The 2ist of August, 1859, was a red-letter Sunday in the Llanddetfiiolen district. On Monday morning the amazing news was blazed abroad that overpowering spiritual forces had descended the previous evening on every congregation in the neighbourhood, viz. those of Cefnywaun, Ebenezer, Dinorwic, Sardis, Rhydfawr, Rehoboth, and Llanrug, as an earthquake shock might simultaneously affect a group of villages in a territory three miles square.
After this, whenever one might go abroad, he would hear the rocks redoubling songs and supplications to the hills, and them to heaven. One told us he was passing through Llanrug at three in the morning and surprised to find the chapel lit up, he went in and found a congregation white-hot with flaming '' rapture," like a bush that burned with fire and was not consumed.
From, 'The '59 Revival', by J J Morgan, page 140.
Not 100% sure this is it.