CAERNARVONSHIRE AND ANGLESEY, 299 
range of mountains, are in some respects unlike the rocks already 
described, but yet clearly of that type and age, with the exception 
of some dykes which haye been intruded amongst them. The main 
masses are of a highly felspathic type, chiefly old rhyolitic lavas and 
breccias, which have undergone subsequent change. The rocks 
No. II., described by Professor Bonney in the Appendix, are from 
the Hill range. In the spur of the Hifl to the south-east felspathic 
breccias and schistose felspathic rocks occur. Near the centre horny- 
looking quartz-felsites of the Llyn-Padarn and Moel-Tryfaen type, 
showing a bedded appearance (perhaps only a parallelism of joint- 
ing), are seen interbedded with rocks of the hilleflinta type. 
Further westward quartz-felsites again prevail. The detached 
mass on the coast is a kind of fine- erained granite, and is supposed 
by Professor Bonney (Appendix, } No. III.) to be of igneous origin. 
Upper Cambrian rocks, chiefly of Tremadoc age, are faulted against 
these masses, and are in no cases altered except near the dykes of 
intrusive rocks (gabbros, &c.). Lower Tremadoc fossils (Wesewretus, 
&e.) were found in beds at Pont-rhyd-Goch, a mile north of Four 
Crosses. 
The rocks forming Nevin and Boduan mountains, as well as some 
masses near Pwllheli, and to the north and south of Llanfihangel 
Bachellaeth, are of the same type as those described from the Eifl 
range (vide Appendix, No.1Y., from Boduan mountain). They are 
in the main lava-flows, breccias, or hiilleflintas, and show no cha- 
racters indicative of intrusive rocks. ‘hey are highly metamor- 
phosed, and are traversed by many intrusive dykes ; but the flow- 
structure is frequently quite apparent in the field, and a bedded 
appearance commonly shown. ‘The surrounding sedimentary rocks 
are also quite unaltered in contact with them. ‘These, therefore, we 
may I think, without fear, also place with the Pre-Cambrian 
rocks, and in their general characters they seem allied to the Ar- 
yonian. Some few of the less altered brecciated rocks found here 
and there in near association with the quartz-felsites may belong to 
the Pebidian group. The rocks, No. V. in the Appendix, are from 
the mass to the east of Nanhoron; and though in the field they look 
almost as if bedded and intermediate in character between Dimetian 
and Arvonian rocks, yet on microscopical examination this view 
does not seem to be borne out, and Professor Bonney thinks they 
are of intrusive types. 
It is satisfactory to find, however, that undoubted Dimetian rocks 
ovcur not far south of this point, and that the great mass, called on 
the Geological Survey maps Rhos Hirwain syenite, is of this nature. 
Both in the field and under the microscope the Dimetian character 
of this rock is undoubted; and there can be no difficulty in recog- 
nizing at once that we have here an old ridge of these rocks such as 
is found at Caernarvon to the north and St. David’s to the south. 
Its position here also is most instructive, as bearing out our views 
expressed with regard to some of the other areas. 
In crossing the ridge from east to west, starting from Sarn 
Meyllfeyrn, after passing some masses of intrusive gabbros with 
