ROCKS OF PEMBROKESHIRE. 517 
rocks, the halleflintas, and the old rhyolites of the South Pembroke- 
shire areas is proved beyond doubt to be Pre-Silurian (Upper Silurian 
of Survey), and by every reasonable probability to be Pre-Cambrian. 
¢. Probable Origin and Petrological Characters.—That many of the 
rocks included in the Arvonian group must have had a volcanic 
origin is certain. Acid lavas and ashes prevail in it in all the Pem- 
brokeshire areas; but associated with these occur some indurated 
argillites and quartz rocks. The doubtful rocks called halleflintas *, 
so far as the microscopical and field-evidence can at present 
decide the question, appear to be mainly made up of fine volcanic 
ashes or muds. Sometimes fragments, chiefly of quartz, are recog- 
nizable in considerable abundance in parts of a slide, while the 
other portions are homogeneous. The rock appears as if it had 
been in a colloidal condition, but with fragments still remaining in 
parts unchanged. 
Of course another very natural view concerning them would be 
that they were originally rhyolites which had subsequently undergone 
great change. As the old rhyolites, however, which are found in 
association with these rocks still retain marked evidences of their 
original character, it seems a fair presumption to think that the 
hilleflintas had some other origin. These halleflintas were first 
recognized by Mr. T. Davies, when examining some slides for me 
for the paper communicated to the Society in 1879, as being of the 
peculiar nature characteristic of the rocks so well known under 
that name in Sweden. There, I believe, they are classed as sedi- 
mentary rocks. In Pembrokeshire they are mainly limited to 
the Arvonian group, and are found in each of the areas where 
rocks of that age are exposed. They are particularly well shown 
in the gorge of the Cleddau, between Haverfordwest and Fishguard, 
about Trefgarn, and also at and to the east of Roch Castle. But 
in the areas between the latter points, named Dudwell and Plum- 
stone Mountains, true breccias, beds of quartzo-felspathic ash, and 
old rhyolites are the prevailing rocks (Notes 80 & 81). The halle- 
flintas will be fully referred to by Mr. Davies in Notes 73 to 79, and 
also those parts of them containing the peculiar replacements of fel- 
spar crystals by quartz, noticed by Prof. Blake in a slide from Roch 
Castle, and referred to by him in his paper read before the Society 
in January last +. Mr. Davies supposes these replacements to have 
taken place mainly in the brecciated portions; and as these rocks 
have been greatly fissured and completely permeated by secondary 
quartz, such a change could there readily take place. A fine 
quartzo-felspathic ash, showing distinct lines of lamination, makes 
up a considerable proportion of this group in Ramsey Island, in 
association with breccias and old rhyolites (Notes 12-14). The 
descriptions given in my previous papers as to the special characters 
* The term halleflinta is used generally for a compact rather flinty-looking 
rock, which in chemical composition does not differ much from one of the more 
acid varieties of rhyolites, and may be either one of these considerably changed, 
or an altered sedimentary rock. 
+ Quart. Journ. Geol. Soc. vol. xl. p. 308. 
