Keynes's rebuttal to the junk-mail-is-commerce hypothesis

The junk-mail industry defends its practices in a letter to the Smithsonian magazine by saying, that an $800 billion mailing industry that employs 9 million people isn't junk — it's commerce. Seth responds with this great gedankenexperiment from John Maynard Keynes:

If the Treasury were to fill old bottles with bank-notes, bury them at suitable depths in disused coal-mines which are then filled up to the surface with town rubbish, and leave it to private enterprise on well-tried principles of laissez-faire to dig the notes up again (the right to do so being obtained, of course, by tendering for leases of the note-bearing territory), there need be no more unemployment and, with the help of the repercussions, the real income of the community, and its capital wealth also, would probably become a good deal greater than it actually is.