File:The Quarterly journal of the Geological Society of London (12893678443).jpg

354
DE. C. CALLAWAY ON THE OKIGLN OF
.Aug. 1897,
parallel with the foliation are several veins of grey fine-grained
rock, some of which are a kind of halleflinta, while others are
foliated. They range in breadth from several inches to less than a
line. Some of the thicker veins show a gradation within an inch
or so between halleflinta and gneiss. Similar veins appear in the
floor of the quarry, and strike across the road, sometimes branching.
They thin out to a point. It is incredible that this grey rock should
be sedimentary, and it is almost equally difficult to believe that it
can be older than the diorite in which it is enclosed.
Fig. 1. — Gneiss of primary injection, Llanyaffo.
w - _______ E-
In thin sections some of the felsite (or halleflinta) exhibits the
minutely-granulated structure of the grey veins at Llangaffo

but
it contains numerous granules of epidote, the effect, perhaps, of
the chemical action of the enclosing diorite. Junction-specimens
show this felsite to be interlaminated with bright, clear horn-
blende (whether by veining or infiltration I cannot say), and
sometimes we observe the hornblende to alternate with fresh, clear
plagioclase in long prisms lying in different directions. It would
thus appear that in this locality there has been an actual fusion of
some of the constituents, resulting not only in the generation of
new minerals, but in the regeneration of the hornblende.
Turning into the field close at hand, we come to the typical
section (p. 356) of felsite graduating into grey gneiss, and we are
thus led to infer that the veins in the road proceed from the same
Section at Forth Gwyfen.
On the shore at this locality there is a very clear exposure of a
striped gneiss similar to the section shown in fig. 1. Grey gneiss
and dark-green schist run into each other in numerous thin bands.
The gneiss passes in places into halleflinta, and veins of the grey
rock, in the form of either halleflinta or gneiss, are also seen in the
green schist at a distance from the junction of the two rocks. In an
easterly direction the grey veins come in more and more abundantly,
till we reach a large mass of grey