414 
IIARKER : PETROLOGICAL NOTES. 
i 
grains. Sometimes a number of these little quartz-grains within the 
same felspar crystal have the same optical orientation; and elsewhere 
we find in the heart of a felspar plate a little ronnd patch of delicately 
branching micropegmatite. The felspar itself shows minute but rather 
shadowy twin-striation. The cross-striation characteristic of microcline 
is very general, but is sometimes confined to a portion of a crystal, 
usually the interior. The clear quartz has traces of minute fluid- 
pores, usually without bubbles. It invariably shows between crossed 
Nicols the strong ''strain-shadows" or "spectral polarisation" 
indicative of mechanical forces. 
No. 253 is a very fine-gi'ained grey granite, rather weathered. 
No. 158, of rather coarse grain, shows lustrous black mica, pink 
felspar, and greyish glassy quartz, and in a hand-specimen looks very 
like the rocks described in my former notes [942, 943]. The only 
other granitic rock sliced is from a boulder near the headland east of 
South Sea Landing, and is a biotite-granite of moderately coarse 
grain. The pink felspars are often in good crystals with Carlsbad 
twinning; the dark mica tends to aggregate in little nests or patches; 
the irregular quartz-grains are gi'ey to reddish in colour. 
Micro. [1047] Though the hand-specimen has no gneissic 
character, a slice shows under the microscope decisive evidences of 
crushing. The mica is often bent or buckled along " ghding-planes," 
and many flakes are torn to pieces. This mineral is mostly altered 
into a bright green, dichroic, and fairly birefringent substance agreeing 
with chlorite, and flakes cut nearly parallel to the basal cleavage 
exhibit many minute rutile-needles arranged parallel to three 
directions at angles of 60° to one another. The felspars are mostly 
microcline and microperthite, with a finely striated acid plagioclase. 
Of these the microline is, as usual, of later formation than the quartz 
of the rock. The last-named mineral builds clear grains, which 
between crossed Nicols give a somewhat indistinct extinction 
(" strain-shadows "). Other features in the slice pointing to the 
effects of crushing are an occasional veined and " cataclastic " 
structure, and the occurrence of little flakes of secondary white 
mica in the heart of the felspar crystals. 
Quartz-porphyries of various kinds are rather abundantly met 
J 
