262 
C. Fj. S. Davjs 
3. Biotite-Epidote 
These thin veins (maximum width two feet) are, unlike the dykes irreKu 
lar and not persistent. ’ 
The ^-eln material is aphanitic, green or black and flaky, evidently eon- 
taining abundant biotite, Indeed, it consists of green biotite (largely con- 
verted to chlorite) and very pale green epidote. Granite inclusions in the 
vein are cut by stringers of biotite and epidote. Albite twin lamellae of the 
felsiiar in the xeuolithic rock are bent and fractured, but thi' felspar is clear 
and nnsericiti.sed and may have recrystalliscd before the.se deforming move- 
ments took place. 
4. Porpln/ritie Epidioritr. 
This type is found only near Ellis Brook, where it intrudes the Cardiip 
sediments. Porphyritii' epidiorites occur at various jilaces (Wongong, Car- 
diip, Muudijong) farther south, and a similar non-])orphvritic rock at Arma- 
dale (Prider, 1!I41, p. 48). 
It is a ilark, weathered rock made uji of laths of felsiiar, up to 5 mm. 
long, set in a line-grained ground. Prom a section, it is seen that felspar is 
developed as stout prisms (of all sizes from i /2 mm- to 5 mm.) which form 
a coarse network. The felspar is near albite, contains abundant ehloritic in- 
clusions and IS little altered except for slight kaolini.sation. The ferromag- 
uesiaii constituent is now green to brown biotite, forming decinssate aggre- 
pte.s, tilling the interstices between the network of albite prisms. Magnetite 
is fairly coiainoii. ^ 
Small iuiin^s of barite have been found near a pit in the porph^Titie 
epidiorite. The mineral is associated with veins of strained quartz, and is 
largely massive (g:rain size 0.3 mm.), but is also developed as tabular cry- 
stals up to 5 cm. long’. As this is the sole occurrence of barite in association 
with any igneous rock in the area, the porphyritic epidiorite is probably the 
source of the barite in the (^ai’dup sandstones at Gosnells. 
V. GEOLOGICAL HISTORY. 
There is no record in this area of any event prior to the emplacement 
of a granite batholith in middle Proterozoic time (Clarke, 1930, p. 160),. 
although a little to the south large masses of earlier, more basic rocks are 
preserved (Prider, 1941). These have been largely digested by the granite* 
at Gosnells, and only a very small (juantity of them is found as xenoliths.. 
End-liquids circulating in Joints tormed in the cooling* mass crystallised 
as dykes of pegmatite and aplite. Stresses at a later stage fractured the 
granite over a wide zone, and quartz was deposited from hydrothermal solu- 
tions over this zone and in many smaller fractures. 
There followed a period of erosion and deposition of a normal sequence 
of sediments (the Cardup series) which have undergone very little anamor- 
phism. Although only a small thickness of sediments is exj^tosed at Gosnells, - 
several hundred feet are exi>osed farther south, and a considerable thickness 
may underly the surface rubble even at Gosnells. 
All pre-existing rocks were invaded by basic dykes of (Clarke, 1930,. 
table p, 187) late pre-Cambrian age. These dykes now contain the lower- 
grade mineral hornblende instead of jiyroxene. In a porphyritic basic dyke,, 
a segregation of barite was formed. At the same time, probably, baritie 
solutions entered the more porous basal beds of the Cardup series. 
