How things become popular is never less than a mystery, and in the age of the post-genre ecosystem, of the YouTube sensation, what becomes popular isn’t nearly as legible as it once was. What was once fringe is now mainstream, what was mainstream is now fringe.  This is fraught territory for regular old rock music, which is the least tumultuous and least innovative of all genres (yes, even country). Hip-hop and dance are far more essential to the grammar of pop than rock has been for years — it is an atrophying relic, dying on the vine. And yet it persists, its young comers finding ways to thrive out of the new limelight. Down-the-center rock is the new underdog, steadily and studiously uncool, and maybe the last genre that can truly move in silence.  That’s certainly how it happened for Bastille, the British band fronted by Dan Smith that headlined at Webster Hall on Wednesday night. It’s experienced a sudden burst of success in recent months, largely on the strength of “Pompeii,” currently No. 10 on the Billboard Hot 100, as stealthy a pop hit as has been seen in recent memory.  Bastille is a relatively toothless, relatively harmless band that takes 1980s New Romantic impulses and softens them up considerably until they resemble the overblown British soft arena-rock of the 2000s.  “Pompeii,” which adds a touch of “So”-era Peter Gabriel, is somehow both cheery and stoic. It was the fourth single — in England, at least — from “Bad Blood” (Virgin), the debut Bastille album, which was released in this country in September.  A month later, St. Lucia released its debut album, “When the Night” (Neon Gold/Columbia). St. Lucia is the project of the South African singer-songwriter Jean-Philip Grobler, who takes his early and mid-1980s more literally than Mr. Smith does.  These are albums that landed with only the faintest thud, in a fashion that somehow obscured the bands’ actual popularity. Bastille took off in England last year, but more slowly here, and St. Lucia — without a hit to its name — played the first of four New York shows on Jan. 16 at the Music Hall of Williamsburg.  How the two bands translated their albums for the stage varied, and showed their differing ambitions. St. Lucia turned even more digital in its numbing set of faux new wave, its performance the quietest noise that can be made by five people striving to make a racket. Bastille was warmer, more expansive, and clearly indebted to the Coldplay school of plush anthems.  Which is to say that Bastille has its sights set on bigger things. “Bad Blood” is a sometimes catchy but ultimately fairly brittle album, and Mr. Smith is a singer with character in his voice but without much power. But he did his best during this show to seem larger: pounding away at a pair of floor toms for emphasis, and on one song, “Flaws,” walking through the crowd, engendering plenty of hero worship.  A pair of new songs were both more aggressive than those from “Bad Blood,” as if Mr. Smith understood that it would take more durability to survive at the higher altitude his new fame was landing him. (Bastille will perform on “Saturday Night Live” this Saturday.)  By contrast, St. Lucia is more narrowly focused. The writing is crisp but dull, and there is an intense similarity among the songs. In concert, St. Lucia approximated some of the unrestrained and grating glee of Passion Pit, and leaned heavily on drums straight out of the “Breakfast Club” soundtrack. The result sounded like a collection of the most polite Depeche Mode songs ever, though a couple of them — “We Got It Wrong” and “September” — had real brio.  Both of these bands force an understanding of success that has little to do with accomplishment. They’re derivative, lightweight and static, and that may have everything to do with why they’ve managed to find audiences.  They showed their hands, though, when performing R&B covers during their concerts. In both cases, the songs were utterly denuded, the life drained out of them — City High’s “What Would You Do?,” for Bastille, and Rufus and Chaka Khan’s “Ain’t Nobody,” for St. Lucia, sung as a duet with the woman handling the band’s sound. She won, handily.  A version of this article appears in print on January 25, 2014, on page C1 of the New York edition with the headline: Quietly Ascending Rock’s Peaks. Order Reprints|Today's Paper|Subscribe
In recent New York City concerts, Bastille and St. Lucia trod different, if equally familiar, musical routes on their way up.