A. R. Andrew—The Dolgelley Gold-belt. 165 
from + up to 2 inches thick. In the lower divisioa, however, they 
increase in individual thickness to 2 or 3 feet, and occur as hard ribs 
sufficiently resistant to form surface features, and numerous enough 
to make up 25 per cent. of the actual thickness of the formation, as it 
is exposed in the Bontddu Glen and in the bed of the Mawddach 
River near the Cefn-deuddwr Gold Mine. The very great pre- 
dominance of these ribs in the Lower Maentwrog is the lithological 
character which has been utilized by Lapworth and Wilson in 
separating the Maentwrog Beds into— 
(6) Pen Rhos Beds or Upper Maentwrog. 
(a) Vigra Beds or Lower Maentwrog. 
(a) Vigra Beds.—The Vigra Beds are characterized by the large 
proportion of siliceous grit bands. In hand-specimens these grits 
are very fine-grained, homogeneous in texture, and light grey in 
colour; they resemble in appearance a fine-grained quartzite, and the 
similarity is increased by their extreme hardness. 
Under the microscope it is seen that these bands are even and 
compact grits, the quartz grains of which are about 35 inch in 
diameter. The visible minerals are quartz, muscovite, and a little 
felspar and pyrites. The grains of quartz are but slightly rounded, 
and a few may show crystalline faces: the quartz constitutes quite 
95 per cent. of the total bulk of the rock. The microscopic structure 
of the flagey beds in the Vigra Group is similar to that of the grit 
bands, but the flakes of muscovite are much more numerous and are 
arranged more or less parallel to the bedding. 
The shales, slates, and flags with which the grit bands are inter- 
leaved are in the main similar in lithological characters to the slates, 
etc., in the overlying Pen Rhos Beds, but the harder ribs seem to 
a great extent to have protected the shales from cleavage, with the 
result that well-cleaved slates are seldom found. The colour of these 
interbedded shales and flags is dark grey and dark blue; the 
proportion of iron in them is somewhat less than in the Pen Rhos 
Beds, and the characteristic reddish weathering of the latter is 
generally wanting. Where exposed to atmospheric agencies the 
shales and flags “weather easily from the outstanding “grit bands. 
Many of the flag gy beds and a few of the grit bands ‘weather more 
readily along certain lines which are not visible in the unweathered 
specimen, and the weathered surface becomes covered with curling 
ridges and furrows, which rather resemble tattooing. The type 
locality for these Viera Beds is on Vigra Hill, which lies immediately 
north-west of Bontddu village. Good exposures are seen along the 
road which leads from Bontddu Post Office to the St. David’s Gold 
Mill, and in the Bontddu stream parallel to this road. Other good 
exposures are in the bed of the Afon Cwm-Mynach above the 
Cambrian cottage, and in the bed of the Mawddach River upwards 
from its junction with the Afon-Eden. 
(6) Pen Rhos Beds.—The Pen Rhos Beds are 1600 feet thick. They 
consist of shales and slates, always dark blue or black in colour; they 
are well cleaved, the cleavage usually being strong enough to efface 
the planes of bedding. They contain a few bands of siliceous grit, 
