HY W. N. BENSON. 543 



cleaves faiily well, showing a few glittering mica flakes on 

 cleavage. The spots in it are quite black, and generally elliptical; 

 and contain a good deal of mica. Microscopically, it differs from 

 the knotted slates in that there is less carbonaceous matter; the 

 biotites are chloritised and stained yellow with limonite; and 

 muscovite occurs in large flakes lying across the general direction 

 of schistosity. Small oxidising grains of magnetite occur ; 

 tourmaline and r utile as before. 



Sixth Group : Chiastolite Slates. — These have so far been 

 found only in the railway cutting just east of the large western 

 granite vein, and about 200 yards east of the 164|-mile peg. 

 The specimens obtainable in the old railway cutting are better 

 than those in the new. The slate appears first about two yards 

 from the granite vein, and extends to 20 or 30 yards from it, the 

 crystals becoming smaller as we pass outwards. In hand- 

 specimen it is rather like the green slate, and has a fairly good 

 cleavage. There are dark green rectangular prisms in it up to 

 one-quarter of an inch in length, and surrounded by a bleached 

 halo (Plate xxiii., figs. 5, 6). Sometimes instead of the prism-form 

 the black" patches are in the shape of crosses, and may be with 

 or without definite outline. Microscopically, the green main 

 portion of the slate is seen to be very fine-grained, with rather 

 larger crystals of biotite than usual, which are strongly pleochroic; 

 though sometimes they may be partly chloritised. Sericite is 

 scattered in wisps over the slide; there are some flakes of mus- 

 covite and a little magnetite. Rutile and tourmaline occur in 

 their usual forms. Some andalusite is present as irregular grains. 

 The bleached halos of the dark patciies are composed of quartz, 

 sericite, and muscovite only. The dark patches consist of a 

 closely felted aggregate of biotite, muscovite, and a little quartz. 

 I believe them to be decomposed chiastolite cr3^stals. 



The seventh group is that of the Mica Schists. These are the 

 most metamorphosed of all the slates that occur in the district, 

 and vary considerably in hand-specimen. Sometimes they are 

 only distinguishable from the green slates by their coarser grain, 

 and large flakes of muscovite. In this form they are generally very 

 43 



