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CAMBRIAN SERIES IN N.W. CAERNARVONSHIRE. 311 
mass of the conglomerate, which he describes, on the summit of the 
hill, contains pebbles of the same purplish quartz-felsite, generally 
from about 2 to 4 inches diameter, but sometimes a foot or more, 
together with angular fragments of purple slate. On the western 
side about one fourth of the fragments are felsite; the remainder 
slate, green and purple, and a dull-green grit resembling one in the 
underlying series. The fragments of purple slate are rather more 
numerous on the eastern side. The strike of the conglomerate 
appeared to be about H.N.E. and W.S.W. ‘The cleavage dipped at 
a high angle to 8.8.W., and the bedding, as it seemed to us, dipped 
to the N.N.W., also at a high angle; but as many of the smaller 
pebbles had been twisted so as to have their longer axes to the 
planes of cleavage, it was difficult to be sure of this. I have ex- 
amined microscopically two specimens from the adit ; but it will save 
time to describe them with the specimens from Llyn Padarn. The 
quartz-felsite was abundant in the loose blocks below Rhos Tryfaen ; 
but as I did not meet with a good exposure of live rock, I have not 
examined it microscopically. In general character, however, it is 
identical with those described hereafter. 
We crossed the massif again by the road from Bethgelert to Caer- 
naryon. The felsite is well exposed left of the road north of Bettws 
Garmon. Here it is rather pale in colour, and shows traces of 
cleavage. Subangular patches of a darker colour suggest at first 
sight included fragments ; but a closer examination shows that they 
are merely due to slight variations in structure. This somewhat 
streaky or mottled aspect is not uncommon in modern trachytic 
lavas*. Microscopic examination shows that the rock has a crypto- 
crystalline ground-mass of rather variable character, the structure 
being sometimes exceedingly minute, containing crystalline grains 
of quartz and felspar—part is orthoclase, closely resembling sani- 
dine, part plagioclase. Microlithic enclosures are common in the 
felspar, some looking as if they had once been glassy. There are a 
few grains of iron peroxide and some disseminated opacite. <A por- 
tion of the slide shows flow-structure very clearly. The rock seems 
to have been crushed ; a very minute gold-coloured mineral appears 
in the cracks, and is disseminated about the slide. 
Llyn-Padarn District. 
We examined the quartz-felsite on both sides of the lake, on the 
western along the highroad, as well as by the railway. ‘The litho- 
logical details are given below, and with them, in order to save time, 
I have included a description of specimens from the northern end of 
the second massif. The following facts, however, noted in the field, 
of themselves, I think, are hard to reconcile with any theory of 
metamorphism or intrusion. 
(1) The streaky structure so characteristic of lavas (especially of 
an acid type), as well as the platy jointing common in igneous rocks, 
may be seen not seldom in the quartz-felsite about Llyn Padarn. 
The most striking example is in a quarry by the roadside north of 
* The “piperno” of the Phlegrzan Fields is a marked Se 
Y 
