306 REV. J. F. BLAKE ON THE 
of a small quantity of graphite) which also runs in a kind of 
horizontal vein through the porphyry. This rock has a very strong 
resemblance to those portions of the felsite below Nun’s Chapel 
which are destitute of spherulites. There is the same crystalline 
ground-mass, though the Ramsey rock is clearer and has less 
secondary quartz, and the same abundance of pyrites and of viridite. 
In the Porth-Hayog examples, however, there are more of the 
rounded quartz crystals, as in the Church-School quarry, and still 
larger crystals of felspar of either kind. There can be no doubt we 
are dealing with a rock of the same group. 
On the other side of the fissure, which is undoubtedly a line of 
fault, we have vertical glassy-looking rocks with a well-marked 
banded structure. This I at first thought due to the pressure and 
dragging which were concerned in the faulting ; but the microscopic 
structure rather contradicts this. Innumerable lines are seen under 
ordinary light running somewhat irregularly parallel to each other ; 
but under crossed Nicols the minute crystals are often quite 
independent of these, though others which seem to be felspar 
have their axis in the same direction. We must regard this, then, 
as exhibiting a typical fluidal structure, with subsequent devitrifi- 
cation. The truly crystalline character of the constituents, and 
their only partial dependence on the parallel lines forbids the idea 
of bedding. 
This rock is succeeded by a series of agglomerates, ashes, and 
tuffs, finally bounded by a broken fault which has let down the 
fossiliferous Arenig rocks into their midst. The only points of 
interest in this ashy series are the intrusion into it of a tongue of 
true amygdaloid, in which nothing but narrow felspar crystals 
in an isotropic ground-mass is now seen (but the cavities and cracks 
are lined and sometimes even filled with secondary quartz), and 
the occurrence amongst the tuffs of a bed of black peperino. 
Such a group of rocks as this is as far removed as possible from a 
sedimentary series, and is typically volcanic. 
The north side of Porth Hayog forms the southern boundary of 
the mass which continues hence to Aber-Mawr and culminates in 
two peaks called carns. All that is seen of this in Porth Hayog 
consists of the felspar porphyry, in parts rotted by trickling water 
into an arkose. The same material forms the bulk of the carns, 
which are likewise surrounded on the eastern side by a broad band 
of ashy rocks becoming in places a veritable breccia, but composed 
essentially of the materials of the felsites or porphyries. In Dr. 
Hicks’s collection in the museum of the Geological Society is a speci- 
men with a fragment of the banded rock. On the eastern side, in 
limited areas which seem to contract towards the summits of the 
carns while expanding to form distinct headlands, are the most inter- 
esting rocks in the whole district. The most southern is distinctly 
glassy in aspect, and shows even on the large scale lines of flow 
which are occasionally contorted in the manner characteristic of 
semiliquid matter subjected to varying forces. Under the microscope 
with polarized light the glass is seen to be devitrified; but with 
