Eloquent first novel in Wales; On the Black Hill, by Bruce Chatwin. New York: The Viking Press. 249 pp. $14.75

The author of that fine travel book ''In Patagonia'' (1977) has now produced an eloquent first novel, which encompasses a near-century of social and familial change within the story of a strange, limiting yet liberating symbiotic relationship.

Benjamin and Lewis Jones, identical twins, grow up on and never grow away from their family's farm in Wales, on the English border. Shaped and inhibited by their parents' helpless contention, they become mutually dependent. Benjamin is the sensitive introvert; Lewis, the sturdy, diligent soul who patiently abides his brother's clinging need for him. Through two world wars, illnesses, losses, and various relationships with neighbors, we observe how their ''cramped and frugal life'' nevertheless incarnates the biblical vision of that ''city on a hill'' which promises perfect contentment.

Essentially derivative (from Hardy, Lawrence, Arnold Bennett, Patrick White), this story is an extraordinary act of homage to a lost but not forgotten literary tradition.