540 H. HICKS ON THE PRE-CAMBRIAN 
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, furnishing 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-llesugn 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-llesugn 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. Porth-lisky and Ramsey-Sound Area. 
On the west side of the Dimetian the Pebidian beds lie at a high 
angle with a dip towards the N.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 
Fig. 13.—Sketch of Cliffs in Hast Harbour, Porth-lisky, showing 
contact of Pebidian with Dimetian, and line of Fuult between. 
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