ROCKS OF CAERNARVON. 693 
Twt-Hill series, and was of Cambrian age; the speaker inclined at 
present to the latter view. 
Prof. Bonnry observed that he had not committed himself to more 
than two series, and that the views which he had stated in his paper 
had been founded not only on field-work but also on the microscope. 
With regard to the minor points in Prot. Hughes’s paper, he thought 
there was no doubt evidence of fragmental structure in rock 
from the heart of the T'wt-Hill mass; he did not think that 
colour was of much importance in the classification of felsite; and 
he thought that the district N.K. and E. of Brithdir, which Prof. 
Hughes said he had not worked out, was the most important part. 
He thought that evidence, both in the field and with the microscope, 
showed the beds of the Tairffynnon district to be very different 
from the true Cambrian conglomerate; and that the grit from 
Fachell to Beulah Chapel was altogether different from that at 
Careg goch—the former being a felsite-grit, little altered, the latter 
a quartzite. Lastly, he maintained that the conglomerates in the 
grit near Twt Hill were totally different from the Cambrian con- 
glomerate, were much more highly altered, were in true sequence 
with the beds below, and, so far, older than the Cambrian con- 
glomerate. 
Prof. Hueuxs pointed out that Dr. Hicks had mistaken his views. 
In reply to Prof. Bonney he stated that he thought that the im- 
portance of working out a true sequence among the beds was so 
great as to warrant him in carefully studying that and the effect 
of faults. He had not neglected the microscopic characters of the 
rocks, and slices of all the more doubtful or important rocks were on 
the table; but his chief reliance had been placed on the behaviour 
of the great masses in the field. 
