For AC/DC, too, it’s been a rugged season. Last year it was announced that rhythm guitarist Malcolm Young, 62, Angus’s brother, was suffering from dementia and could no longer continue in the band. Grotesque irony, that Malcolm, author of the most unforgettable riffs in rock and roll, should now be unable to remember them. Then Phil Rudd, the drummer whose disco-pistoned simplicity drove AC/DC from 1975 to 1983 and again from 1994 to 2014, was arrested last November and later convicted of drug possession and threatening to kill a former employee. Chris Slade is now on the drums, while Malcolm’s slot has been filled by his 58-year-old nephew, Stevie. (The Youngs are a clannish crew.) Stevie Young fits right in, presenting next to Angus a spectacle of withered and slightly vicious consanguinity. But Malcolm is irreplaceable: the huge, benign tensions he summoned on the fretboard of his Gretsch, the anti-chords called into being by his chopped super-chords, his grimly joyful face and grimly twitching body. Then again, replacing the irreplaceable is what AC/DC does. When Bon Scott, the original singer, died in 1980 at the age of 33, after choking on his own vomit in a parked car, it took the band mere weeks to hire Brian Johnson and start recording Back in Black.

As dusk falls and the air cools, the great bowl of the Gillette scintillates with the restless, insectile blink-blink of a thousand pairs of toy devil horns—tiny red lights, everywhere. You can get your horns at the concession stand, all part of AC/DC’s jolly postmodern diabolism: “Hell Ain’t a Bad Place to Be,” etc. And there on the big screen is Angus, with his pale, bare shins and his gibbering kneecaps and his head going up and down, up and down, in a contemplative frenzy, a 60-year-old man dressed like a schoolboy. He strikes his black Gibson, right arm lifting away from it in tribute. It takes a second for the sound to reach us, and in that time lag is the span of our adoration. This is the scalar difference between Motörhead and AC/DC: Lemmy has always spoken profoundly and poetically to his constituency of banged-up bikers and disaffectees; Angus is global.

But when it’s done—and it’s almost done—there will be no more Anguses, no more Lemmys. The bloody-minded, death-demolishing longevity of AC/DC and Motörhead cannot be counterfeited or repeated. Lemmy once roadie’d for Jimi Hendrix; these days, retiring postshow to his tour-bus bunk, he reads P. G. Wodehouse. His mic stand, for 40 years, has been higher than his head, microphone angled downward, the better to catch the heavy-metal plasma shooting from his larynx. And now he’s disappearing into the dark wings of the stage, taking with him his grave-digger wit and his gnashing bass and the gorgeous, ruinous momentum of his music.

You’re not supposed to go gentle into Dylan Thomas’s good night. Rage, rage, and all that. But it makes you gentle, the going. It takes your strength. The night after Salt Lake City, Motörhead canceled a show in Denver. In Austin, four days later, Lemmy left the stage after three songs. Lem-my! Lem‑my! I can hear the crowd, the lowing, forsaken mob, letting off steam and honoring his frailty. In the middle of all this, Motörhead’s new album, Bad Magic, was released. There’s a track on it called “Thunder and Lightning”: Stand on the stage, promises made / Under the blade, scratching and biting. How hard it must be, and what strange grandeur in the effort, to try to keep those promises. To stay standing, even as the blade comes down.