The Contact Between tfte Granitic Rocks and the Cardup Series 
AT Armadale. 47 
The biolite is a brownish tureen variety witli pleochroism X pale yellow- 
green, Y = Z deei) brownish-green, and absorption X Y := Z. The 
axial angle is very small (almost nniaxial) and j8 ~ 1.628 ± .002. In 
parts of the rock it oeeurs in clotted aggregates with a decussate structure, 
appearing to be either xenolithic or replacing ferromagnesian phenocrysts. 
The ani]dhbo]e is a pale greenish, slightly pleoehroie uralite with 
,Z /\ c — 16°. The plagioclase has mostly been replaced by granular 
epidote and zoisite, but rare small laths of iintwinned albitic plagioclase 
remain. 
This rock, in its high biotite content, differs considerably from the 
uralitised (luartz dolerites of the area. An examination of specimens col- 
lected from across the dyke showed that the edges are very rich in biotite, 
which is present to the exclusion of the amphibole, the rock consisting of 
biotite, epidote (and zoisite) and minor amounts of Ieucox(‘ne, albite and 
quartz. The biotite here is in flakes to 0.25 mm. diameter, which is large 
compared with Oakes in the central parts of the dyke, and is often aggre- 
gated into clots. At three inches in from the edge the structure i>s the same 
with clotted biotites in a ground of biotite, epidote, zoisite and amphibole 
with accessory leucoxene and quai’tz. In the centre of the dyke the grain 
is slightly coai-ser and the main ferromagnesian is the pale bluish-green 
amphibole which is in excess of the brown-green biotite. Angular grains 
of quartz are also more abundant. The mineralogical variation throughout 
the dyke may be shoAvn by the following estimates of the mineralogical com- 
position of the different rocks examined (figures quoted are volume- 
percentages) : — 
Edge of dyke. 
Biotite ... ... 65 
Amphibole ... ... Nil 
Epidote zoisite ... 30 
Leucoxene ... ... 1 
Albite -j- quartz ... 4 
3in. in from edge. Centre. 
50 22 
15 40 
3(^ 30 
1 1 
4 7 
An analysis of the siieeimen from 3 inches from the edge of the dyke 
yielded the result shown in Table V. It will be seen from this table that 
chemically this rock appears ti) be more closely relate<l tit the dilorite-albite 
epidiorite than to the quartz dolerite, the pnly diff'erenee between analyses 
1 and 2 being in the alkalies, the biotite epidiorite being exceptionally rich 
in ])otash while in the chlorite-albite epidiorite soda is considerably in excess 
of the potash — it may be noted here however that certain specimens of the 
chlorite-albite e])idiorite that were examined showed considerably more biotite 
than the specimen analysed. 
Wiseman (1934, p. 401) has noted the })roduction of biotite in the 
peri]iheral ])arts of an epidiorite sill at Loch Fyne, Scotland, and he con- 
siders that both chlorite and biotite have been produced during the shearing 
of the epidiorite mass during which process some potash has been intro- 
duced by freely circulating solutions thus leading to the formation of biotite. 
In the Armadale rock there is but little evidence of shearing but the biotite 
rich peripheral parts of the dyke* are similar to the occurrence described 
by Wiseman. This dyke is intrusive through a considerable mass of the 
biotitic hornfels in the south-eastern corner of the quarry and may have 
picked u)) some potash from this source. However the complete absence 
