702 DESCKIPTIVE GEOLOGY. 



ing labradorite, from their extreme delicacy of hue. It was hoped that a 

 careful microscopical analysis of these feldspars would throw some light 

 upon their composition ; but Professor Zirkel,^ in his report, says: "The 

 blue color is much more intense even than that of the famous 'labradoriz- 

 ing' feldspar from Frederiksvarn in South Norway. But while the luminous 

 shimmer of the latter is connected with the numerous brown and violet- 

 black laminse and needles, which are microscopically interposed in its 

 mass, the same strange bodies being also present in the proper labradorite 

 from the coast of Labrador, and from Kiew, Russia, in the feldspars of these 

 rhyolites, no strange particles can be detected, neither nee'dles, nor plates, 

 nor grains, nor a dust-like powder, nor glass- or fluid-inclusions. * * * 

 The cause of this strange blue color must for the present, therefore, remain 

 uncertain. It appears in the sanidin of some more eastern rhyolites. If 

 the mineral which shows the color were plagioclase instead of sanidin, the 

 phenomenon could easily be explained as a freak of polarization, produced 

 by the passage of broken rays from one lamel into another whose planes 

 of vibration do not correspond." 



A sufficient amount of this iridescent feldspar was obtained by Mr. 

 R. W. Woodward for the purpose of chemical examination, the analysis 

 yielding: 



Silica 66.00 



Alumina 18.74 



Ferrous oxide 0.35 



Lime 0.42 



Magnesia 0.25 



Soda 6.26 



Potassa 7.98 



100.00 



It is evident that the composition agrees closely with the common sani- 

 din found in trachytes and rhyolites, with perhaps a somewhat large per- 

 centage of soda. 



' Microscopical Petrography, vol. vi, 183. 



