In what seems to be a "floating" verse in a version of the song "Jim along Josie" the word bart appears.

Hitch the oxen to the cart
Drive to town and get a load of bart.   
Some was black and some was blacker
Some was the color of chaw tobacca.

Does anyone know what bart is? I note that Black Bart (Charles E Boles) the highway man was a bit of a wag. Is his name based on the same mystery substance?
My informant says he is sure it isn't bark.