The trial workings are on the banks of the Afon Ogwen close to Coetmor Bridge at Bryn Bella. The adit on the west bank extends some 20 metres and the one on the east bank around 6 metres.
In his 1802 Observations of the Snowdon Mountains, William Williams noted:
In the body of this hill [Dinas] there are branches, or, as they are termed by the miners, strings of fine copper, and some lead ores: trials have been made there at different times, but they did not answer the expence.
These veins cross the river to Coetmor Demesne, where spirited trial was made between the years 1760 and 1770, and a good deal of ore, both copper and lead, dug out of the place, but not so much as enabled the venturers to pocket any profit. However, as there are so many strings of ore to be seen entering the hill close to the river side, it is not improbable but that a large quantity is somewhere lodged in the bowels of it.
And David Bick (2003) has the following to say:
Another mine of doubtful whereabouts is COED Y DINAS, the scene of spirited trials between 1760 and 1770 for copper and lead. According to Williams a good deal of both ores were raised, but without profit. Another author, presumably referring to the same site, gives a similar account. ‘About 1760 and later, attempts were made in Coed y Dinas for copper by Cornish miners, where candles were not extinguished night and day for seven years. Levels were made from the Ogwen to go under Coetmor’s land. Pits were made from these, which are now full of water… About 1802 Lord Penrhyn made attempts from the bed of the Ogwen on the same vein, going under Pen Dinas, but with little success’.
A likely area for the operation is the wooded bank of the river below Coetmor Bridge where a level was reported about 1810.
William Williams, Observations of the Snowdon Mountains (Google Books)
David Bick, The Old Copper Mines of Snowdonia, 3rd Edition, 2003, Landmark Publishing (ISBN 1843060752), p. 120.