Fig. 1.—Diagrammatie Section from South side of Porth Seli, Whitesand Bay, to Pen-y-Cyfrwy, S.E. of Nun’s Chapel. 
Cambrian. 
Pebidian. 
Dimetian. 
For explanation, see Index to Map, Pl. XXTY. 
Pebidian. 
Cambrian, 
H. HICKS ON THE PRE-CAMBRIAN 
breccias are undoubtedly older than the 
agelomerates, and are therefore placed in 
the map (Pl. XXIV.) in the Arvonian series. 
In the district to the west of the valley to- 
wards St. David’s there are no exposures 
of the rocks beyond the agglomerates which 
form the west hill. The only evidences we 
have as to the characters of the rocks in 
this area are those which have been ob- 
tained from the few wells that have been 
sunk. At the wells sunk at the Windmill, 
and at Glas-fryn, to the north, the only 
rocks found were fine-grained quartz-fel- 
sites like those found near the Church 
School, and some dykes of porphyritic 
quartz-felsites and diabase. I took pains 
to watch these wells carefully when they 
were dug. Last year, from another well 
opened in this area, a few hundred yards 
to the south of the Windmill, at a new house 
which is being built on the side of the. 
road leading to Caerfai, it has been shown 
that quartz-felsite breccias traversed by 
porphyritic quartz-felsites occur at that 
point. These breccias are of so compact a 
character that were it not for the occasional 
presence of foreign fragments in them, they 
could almost be classed with flows which 
had been much crushed (Note 16). These 
breccias I am inclined to consider as of 
Arvonian age, as they are undoubtedly be- 
low the agglomerates which seem to me to 
mark the base of the Pebidian on both sides 
of the ridge. The large proportion of a 
dirty quartz like that characteristic of the 
Dimetian, in many of these Arvonian 
breccias, seems to indicate that the Di- 
metian was nearly in its present condition 
before the Arvonian breccias were accumu- 
lated. 
2. Nun’s-Chapel District and Area S. of ~ 
St. David's. 
The Cambrian rocks are well exposed along 
the coast between Caerbwdy Valley and 
Nun’s Chapel; and the succession described 
to the S.E. of Caerbwdy is equally clear on 
the west shore of that creek up to the Mene- 
vian beds, which are the highest found on 
