324 ‘7, G. BONNEY AND F. T. S. HOUGHTON ON THE METAMORPHIC 
with afew of jasper), the matrix being a purplish-brown quart- 
zose grit, apparently not more highly altered than those at the 
base of the Cambrian series. Strike difficult to obtain, but pro- 
bably W.S.W. About a mile beyond this, along the road, is a 
quarry * on the left hand, showing rock of the normal Twt-Hill 
type, but rather fine in texture; and further on, in a farmyard, 
“ Parciau Isa,” a similar rock. The same may also be seen off the 
main road, halfway between Griffith’s Crossing (Llanfair-is-gaer of 
map) aud Port Dinorwig. 
Rock of the same general type, granitoid, and consisting mainly 
of quartz and felspar, with sometimes a slight approach to a foliated 
structure, extends to ‘‘ Elim Chappel,” a little beyond Port Dinorwig. 
in this place, where the railway crosses the main road, is a good 
exposure of the above rock. Mr. Houghton, by following a footpath 
leading from the Bangor road and crossing the railway {, found some 
decomposed rock, probably of the same character; and then behind 
the northernmost buildings of ‘“‘ Pen-r-allt” a felsite, and on the 
hill above that a purplish quartz-felsite of the normal aspect (¢. 9. 
like that of Brithdir). He traced the felsite along this ridge; but 
after crossing a slight valley, and ascending another ridge over- 
looking the Straits, found again the Twt-Hiull rock ¢. 
He observed in a quarry by the road running up from “ Ferry 
House” a rather compact grey variety of the metamorphic rock, and 
at the N.W. end of it a green rock. A little green rock intercalated 
in the metamorphic occurs on Pen-r-allt on the northern side. The 
normal quartz-felsite occurs at the corner, ‘“ Pant yr fallan fach;” 
and to the N., behind Pant yr fallan, is a very pretty variety. 
Granitoid or gneissoid rock may be traced for some distance on 
the ieft-hand side of the main road to Caernarvon, where it runs 
parallel with the railway. This is here faulted against dark shaly 
Carboniferous rock. It may also be seen at the top of the steps 
behind the Arvon inn, between a school and chapel (felsite is ex- 
posed on the hill-side about a furlong to N.H. along the road); and 
rock of a gneissoid type occurs where the road (joined by that coming 
from the Ferry House) turns sharp to §.H., and by the side of a 
ravine in this latter direction. On the left bank of the same ravine 
it is again exposed, near an-y-maes ; but on crossing the stream 
we find a rotten felsite exposed near Tan-y-perthi, and a little 
further on, by a farmhouse, we are on a knoll of the characteristic 
quartz-felsite. To the 8. of Tan-y-maes, in a field, is a fine greyish 
bedded grit (sometimes brownish); some four yards or more in 
thickness are exposed, and the dip is about 30° to 8.E. This 
rock seems hardly more altered than many Cambrian grits; still, 
as it can be nearly matched from bands intercalated in some of the 
conglomerates, and rock of the granitoid type is exposed in the 
* On map 8,"N. of first “1” in Pengelli. 
+ Running between words “ bush” and “ inn” on map. 
{ The specimen is a gneissose rock, consisting of quartz, felspar, and a dark- 
green mineral, seen also at T'wt Hill, and on close examination shows indications 
of a fragmental structure. 
