HERE'S THE OBVIOUS : the Jay-Z-Linkin Park mash-up at Hollywood's Roxy last July and the corresponding studio recordings aren't the historic events MTV touts them to be. Less obvious is the fact that rock and rap have been colliding since hip-hop's earliest years. Legendary b-boy breakers like the Bronx's Rock Steady Crew got down to rock breaks, along with disco, funk, and Latin soul. In the 1980s, sociologically minded hip-hop record pioneer Afrika Bambaataa amused himself by catching avowed rock haters freaking out to cut-up Monkees, Mick Jagger, and Grand Funk Railroad.

For sheer visibility, pop influence, and middling innovation, however, Run-DMC and Aerosmith's remake of the latter's "Walk This Way" is probably the most relevant precursor to Jay-Z and Linkin Park's Collision Course (Warner Bros.). Both meld metal riffs, hard beats, full-throated rock choruses, and streetwise raps with just the right mix of predictability and panache to sell rebel pop by the millions.

But where Run and Aerosmith stirred rap's first mainstream audiences, Jay and Linkin promise to make their already gargantuan constituencies even larger. Where Run and Aerosmith were making a huge crossover leap and boosting a flagging career, respectively, Collision finds two of today's biggest Billboard-busters collaborating at the top of their game. Counting the million-some Collisions shipped since Nov. 30, the two combined have racked up more than 40 million sales. For better or worse, it might stand as the most salient rock-rap fusion for some time to come. Few will, but many should, consider how the years of intergeneric crossfire embodied by Linkin's 2000 LP, Hybrid Theory, preceded 2004's inescapable Collision with a million invisible sparks of mutual influence.

"When I first [heard Linkin vocalist Mike Shinoda] ... I was like, 'That guy has to be influenced by Rakim,' " Jay says in an interview included in Collision's CD-DVD package. Sort of – it depends on how well you can coach your ears into placing Shinoda's smooth, apocalyptically abstract rhymes and Rakim's even smoother, tenaciously concrete raps on the same spectrum.

The point is, there was a kernel of hip-hop in Linkin and of rock in Jay before their four-day project last summer. Shinoda aside, sample-happy DJ Joseph Hahn gave Linkin's occasionally claustrophobic industrial sound an airy, unpredictable edge. Admittedly no Dälek or M.O.P., to the loss of his own painfully monocultural career, Jay has made some forays into rockish territory ("99 Problems" with Rick Rubin, his apparent or unofficial approval of rocked-out Black Album mash-ups). Collision is less about two genres colliding for the first time than about the world's top rock-rap hybrid and rock-friendly rap hustler filling out the less-than-liminal space between them.

Whether it succeeds depends on how mashed-up you like your mash-ups (and, of course, the blockbuster singles Jay and Linkin purposely picked). Their self-professed goal was to out-mash the tech-savvy DJs mixing recent bootlegs like "Smells like Booty" (Nirvana versus Destiny's Child). The experiment works best when producer Shinoda's pairings successfully bounce Jay and Linkin tracks off each other. "Points of Authority/99 Problems/One Step Closer" plods compared to Rubin's original, but Linkin's unbelievably crunchy guitars and melodic screams smash all the right antiauthoritarian notes. Linkin angst deconstructs Jay's deadpan bravado on the album's best track, "Numb/Encore," the second verse of Jay's cool farewell falling away to Chester Bennington's crystalline chorus, which bespeaks the vulnerability and regret Jay can never admit ("Tired of being what you want me to be / Feeling so faithless, lost under the surface ...").

But such moments are few and far between. Like most real-life revolutions, Collision has brilliant flashes but ultimately falls short of the rhetoric, too often settling for self-satisfied juxtaposition and interpolation. "Big Pimpin'/Papercut" and "Izzo/In the End" leave Timbaland's and Kanye West's beats largely intact – a lazy oversight, considering how nicely Shinoda deployed warmed-up "Numb" organ stabs and guitars in lieu of the tinkling piano and trumpets of West's "Encore." Hearing Jay rap at 135 bpm on "Jigga What/Faint" and Bennington croon soulful hooks in major keys (instead of being his usual, alienated self in minor ones) is alternately exhilarating and moving. But in the end, the tracks are barely as sonically daring and thoroughly reimagined as last year's best mash-up, DJ Danger Mouse's The Grey Album.

Perhaps Jay and Shinoda weren't inspired enough to transcend their original material. Perhaps we shouldn't expect more from these masters of art as commerce, who make you feel square for pointing out the obvious. Maybe mash-ups are gimmicks best suited for commercial and party uses. The Grey Album, after all, treads the line between DJ composition and mash-up. Played for invite-only fans expecting straight-up Linkin or Jay sets, the live Roxy show on Collision's DVD makes the studio cuts seem cold and self-contained by comparison. It's gripping to see Linkin fans rap alongside or quietly appreciate Jay's lyrics, or Jay fans nod their heads to "Points of Authority."

"We mixed the music, but we also mixed the crowd," Shinoda says, as Jay jokes, "We saved the world that night." Not exactly – but it does reflect a mashed-up world that Bambaataa and hip-hop's founding fathers would have recognized, and smiled on, before reaching for the next beat.