and their Contact Zones. 33 



milk-white, and in it, so far as I could observe in the absence 

 of measurements, the two most perfect cleavages are not 

 quite perpendicular to each other. The basal cleavage 

 reflects light in numerous planes very slightly inclined to 

 each other. The lustre in this cleavage is slightly more 

 pearly than in orthoclase. Samples examined before the 

 blowpipe gave strong potassium flame and moderate sodium 

 flame reactions, and a quantitative examination showed a 

 considerable amount of potassium. Pieces from the basal 

 cleavage, examined as thin slices, proved to me that this 

 felspar varies much in structure. In one example, half, at 

 least, shows the usual characters of orthoclase in which 

 the internal structure is not quite homogeneous. The 

 other half is traversed by irregular bands of triclinic 

 felspar, in which the composition face is across the 

 direction of the bands. The angle of obscuration in these 

 bands is from 2 J to 3° on each side of the composition face. 

 The portions alternating with these bands of triclinic 

 felspar become obscured in a direction, according to my 

 measurements, which x were not quite satisfactory, of 14° 

 against the edge o — ii, supposing the felspar to be ortho- 

 clase. This ground mass is always more or less cross- 

 banded, precisely in the manner of Amazon stone ; and 

 where this structure was well developed, I obtained some 

 measurements of the angle which oave me 15° 30'. This 

 indicates micro cline. 



A second piece, prepared from a sample which had the 

 discordant cleavage planes on the basis, showed me that the 

 whole was traversed by very narrow and somewhat faint 

 lines of twinning. In places, however, where I could obtain 

 measurements I found that portions became obscured 

 when the plane of polarisation of one of the nicols was 

 parallel to the composition face, while other lamellae required 

 to be rotated over an angle of o° with that direction before 

 becoming obscured. These observations indicate that the 

 first example has the structure of perthite, with this 

 difference, that potash felspar is in its triclinic form. The 

 second example appears to consist of alternating lamellae of 

 albite and orthoclase. 



A sample of the pale red coloured felspar prepared from 

 the basis showed only the cross-banded structure character- 

 istic of microcline. 



This felspathic vein is one of a numerous class which 

 occur in the Omeo district, traversing the crystalline schists. 



