544 H. HICKS ON THE PRE-CAMBRIAN 
thickness of d@ and with the addition of a part of series ¢. Here 
again the evidence of a regularly ascending succession in the 
Pebidian, but broken by a few faults, is perfectly clear, and there 
is no indication of an isocline. The beds dip regularly in one 
direction at a high angle, and the alternations are well marked 
and easily traced. The crags of Croeswdig, Trelethin, Rhosson, and 
Trefaiddan, consist of basic tuffs, dykes, and sheets. No. 29 occurs 
west of Rhosson rock, almost under the conglomerate. Further evi- 
dence of this fact is furnished in several sections to the north. The 
only one to which I need refer, however, though I have collected evi- 
dence all over the district, is the one to the north of St. David’s, in 
the Allan valley. Clegyr Foig rock has been frequently mentioned ; 
it consists mainly of basic sheets or dykes like those at Pen-y-foel, 
which occur in the felsitic tuffs, series 0. 
7. St. David’s and to the North-east, and Allan Valley to the North. 
The ridge of St. David’s and to the north-east of that city consists 
mainly, as already stated, of quartz-felsites (spherulitic) and of 
numerous dykes of porphyritic quartz-felsites and some breccias. 
These rocks are found at various points all along the area coloured as 
Arvonian to the N.E. of Lianhowel. In the quarries on the hillside 
to the N.E. of the Cathedral, and in the Church- and Board-Schools 
quarries, they are well known. There are many minor exposures 
also about the town. To the N.E. of St. David’s I have collected 
specimens in exposures near Dowrog Pool (Note 11), at Harglodd- 
issaf, Hendre, Carfarchell, &c. I have also recently received some 
beautiful specimens of a porphyritic quartz-felsite from Trefadog, 
through the kindness of my friend Capt. Griffiths of St. David’s. 
So far, therefore, as this portion of the area can be examined, the 
quartz-felsites form a very continuous belt in a N.E. direction from 
St. David’s to the great fault above Bryn. ‘The north edge of this 
belt is faulted probably at many points, and it is hidden greatly by 
the moors which extend along in that direction. The exposures of 
the Pebidian rocks along the area show them everywhere dipping 
away from this central belt. The Allan Valley to the north of 
the Cathedral gives a very fair section of the Pebidian rocks, dip- 
ping continuously to the N.W. Immediately north of the Board- 
School quarry the porcellanites, Note 23 (like those in Caerbwdy 
Valley to the 8.W. of the central belt), are seen with a moderately 
high N.W. dip. Between these and the quartz-felsites there is, I 
| believe, a fault which has dropped the lowest agglomerates as at 
Porth-lisky. The next beds in a N.W. direction are some greenish 
and purplish felsitic breccias (Note 23), and some argillites belonging 
to series 6. These are succeeded by quartz-felsites in a schistose 
state, and some acid breccias. The may be examined in quarries 
opened lately on the west side of the river, in the fields belonging to 
the new vicarage-house at Penrhyw; also on the east side, in quarries 
in a field nearly opposite, through which the path leading towards 
Emlych passes (Note 32). Beyond this, at and about Emlych, there 
