i 
E.. B. Tawney— Woodwardian Laboratory Notes. 209 
to the S. end of the Penarfynydd Promontory at Trwyn Talfarach—- 
indicated as Greenstone on the Survey Map—I found at least two 
distinct varieties of rocks: the blacker one I imagined to be the one 
I was seeking; but on comparing it with the slide described by 
Prof. Bonney, it is seen to differ, as he points out, by the greater 
abundance of olivine and the absence of felspar. Hence the question 
whether it has a separate existence or its relations to the rocks 
_ described below must be left to future investigation. 
First, round Plas Rhiw and Treheli, where worked for “sets,” it 
is a gabbro-like rock of medium grain. This continues through 
the ridge of Mynydd y Graig, not varying much locally with one 
exception. If one classed it by the outward appearance, it would 
probably be called gabbro from its brown colour, in contrast with the 
pronounced green colour of most of the diabases of the district. 
Microscopical examination shows, however, that the pyroxene has 
not well developed pinacoidal cleavage as typical diallage should 
have. To be logical we must therefore place it with the diabases. 
The second rock is very similar in appearance to that collected by 
Sedgewick from “below Penarfynydd,” but which we were not 
fortunate enough to identify in siti. It is an olivine-diabase, and 
holds over one side of Mynydd Penarfynydd ridge, being of suffi- 
ciently different appearance to the diabase, which forms the other 
half of this promontory. 
It will be well first to describe its position with more detail. 
Mynydd Penarfynydd is a ridge about three-quarters of a mile long, 
running N.N.E —S.8.W., coloured Greenstone on the Geological Map 
and connected withthe rest of the greenstone mass by a narrow 
link. This connexion is of diabase, the extreme end of the pro- 
montory on the 8. is also of the same diabase, as is also apparently 
the whole E. face of the ridge. Roughly speaking, the blacker or 
serpentinous olivine-diabase forms the W. slopes of the ridge from 
the crest downwards. It is seemingly then bounded by the diabase 
on two sides in such a way as to suggest that one is intruded into 
the other. As the ridge is covered by turf and heather, it is only 
possible to trace the line of junction by the blocks which occasionally 
protrude from the soil, many of which seem detached, but the line 
probably corresponds with the crest of the ridge. 
In places where the olivine-diabase crops out it has a most delusive 
resemblance to a bedded rock—harder bands standing out with 
depressions between—the strike of these bands being N. 20° E., and 
dip at a moderate angle. On the W. slopes of the ridge this is very 
well seen also, little cliffs of olivine-diabase there weathering into 
apparent thick and thin beds, with planes of coarser and finer 
crystals, or harder and softer bands, which produce the effect of a 
weathering sedimentary series. 
At the time I was disposed to think that the olivine-diabase had 
come up obliquely as a thick dyke inclosed between the diabase on 
the one side and the fossiliferous beds on the other, and that the 
harder and softer planes were parallel to the supposed inclined walls ; 
but the case can scarcely be considered apart from others which 
DECADE II.—VOL. VII.—NO. V. 14 
