BY H. I. JENSEN. 141 



the plane of the optic axes (a & c) lies in the zone c, y, x. 

 Sections perpendicular to the acute bisectrix frequently show the 

 brushes so close together as to appear like a black cross. Hence 

 the sanidine is almost uniaxial. It appears to be slightly negative 

 in optic sign. When lath-shaped sections occur they usually 

 show Carlsbad twinning and low extinction angles. Angles up 

 to 12° have been observed. It may therefore be concluded that 

 some, at least, of the sanidine is anorthoclase (soda sanidine). 



In my section of Cooran comendite one sanidine phenocryst 

 had a border of albite showing pericline twinning. 



In my sections of Ngun-Ngun pantellarite, I have identified 

 the felspar phenocrysts as anorthoclase. They are frequently 

 tabular pai*allel to b, and the faces n and m are here developed 

 at the expense of a; and y, which characterise orthophyric felspars. 

 Simple twinning on the Carlsbad and Baveno plans is common. 



Lath-shaped sanidines with the characteristic cross-cracking 

 (parallel to a) are beautifully developed in the Round Mountain 

 trachyte. 



In the Beerwah trachyte both columnar and tabular habits are 

 represented in the felspars. Those of both habits have developed 

 the faces C, B, m, y, x, In the light blue silky trachyte from 

 this locality all the felspar has the properties of sanidine. The 

 R.I. is / ■ Canada balsam; Opt. Sign — ve - } axial plane X 7 ' to b; 

 tt = a; b near c. The axial angle is very small, and the interfer- 

 ence figure is consequently nearly uniaxial. The extinction 

 angle is occasionally as high as 12° on b/c. Hence this is a soda- 

 orthoclase. In a somewhat yellowish-grey variety of Beerwah 

 trachyte lath-shaped oligoclase has been found present as well as 

 anorthoclase. 



In the microgranular comendites of Tibrogargan, Beerburrum, 

 Trachyte Ra., &c, and the fine-grained orthophyric comendites of 

 Mt. Ewin, &c, as well as in the groundmass of most of the others, 

 the felspar is so fine-grained and often so allotriomorphic that its 

 exact nature cannot be determined with the high power of a ^in. 

 objective. From its glassy appearance and low R.I. and D.B., 

 we can merely say that it is a variety of sanidine. 



