290 H. HICKS ON A NEW GROUP oF 
Hill, the base beds of the Pebidian, which are made up chiefly of 
masses of Dimetian rocks, of quartz-felsites, spherulitic felstones, 
and halleflintas, and all im the condition in which they are now 
found composing the underlying ridges. From this evidence it is 
tolerably clear that the position of the Arvonian or hilleflinta group 
is intermediate between the Dimetian and the Pebidian, and that 
there is, at least in this area, very clear proot of unconformity, 
and hence of a lapse of time having intervened. 
From this point to near Llanhcwell there are but few exposures ; 
but it is clear from the rocks that I have been able to examine that 
they are much of the same character as those nearer St. David's, 
an alternating series of true halleflintas, breccias, and quartz-felsites 
usually striking across the axis. Near Caervoriog Bridge, the Pebi- 
dian agglomerate is made up in some cases almost entirely of masses 
of halleflintas. At and about Llanhowell the rock is of a lighter 
appearance than further south, and a section has been examined 
(No. 8, Appendix). 
Beyond this, and extending as far as the great HE. and W. fault, 
hallefiintas and breccias, with some quartz-felsites, are again the pre- 
vailing rocks (vide No. 9, Appendix). 
All along the flanks of this axis on either side and dipping away 
from it are found either Pebidian or Lower Cambrian rocks, the 
former everywhere along the N.W. and nearly along the whole 
of the §.E. side. The N. end is cut off by the HK. and W. fault, 
and Upper Cambrian and Lower Silurian rocks have here been 
brought against it. Two other masses of Arvonian rocks are found 
in a line to the east of the north end of the ridge; and these in their 
lithological characters are almost identical with those described from 
the axis. 
The prevailing characters in the three formations of Pre-Cambrian 
rocks now made out in Pembrokeshire may be briefly defined as 
under :— 
a. Micaceous, talcose, and chloritic schists, with slaty and 
. q z . . . . 
| massiye green bands containing epidote, serpentine, 
Pebidian ... 4 &e. 
| 6. Tuffs, indurated ashy shales, breccias, silvery schists, 
porcellanites, conglomerates, and agglomerates. 
Aryonian... Breccias, halleflintas, and quartz-felsites. 
gilt He Quartzose rocks, granitoid gneiss, and compact granitoid 
Dimetian.. 5 eae 
rocks with bands of crystalline limestone. 
Speaking generally, it may be said that the state of alteration or 
metamorphism exhibited by the rocks in each of these formations is 
in proportion to their age, the Dimetian being the most highly 
altered, and the Pebidian the least so. 
This, however, will not apply to cvery member in each group ; for 
some materials, as is well known, tend to change more readily than 
others. For instance, deposits which are of chemical origin, or 
even partially so, naturally assume a more crystalline appearance 
than those entirely made up of detrital materials. Again, sediments 
largely made up of volcanic materials readily put on a metamorphic 
