E.. B. Tawney— Woodwardian Laboratory Notes. 19. 
were fluid-inclusions with moving bubbles. The rock may be classed 
with the felsites. 
[Cl. 1.] Sedgwick collection (lc. p. 458).—The mass H. of 
Llanaellaiarn has been already described by Prof. Bonney as rhyolite. 
It has been stated above that this group of igneous rocks has been 
mapped by the Geological Survey with the same colour as the Hifl 
hills, which are indicated as ‘ syenitic-felspar-porphyry,” and desig- 
nated as intrusive. Dr. Hicks has, however, in his sketch-map 
and description of the Pre-Cambrian outcrops (l.c. p. 297, 299), con- 
sidered all these not as intrusive, but as Pre-Cambrian rocks. 
We have now to add that our observations led us to note an altera- 
tion of Cambrian slates at different spots round the hills, where the 
shales can be seen sufficiently close. We traced the indurated shales 
S.E. of Bwlch Mawr, again N. of Girn Goch, while similar indurated 
beds may he found in the narrow col between the two Hifl hills. 
Guided by Sedgwick’s MS. Journal of 1831, I found the junction 
of porphyry with the slates on N. side of Girn Goch, above and to 
the right of a small waterfall, viz. at the top of the screes; the two 
rocks are here within a foot of each other; the slates are much indu- 
rated and crushed, being converted to a hard greenish-grey horn- 
stone-like rock scarred by ferruginous veinlets. 
There is therefore little doubt that the Geological Survey are 
perfectly correct in their mapping. The igneous rock must be later 
than the slates into which it is intrusive. As to age, it is probably 
Post-Arenig; but not having found fossils, I cannot give any exact 
date. 
Continuing in a southward direction, we come to the Hifl and 
Nevin groups. The rocks of the Hifl hills were described by Prof. 
Bonney (l.c. p. 805), from Dr. Hicks’s specimens, as quartz-felsite 
on §.H. side, rhyolite on EH. side, and N.W. of the Hifl near the 
coast (Trevor quarry) as possibly a granite-porphyry. The rock of 
the smaller Hifl (Tre’r Ceiri) encampment hill is also a felsite of 
pinkish-grey colour and minutely granular ground. Under the 
microscope it is seen to consist of a microcrystalline mosaic, with 
larger patches of quartz of irregular outline, never rounded crystals ; 
the few larger felspars are chiefly plagioclase however. 
Dr. Hicks’s specimen [III. l.c. p. 805] seems to have come from 
the Trevor so-called granite-quarries. The rock there varies; I 
have one darkish grey variety outwardly similar to that of Bwlch 
Mawr, except that there are dark greenish specks in it; others are of 
lighter tints, some pinkish, some light grey. The larger felspars are 
about 4 inch long; no separate quartz is visible to the eye. 
Though a granite-porphyry, there is a good deal of plagioclase in 
my slice among the larger crystals. A detached mass 8. of Vortigern’s 
valley (Nant Gwrtheyrin) is a rock with slightly different aspect. 
Its colour is pale grey, it has fewer separate felspars, but a finely 
granular texture throughout; it is well seen in the “set ” quarries. 
Microscopic examination shows it to be quite akin to the Trevor 
rock, the ground crystalline with abundance of small clear quartz 
grains mixed with felspar. Being less decomposed, we are able to 
