The Mabinogion, tr. by Lady Charlotte Guest, , at sacred-texts.com
THE extraordinary property of producing rain, when spilt upon a stone, is attributed to the waters of Llyn Dulyn, in Snowdon, according to the following account, which is translated from the Greal, a Welsh Magazine, published in London, 1805.
"There is a lake in the mountains of Snowdon, called Dulyn, in a rugged valley, encircled by high steep rocks. This lake is extremely black, and its fish are deformed and unsightly, having large heads and small bodies. No wild swans are ever seen alighting upon it (such as are on all the other lakes in Snowdon), nor ducks, nor any bird whatever. And there is a causeway of stones leading into this lake; and if any one goes along this causeway, even when it is hot sunshine, and throws water so as to wet the furthest stone, which is called the Red Altar [yr Allawr Goch], it is a chance if it do not rain before night. Witness, T. Prys, of Plas Iolyn, and Sion Davydd, of Rhiwlas, in Llan Silin."