SERIES BETWEEN TWT HILL AND PORT DINORWIG. 325 
next field, it seems more probable that this belongs to the oldest 
series, and is not some faulted Cambrian. 
Mr. Houghton has traced the grit for some distance towards the 
S., beginning with this hill between Gareg goch and Tyddyn bach. 
It occurs at the 8.1. end of the slight knoll on which stand the 
Tan-y-maes cottages, and when traced round this in a N.W. direc- 
tion becomes felspathic, till it assumes the usual type mentioned 
above*. On the E. slope of a hill N. of a lane leading towards 
Griffith’s Crossing from Pen-Rhos farm, the quartzite occurs inter- 
bedded with a felspathic rock; and on the S.W. slope of the same 
hill is the granitoid rock, which also occurs in other places nearer 
to that railway station. 
It results, therefore, that this northern strip of ‘ quartz-por- 
phyry” consists of two very different groups of rocks, divided roughly 
by a line drawn on the map from the left side of the ‘* C” in Elim 
Chapel, between the two “‘n”’s in Bush Inn, between the “r” 
and “alt” in Pen-r-alt, and the “ y” and “ g” in Tan-y-grisrau, 
and probably continued across the patch in the same (S.W.) direc- 
tiont. To the N.K. of this division we have the quartz-felsite 
already described; to the S.W. a group of metamorphic rocks, 
which vary in character from quartzose conglomerates to granitoid 
rocks poor in mica, some of which have considerable resemblance to 
the so-called granite of Ty-Croes, Anglesey, and are probably very 
much older than the quartz-felsite. If there is no faulting, the 
Twt-Hill group is the highest of the metamorphic series, the dip, 
where observed, being approximately 8.S.K. As the length of the strip 
is about 41 miles, 2 of the whole area, coloured red on the map, 
belongs to the Pre-Cambrian (Dimetian) rocks; and the thickness 
of the series, if we may suppose there is no repetition by faults, can 
hardly be much less than 5000 feet. 
Discussion. 
The Presrpent had made microscopical sections thirty years ago 
of the rocks of Caernarvonshire, in illustration of the question of 
the origin of slaty cleavage. When he commenced his work, ridicule 
was thrown on the application of the microscope to the study of 
rocks ; but the papers of the evening had fully shown that this mode 
of study was now regarded in a very different light. 
Prof. Huenrs asked for better proof than had been given by Dr. 
Hicks that his three series are unconformable with one another, 
He thought that Dr. Hicks’s identification of some of the patches 
* The exact boundary will require re-examination at Tan-y-Maes, for I find 
that a specimen which, in the field, I had regarded as a variety of the granitoid 
group is really a spherulitic quartz-felsite, quite unlike, however, any that I 
haye yet scen in this district. It is remarkable how.similar the rocks of these 
two groups now and then are in the field, though under the microscope it 
would hardly be possible to imagine any two things more different.—T. G. B. 
+ Outcrops and pits appear to be wanting in the remaining part of this. A 
rock resembling rotten felsitic grit is just exposed on the road between Bethel 
and Llanddeiniolen rectory, and beyond Gors bach is true quartz-felsite. 
Q.J.G.8. No. 138. Z 
