Sometimes it's a shock to remember that John Denver was once one of the most popular recording artists in America, vying with the likes of the Eagles and Elton John for the most No. 1 hits in a given year and the most consistently successful albums. But that was a couple of musical generations ago - in the mid-1970s. The mainstream audience is capricious; tastes change.

Denver, on the other hand, hasn't. Much. As his new album, "The Flower That Shattered the Stone," demonstrates. Denver's folk-pop voice continues to be strong, clear and appealing; images from nature - panoramic vistas and noble beasts - still inhabit most every song; and the human spirit remains an important lyrical consideration.But something's on the wane. His sense of fun? Certainly there's no "Thank God I'm a Country Boy" or "Grandma's Feather Bed" here. His folk roots? We do get a few down-home-ish arrangements on the new album, but overall the music seems so . . . pristine. The sentiment's . . . measured. The song-to-song flow's rather . . . plain.

Which is not to say "The Flower That Shattered the Stone" lacks interest entirely. After the OK title song, the succeeding tracks show occasional variety. "Wish You Were Here (Postcard du Paris)," for instance, is a characteristically showy Jimmy Webb number, full of scene-setting description (Europe, in this case) and exuding Webb's palpable sense of loss. And just when you think Denver should folk things up a bit he does, moderately, with "High, Wide and Handsome."

And he follows that with the collection's epic, "Eagles and Horses (I'm Flying Again)." Familiar as the song's themes and images are, "Eagles and Horses," with full orchestra and a warbling flute, is grand.

"I had a vision of eagles and horses," Denver sings, "High on a ridge in a race with the wind/Going higher and higher and faster and faster/On eagles and horses I'm flying again."

Another fine song is "The Gift You Are," featuring a children's chorus. And at the end we get a reprise of the title song, with Japanese vocals at one point by Kosetsu Minami. John Denver must still have a loyal following in Japan, and he's tipping his hat.

DAN FOGELBERG; "The Wild Places" (Full Moon-Epic); produced by Dan Fogelberg. * * *

Fellow nature-loving Coloradan Dan Fogelberg revives and updates his rock-folk-pop style rather more successfully than Denver on his varied and enjoyable new album, "The Wild Places."

Those who've heard his remake of the old Cascades hit "Rhythm of the Rain" will testify that this is top-notch Fogelberg. "Rhythm" itself sways with an appealing Memphis arrangement starring sax, horns and surprisingly blue-eyed-soulful vocals. And as the song winds down we hear familiar phrases from the Beatles' "Rain."

But really, the gently soulful "Rhythm of the Rain" is not typical of "The Wild Places." The album opens with a synthesizer-and-percussion instrumental, "Aurora Nova," which segues into the title track, a paean to mountain wilderness. In fact, keyboards and synthesizers set the early aural mood of the collection, through songs like "Forefathers" (a companion to the Fogelberg hit "Leader of the Band") and "Anastasia's Eyes," a lovely tribute to a loved one.

Besides "Rhythm of the Rain," "Lovers in a Dangerous Time" seems most atypical for Fogelberg, with a forceful rock beat and an insistent semi-gruff vocal delivery. What's also unusual for this well-known singer-songwriter is that "Lovers" is another cover, written by Bruce Cockburn.

The tastefully diverse songs on "The Wild Places" range from romantically delicate ballads to spirited rockers and downright apocalyptic warnings. Although flavored by occasional guest woodwinds, percussives (including congas and Indian drums) and harmony vocals, several of the tracks are virtual solo creations, with Fogelberg handling keyboards, guitars and computerized drums as well as the singing. In short, this is vintage Fogelberg with a few inventive twists to get - and keep - our fickle attention.