Apologies if you consider this as thread drift but Steve Gardham's post at 03.33 sent me to my bookshelf and there I found The Penguin Book of American Folk Songs with notes by Alan Lomax.
On the first page it states the following;

"Unless otherwise stated the songs in this book are drawn from previously copyrighted sources and, as composite versions, have been adapted and arranged by Alan Lomax, by John A Lomax or by other editors. They may not be reprinted, recorded, or used for commercial purposes without the written permission of the authors, through Ludlow Music Inc., Columbus Circle, New York, or Essex Music Ltd, London".

One song included here is what I believe was the first song recorded by the Carter Family in 1927. In which case it would have been copyrighted by Ralph Peer and A.P Carter. It would therefore still have been in copyright I believe, Whether it was written by Carter is questionable I believe. To me without checking it against the Carter recording I cannot see how it has been adapted.