292 PROCEEDINGS OF SECTION C. 
frequently, however, the nodules consist of colourless sub- 
stances... Large aggregates of granular or even radiating 
quartz are seen, sometimes without any regular external 
boundary, sometimes plainly, and without doubt showing 
the form of felspar. These are replacement metamorphoses 
of quartz after felspar, of such beauty as I only know in 
quartz porphyries. Between these pseudomorphoses of 
quartz after felspar there are roughly-radiate bundles and 
spherulitic crystals of felspar, which, from their optical 
behaviour, clearly belong to orthoclase or andesine. They 
are partly converted into sericite, and, when this happens, 
the nodules can be scratched with a knife. Finally, the 
nodules are much intersected by veins of quartz, the fillings 
of cracks in the rock. Iron ores are absent; but from the 
often quadratic and trigonal outlines of the quartz aggre- 
gates, I believe we must concludé that the ores have been 
removed, and their place taken by quartz. After all said 
and done, I regard the rock as a characteristic variolite, but 
certainly in a much-altered state.’’* 
Basatrt. 
This rock is the effusive form of gabbro, and its mineral 
constituents are lime-soda felspar, and pyroxene, with or 
without olivine. As in other groups, the aim in classifying 
should be to get behind mere structure, and found sub- 
divisions upon mineralogical constitution. The time 
honoured divisions of dolerite, anamesite, basalt, applied to 
this group may well be allowed to lapse; the terms are of 
merely textural significance, and may be used in describing 
any of the sub-groups, but not in defining them. The sub- 
groups met with in Tasmania are :— 
1. Olivine-basalt-.252). 20<..5.> *Passinm: 
2. Hypersthene-basalt ... ... ... Circular Head. 
The Tertiary basalt which has so widely overspread the 
Island is olivine-bearing. The olivine is extremely abundant 
in some varieties, usually in rounded or corroded crystals, 
but occasionally perfectly idiomorphic, as in the vitrophyric 
lavas of Sheffield, &c. Serpentinisation is frequent. 
Every variety of structure is represented—hypocrystalline, 
vitrophyric, and hyaline. Near Lefroy the basalt is often 
intersertal in structure. In many parts of the Island 
(Waratah, Hampshire, Conara, Benham Plains, Bothwell, 
&c.) it is highly vesicular. It is vitrophyric and hyaline at 
vee ateeene on the Mineral Fields between Waratah and Corinna, 
b 
