ROCKS OF CAERNARYVON. 685 
Fig. 5.—North-east end of Field Quarry, Twt Hill, Caernarvon. 
c'. Lower Caernarvon beds. 
* Bed of fine-grained quartzose rock, 
On the two bosses which occur N. and N.E. of Twt Hill there is 
a brownish or yellowish-pink felsite, generally showing a platy 
structure. This should be distinguished from the Twt-Hill rock. 
In the large quarries on the N.W., facing the Straits, there is an 
altered grit. If the more obvious divisional planes represent 
bedding, these dip in a northerly direction, and are the highest beds 
there seen ; they would necessarily be thrown back out of sight by 
the fault seen in the roadside quarry, fig. 3. 
Close to the great fault which brings on the Carboniferous rocks 
near Tycoch there is a crystalline grey felspathic rock, much 
crushed and traversed by slickensides, the surfaces of some of which 
are glossy, black, and anthracitic-looking. This rock probably 
belongs to some part of the Caernarvyon series. 
I. ii. Crug Beds. 
On the S.E. side of the ridge we find strong proof that a group of 
beds, somewhat similar to the Twt-Hull rock in essential characters, 
but finer, alternates with felsite-lke rocks or passes into them ; and 
though nothing like dip can be made out in them, the strike, as 
followed along the ridge (see map, Pl. XX XVI.), from about half a 
mile N.E. of Pengwrach to the brow of the hill above Dinorwig 
station, shows that they run nearly with the trend of the hill, 
with perhaps a little more north in them, and generally somewhat 
dragged over towards the fault along the Carboniferous beds. 
On the boss N.W. of Pengwrach we find on the N.W. side crystal- 
line beds, like the finer portions of the Twt-Hill group; and on the 
S.E. side of the same boss a felsite-like rock. A precisely similar 
boss occurs close to Crug, with a coarser rock on the N.W. and a 
felsite-like rock on the 8.E. In the road between the two places 
rock of the coarser variety is exposed. 
In the field N.W. of Bryn finer and coarser varieties of the same 
kind of rock are exposed in bosses projecting through the soil, while 
immediately over the wall on the N.W. the coarse granitoid rock of 
Twt Hillisseen. A grey erystalline felspathic rock much seamed 
by quartz veins forms the most prominent point in this field on the 
N.E. of the house. 
Still further on to the N.E. in the same line, in the lower part of 
the little valley which runs down from below Tynymaes to about 
half a mile E.N.E. of Plas Llanfair, there are alternations of beds of 
finer and coarser crystalline rock, which must be referred to this 
group. One pink bed we find again, almost identical in character, 
Q.J.G.8. No. 140, 3A 
