File:The Quarterly journal of the Geological Society of London (13889246257).jpg

540
H. HICKS ON THE PRE-CAMBEIAlf
Evidence bearing on another important question already touched
upon is furnished in the face of the cliffs here, as not only has the
Dimetian been penetrated in all directions by diabase dykes, but also
by dykes of porphyritic quartz-felsites,like those described in Porth-
clais farmyard, in the fields near Porth-melyn, and as cutting across
the Pebidian rocks near Nun's Chapel, famishing additional proof
against the view that the porphyritic felsites are in any way to be
associated with the Dimetian rocks as peripheral masses. A slide
from a specimen taken from one of these dykes in the Dimetian at
Ogof-Uesugn is described in Note 7. I have not found any of
these porphyritic quartz-felsites cutting actually into the Cambrian
rocks, but diabase dykes, differing somewhat, however, in appearance
from the majority of those found in the Pre- Cambrian rocks, occur
plentifully in them. The alteration produced by the diabase dykes
in the beds they cut across, and the welding which takes place,
may be studied with advantage in the cliffs along the coast between
Ogof-Uesugn and Porth-clais. One of these dykes is about 40 feet
across, and the alteration produced by it in the rocks on either
side is most marked.
5. PortTi-lisky and Ramsey -Sound Area.
On the west side of the Dimetian the Pebidian beds lie at a hio-h
angle with a dip towards the jST.W. Immediately at their point of
contact with the Dimetian there are the usual indications of a
fault, and doubtless a considerable thickness of the lowest beds has
been dropped by it. That these are some of the lower beds of the
Pebidian, not to be confounded with those under the conglomerate
in Ramsey Sound, with which they are correlated by Dr. Geikie,
is perfectly clear from the stratigraphical evidence here and at many
other places in the area further north ; the microscopical evidence
also is most conclusive on this point. The sketch, fig. 13, shows
Pig. 13. — Blcetch of Cliffs in East Harbour^ Porth-lislcy ^ showing
contact of Pehidlan ivith Dimetian, and line of Fault between.
F. Fault.