IGNEOUS ROCKS OF TASMANIA. 2E 
granite masses, and as the quartz-porphyry facies of granite. 
The most important development of these rocks is along 
the axis of the West Coast range, where they are deposi- 
tories of copper ores. They prevail on Mts. Darwin, Jukes, 
Owen, Huxley, Tyndal, Murchison, and at the Red Hills, 
usually flanked by slates and schists, but on Mt. Darwin 
and on the east side of Mt. Farrell in contact with granite. 
The exposures of granite close to the quartz-porphyries are 
significant in connection with the genesis of the latter, but 
the whole line requires careful examination before anything 
decisive can be said. Associated with the copper deposits 
are large lenses of magnetite and hematite. 
These rocks present varied types of structure. There is 
the granophyric type, in which the ground-mass is_holo- 
crystalline, and consists of isometric forms of alkali felspar 
and quartz mingled with granophyric intergrowths of quartz 
and felspar. Porphyritic crystals of pinitised felspar are 
sparsely scattered through this ground-mass. A frequent 
feature in the ground-mass is the presence of a kernel of 
clear quartz in the centre of a felspar individual. Occasional 
small crystals of biotite are noticeable, and aggregations of 
secondary muscovite. 
On Mt. Huxley some of the rock is not gr anophyti ic, but 
a holocrystalline granular admixture of quartz and felspar, 
with phenocrysts of quartz and alkali and lime-soda felspars. 
The same feature is sometimes noticed at Mt. Jukes. 
There is a type, too, in which the ground-mass assumes a 
felsitic aspect, and the felspar and quartz phenocrysts float 
widely apart. 
The forms with porphyritic quartz may be called quartz- 
porphyry; those with porphyritic felspar only, felsite-por- 
phyry; those without visible porphyritic crystals, felsite. 
The holocrystalline granophyric forms may be termed 
granophyres; the holocrystalline granite porphyry-like 
forms, microgranites; and generally, forms with felsitic 
ground-mass, felsophyres. 
Dykes of topazised and tourmalinised quartz-porphyry, 
carrying cassiterite, traverse the Silurian slate at Mt. Bischoff. 
Topaz and tourmaline have largely replaced the felspathic 
constituent of the rock, and a silicification of the ground- 
mass characteristically accompanying the introduction of 
tin has also taken place. Where the dyke-rock has escaped 
the pneumatolytic action, as in the North Valley, pheno- 
erysts of quartz and felspar in a felsitic matrix give a clue 
to the nature of the original porphyry. The silicified rock 
is not always topazised, and is then a modified quartz-por- 
phyry, in which substitution of silica has taken place. At 
