METAMORPHIC SERIES BETWEEN TW HILL AND PORT DINORWIG. 321 
26. On the Mutamorruic Surtus between Twr Hint (Cazrnarvon) and 
Port Dinorwie. 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, and F. T. S. Hoventon, Esq., 
B.A., Scholar of St. John’s College. (Read February 5, 1879.) 
Tus interesting group, which on the map of the Geological Survey 
is not divided from the “ quartz-porphyry,” has already been briefly. 
noticed by Prot. Hughes and Dr. Hicks*. They assert that the 
rock is not igneous, and lithologically much resembles the Dimetian 
of St. David’s. That view is fully borne out by our observations, 
which, we venture to think, prove conclusively that there is here a 
series of metamorphic rocks much older than the quartz-felsites 
and conglomerates described in the last paper ft. 
The rock exposed on the summit of I'wt Hill is at first sight 
very like a coarse granite, which, however, is almost without a 
Diagrammatic Section (reduced to one plane) of Pit to N.E. of summit 
of Twt Hill. 
= SSeS Jee Z Wi eS 
NS fo og eS, PRISM = IZ 1 aS 
oN A an 2 2.02 Wa Qin! _ LV HV lin 
Sy ads Gu a 
hig 
Z P Iny= =| = 
~\Ze ea) ag qs ee ‘N 
eee SS fRO.\ ZAI IES 
(AN =o 5770 08 9 ON Ne Nee ie 
S33 20% Ab 7 yeaa 
ks ik sas/ MS ME al 7 NLS 
FE E A 
A. Granitoid rock. D. Fine conglomerate. 
B. Same, becoming pebbly. EH. Coarse conglomerate. 
C. Coarse conglomerate. F. Finer conglomerate, in parts more 
like B and A. 
micaceous constituent. We saw nothing absolutely irreconcilable 
with an igneous origin; but still there was in the aspect of the 
rock a something, hard to define, which suggested a metamorphic 
origin. This is particularly noticeable by the wall on the southern 
side of the ridge. Under the microscope it is seen to consist of 
quartz, felspar (rather decomposed), a few granules and grains of 
iron peroxide, and a very small quantity of a micaceous mineral. 
The quartz is so full of minute enclosures as to have a rather 
“ dirty” aspect. A very closely twinned plagioclase felspar is 
* Quart. Journ. Geol. Soe. vol. xxxiv. pp. 142-152. 
+ Accompanied by the party named in the last paper, I examined the 
yieinity of Twt Hill last September, and discovered the conglomerates de- 
scribed below. Mr. Houghton, who remained at Bangor for some time after- 
wards, was able to return and traverse the ground between Caernaryon and Port 
Dinorwig, and to trace the boundary for some distance from that place between 
the quartz-felsite and the older rock, forwarding to me the specimens which he 
had collected. Since the reading of this paper, we have separately traversed 
again most of the ground described in it, revising the work, and making a few 
additional notes.—’. G. B. 
