VOLCANIC ACTIVITY IN SOUTHERN QUEENSLAND. 113 
The concluding effusions appear to have been dense 
andesites and basalts, and although there is not any 
tuffaceous material associated with these latter flows, there 
is, at one point on Rhyolite Range, a very coarse rhyolite 
breccia immediately underlying the andesitic material, 
indicating quite local and intense volcanic explosion 
previous to the andesitic effusions at that point. 
The voleanie activity was characterised by— 
(a) An increasing basicity as it progressed ; 
(b) The amount extruded being inversely propor- 
tional to the basicity. 
(iiM.) -PHTROLOGY: 
As indicated above there are in the thickness of at least 
2,500 feet eZ voleanic material, both flows and tuffs. The 
flows range from acid to basic in character, and the former 
much predominate. Many of the fine-grained rhyolite tutis 
are difficult to distinguish from flows, and recourse to 
microslides is necessary. Except in the Eight Mile Range, 
which is largely dacitic, almost the whole of the voleanic 
material is rhyolitie in character. 
RHYOLITES. 
These are variable, but both lithoidal and fluidal types 
are common, while spherulitic structure is frequently 
developed in a striking manner in the hand specimen. The 
colour varies through pink, lavender, and grey, to white— 
the last especially on the weathered surfaces. Porphyritie 
erystals of quartz, orthoclase, and plagioclase occur, and 
frequently spherulites have developed around the felspar 
crystals. 
The descriptions of the three following rhyolites may 
be regarded as representative of the various flows in the 
several blocks under consideration :— 
Rhyolite from Eight Mile Range, near the Summit, 
Portion 2161, Parish of Wildash. This is a deep lavender- 
coloured rhyolite containing numerous small phenocrysts of 
sanidine and very occasional small rounded crystals of 
quartz in a very fine groundmass. Fluxion structure is 
shown well in the hand specimen. The specific gravity is 
2-628. Under the microscope its holocrystalline character 
is apparent, and one sees idiomorphic phenocrysts of 
R.S.-—I 
