and their Contact Zones. 43 



divergent tufts. Such needles often border the edges of 

 hornblende. 



Such a colourless pyroxene, having the form and optical 

 properties of augite, would be diopside. 



Mica. — Magnesia mica is the only one found in these 

 rocks. It occurs as an essential constituent in the dioritic 

 and granitic rocks at Riley's Creek, at Upper Swift's Creek, 

 at Eureka, and generally in the fall from the various gaps 

 at Upper Riley's Creek towards the Tambo. On examining 

 the forty slices which I have selected as presenting an 

 average of the whole, I find that it occurs with felspars 

 of group 1 three times, with 1 and 2 twice, with 2 once, 

 with 1. 3, and 4 thirteen times, with 2, 3, and 4 once. 

 Magnesia mica, therefore, may be said to show a preference 

 for the potash felspars, microcline, and orthoclase, and in 

 a less degree for the soda felspar albite. In the one 

 instance when it occurs in company with anorthite, it 

 differs somewhat in its characters from those usually seen. 

 Magnesia mica is most frequent in those rocks which lie on 

 the outskirts of the intrusive mass, and more especially 

 near the contact, when there is a distinctly gneissic 

 structure. 



It is not met with as crystals, but as masses or aggregates 

 of bent, twisted, or divergent flakes. It occurs either as 

 disseminated flakes, or as filling in between the various 

 constituents, or enveloping the felspars ; when associated 

 with hornblende it is often intimately connected with it by 

 intergrowth. 



This mica is usually brown or reddish-brown by light 

 transmitted in the direction of the chief axis, and when 

 examined in a direction perpendicular to this by the 

 polariser alone it is strongly dichroic, the dfference in 

 the colours being pale straw-yellow to almost black. 

 Almost all this mica is optically binaxal, but I have 

 observed a few instances where the mica was, so far as I 

 could observe, uniaxal, or binaxal with a very small angle ; 

 and in the slices where this mica occurred I could observe 

 distinctly that one or two minute isolated flakes had 

 hexagonal outlines. I have, however, only met with this 

 in the close-grained micaceous portions of the massive quartz 

 diorites of Riley's Creek, and in the crystalline granular 

 compounds of felspar and mica which constitute the fine- 

 grained parts of the massive quartz diorites of Eureka. 

 Examined stauroscopically the flakes were either uniaxal, 



e2 



