128 WHITE : OATLAND COMPLEX OF IGNEOUS ROCKS. 
The appearance is similar to that of the same species in the 
more basic acid types of rock. Individuals are sometimes pierced 
by thorns of quartz like the green biotite in the acid rock. The 
broad cleavage bands would perhaps evidence a scaly character 
in the altered mineral like the biotite in the plagioclase-augite 
rock at Tolcarn, near Penzance, where it has been altered by the 
granite. 
The included iron -ores are not quite opaque at their extreme 
margins, which show a muddy greenish-brown colour. Where 
crystals of the biotite have been bleached to a greenish-brown 
■colour the iron -ores are not nearly so plentiful. 
Hornblende is found in two varieties, as in the basic rock, 
but the brown variety is usually tinged with greenish shades, 
and it occurs in varying quantity with the pale green variety, 
though the proportion of total amphibole is remarkably constant 
in all the specimens examined, remaining at 21 per cent, to 23 
per cent. The darker variety, with pleochroism = dark green 
— light green — brownish-straw, often lies embedded in the 
quartz while the hornblende surrounding the augites is lighter 
in colour, with pleochroism = bluish-green — brownish-yellow 
and pale straw. 
In the basic xenolith above mentioned the hornblende often 
occurs as large individuals, sometimes twinned. The extinction 
angle is about 15 degrees. 
Augite has characters like those in the basic rock. One 
chloritic pseudomorph appears to have a distinct " herring-bone " 
structure. The outlines are usually regular and hexagonal, and 
the average dimension is 2.5 mm. by 2.5 mm. There are some 
colourless patches of unaltered malacolite. The products of 
alteration are usually uralite, chlorite and calcite, the latter 
along cleavage planes, and a little iron-ore in minute grains. 
In the basic xenolith, particular crystals of augite show 
interesting features. One individual 2.6 mm. by 1.82 mm. has 
within the outer zone of chlorite a patch of uralite about 1.45 mm. 
by 1.45 mm., surrounded by a narrow zone of magnetite dust 
and hornblende with a pleochroism — brownish-green — light - 
green-straw. In the heart of the crystal is a patch of unaltered 
