The Geology and Physiography or the Gosnells Area. 
25 :^ 
The Gosnells xenoliths resemble the hornblencle-biotite-zoisite hornf'els 
xenoliths found at Aimadale (Pridev, 1941). They are, however, much 
more digested by the granite magma. Hornblende has disappeared, and 
apatite is much rarei’. Quartz (and plagioclase in the more assimilated 
rocks) have been introduced, and epidote has recrystallised to coarser 
granules. 
A rather different type of mateiial is the chloritic schist, ^vhicb resembles 
one, but only one xenolith, near the foot of the scarp north of Ellis Brook. 
The former is green, fine-grained and finely banded (about 15 bands 
to the centimetre). The constituent minerals found in thin section are 
quartz, chlorite, magnetite, muscovite, and a little (’pidote and a])atite. 
Alternate bands are rich in <|uartz and in chlorite and magnetite, and the 
minerals are usually elongated parallel to the handing. The ((uartz veinlets 
seen in the field are found, on a small scale, in a thin section. As a result 
of weathering, hematite has been formed from the magnetite and has dis- 
coloured the rock, and eonse(punirly it was imi>ossible to compare specimens 
of the sichist from both sides of sti’cam A. 
An epifliorite dyke alongside the schist contains abundant green chlorite, 
but the schist re.sembi(*s most closely a dark, massive xenolith found near 
Ellis Brook. The xenolith is made np magnetite, biotite, apatite, sericitised 
felspar and mnscovite; a higher grade assemblage than occurs in the schist. 
Sericife is abundant in the schist, to the exclusion of felspar, of which 
it is the alteration j)roduct: apatite is less common; biotite has been con- 
verted to chlorite and quartz has i)een introduced as bands and veinlets. In 
both rocks, the sericite has been partially recrystallised to small Hakes of 
muscovite. 
From its Held occurrence the schist is considered to be a xenolith, 
jmrmcated ])y siliceous solutions derived at a late stage of cooling of the 
gi'anite. Xear ijuartz veins in the area, granite is genei^ally altered to a 
schistose aggregate of low-grade minerals: similar changes in a xenolith 
have produced the schist. 
B. (Granite. 
A hand-specimen of this I’ock is medium, even-gTained and (uther light 
coloured, or tinted red by weathering. The minerals recognisable arc limpid 
(iuartz; i)ink, greenish or white f('Ispar and Hakes of biotite. 
The texture is allotriomorphic granular (or occasionally poikilitic) and 
the minerals found in thin sections are quartz, a plagioclase near alliite and 
some microcline with dark minerals (biotite, chlorite ami epidote) very 
subordinate. Table I. shows the range of mineralogical compo^ition of the 
granite. 
Plagioclase in all the racks sectioned is of }>ositive oi)tical character, and 
in most of them the maximum extinction angle on the albite twin lamellae 
is 12° to 15°, indicating a coni})osition between Al).,, and Ah,,.. The average 
composition is then the same as that ()f plagioclase in the xenoliths, but its. 
range is narrower. 
The ])lagiociase in some of the granites is saussuritised. In most, how- 
ever, it is sericitised, and it has been found in all stages of alteration from 
being perfectly fi'esh to being completely laqilaced by sericite. These altera- 
tions must be primary; they are too widespread and independent of jointing 
and other means of ingress of water to be regarded ns due to weathering. 
Plagioclase in lh(» peumatite aiid a]dite dykes is remarkably fresh. In a 
