Vol. 65.] KOCKS OF THE TOCRMAKEA.DY DISTRICT. 135 



red. A rock from immediately north of Gortbunacullin Farm bridge 

 differs from the usual type, showing a felsitic ground-mass much 

 iron-stained and containing no quartz-phenocrysts. Another (187) 

 near the western boundary of the felsite, due west of Srah Bridge, 

 shows a somewhat banded appearance in the field, and in section 

 is seen to contain a considerable amount of augite, mostly repre- 

 sented by pseudomorphs in cblorite. 



(8) Felsites between Derrassa and Derrindaffderg. — 

 The rocks here are principally of the red type ; quartz is incon- 

 spicuous. A felsitic, instead of an imperfectly-spherulitic, type of 

 ground-mass prevails. Flow-structure is sometimes seen (229), and 

 the quartz-crystals are occasionally shattered as well as corroded. 

 The specific gravities are lower than iisual, those of five rocks near 

 Stream H being respectively 2-67, 2-69, 2*70, 2*70, and 2*71. 



(9) Augite-felsite intrusion of Shangort (279). — In a 

 hand-specimeu this is a dark, fresh-looking rock, with a fine- 

 grained ground-mass in which are sparingly distributed a few small 

 quartzes and well-cleaved felspars. In section the ground-mass is 

 seen to be imperfectly spherulitic, quartz is rather scanty, and the 

 felspar includes plagioclase as well as orthoclase. The augite is 

 sometimes fresh, sometimes shows replacement partly by epidote, 

 partly by chlorite ; there are also pseudomorphs after rhombic 

 pyroxene. The augite-crystals occasionally wrap round and enclose 

 the ends of the smaller felspars. Magnetite is plentiful, and ser- 

 pentine occurs filling small irregular vesicles. The specific gravity 

 of (279) is 2'78. This rock might, perhaps, be classed as a 

 quartz-andesite. 



Summary of the petrological characters of the fel- 

 site s. — Three principal types are recognizable, the green and brown 

 felsite, the red felsite, and the augite-felsite. In the green and 

 brown felsite the ground-mass is, as a rule, imperfectly spherulitic ; 

 in the red felsite it is frequently felsitic, and sometimes shows flow- 

 structure (229J. The principal phenocrysts are quartz and felspar. 

 Quartz is always prominent in the green and brown felsites and, as 

 a rule, shows strong corrosion by the ground-mass ; occasionally, in 

 rocks belonging to the group of red felsites (4, 19, 67) the sections 

 examined do not show quartz-phenocrysts. The quartz-crystals 

 are sometimes shattered (69). 



Of the felspars orthoclase is always present, but it is rarely (48) 

 fresh enough to show good twinning. Plagioclase is sometimes 

 (304 & 279) identifiable by means of the albite twin-lamellse, and 

 would probably be more often recognizable if the felspars were 

 less altered. Augite, either fresh (187) or more or less replaced 

 by epidote (279), serpentine (38), or chlorite (19, 90), is not 

 infrequent. Biotite in a more or less chloritized state was noted 

 at three points. 



