Marshall Chapman's boots are dusty enough to look like they belong to the cowgirl she seems to be - but there's something wrong here. She's hustling her way through the new East Building of the National Gallery pivoting to stare up at the immense Calder mobile that churns overhead, jumping up and matter-of-factly declaring, "I WANT IT!"; then descending into the Dresden exhibit, shuffling through the rooms filled with jeweled artifacts and stopping dead in front front of Rubens' "Quos Ego!," a horse-laden depiction of Neptune commanding a storm to subside.

"Peter Paul," she moans. "You always understood us southern girls." And then, "Don't you just want to pat their noses? They're so cute."

The contradictions are what sum up this cowgirl who can spot a Rubens across a football field and then climb on a stage and wail rock 'n' roll like probably no other woman performing today. Her name and her looks are more masculine than feminine, and when Marshall Chapman straps her Telecaster across her back - step back. This is no Linda Ronstadt poking out from behind hot pants, no Joni Mitchell fawning over lost loves, Southern California-style, and not the kind of voice-only supercharge that Janis Joplin used to craze her audiences with. Chapman sings like a demonic Lauren Bacall, grinds her guitar like Keith Richards of the Rolling Stones, drives her fans into nuts-ball punk lunacy like Patti Smith.

And then she walks off stage, mutters in French, glances at her dainty and proper wristwatch and says, "I ain't read nothin' since 'National Velvet.'"

Marshall Chapman, at 29, has been bouncing around Nashville for about a decade now - first as a student at Vanderbilt (she majored in French and art history, befitting her blue-blooded Spartanburg, S.C., aristocracy), then as a cocktail waitress (fired for playing too much pinball), finally as a writer and performer. Her first album, "Me, I'm Feelin' Free," was decidely country in tone, was university praised but not bought, and spawned several cover versions of her tunes by Crystal Gayle, Jessi Colter and Olivia Newton-John ("My first royalty check," she says, "was for $2.21") A new one. "Jaded Virgin," is a heavily produced rock amalgam that doesn't capture the spunk of her live performances and, she says, "nearly gave me a nervou breakdwon when we were making it. Anyway, getting up on stage is what rock 'n' roll is all about - which is what Chapman will do tonight at the Childe Harold.

Chapman's act, considerably less contradictory than hr demeanor, is generally fullborn rock 'n' roll. Her band cranks up "Honky Tonk Woman," she struts out and picks up her electric guitar, and the five proceed to tear it up. Her music crackles with makes, say, the Stones or the Who so the kind of nasty intensity that exciting to watch and hear; her lyrics are sparse but direct; together they compose a kind of female rock that nobody else - except possibly Patti Smith, whom Chapman is sometimes compared with - is performing. Her best rocker, "Running Out in the Night," about what she calls " a half-night affair," has never been recorded. Another, called "Don't Make Me pregnant," she says she won't record. She performs both.

And then there's "Why Can't I Be Like Other Girls," from the new record:

Back in 1956 I was seven

And the second grade was goin' real slow

I could read and I could write but learning

To be white was nothing that I needed to know

'Cause I'd seen Elvis Presley

And I was runnin' round singin' the blues

And I remember the words my mama said

When I asked her for them blue suede shoes

She said to me

Why can't you be like other girls . . .

What makes Chapman different is the way she blends sentimentality with the sheer freneticism of rock. Unlike poet-turned-rocker Smith, she's not an arty lyricist; she'll make do with a line like, 'I lost my mind and I found you." There's a direct sensibility to her writing and her music.

"WhenI grow up," she says, "I think I want to be Mick (Jagger)."

Which is a far cry from Chapman's birthright - a family that for four generations has operated a huge cotton mill in her home town.It's a part of her past that she struggles to keep in perspective.

On the one hand, she says she was crying last Christmas in Los Angeles (where she was recording the new record) because "it was the first time I was away from home for Christmas." On the other hand, "You go back and your bedroom is the same and the sheets are turned down and . . . it's just wierd.

"There's something about me that's different. When my grand-daddy died he wanted me to take over the mill, not the boys. After college I was living in Boston with this guy and I had an A going into the final with him, and then I just decided I didn't want to live this little housewife life."

She's also tired of Nashville. "You do rock 'n' roll and they can't stand you."

"I think I'd like to live in New York," she says. "I like the energy there. The only nice thing about living in Nashville is that I'm on the seventh floor of an apartment building and I take the elevator down and there's a Chinese restaurant around the corner, just like New York."

She's sitting in the cafeteria under the East Building now, staring at the waterfall that architect I.M. Pei placed behind a window near the dining area.

"If I get rich," she says, "I'm gonna have Pei design a bedroom for me with a waterfall. I'll never have to take another Valium."