ROCK / Talk about your evolution: Dinosaur Jr and Alice in Chains take to the stage

THE 'Junior' qualification was only added after a threat of legal action from a San Francisco outfit called The Dinosaurs, but it fits. J Mascis's coruscating trio is the vigorous young descendant of the dinosaur bands of yore. And it has a far stronger claim than its predecessors to be named after the thunder-lizards.

Outfits like these used to be known as 'power trios', possibly because their leaders were too megalomaniacal to tolerate the presence of a second guitarist. In later times, the pop song returned and the rhythm guitarist regained the ascendancy. Solo

antics were out; it was better to be a team player, to be circumspectly inventive, to support the ethos of the pop song. The model modern guitarist was Johnny Marr.

And now, just a few years later, the throwback horde gathers at the Brixton Academy, seeing the late winter of the century through in a blizzard of feedback. No shred of pop here; not a trace of the eternal formula - verse, chorus, verse, chorus, clever bit, verse, chorus - that served Johnny Marr and Johnny Mercer equally well. This audience has rediscovered the hardcore pleasures of rock; of volume, distortion, and the unexpected. It wants a sensation rarely experienced at a contemporary rock concert; not knowing what is coming next.

The scene is elemental. From the left, Mascis unleashes gales and gouges out shrieks; on the right, the gaunt figure of bassist Mike Johnson sways like a tree on the blasted heath. Unlike a conventional guitar star, Mascis uses sound to replace, rather than to delineate, his body image. His features invisible most of the time under a curtain of hair, he only speaks once throughout the entire performance, to growl 'Turn that goddam spotlight off'. Johnson is the group's visual figurehead; lean, lofty, and classically cheekboned. Despite the refinement of his build and features, there's a touch of the Addams Family's hulking servant Lurch about him.

Dinosaur Jr's music has its subtleties. The problem with it is essentially neurological. By the time feedback gives way to strumming, the auditory system has pulled in its horns, saving itself for the next attack. For a couple of Japanese teenagers,the assault was overwhelming: they crouched at the back, curled into foetal balls.

The rest of the crowd faced a dilemma: how to respond to a performance which evokes the awesomeness and caprice of natural force; a storm with its crescendos, lulls and cross-currents. You can't dance to it, as those at the front must have known in their hearts. You can't even bang your head to it, or tap your feet.

There is nothing to put you in synch with your neighbours. A Dinosaur Jr show, dominated by the supreme anti-poseur Mascis, is geared to make a crowd feel alone.

Magnificent as it intermittently is, Dinosaur Jr's music also cancels itself out. Although Mascis can summon up fantastic forces, he dispels them with an abrupt ending or a swerve sideways into a different register. In laying waste the conventional structures of rock songs, he destroys any sense of direction or resolution. He can do what he likes and he has the toys to do it with, but when boundaries vanish, a sense of purpose tends to evaporate along with them.

And as in Dinosaur Jr's music, so in the upbringing of a generation - a comparison to which Mascis himself has alluded. Whatever his own sense of creative purpose, his audience can draw comfort from a sonic experience which reaches the extremes of freedom, but gets nowhere. Home they go, socks on their heads, reassured that underachievement is actually an existential condition.

(Photograph omitted)