If you listen to the music of the 1920s (for example), there's considerable crossover between: string bands of the time, jug bands, popular songs, country songs, vaudeville songs and early jazz. In one sense or another, it's all "old-time music" - it's all very much in the feel and the interpretation. What's also interesting is the crossover between black and white performances and performers. There are some fascinating compilations on some of the Roots label issues in the late 60s and early 70s - and I must now go and dig out some examples - songs like "Lindy", "Dallas Rag", etc.