When Tracy Chapman sang one of her seven really solid, polished songs at the Merriweather Post Pavilion last night, she sounded like a major talent. On those numbers, her full-toned, grainy alto stood up to her folk-soul models -- Joan Armatrading and Van Morrison -- and her arrangements combined the intimacy of folk music with the forcefulness of rock. Unfortunately, she had to reach beyond those seven gems to fill out her 20-song, 80-minute show, and the drop-off in quality was dramatic.

Chapman's weaker material tends to sound much the same as the melodies flatten out and the lyrics lapse into generalized assertions. By contrast, the stronger songs like "Across the Lines" or "Behind the Wall" boasted distinctive rhythm figures and definite stories. Chapman's backing quartet built "Talkin' 'Bout a Revolution" from a folkie protest song into a rock-and-roll anthem, and Charles Mims added jazzy piano fills around Chapman's flamenco guitar figure for "This Time."

She performed 18 of the 21 songs on her first two albums, adding only a new 12-bar blues number and a calypso-jazz arrangement of Sam Cooke's "A Change Is Gonna Come." Nothing, though, matched the astonishing accomplishment of "Fast Car," which she did with just her voice and guitar.

"We're a South African rock band," Johnny Clegg announced during the opening set by his group, Savuka. Though the band proved quite adept at Zulu warrior dances, township jive chants and soukous guitar fills, Clegg's interracial septet digested these influences and recast them with the impatient, everything-up-for-grabs attitude that defines rock-and-roll.

Clegg and Savuka didn't merely add some synths and English lyrics to an indigenous music; they transformed Zulu township music as thoroughly as the Byrds transformed Appalachian ballads and as Ray Charles transformed gospel, creating a music of future possibilities rather than past traditions.