326 METAMORPIIIC SERIES BETWEEN TWT HILL AND PORT DINORWIG. 
of rock as Pre-Cambrian was premature. He thought that there 
was evidence that the three series (the granitoid, the felsitic, and the 
volcanic) passed into one another. The sequence of these beds it is 
often very difficult to discover, on account of the difficulty of making 
out which of the divisional planes are due to original bedding and 
which to joints, as well as because of the numerous faults which 
traverse the district, which did not seem to have been taken into 
account. He criticised the views of Prof. Bonney as to the direc- 
tion of the dip of the beds seen near Caernaryon. 
Mr. Rourtey thought that the presence of fluidal structure might, 
in some eases, fail to afford decisive evidence as to the rocks haying 
flowed as lavas, since he thought it possible that such a structure 
might also be formed by a kind of convective action in the throats 
of voleanos. He expressed some doubt as to the sedimentary origin 
of hiilleflintas. He thought that the cryptocrystalline character 
of such rocks might be of secondary origin, and due to the passage 
through them of alkaline waters, as suggested by Mr. John Arthur 
Phillips. 
Mr. Davies regarded the hilleflintas as quite distinct from the 
felstones, as the former contained a larger proportion of silica and 
are of very different physical character. 
Mr. Ruttey, in reply, insisted on the widely variable characters 
displayed by the rocks classed together as felstone. 
Mr. Tawney doubted the advisability of coming a new formation 
to include the quartzites near Haverfordwest, and the qnuartz- 
felsites and grits near Bangor, until their relations had been worked 
out more in detail; the proofs of unconformity to rocks above and 
below he also regarded as dubious. 
Dr. Hicxs had proposed the names of the three series for con- 
venience’ sake. He could vouch for their being unconformable in 
many places, as was proved by the test of included fragments ; 
but at other points they might be conformable with one another. 
He pointed out, in opposition to Mr. Rutley’s view, that both Mr. 
Davies and Prof. Bonney agreed as to the distinction of the hilleflintas 
from the felstones. In reply to Mr. Tawney, he stated that the 
Arvonians were separated from the Dimetians below and the Pebi- 
dians above by great masses of breccias. 
Prof. Bonny replied to the remarks of Prof. Hughes that the 
rocks of the granitoid and felsitic series were very distinct from 
one another. He maintained the accuracy of the section at Twt 
Mill. He thought that the occurrence of derived fragments of the 
rocks in question in the overlying beds placed both the intrusive 
and metamorphic theories of the origin of the felsite series out of 
question. In reply to Mr. Rutley, while admitting that a kind of 
flow-structure might be found in sedimentary rocks, he thought 
that the rocks in question were certainly lavas. He thought that 
halleflintas are usually of sedimentary origin. 
