83

masses are pyroxenes of a rich green colour with well-marked parallel
striations. These are hard to distinguish from the olivines when small,
but in many cases occur in large crystals up to 13 inches across. The
olivines have the appearance of broken green bottle glass, whereas the
pyroxenes are not translucent. .

Under the microscope some masses of this material were seen to consist
nearly entirely of olivine, others carry up to 30 per cent of hypersthene.
Feldspar also occurs. Rare constituents of these masses are individuals of
biotite, magnetite, and green spinel. The olivines are in most places clear
and colourless; in others they are shot through with iron oxide in thread-
like films. The hypersthenes are green and pleochroic in tones of green
and golden yellow in thick sections; they are colourless in others. ‘Two
cleavages intersect nearly at right angles, but in many sections cleavage is
poorly developed and the mineral is traversed by irregular cracks. They
are negative in optical character with extinction angles up to 28 degrees.
In certain cases twinning has taken place in narrow strips along one of the
cleavage planes.. Walker and Collins! have described hypersthenes of this

character from the Hill Tracts, Vizagapatam district of India. A feldspar —

was determined as basic labradorite, Abso Anz, in one instance.

The basalt is dark brown, fine-grained, and holocrystalline, with

pilotaxitic texture and amygdaloidal cavities. It is made up of iron ore,
augite, olivine, and plagioclase. Quartz is present in one or two cases.
The magnetite forms 15 to 20 per cent of the slides. The augite is of a
reddish violet colour, in some cases colourless. So many fragments of
olivine scattered through the matrix are clearly derived from the hypers-
thene peridotite that it is difficult to distinguish those that crystallized as a
part of the basalt. These are small, of higher birefringence than other
minerals present, and are stained brown by iron oxide. The plagioclases
are andesines or labradorites in the few cases tested.

Certain fragments of hypersthene peridotite lying in the basalt have
been fused along the edges and sometimes the whole fragment has been
changed. In some places the outer edge of the altered mass is a ring of
violet-coloured augites, in others it has been altered to a highly birefringent
aggregate of small particles. Larger masses are not affected to so great an
extent and in some instances there is no sign of fusion at the contact.

On the northeast and steeper side of the hills (Figure 13, locality 3)
the actual contact between granite and lava is exposed at the foot of the
hill, where it strikes north 10 degrees east and dips 60 degrees to the west.
The granite is shattered and oxidized and the lava is dense at the contact.
It has the appearance of a part of a fissure from which the lava was extruded.
The immediately adjacent cliff which does not show bedding, but an
irregular structure dense toward contact and porous away from it, may be
the side of a lava dyke from which the quartz diorite has been removed.
Across the basalt area 1,500 feet west from this place, an olivine basalt
dyke cuts the quartz diorite striking south 35 degrees east and dipping 60
degrees west. It is 4 feet thick and trends away from the basalt area to
the north, and to the south successive lava flows can be distinguished lying
one over the other. In places there are pillow-shaped masses with layers

like the skin of an onion. Ropy and amygdaloidal lavas are common, but
no true ash beds were observed.

1Walker, T. L., and Collins, W. H., “‘Petrological study of some rocks from the hills of Vizagapatam district,
Madras Presidency.” Records, Geol. Surv., India, vol. XXXVI, pt. 1, 1907, pp. 14, 15.