My version of Peggy Gordon, copped, I think, from Helen Creighton's Nova Scotia collection a long time ago (see Sandy Paton's message above) has a slightly different line:

I will go down into Spencervania, Where the marble stones run black as ink...

Note those stones are running, and ink is liquid, too. As a native Pennsylvanian, anthracite all the way, I figure coal is a good guess...but I'm wondering whether it might not be oil, another "Spencervanian" gift to the world???

On the other hand, "Peggy Gordon" and its cousin "Sweet Primroses" are Nova Scotian, Irish and English by origin, so that could throw the question back toward Carrickfergus and clear across the Big Water.

Maybe the destination wasn't Pennsylvania originally but somewhere in Britain?

.... No firm answers. Just noodling on a line that's always been one of my favorite mysterious bits of crazy sense too.