164 A. R. Andrew—The Doigelliey Gold-belt. 
rigidity enough to support massive arches. The cavities formed 
would be smaller and less continuous, and this difference would be 
represented by the difference in the size of the greenstone sills that 
we see to-day. 
Clogau Slates.—These consist of a thickness of 250 feet of well- 
cleaved slates, dark-blue and black in colour. The streak obtained 
by scratching, however, is never really black, and as the beds in 
many places are greyish blue it seems difficult to justify the epithets 
‘jet-black’ and ‘intensely black’ applied by nearly all those who 
have previously referred to them. The slates weather to a dull 
crumbly ochreous yellow colour on the surface. Their dark internal 
colouring is due to organic matter and to iron sulphide, both being 
present in greater abundance than in the correspondingly black slates 
of the Pen Rhos Beds. On analysis I found that the Clogau Slates 
contained the following :— 
Organic matter 2 5 ; : : : : 5°78 
Tron in the form of sulphide c : : : ; 21°88 
Not determined ‘ : : : : : : 72°34 
100-00 
The Clogau Slates do not give rise to surface features, and are seen 
to advantage only in the stream courses, especially in the bed of the 
Afon Gamlan, above its confluence with the River Mawddach. 
Further up the Mawddach, close to the Tyddyn-Gwladys mine, 
they cut across the river and are well exhibited. Other good 
localities are found in the course of the Bontddu stream from the 
St. David’s Gold Mill to the Pont-ty-glan-afon; in the workings of 
the Clogau lode and on the Clogau Hill; and at Aber-Amffra Harbour 
close to Barmouth, where the beds are exposed on the roadside. 
The Clogau Slates are interleaved with numerous thin pyritous 
bands, usually about 4 inch in thickness, which are rarely discernible 
at outcrops on the hillside, but are quite plain in the stream-courses. 
When fresh these bands are but slightly paler in colour than the dark- 
blue slates with which they are bedded; but on weathering they turn 
white and show up very distinctly on the smooth water-worn beds of 
the streams. ‘The cause of this peculiar weathering is probably the 
much greater proportion of pyrites contained in the white bands. 
On weathering, the pyrites sets free ferrous sulphate and sulphuric 
acid, and the latter may dissolve out some of the coloured constituents 
of the slate. 
The Clogau Slates are interbanded with many sills of intrusive 
greenstone: a few of these are massive, ranging up to 100 feet in 
thickness, but the majority are much thinner, often as thin as 2 or 
3 feet. The strike of the thinner sills is irregular: they show 
a tendency to cross a few of the planes of bedding, and then resume 
their original course. 
Maentwrog Beds——These lie above the Clogau Slates and have 
a thickness of about 2700 feet: they consist of dark-grey, dark-blue, 
and black slates and flags. There occur throughout them numerous 
bands of fine-grained siliceous grit. In the upper division of the 
Maentwrog Group, these are present only as thin wavy white layers, 
