As mass production has to be accompanied by mass consumption, mass consumption, in turn, implies a distribution of wealth -- not of existing wealth, but of wealth as it is currently produced -- to provide men with buying power equal to the amount of goods and services offered by the nation's economic machinery. [Emphasis in original.]
Instead of achieving that kind of distribution, a giant suction pump had by 1929-30 drawn into a few hands an increasing portion of currently produced wealth. This served them as capital accumulations. But by taking purchasing power out of the hands of mass consumers, the savers denied to themselves the kind of effective demand for their products that would justify a reinvestment of their capital accumulations in new plants. In consequence, as in a poker game where the chips were concentrated in fewer and fewer hands, the other fellows could stay in the game only by borrowing. When their credit ran out, the game stopped.
-Marriner S. Eccles, Franklin D. Roosevelt's Chairman of the Federal Reserve from November 1934 to February 1948, detailed what he believed caused the Depression in his memoirs, Beckoning Frontiers (New York, Alfred A. Knopf, 1951).