116 WHITE : OATLAND COMPLEX OF IGNEOUS ROCKS. 
felspar round the borders exactly resembles that round the 
plagioclase in the Foxdale and Dhoon granites. 
The decomposition product in addition to amorphous matter 
is mostly white mica, but much epidote occurs in places, often in 
veins which completely traverse some of the crystals. The 
flakes of white mica often project from the turbid patches into the 
clear felspar, and are sometimes completely surrounded by it. 
There are inclusions of apatite usually surrounded by iron 
ore in small quantity, and iron-ore in octahedral crystals. One 
large enclosure is an aggregate of dusty-looking greenish-brown 
sphene with ilmenite, calcite and chlorite. Not only is plagio- 
clase moulded on the orthoclase, but occasionally the former 
encloses in its centre small but well shaped crystals of the latter 
with Carlsbad twinning. 
The gravimetric proportion of plagioclase in these specimens 
is variable, showing, in the sections examined, 39.2 per cent., 26 
per cent., 36.5 per cent., 25.4 per cent., and 41.2 per cent, respec- 
tively of the total mineral content. 
Biotite in greenish pseudomorphs of chlorite, with slight 
pleochroism, low birefringence and straight extinction, occurs 
with hexagonal basal sections, and longitudinal sections the ends 
of which are both ragged and granular. Sometimes the colour 
is greenish-brown, and the pleochroism becomes more marked, 
showing green, greenish-yellow and very pale yellow. The edges 
are semi-opaque with what at first sight appears to be magnetite 
dust, but under a higher power this seems to be due to minute 
granules of epidote which mutually interfere. The presence of 
these microscopic grains in the interior of the mineral probably 
gives rise to the browTiish -olive colour that is occasionally pre- 
sented by sections under crossed nicols. Again, crystals are often 
bleached, and what seem to be thorns of quartz often appear 
between the lamellae. 
Next to chlorite, epidote is the principal product of altera- 
