IGNEOUS ROCKS OF TASMANIA. 283 
Mt. Huxley or the felsites of Mt. Read. The decomposed 
clayey portion on the King River is sluiced for gold. The 
rock contains porphyritic alkali and plagioclase felspars, 
augite, and quartz. Such quartz-porphyries are not common. 
For some time this rock has been considered to be a syenite 
porphyry, of which it is evidently a near ally. 
GABBRO-PERIDOTITE SERIES. 
This series is essentially connected with the preceding, 
notwithstanding a continuous increase of basicity. The 
foyaitic rocks are higher in 810, and in that respect, but 
in that only, are intermediate between granite and gabbro. 
They are, however, never found associated with these, and 
must be regarded as being more distant from them than 
either is from the other. Accordingly, granite often assumes 
a gabbro facies, but never an elaeolitic one, and in fact a 
continuous series exists of granite, syenite, diorite, gabbro, 
with various passage-rocks difficult to define. Ii follows 
that the three divisions, granite, gabbro, elaeolitic rocks, are 
not equivalent in value, the two former being connected 
by passage-rocks, while the nepheline series is absolutely 
independent, as far as we know. 
GABBRO. 
Geologically, plutonic: chemically with Ca> (Na + K): 
mineralogically, consisting of basic lime, soda felspars (labra- 
dorite-bytownite-anorthite), + pyroxene (monoclinic or 
rhombic), with or without hornblende or olivine: structur- 
ally, like granite. Si0,, mostly between 44 and 54 per 
cent. The subdivisions of the family are mineralogical, and 
its representatives in Tasmania are— 
(1) Gabbro proper = diallage + plagioclase. 
(2) Norite = plagioclase + rhombic pyroxene. 
(3) Olivine norite = plagioclase + rhombic pyroxene + 
olivine. 
(4) Quartz-norite = plagioclase + rhombic pyroxene + 
quartz. 
Modifications. 
(5) Saussurite gabbro. 
(6) Flaser gabbro. 
(7) Gabbro amphibolite, Zobtenite. 
‘3) Gabbro-diorite. 
Gabbro has not been found in the southern or eastern 
parts of the Island. It occurs at the Heazlewood, Forth, 
near Beaconsfield, Dundas, and Mt. Heemskirk, and always 
carries with it a development of pyroxenite; and these 
