Geology of NiUahcootie. 295 



from the foot of Wild Dow Creek Falls shows rounded and cor- 

 roded crystals of quartz, with numerous oorroded felspars set in 

 a micro-crystalline ^roundmass. The felspars are very much 

 altered, some beinjjf so much so that it is impossible to deter- 

 mine even whether they are orthoclase or plagioclase. One of 

 the less altered phenocn^sts is andesine approximating to the 

 composition AlipAn^. OrtTioclase is almost certainly present, as 

 some of the kaolinized felspars, showing not the slightest trace 

 of twinning, contain a micro-perthitio intergrowth of a fresher 

 felspar, which is proba,bly albite. Flakes oi biotite are not un- 

 common, some being fairly fresh, while others are partially or 

 completely altered to a strongly pleoohroic chlorite, which gives 

 a change from a brownish green to a light whitish green. In- 

 clusions of apatite and magnetite are fairly abundant. The 

 groundmass is granular and consists of quartz and felspar with 

 minute flakes of ohloritized mica. Using Becke's method the 

 felspar is found to have a refractive index lower than that of 

 quartz, and different sections show refractive indices greater, 

 equal to, and less than that of eanada-balsam, so that we evi- 

 dently have orthoclase and an acid plagioclase both present. 



A section cut from a specimen obtained from the summit of 

 Mount Samaria, alongside the Ttig, st-ation, shows somewhat 

 similar character, but is much finer in the gi-ain. The ground- 

 mass is felsitic, and consists of quartz and felspar, with minute 

 flakes of mica. The felspar microlites are too small to deter- 

 mine. Quartz intenseh' oorroded and embayed, and both ortho- 

 clase and plagioclase occur as phenociysts. Flakes of mica are 

 fairly abundant, and garnet, sm-rounded by a ring of chloritized 

 mica, is present. 



(D). — The Nomenclature of the Rocks. 



Most of the more acid plutonic rocks of Victoria come under 

 the head of grano-diorite, as they have basic felspar in excess 

 of the alkali felspar. In the rock described in this paper the 

 orthoclase is about as plentiful as the plagioclase, and moreover 

 the plagioclase in this case is andesine and not labradorite, the 

 common felspar of grano-diorite. Biotite is not nearly as abun- 

 dant in this rock as in the grano-diorites of Harcourt and else- 

 where. 



