302 Prof. T. G. Bonney—Welsh Pre-Cambrian Rocks. 
distance in such exact succession? Further, this fault must bend 
round in a very odd manner, for I have followed the grit to a spot 
on the steep hill-side overlooking the above-named valley (to me 
the Survey Map, though less detailed, seems more correct here than 
that given by Prof. Hughes) where the grit is conglomeratic, as in 
some places near the entrance to Brithdir Farm. Again, on both 
sides of the fault, slate appears to succeed the grit. Next I have 
detected the peculiar agglomeratic rock of Tairffynnon in the field 
on the top of the hill, to the north.t. Of the identity of these rocks 
there can be no doubt, and their structure, full of lapilli of various 
kinds, and angular fragments of rhyolitic rock (obviously not 
transported from far, so that they might even be true volcanic 
deposits), differs very widely from that of any acknowledged 
Cambrian conglomerate which I have examined. All these I have 
studied microscopically, and this is one of the questions where such 
evidence is of high value. Further, I have now traced the peculiar 
_rock of Cae Seri as far as Bryn Dreiniog Farm, perhaps two-thirds of 
a mile, showing it to be a very persistent bed. This extensive 
group of rocks—distinguished microscopically by the presence of 
lapilli and rhyolitic fragments—which we find at intervals over the 
country” about Tairffynnon, Perfyddgoed, Cae Seri, and the ridge on 
the eastern side of the Carnarvon road as far as the Poorhouse, is far too 
thick to represent the true Cambrian conglomerate with its rounded 
pebbles of felsite and miscellaneous rocks, and, as above shown, is too 
constant in the succession of its varied members to be regarded as 
that conglomerate repeated by numerous faults. May we not also 
express a little surprise that these faults should have always so 
cleverly cut out the characteristic Bangor beds, over this wide area, 
and have simulated a true succession ? 
I pass now to the western side of the above-named fault. Here, as 
described, I can track the felsitic grit for a considerable distance, and 
have picked up in a little spinney (all but opposite to the junction 
of the Birthdir lane with the main road) a rock, which I think we 
may safely regard as representing part of the Tairffynnon series. 
Afterwards I confess I am utterly perplexed, though I do not find 
that Prof. Hughes’s arrangement helps me more than my own. On 
his theory we have all the Bangor series cut out. On mine the 
disappearance of almost everything above the purple felsite grit is 
no less perplexing. On both, the position of the thick bed of con- 
glomerate, extending from the east entrance of the western tunnel 
at Bangor to the back of Belmont House, is very anomalous. JI find 
a conglomerate, identical as it seems to me with this, exposed in a 
byway, near the gate of Gorphwysfa, leading down to a small 
farm, and on the other side of the felsite. More than once I have 
asked myself whether this conglomerate may not be identical with 
that of Brithdir? But whichever it be, its position is most per- 
plexing. This district seems to me to be completely broken up by 
faults, and far less regular than that on the eastern side of the 
1 On map, about half-way between 7 in Trawscanol and Zin Tairffynnon. 
® Part of that which Prof. Hughes admits he has not worked out, Quart. Journ, 
Geol. Soc. vol. xxxiv. p. 687; the key of the position as it seems to me. 
