MH 
BASE OF THE CAMBRIAN SERIES IN N.W, CAERNARVONSHIRE. 309 
25. On the Quarrz-FELsItE and AssociatEeD Rocks at the base of the 
CamBrian SeRres tn NortH-wesrern CanRNarvonsHire. By 
Rey. T. G. Bonnny, M.A., F.R.S., Sec. G.S., Professor of 
Geology at University College, London, and Fellow of St. 
John’s College, Cambridge. (Read February 5, 1879.) 
[Puatr XIIT.] 
Tuts subject has already been brought to the notice of the Society 
by communications from Prof. Hughes and Dr. Hicks, published in 
the last volume of the Quarterly Journal*. These vigorous and 
valuable sketches still leave many details, especially with regard to 
the great masses of quartz-felsite (or quartz-porphyry) in the vici- 
nity of Bangor, Caernarvon, and Llyn Padarn, to be filled in by 
future observers, so that I venture to hope the following notes may 
be useful as a small contribution to the subject. I spent several 
days in September 1878 in working carefully over parts of the 
district, and the conclusions formed in the field have been tested 
by subsequent study of a series of slides, prepared for me, as usual, 
by Mr. F. G. Cuttell. The result has been that I differ in several 
respects from the views recorded on the maps of the Geological 
Survey, and in the Memoir on the Geology of North Wales. 
But, while expressing a very decided difference of opinion on 
eertain points, I gladly bear testimony to the general excellence of 
the maps and the value of that Memoir. It needs but a few days’ 
work in North Wales to show how arduous a task the survey must 
have been. It should also be remembered that this was executed 
before the microscope was generally applied to lithological study. 
That has rendered clear much which was previously uncertain, and 
has given to the student a firm basis of ascertained facts wherefrom 
to educate his eye and his perceptions. Itis therefore not surprising 
that the officers of the Survey should have been occasionally misled 
by the superficial aspect of the rocks, when no more searching tests 
could be applied. 
As may be seen by a glance at the Survey Map, the Cambrian 
strata between the base of the mountains of North Wales and the 
Menai Straits appear to rest upon, or be cut off by, a great lenticular 
mass of quartz-felsitet, which extends in a north-east direction 
across the lower end of Llyn Padarn. A second and smaller massif 
is represented as extending from the town of Caernaryon to near 
* Vol. xxxiy. pp. 137 & 147. 
+ I was accompanied by several members of my geological classes, past and 
present, some of whom, especially Dr. R. D. Roberts, Mr. Houghton, and 
Mr. Marr, rendered me very valuable assistance; and on one occasion we had 
the advantage of being conducted by Prof. Hughes. Since the reading of the 
paper I have briefly revisited the district. 
t Called quartz-porphyry in the memoir. Notwithstanding the weighty au- 
thority in favour of this term, I think the objections to the word porphyry (on 
account of its vagueness) are so strong that I prefer to use quartz-felsite, 
Q.J.G.8. No. 188, ¥ 
