IGNEOUS ROCKS OF TASMANIA. 275 
the granite consists of orthoclase (dominant) + plagioclase 
+ green hornblende (no mica). It contains some large 
crystals of sphene, and some radiated chlorite. 
Hornblende-granite has also been seen in boulders at the 
Rosebery Mine on Mt. Black. 
DYKE AND COMPLEMENTARY ROCKS OF THE 
GRANITE SERIES. 
In the tin districts of the East Coast dykes of pegmatite 
traverse both the ordinary biotite-granite and the greisenised 
modification of it. These dykes usually carry cassiterite. 
Their constituent minerals are quartz, orthoclase and micro- 
cline felspars, with accessory muscovite. The broad cleavage 
surfaces of the felspar show irregular sections of quartz, 
denoting inter-growth, and constituting the graphic or peg- 
matitic structure. Naumann applied the term pegmatite . 
to extremely coarse granite. In Tasmania we have an 
exceedingly coarse variety of granite at the Fly-by-Night 
Creek near Gladstone, but the graphic structure is absent, 
and it ought not to be called a pegmatite. Some of the peg- 
matite dykes on Mt. Cameron carry crystals of topaz. The 
origin of pegmatite is evidently to be sought in what Vogt 
calls the eruptive after-action of granite, commonly known 
as pneumatolytic.. Good examples are to be: found near 
Lottah, on the Blue Tier. Descending from the township 
to the Laffer Mine, and at the mine itself, coarse pegmatite 
is seen in biotite-granite. It is also visible on the road- 
saddle between the Crystal Hill and Liberator mines in tin- 
granite, and pegmatitic segregations are extremely common 
in the tin-stone at the latter mine. In pegmatites it will 
be noted that the crystallisation of the quartz was not sub- 
sequent to, but synchronous with, that of the felspar. 
A peculiar differentiation product (or intrusion) is a 
granite-porphyry, rich in biotite, in biotite-granite on the 
Corinna Road, 5 miles from Waratah. <A panidiomorphic 
matrix of clear orthoclase and quartz is crowded with crys- 
tals of biotite. Strictly speaking, primary quartz is very 
rare in true minettes, and hence, as a rule, they are placed 
among the dyke-rocks of the sub-acid series. In this case 
the abundance of biotite suggests at the first glance the 
thought of minette, but quartz is far too plentiful. The 
rock appears rather to be a variety of granite porphyry. A 
somewhat similar occurrence is to be noted at Weldborough, 
only there muscovite is equally abundant with biotite, per- 
haps even dominant, and the rock is finer-grained. 
