v 



+ 



6-2° 



n 



85° 41' 









+ 



33-0° 



5 



89° 27' 



§ 





— 



11-9° 



6 



88° 59' 



II 



...- 



— 



13-0° 



* We are indebted to Mr. E. S. Larsen, jr., of the Geophysical Laboratory, 

 for an independent measurement of the optic axial angles of the entire set. 

 His results are practically the same as those in the table, the greatest differ- 

 ences being 20' on a less favorable plate. 



f Too much stress cannot be placed on these extinction angles, since the 

 plaies were cut only approximately normal to the bisectrices and were out a 

 number of degrees in certain instances. 



\ Plate cut normal to the obtuse bisectrix. 94° 18' measured. 



£ 90° 33' measured. |9l° 1' measured. 



