in Labrador Felspar. 331 



the cleavage plane. The pearly light reflected from this speci- 

 men seems to be owing to a want of homogeneity in the mineral, 

 in virtue of which portions of different refractive densities are in 

 contact. The existence of such a structure is clearly proved by 

 the great nebulosity that accompanies the images of luminous 

 objects, and by the dimpled surface of its cleavage planes, when 

 examined by the microscope. This variety of felspar differs as 

 widely from the common Labradorite, as Chalcedony does from 

 Quartz, and the distinctive character arising from its heteroge- 

 neous structure is as easily appreciated. 



In a fine specimen of felspar belonging to Mr ALLAN, there 

 are, besides the plane of changeable colour, two other planes, 

 which reflect a silvery white light from long and narrow paral- 

 lelograms. Each of these last planes is formed of portions not 

 accurately parallel to each other, and hence the reflected light is 

 divided into separate masses. These masses are bounded by the 

 prismatic colours, which disappear when the trace of the plane 

 of reflection is parallel to the common section of the reflecting 

 plane and the surface of the specimen, and reach their maximum 

 when these lines are at right angles to each other. Hence, the 

 prismatic colours are produced by the prism of felspar bounded 

 by that surface, and by the plane that reflects the silvery tints. 

 By ascertaining the angle of a prism of felspar which connects 

 the maximum prismatic tints, we obtain the inclination of the 

 reflecting plane to the surface of the specimen. 



In many specimens of felspar, I have observed with the micro- 

 scope minute crystals and very small spheres of a metallic sub- 

 stance, which I have no doubt is titanium, and which has pro- 

 bably given rise to the peculiarities of M. PESCHIER'S analysis. 



VOL. XI. PART II. T t 



