VOLCANIC ACTIVITY IN SOUTHERN QUEENSLAND. 115 
phenocrysts give much evidence of having been attacked by 
the matrix, as the edges are rounded and sometimes the 
erystals are embayed. The felspar phenocrysts sometimes 
form composite masses up to 4 mm. in diameter, but their 
average length is about 2 mm. The felspar phenocrysts 
give an extinetion such as to indicate medium andesine 
(Ab,;—An,,). The groundmass is hemicrystalline and 
hyalopilitic, with crystals of quartz and felspar, together 
with smali rod-like crystals of a ferromagnesian mineral 
which was perhaps originally augite but is now chlorite. 
Density 2-650. (Micro. 570.) 
Dacite trom the centre of Portion 2274, Parish of 
Wildash, Hight Mile Range. In the field this rock was 
difficult to distinguish from the first rhyolite described. It 
is deep lavender in colour, with small phenocrysts of pink 
felspar. Under the microscope the phenocrysts are seen to 
oceur up to 1 mm. in length and to show multiple twinning. 
It is difficult to determine them, but they appear to be of 
the andesine-oligoclase type. The groundmass is pilotaxitic 
and contains abundant small allotriomorphie crystals of 
quartz and felspar set in amongst the cryptocrystalline 
material. Density 2-609. (Micro. 455.) 
AANDESITES. 
The andesites vary from light-grey to purple in colour 
and from markedly porphyritic to non-porphyritic types. 
The three following rocks will serve to illustrate the 
important characteristics of the andesites :— 
Andesite from the top of Rhyolite Range, due east of 
Bald Mountain. In the hand specimen this is a purple- 
brewn in colour with abundant phenocrysts of plagioclase 
of various sizes but frequently equidimensional. Under the 
microscope the phenocrysts of plagioclase show marked 
twinning and are determined as medium andesine (Ab,.— 
An,;), from the extinction angle. Some of the felspars 
show definite zoning. The groundmass is hyalopilitic and 
fluxion structure is very evident. Little other than minute 
crystallites of plagioclase and glassy material may be seen 
in the base. Abundant granules of ilmenite, partly 
weathered into leucoxene, occur through the field. Density 
2-691. (Micro. 447.) 
