CAMBRIAN SERIES IN N.W. CAERNARVONSHIBE, olla 
grains (angular and rounded) of quartz, felspar (some plagioclase), 
and a rock resembling the quartz-felsite with much interspersed 
ferrite. 
Descending from Brithdir in a north-east direction towards the 
Bangor road, we pass a greenish slaty rock, and on reaching the 
bed of the valley find shallow excavations in a similar rock in the 
angle between the main and the by-road. ‘This latter is much 
crushed and jointed, as if some intrusive rock were near; but the 
dip (which varies) seems to be H.N.E., or a little to the east of 
that, at a high angle*. Bearing to the east we find a quarry among 
woods at Tair-ffynnon. The rock here varies from a slaty grit to 
an agglomerate or conglomerate of fragments of quartz-felsite and 
slate in a green matrix. I have examined slides of each variety. 
They consist of lapilli or fragments of igneous rock, broken crystals 
of felspar and quartz, and bits of slaty rock. Some fragments 7, 
especially in the coarser variety, have a general resemblance to the 
quartz-felsite of the adjoining massif, but most of the igneous con- 
stituents are rather different. There are of these three well-marked 
varieties :—(a) glassy or nearly glassy, the base often opaque from 
abundant opacite, with many acicular felspar microliths and spots of 
viridite, probably replacing augite or hornblende; (6) a more glassy 
and transparent rock, with acicular microliths and larger felspar 
crystals, up to 0:03 in. diameter, sometimes certainly plagioclase § ; 
(c)a fairly clear rock, with glassy aspect and partial cryptocrystalline 
structure ; iron peroxide in larger, but less frequent, grains. Most 
ot these are probably lapilli. The finer rock is chiefly composed of 
fragments; the coarser has a good deal of matrix, cemented by 
viridite and a very pale green mineral, which, with crossed Nicols, 
shows fibrous structure and bright golden-green and orange-red 
colours. The materials of this rock are largely derived from volcanic 
ejecta, and some apparently have not come from far; but I do not 
think it bears decisive evidence of contemporaneous volcanic action. 
Further to the east, by ascending a hill on the northern side of 
the road, we find both inside and outside the wood a great quantity 
of a similar rock. Its microscopic structure also corresponds, some 
of the included fragments showing a very distinct flow-structure. A 
little more to the east, and nearer Perfyddgoed House, the rock be- 
comes so compact that it might be easily mistaken for a quartz-felsite 
(greenish grey). Microscopic examination, however, shows it to be 
beyond doubta clastic rock, with many small subangular quartz 
grains, and minute rolled fragments of various compact rocks, some 
of which are certainly of igneous origin. One is almost a mass of 
opacite. 
To the north-east we find other fine felsitic grits and ashy-looking 
rocks, and at Cae Seri a coarse conglomeratic rock, somewhat resem- 
bling that of Tair-ffynnon, but with more slaty fragments of a dull 
* On the opposite side of the fault a purplish slate seems to succeed the grit 
by the Caernarvon Road, and a prominent knoll to the N.E. consists of slaty 
rock dipping a little N. of E.N.E. at an angle of 50°. 
+ Plate XIII. fig. 2. { Plate XIII. fig. 4. § Plate XIII. fig. 5. 
