"BLOOD MERIDIAN'' comes at the reader like a slap in the face, an affront that asks us to endure a vision of the Old West full of charred human skulls, blood-soaked scalps, a tree hung with the bodies of dead infants. But while Cormac McCarthy's fifth novel is hard to get through, it is harder to ignore. Any page of his work reveals his originality, a passionate voice given equally to ugliness and lyricism. Over the past 20 years the brutality of his subjects may have kept readers away, but the power of his writing has earned high critical repute. Three early novels, in fact - ''The Orchard Keeper,'' ''Outer Dark'' and ''Child of God'' - have been reissued in the Ecco Press series, ''Neglected Works of the Twentieth Century.''

This latest book is his most important, for it puts in perspective the Faulknerian language and unprovoked violence running through the previous works, which were often viewed as exercises in style or studies of evil. ''Blood Meridian'' makes it clear that all along Mr. McCarthy has asked us to witness evil not in order to understand it but to affirm its inexplicable reality; his elaborate language invents a world hinged between the real and surreal, jolting us out of complacency.

Loosely based on historical events, the novel follows a fictitious 14-year-old called only ''the kid'' - born in 1833, exactly 100 years before the author - as he drifts through the Southwest. He soon joins an outlaw band of Indian hunters who have been hired by a Mexican governor to return Apache scalps at $100 apiece. These misfits - including an ex-priest, a man with initials tattooed on his forehead and a mysterious, erudite judge named Holden - have a taste for blood and death that Mr. McCarthy seems to revel in.

Grotesque descriptions are alleviated by scenes that might have come off a movie screen. Indians pass through the novel like extras in a Fellini film, ''wardrobed out of a fevered dream with the skins of animals and silk finery . . . one in a stovepipe hat and one with an umbrella and one in white stockings and a blood stained weddingveil.'' The kid's terseness is a mild parody of B-movie westerns. Looking at a severed head, ''he spat and wiped his mouth. He aint no kin to me, he said.'' The horrifying details stick in our minds, however, while the surreal elements melt away. That imbalance is a problem, for Mr. McCarthy's emphasis is not on the violent set pieces but on the characters' reactions to them. The kid recedes into the background as the judge comes forward, in scene after scene sounding the novel's major themes and hinting at the author's strategy. Half-naked, the judge sits among the others by the fire ''like an icon'' and pontificates. One who observed a conflict between two enemies ''expressed the very nature of the witness and . . . was no third thing but rather the prime, for what could be said to occur unobserved?'' Pointing to the surrounding Indian ruins he announces, ''Here are the dead fathers'' against whom their descendants define themselves.