516 H. HICKS ON THE PRE-CAMBRIAN 
Dayies in notes 17 and 18, and I shall have occasion again further 
on to refer to them. 
In the Pebidian breccias on the west side of the ridge, frag- 
ments of these old rhyolites, of the hilleflintas, and of the indurated 
argillites are constantly found; but probably the most important 
series to prove that the rocks I call Arvonian are older than those 
named Pebidian are the great agglomerates to the north and south 
of the Llanhowel ridge. At Llanhowel and reaching northwards 
to Carnymyl old rhyolites with flow-structure and sometimes sphe- 
rulitic, alternating with bands of hialleflintas and fine acid breccias, _ 
are constantly met with. These are flanked at Treglemais (as is 
-especially well seen in a field at a point in the Ordnance map 
indicated by the letter ¢ of Penbont) by agglomerates containing 
fragments of all those found in the axis, many in a rounded or 
rolled condition. On the south side of the ridge in the Caerforiog 
Valley, as shown in the map, similar agglomerates are found con- 
taining large fragments of like character to those above described. 
The evidence at these points that the rocks forming the central part 
of the ridge must be of older date than the agglomerates is in my 
opinion most conclusive, and the latter are, as far as can be made 
out, the oldest Pebidian rocks in the district. The beds which 
immediately succeed the latter are porcellanites like those on both 
sides of the ridge at St. David’s. The peculiar grouping of the 
rocks found in each of the Arvonian areas is an interesting point, 
and will be further referred to in speaking of their probable origin 
and special petrological characters. 
b. New Areas in Pembrokeshire.—Since my paper in 1879 was 
read, | have examined an area in South Pembrokeshire not far from 
Milford Haven, and to the east of a place called Rosemarket, where 
rocks of a type in many respects resembling those in the Arvonian 
group occur. It is only a mile or so to the south of the Johnston 
ridge, where, as I have already pointed out, rocks of the Dimetian 
type occur ; between these also another short ridge of granitoid rocks 
is found. It therefore appears tolerably clear that by the combined 
influence of denudation and faulting a considerable portion of the 
old Pre-Cambrian floor has become exposed in South Pembrokeshire, 
and that wherever it occurs in these areas the types of rocks forming 
it are similar. Three well-marked areas of elevation are thus indi- 
cated in Pembrokeshire—viz. (1) the district of St. David’s proper ; 
(2) that of Brawdy, Hayscastle, Roch, and Trefgarn towards the east ; 
and (3) that of Talbenny, Johnston, and Rosemarket in the south. 
The Paleozoic rocks are troughed, folded, and faulted-in between 
these ; and wherever the basal rocks of the Paleozoic sediments are 
exposed in these areas it is found that they contain materials which 
could only have been derived from rocks similar to those we claim to 
be in these areas of Pre-Cambrian age. I was particularly struck 
with the undoubted arkose character of the grit of Llandovery age in 
the Johnston and Rosemarket area, and small fragments of the 
granitoid rocks and of those of the Arvonian type occurred also in 
it in considerable abundance. The age therefore of the granitoid 
