"...As London is the market of England, to which the best of all things find their way, so Rome was the market of the Mediterranean world; but there was this difference between the two, that in Rome the articles were not paid for. Money, indeed, might be given, but it was money which had not been earned, and which therefore would come to its end at last. Rome lived upon its principal till ruin stared it in the face. Industry is the only true source of wealth, and there was no industry in Rome. By day the Ostia road was crowded with carts and muleteers, carrying to the great city the silks and spices of the East, the marble of Asia Minor, the grain of Africa and Egypt; and the carts brought nothing out but loads of dung. That was their return cargo...."
-- Winwood Reade, _The Martyrdom of Man_ (1872)

For "dung", to bring this up to date, read "Treasury bills".