316 T. G. BONNEY ON THE BASE OF THE 
nized. Besides these are fragments, generally rather irregular in 
form, of a slaggy aspect like a glass (devitrified), rendered more or 
less opaque by opacite, which I have no doubt are of volcanic origin. 
The matrix in which the above are imbedded is sometimes rather 
streaky, composed probably of minute fragments of felspar crystals 
and felspathic mud, now considerably altered, and sometimes inter- 
spersed with opacite and filmy patches of viridite. The quartz 
grains are sometimes crushed by the pressure which the rock has 
undergone, and the fragments of included rock are often rather 
flattened and distorted. Some, however, are distinctly rounded ; and 
all have probably been transported py water, though many are still 
fairly angular. 
In one of the specimens from the thickest part of the conglomerate, 
on the east side of the lake, I observed a part of a pebble which 
struck me as bearing a great resemblance to the peculiar rock of 
Twt Hill, Caernarvon*. This I have examined microscopically, and 
feel no doubt of the identity. This identification is of great interest, 
as it shows that some area of this granitoid rock was undergoing 
denudation, and was approximately in its present mineral condition, 
when the Cambrian conglomerate was being formed. 
The matrix and some of the included fragments of the slides from 
the conglomerate between the greenstone dyke and the quartz-felsite 
are rather more altered than the rest; but the changes are more of 
the nature of those brought about by water and pressure at moderate 
temperatures than any thing like “ melting down.” At any rate the 
clastic character of the rock is still perfectly distinct. 
Bangor District. 
Proceeding now to the eastern edge of the northern mass of quartz- 
felsite, and commencing near Brithdir Farm7, we find exposed in 
the road and elsewhere a grit composed of well-worn fragments of 
quartz-felspar and quartz-felsite. It is generally of a mottled 
purplish-red colour (very like the neighbouring quartz-felsite) with 
grains about 5), inch in diameter, but becomes in places conglome- 
ratic, with subangular pebbles up to 1 inch diameter. It weathers 
a reddish-buff colour, and one knoll in a field close to the road has 
a certain superficial resemblance to the rock of Twt Hill. The rock 
clearly derived its materials from the adjoining quartz-felsite. We 
find a similar, but rather finer and more quartzose grit exposed 
above the quartz-felsite on the northern side of the Bangor-Caer- 
narvon road in the adjoining valley, and again, to the north, behind 
Tafn-Newydd Farm, and another adjoining the felsite exposed in the 
road about half a mile to the N.E. by “ Beulah chapel”t. The last 
I haye examined microscopically, and found it confirm the opinion 
stated above as to the source of its material, as it made up of 
* See next paper. Prof. Hughes also found a similar pebble in the vicinity 
of Llanddeiniolen, Joc. cit. p. 140. 
+ Just south of the first letter “x” in that name on the map. 
t A similar pale grit also oyerlies the felsite near Llanddeiniolen. 
