ROCKS OF PEMBROKESHIRE, 519 
date than the Cambrian ; that they had been uplifted to form shore- 
lines, and very much in their present mineral conditions, before any 
of the Cambrian sediments were deposited. 
Their age as compared with the Dimetian and Arvonian groups is 
almost equally clear; for materials from each of those groups appear 
to be present in considerable abundance in the lowest beds of the 
- Pebidian, according to the evidence furnished by microscopical ex- 
amination. The quartz and felspar fragments, as will be mentioned 
by Mr. Davies in Notes 20, 22, &c., have all the characters which 
we consider peculiar to the Dimetian; and fragments of the hille- 
flintas, of the compact quartzo-felspathic ash-beds, of the indurated 
argillites, and of the old rhyolites of Arvonian type occur in them 
in abundance. The section at Porth-lisky is also very clear in 
showing some of the lower beds of the Pebidian overlying the 
Dimetian. 
b. Origin and Petrological Characters.—The special characters 
exhibited by this group have been very fully referred to in my 
previous papers, and Dr. Geikie, in his paper, says that he accepts 
‘generally the lithological descriptions” given in my papers. A 
very large proportion of the rocks in this group are of volcanic 
origin, but along with these there are some which offer ample evi- 
dence to show that ordinary marine sediments were also deposited 
during the period. On the Geological Survey maps the Pebidian 
rocks are coloured either as altered Cambrian or as intrusive green- 
stones. The Director-General, in his paper, p. 284, says that his 
predecessor, Sir A. C. Ramsay, in his original map and section, and 
in his early MS. report on the St. David’s area, recognized the vol- 
canic nature of these rocks, but that he “ afterwards allowed this 
view to be set aside in favour of the opinion that the peculiar 
bedded rocks on the west side of the granite ridge are altered Cam- 
brian strata through which intrusive ‘greenstones’ have been in- 
jected.” He further states that it is the latter view which is 
expressed upon the second and latest edition of the Survey map 
and section, and acknowledges “that in this respect the present 
map and section are seriously in error, and that Dr. Hicks deserves 
the thanks of geologists for having, as it were, rediscovered pro- 
bably the oldest group of Paleozoic volcanic masses yet known in 
this country.” ‘The only maps and sections of the Survey available 
to me were those referred to above by the Director-General as 
seriously in error, and I had no means of knowing Sir A. Ramsay’s 
previous interpretation. It is, however, satisfactory to think that 
they are now likely to be corrected on that point, though I con- 
sider that this is but a trifling matter as compared with the other 
corrections necessary to make the maps intelligible, or to give in 
any way a fair interpretation of the knowledge which has been 
gathered together concerning the rocks of this area during many 
years past. 
c. Thickness of the Growp.—The thickness assigned by me to 
this group has been contested by the Director-General, and in his 
paper he has endeavoured to show that the same beds are repeated 
