In the second installment of their seminal cross-cultural experiment, the Nitty Gritty Dirt Band move on from the roots of country music to its next generation. Contemporary voices like Levon Helm, Bruce Hornsby, John Prine, and John Hiatt join all-time greats like Jimmy Martin, Roy Acuff, Earl Scruggs, and Johnny Cash on a set of 20 songs that includes gospel and bluegrass traditionals as well as modern-day spins on Cajun, country, and folk. The musicianship is terrific, featuring Nashville pros like Jerry Douglas and the late great Vassar Clements, and there’s not a phoned-in vocal in the bunch. Bluegrass legend Jimmy Martin attacks “I’m Sittin’ on Top of the World” with his usual infectious enthusiasm, Helm has rarely sounded as soulful as on “When I Get My Rewards,” and John Hiatt’s duet with Rosanne Cash, “One Step Over the Line,” ranks with the best of both artists’ work. Heck, even John Denver (John Denver!) turns in a likable, understated performance on the minor country-radio hit “And So It Goes.” No one’s making history this time around, but Circle, Vol. Two forms a tuneful branch on the country/rock family tree.