Reports & Proceedings—Liverpool Geological Society. 91 
of the area, near Llangynnog, in the Tanat Valley, four bands of 
spherulitic rhyolite occur, associated with acid tufts (‘‘ Lower Tuffs’’). 
They occupy a stratigraphic position at the base of the Bala series, 
and are correlated with the Snowdonian lavas of the Capel Curig — 
Dolwyddllen group. 
On the extreme easterly margin andesitic lavas and tuffs have been 
extruded from local vents, of which remnants are recognized. They 
comprise a series of hornblende-porphyrite, pyroxene, hornblende, 
and enstatite-andesites, with tuffs, breccias, and agglomerates of 
similar composition. These lavas represent the latest phase of 
eruptive activity in the Berwyn area on or above the horizon of the 
Bala Limestone, and they are probably related to the andesitic lavas 
and tufts of the Breidden Hills. 
The oldest volcanic rocks of the region lie in the centre of the 
Berwyn anticline, near Llanrhaidr yn Mochnant (the Craig y Glyn 
group). They are thin flows of spherulitic rhyolites, interbedded 
with calcareous acid tuffs, and are petrographically similar to the 
Llangynnog rhyolitic series, but of much greater age. They are 
associated with fossiliferous Llandeilo sediments, but it is considered 
that they may possibly be rather of Upper Arenig age. 
The intrusive intermediate series occurs only in the north-eastern 
and south-western corners of the district, in the valleys of Glyn- 
ceiriog and the Hirnant respectively. In the former locality the sill 
has been injected into Bala slates and grits, and overlies the middle 
tuff band. It isa hemicrystalline, vesicular rock, a keratophyre or 
soda-trachyte, composed of oligoclase-andesine felspar, pyroxene, and 
_ secondary products. 
The second type forms a series of five intrusive bands, hitherto 
mapped as voleanic ash, in the much faulted tract south of Llangynnog. 
These soda-rich rocks are hemicrystalline with an original glassy 
base, composed of oligoclase-andesine plagioclase and subordinate 
"pyroxene, now represented by pseudomorphs and interstitial chlorite. 
The structure varies from variolitic to insertal. The rocks resemble 
certain tholeiites, but do not correspond exactly with any rocks 
hitherto described, and have accordingly been provisionally termed 
«« Hirnantite”’. 
On the northern and westerly borders and in the central part of the 
district a number of basic intrusions form well-marked sills in the 
areas characterized by great dynamic pressure. They are well 
exposed in the following localities: Pen-y-bont (Llansaintffraid D.C.), 
Spring Hill (Pandy Glynn), Cwm Dwyll, Carnedd-y-ci (Llandrillo), 
Cader Berwyn and Llyn-llyn-caws, and Miltir-gerig. They are 
dolerites in various stages of preservation, frequently bearing analcite, 
and agree in many respects with the basic sills of Carnarvonshire. 
They are, however, exceptionally interesting by the evidence of 
albitization which they present, a process which has not hitherto 
been recorded for the rocks of North Wales. In one instance 
(Carnedd-y-ci) albite-dolerite is associated with quartz-keratophyre 
under circumstances which appear to suggest an example of a 
composite intrusion. 
