320 BASE OF THE CAMBRIAN SERIES IN N.W. CABRNARVONSHIRE. 
be made; but still, comparing the two sections described above, 
where the first quarry on the Bangor road in conglomerate is 
probably near the horizon of the Tair-ffynnon group, and the Poor- 
house rock not very far away from some part of the Cae Seri, there 
seems a strong probability that we are not far wrong. The thick- 
ness of the series must be very great, unless there is considerable 
repetition by faults (and of this I have seen no evidence); it can 
hardly be less than 3000 feet, and is probably more. 
It is probable that the thickness of the sedimentary beds about 
Llyn Padarn and Moel Tryfaen (where the quartz-felsite must be 
very thick) 1s much less. The little which we now see of the beds 
between the felsite and the Cambrian conglomerate reminds us rather 
of the upper part of the Bangor group; and though doubtless some 
beds are cut out by the boundary fault, the impression which I 
formed on the ground was that the thickness of these was not very 
ereat*. If a volcanic district, such as Jorullo, for example, were 
slowly subsiding, the accumulating sediments would form so as to 
take a sort of cast of the igneous features, and be thickest where 
they were least prominent (and so thinnest). 
The quartz-felsite rocks, however, did not wholly disappear below 
water till after the formation of the Cambrian conglomerate7. 
Appearances suggest that there is a marked physical break between 
this and the subjacent sedimentary series (Pebidian of Dr. Hicks); 
but though I believe there is very great unconformity between the 
latter and the quartz-felsite in certain places, I think it quite possible 
that the subsidence may have begun, as would be natural, after the 
last volcanic outburst, and so the whole, notwithstanding the break, 
may really belong to one and the same series—the Pebidian. 
(For the Discusston on this paper, see p. 320.) 
EXPLANATION OF PLATE XIII. 
Fig. 1. Quartz-felsite from quarry near Cwm-y-Glo, showing flow-structure and 
quartz grains. 
Fig. 2. Part of a fragment of quartz-felsite, showing irregular fluidal structure 
&c., from the coarser conglomeratic rock of Tair-ffynnon (the blank 
part is a crack in the slice). 
Fig. 3. Fragment of quartz-felsite (a), showing fluidal structure, with quartz, 
slaty rock, and decomposed felspar, from rock at the back of the Poor- 
house near Bangor. 
Figs. 4 & 5. Portions of the finer conglomeratic rock of Tair-ffynnon, showing 
fragments of igneous rock (4, 0, c), quartz (d), &e. 
All these are magnified 50 diameters. 
* Since the above was written, I have visited Llanddeiniolen and there seen, 
near Fachell, quartz-felsite, felsitic grit, green slaty rock (slight exposure only, 
but apparently i st¢w) and Cambrian conglomerate in close sequence. 
+ I haye not at present identified them with certainty at any higher level. 
