166 A. R. Andrew—The Dolgelley Gold-belt. 
but compared with the Vigra Beds these are very rare indeed. 
Owing to the relative softness of the Pen Rhos Beds they do not 
form prominent surface features, except where hardened by contact 
with igneous intrusions. The dark colour is due to finely disseminated 
organic matter and pyrites, the latter of which weathers to iron oxide 
and gives the slates a characteristic red coloration on exposed 
surfaces. On fresh specimens of the slates, cubes of pyrites may be 
seen up to half an inch in size. On analysis these slates gave me 
the following :— 
Organic matter é ; é , - 5 c 3°90 
Iron in the form of sulphide .. : : . . 12°48 
Not determined ° : 3 ; : : ‘ 83°62 
100-00 
The Pen Rhos Beds are best seen on the Pen Rhos Hill opposite 
the Tynygroes Inn, 5 miles up the Mawddach Valley from Dolgelley. 
Numerous sections are also exposed in the road cuttings along the 
turnpike road from Llanelltyd to within 2 miles of Barmouth. 
Throughout the Maentwrog Group there are numerous igneous 
intrusions; these belong mostly to the so-called ‘ greenstone’ of 
H.M. Survey, but some of them are hornblende porphyries. They 
are mostly sills, with a great extent in strike and dip, but never 
more than 100 feet in thickness. 
Ffestiniog Group.—This group consists of over 38000 feet of grey 
and grey-green flags, with a few black shales and flags; they are 
hard and resistant and usually form fairly high ground. Among 
these grey and grey-green Ffestiniog Flags there are numerous 
harder ribs from 18 inches down to 1 inch and less in thickness. 
These are of two types: many of them are greywackes of medium 
grain; many are compact, fine-grained, and very siliceous grits, 
similar to those which constitute so large a proportion of the Vigra 
Beds described above. Where the shales and flags are dark-coloured 
they contain a large amount of iron, and their weathering causes 
the rock to assume reddish and dun colours. They are well seen 
along the road which runs past the mill of the Glasdir Copper Mine 
up the south bank of the Afon Las as far as Pont Llamyrewig. 
Good sections also occur in the workings and at the outcrops of the 
Glasdir Copper Mine, and the gold mine of Ffridd Goch. Many 
very massive igneous intrusions occur among the Lower Ffestiniog 
Beds, such as those forming the hills behind Penmaenpool, and 
also the heights of the Precipice Walk, Ffridd Goch, and Moel 
Cerniau. 
STRUCTURE. 
The Dolgelley Gold-belt lies on the east and south flanks of the 
well-known Harlech dome of Merionethshire. The great north and 
south sag between its two main arches, and which Lapworth and 
Wilson have followed from Moel Gedog, south of Talsarnau in a 
broken line through the Harlech uplands, continues into this country, 
and enters the Barmouth estuary close to the Caerdeon Vicarage. 
From the neighbourhood of Gwynfynydd southwards to near Llanelltyd, 
the general strike of the beds is about 10° east of north. From 
