IV.—On the “Lingula Flags,” or “Festiniog Group” of the Dolgelly District

Extract

The accompanying section shows the succession of beds from the Harlech grits, near the sixth milestone on the road from Dolgelly to Trawsfynydd, across their strike in a south-easterly direction, to the lower ash-bed of the Arenig group at the farm of Blaenau, five miles north-east from Dolgelly. In this section all the beds occur in regular sequence, although they are invaded by many intrusive masses and dykes of diabase; only the larger of which are shown in my section, as it would be impossible, on so small a scale to depict the innumerable protrusions of this rock that occur in the district. The section crosses, at Moel-Cors-y-garnedd, the southern flank of the mountain Rhobell-fawr, which is the largest mass of diabase in Wales. All along the eastern side of Rhobell-fawr the strata are completely inverted, as shown in diagram, so that the Festiniog beds (5 and 6) lie above the more recent Dolgelly beds (7 and 8), from underneath which the Tremadoc beds (9) come up, overlying the beds of slates and ashes belonging to the Arenig Group (10 and 11).