ROCKS OF PEMBROKESHIRE, 529° 
In the next creek in this area, called Porth-melyn, to the south- 
west of Nun’s Chapel, the Cambrian conglomerates contain more of 
a true arkose material than perhaps in any other part of the district. 
The yellowish decomposing substance, mainly felspar, so abundant 
in the conglomerates here, and in the immediately overlying sand- 
stones, has given the name of melyn (yellow) to the creek. A slide 
made from some of this is described by Mr. Davies, Note 49. This is 
the only peint in the district where I supposed, as shown in my pre- 
vious maps, that the Cambrian conglomerate was in contact with the 
Dimetian without the intervention of a fault. The actual junction 
was not seen, but the matrix of the conglomerate and the material 
forming the sandstone, seemed to point to such a conclusion. Now 
that more attention has been given to the Cambrian series in this 
creek, it is abundantly clear that the Dimetian was the main source 
from which the finer sediments here were derived. Small pebbles 
also of typical Dimetian in any number may be found in the grit, 
and an arkose so identical in general appearance with the Dimetian 
itself is found on the west side of the creek, at a point called 
Chanter’s Seat, that it would be impossible, I think, for any one 
to doubt about its origin. The nearest point to the conglomerates 
at which I have been able to detect the Dimetian, and that only 
because of a deep drain made in the field above Porth-melyn about 
three years ago, would be at a distance of about thirty yards. This 
drain was particularly important and interesting, in that it exposed 
two well-marked dykes of a porphyritic quartz-felsite (Note 10) 
identical in character with the one found in the cliffs to the south 
and south-east of Nun’s Chapel, which is indicated in Dr. Geikie’s 
section as an arm sent out from the Dimetian, here actually cutting 
through the Dimetian, with as sharp lines of demarcation as are 
usually observed where dykes cut through any of the newer beds. 
I took pains to notice these carefully at the time, and to secure 
specimens. The drain reached the whole length of the second field 
to the north of Porth-melyn, with an average depth of from 4 to 6 feet, 
therefore deep enough to expose good sections. A small part of this 
drain is still open at the upper end of the field, where a portion of 
the quartz-felsite dyke 1s exposed, as also a dyke of diabase. The 
Dimetian was of the ordinary type, quite in its natural condition, 
between these dykes and the Cambrian conglomerates. The Cam- 
brian rocks exposed on the west side of Porth-melyn are identical 
with those found in each of the headlands on this part of the coast, 
but the highest beds (No. 5) in the section (fig. 4), map, and plan 
(fig. 3), are specially interesting, as having yielded the richest 
fauna belonging to that series anywhere in the district. The species 
Plutonia Sedgwieku, Conocoryphe Lyell, Paradoxides Harknessit, 
&c. were first found here. The fossiliferous beds are of a rather 
dark grey colour, and cross the point where the figure 5 is placed 
in the section, fig. 4. The beds below are mainly grits and sand- 
stones, and it is in these that Dimetian pebbles occur so abun- 
dantly. The place marked Chanter’s Seat (fig. 3) can be readily 
examined, and the beds at this point are very rough grits, almost 
