Sir Geraint Evans, the Welsh coal miner's son who became one of the world's leading baritones, died yesterday. He was 70. Sir Geraint died at Bronglais Hospital, in Aberystwyth, Wales, where he was admitted eight days earlier after a heart attack at his home in North Wales, the hospital said. He served as principal baritone at Covent Garden for 36 years, from his debut in 1948 until his emotional final performance in Donizetti's "L'Elisir D'Amore" June 4, 1984. He was best known for his portrayal of Verdi's Falstaff, which he first performed in 1957 at the Glyndebourne music festival in England.